Medical Lad

Cell Physiology । Medical Lad

2020-08-05T08:22:00.004+05:30

Structure of the cell

INTRODUCTION

Cells are basic units of life. All living organisms are com- posed of many blocks of cells. Each single has all the characteristics of life. Cell is defined as structural and functional unit of the living body. The space outside the cell is called extracellular space.

General Characteristics of Cell

Each cell in the body:

1. Needs nutrition and oxygen.

2. Produces its own energy necessary for its growth, repair and other activities.

3. Eliminates carbon dioxide and other metabolic wastes.

4. Maintains the medium, i.e, the environment for its survival.

5. Shows immediate response to the entry of invaders such as bacteria or toxic substances into the body.

6. Reproduces by division. There are some exceptions such as neuron, which do not reproduce.

TISSUE

Tissue is defined as the group of cells having similar function. There are many types of tissues in the body. All the tissues are classified into four major types which are called the primary tissues.

Primary Tissues

1. Muscle tissue: Skeletal muscle, smooth muscle and cardiac muscle.

2. Nervous tissue: Neurons and supporting cells.

3. Epithelial tissue: Squamous, columnar and cuboidal epithelial cells.

4. Connective tissue: Connective tissue proper, cartilage, bone and blood.

ORGAN

An organ is defined as the structure that is formed by two or more primary types of tissues, which execute the functions of the organ. Some organs are composed of all the four types of primary tissues.

Types of Organs

Organs are of two types, namely tubular or hollow or- gans and compact or parenchymal organs. Some of the organs in the body are brain, heart, lungs, stomach, intestine, liver, gallbladder, pancreas, kidneys, endocrine glands, etc.

SYSTEM

The organ system is defined as group of organs that work together to carry out specific functions of the body. Each system performs a specific function. Digestive system is concerned with digestion of food particles. Excretory system eliminates unwanted substances. Cardiovascular system is responsible for transport of substances between the organs. Respiratory system is concerned with the supply of oxygen and removal of carbon dioxide. Repro- ductive system is involved in the reproduction of species. Endocrine system is concerned with growth of the body and regulation and maintenance of normal life. Musculo- skeletal system is responsible for stability and movements of the body. Nervous system controls the locomotion and other activities including the intellectual functions.

STRUCTURE OF THE CELL

Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane. Cell body has two parts, namely nucleus and cytoplasm sur- rounding the nucleus.

Thus, the structures of cell are of three categories:

l. Cell membrane.

II. Cytoplasm.

III. Nucleus.

CELL MEMBRANE

Cell membrane

Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or plasma- lemma. This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). The cell membrane is a semipermeable membrane. So, there is free exchange of certain substances between ECF and ICF. Thickness of the cell membrane varies from 75 to 111A.

COMPOSITION OF CELL MEMBRANE

Cell membrane is composed of three types of substances.

1. Proteins (55%).

2. Lipids (40%).

3. Carbohydrates (5%).

STRUCTURE OF CELL MEMBRANE

On the basis of structure, cell membrane is called a unit membrane or a three-layered membrane. Electron microscopic study reveals three layers of cell Major membrane, namely, one central electron-lucent layer and two electron-dense layers. The two electron-dense layers are placed one on either side of the central layer. The central layer is a lipid layer formed by lipid sub- stances. The other two layers are protein layers formed by proteins. Cell membrane contains some carbohydrate molecules also. 2. "Danielli-Davson model' was the first proposed basic accepted by scientists for many years. This model was basically a 'sandwich of lipids' covered by proteins

Structural Model of the Cell Membrane

1. Danielli-Davson model

Danielli-Davson model was the first proposed basic model of membrane structure. It was proposed by James F Danielli and Hugh Davson in 1935. And it was on both sides.

2. Unit membrane model

In 1957, JD Robertson replaced 'Danielli-Davson model' by 'Unit membrane model' on the basis of electron mi- croscopic studies.

3. Fluid mosaic model

Later in 1972, SJ Singer and GL Nicholson proposed The fluid mosaic model'. According to them, the membrane is a fluid with mosaic of proteins (mosaic means pattern formed by arrangement of different colored pieces of stone, tile, glass or other such materials). This model is accepted by the scientists till now. In this model, the pro- teins are found to float in the lipid layer instead of forming the layers of the sandwich type model.

Lipid Layer of the Cell Membrane

Lipids of the cell membrane

The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell.The materials dissolved in lipid layer also move to all areas of the cell membrane.

Major lipids:

1.Phospholipids

2.Cholesterol

1. Phospholipids

Phospholipids are the lipid substances containing phosphorus and fatty acids. Phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion. repelled by water (hydrophobic). Head portion is the polar end and it is soluble in water and has strong affinity for water (hydrophilic). Tail portion is the non-polar end. It is insoluble in water and Two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. Hydrophilic head portions of outer layer face the ECF and those of the inner layer face ICF mi- (cytoplasm).

2. Cholesterol

Cholesterol molecules are arranged in between the phospholipid molecules. Phospholipids are soft and oily lipids in the membrane. So, cholesterol is responsible for structures and cholesterol helps to 'pack' the phospho- the structural integrity of lipid layer of the cell membrane.

Functions of Lipid Layer in Cell Membrane

Lipid layer of the coll mombrane is a semipermeable membrane and allows only the fat-soluble substances to pass through it. Thus, the fat-soluble substances like oxygen, carbon dioxido and alcohol can pass through this lipid layer. Tho water-soluble substances liko glucose, urea and electrolytes cannot pass through this layer.

Protein Layers of the Cell Membrane

Protein layers of the cell membrane are electron-dense layers. These layers cover the two surfaces of the contral lipid layer. Protein layers give protection to the central lipid layer. The protein substances present in those layers are mostly glycoproteins.

Protein molecules are classified into two categories:

1. Integral proteins or transmembrane proteins.

2. Peripheral proteins or peripheral membrano proteins.

1. Integral proteins

Integral or transmembrane proteins or intrinsic proteins are the proteins that pass through entire thickness of cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane.

2.Peripheral proteins

Peripheral proteins or peripheral membrane proteins or extrinsic proteins are the proteins, which aro partially embedded in the outer and inner surfaces of the cell mem- brane and do not penetrate the cell membrane. Peripheral proteins are loosely bound with integral proteins or lipid layer of cell membrane. So, these protein molecules dis- sociate readily from the cell membrane.

Types of peripheral proteins

Peripheral proteins are of two types:

i. Intracellular peripheral proteins, which are located on inner surface of cell membrane. Examples are receptors, transport proteins and some enzymes.

ii. Extracellular peripheral proteins that are situated on outer surface of cell membrane. Examples are anti- gens, adhesion proteins and some enzymes.

Functions of Proteins in Cell Membrane

1. Integral proteins provide the structural integrity of the cell membrane.

2. Channel proteins help in the diffusion of water-solu- ble substances such as glucose and electrolytes.

3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport.

4. Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane.

5. Receptor proteins serve as the receptor sites for hor- mones and neurotransmitters.

6. Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane.

7. Some proteins act as antigens and induce the pro- cess of antibody formation.

8. Some proteins called cell adhesion molecules are responsible for attachment of cells to their neighbors or to basal lamina.

Carbohydrates of the Cell Membrane

Some of the carbohydrate molecules present in cell mem- brane are attached to proteins and form glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids. Carbohydrate molecules form a thin and loose cover- ing over the entire surface of the cell membrane called

Functions of Carbohydrates in Cell Membrane

1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell.

2. Glycocalyx from the neighboring cells helps in the tight fixation of cells with one another.

3. Some carbohydrate molecules function as the recep- tors for some hormones.

FUNCTIONS OF CELL MEMBRANE

1. Protective Function- Cell membrane protects the cytoplasm and organelles present in the cytoplasm.

2.Selective Permeability- Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier to other substances.

3.Absorptive Function- Nutrients are absorbed into the cell through the cell membrane.

4.Excretory Function- Metabolism and other waste products from th cell are excreted out through the cell membrane.

5.Exchange of gases-Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane.

6. Maintenance of shape and size of cell- Cell membrane is responsible for the maintenance of shape and size of the cell.

Read more about cell - Organelles Of Cell And Gene

Source K Sembulingam's physiology

Kidney Anatomy

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Internal structure of the kidneys seen on a dissected specimen

INTRODUCTION

The closely packed structure and numerous functions of the kidney illustrate the beautiful workmanship of our creator. It not only applies to the kidney but to each and every part of our body.

Synonyms

The kidney are also called renes from which we have the derivative renal and nephros from which we have the terms nephron, nephritis, etc

Definition

Kidneys are a pair of excretory organs located on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum. They remove waste products of metabolism and excess of water and salts from the blood, and maintain its pH.

Location

Location of the kidneys

The kidneys occupy the epigastric, hypochondriac. lumbar and umbilical regions. Vertically they extend from the upper border of twelfth thoracic vertebra to the center of the body of third lumbar vertebra. The right kidney is slightly lower than the left, and the left kidney is a little nearer to the median plane than the right.

The transpyloric plane passes through the upper part of the hilus of the right kidney, and through the lower part of the hilus of the left kidney.

Shape, Size, Weight and Orientation

Position of the kidneys from anterior aspect

Each kidney is about 11 cm long, 6 cm broad, and 3 cm thick. The left kidney is a little longer and narrower than the right kidney. On an average the kidney weighs 150 g in males and 135 g in females. The kidneys are reddish brown in color.

The long axis of the kidney is directed downwards and laterally, so that the upper poles are nearer to the median plane than the lower poles. The transverse axis is directed laterally and backwards.

In the foetus the kidney is lobulated and is made up of about 12 lobules. After birth the lobules gradually fuse, so that in adults the kidney is uniformly smooth. However, the evidence of foetal lobulation may persist.

EXTERNAL FEATURES

Each kidney is bean-shaped. It has upper and lower poles, medial and lateral borders, and anterior and posterior surfaces.

Two poles

The upper polé is broad and is in close contact with the respective suprarenal gland. The lower pole is pointed.

Two Surfaces

The anterior surface is said to be irregular and the posterior surface flat, but it is often difficult to recognize the anterior and posterior aspects of the kidney by looking at the surfaces. The proper way to do this is to examine the structures present in the hilum as described below.

Two Borders

The lateral border is convex. The medial border is concave. Its middle part shows a depression, the hilus or hilum.

Hilum

The following structures are seen in the hilum from anterior side to posterior side.

1 The renal vein

2 The renal artery

3 The renal pelvis, which is the expanded upper end of the ureter.

Examination of these structures enables the anterior and posterior aspects of the kidney to be distinguished from each other. As the pelvis is continuous inferiorly with the ureter, the superior and inferior poles of the kidney can also be distinguished by examining the hilum. So it is possible to determine the side to which a kidne belongs by examining the structures in the hilum. Commonly, one of the branches of the renal artery enters the hilus behind the renal pelvis, and a tributary of the renal vein may be found in the same plane.

RELATIONS OF THE KIDNEYS

The kidneys are refroperitóneal organs and are only partly covered by peritoneum anteriorly.

Relations Common to the Two Kidneys

1 The upper pole of each kidney is related to the corresponding suprarenal gland. The lower poles lie about 2.5 cm above the iliac crest.

2 The medial border of each kidney is related to:

a. The suprarenal gland, above the hilus, and

b. To the ureter below the hilus.

3 Posterior relations: The posterior surfaces of both kidneys are related to the following.

a. Diaphragm

b. Medial and lateral arcuate ligaments

c. Psoas major

d. Quadratus lumborum

e. Transversus abdominis

f. Subcostal vessels

g. Subcostal, iliohypogastric and ilioinguinal nerves.

In addition, the right kidney is related to twelfth rib, and the left kidney to eleventh and twelfth ribs.

4 The structures related to the hilum have been described earlier.

Other Relations of the Right Kidney

Posterior relations of the right kidney

Anterior Relations

1 Right suprarenal gland

2 Liver

3 Second part of duodenum

4 Hepatic flexure of colon

5 Small intestine.

Out of these the hepatic and intestinal surfaces are covered by peritoneum. The lateral border of the right kidney is related to the right lobe of the liver and to the hepatic flexure of the colon.

Other Relations of the Left Kidney

Vertical section through the posterior abdominal wall showing the relationship of the pleura to the kidney

Anterlor Relations

1 Left suprarenal gland

2 Spleen

3 Stomach

4 Pancreas

5 Splenic vessels

6 Splenic flexure and descending colon

7 Jejunum.

Out of these the gastric, splenic and jejunal surfaces are covered by peritoneum. The lateral border of the left kidney is related to the spleen and to the descending colon.

CAPSULES OR COVERINGS OF KIDNEY

Transverse section through the lumbar region showing the capsules of the kidney

The Fibrous Capsule

This is a thin membrane which closely invests the kidney and lines the renal sinus. Normally it can be easily stripped off from the kidney, but in certain diseases it becomes adherent and cannot be stripped.

Perirenal or Perinephric Fat

This is a layer of adipose tissue lying outside the fibrous capsule. It is thickest at the borders of the kidney and fills the extra space in the renal sinus.

Renal Fascia

The perirenal fascia was originally described as being made Posterior layer was called fascia of Zuckerkandal and anterior layer as fascia of Gerota. These two fasciae fused laterally to form lateral conal fascia. According to this view, lateral conal fascia continued anterolaterally behind colon to blend with parietal peritoneum. But lately it has been researched that the fascia is not made up of fused fasciae, but of a single multi- laminated structure which is fused posteromedially with muscular fasciae of psoas major and quadratus lumborum muscles. of two separate layers. dn The fascia then extends anteromedially behind the kidney as bilaminated sheet, which divides at a variable point into thin layer which courses around the front of

kidney as anterior perirenal fascia and a thicker posterior layer which continues anterolaterally as the lateral conal fascia.

It was believed earlier that above the suprarenal gland the anterior and posterior perirenal fasciae fuse with each other and then get fused to the diaphragmatic fascia, but research presently demonstrates that superior aspect of perirenal space is "open" and is in continuity to the bare area of liver on the right side and with subphrenic extraperitoneal space on the left side.

On the right side at the level of upper pole of kidney, anterior fascia blends with inferior coronary layer and bare area of liver. On the left side, anterior layer fuses with gastro- phrenic ligament.

Posterior layer on both right and left sides fuses with fasciae of muscles of posterior abdominal wall, i.e. psoas major and quadratus lumborum as well as with fascia on the inferior aspect of thoracoabdominal diaphragm. Medially the anterior layer is continuous from one to the other kidney and the posterior layer is attached either side of vertebra. Below both the layer extend along the ureter and fuse with iliac fascia.

Pararenal or Paranephric Body (Fat)

It consists of a variable amount of fat lying outside the renal fascia. It is more abundant posteriorly and towards the lower pole of the kidney. It fills up the paravertebral gutter and forms a cushion for the kidney.

STRUCTURE

Naked eye examination of a coronal section of the kidney shows:

1 An outer-reddish brown cortex.

2 An inner-pale medulla.

3 A space-the renal sinus

The renal medulla is made up of about 10 conical masses, called the renal pyramids. Their apices form the renal papillae which indent the minor calyces.

The renal cortex is divisible into two parts.

a. Cortical arches or cortical lobules, which form caps over the bases of the pyramids.

b. Renal columns, which dip in between the pyramids.

Each pyramid along with the overlying cortical arch forms a lobe of the kidney.

The renal sinus is a space that extends into the kidney from the hilus. It contains:

a. Branches of the renal artery.

b. Tributaries of the renal vein.

c. The renal pelvis.

The pelvis divides into 2 to 3 major calyces, and these in their turn divide into 7 to 13 minor calyces. Each minor calyx (kalyx = cup of A flower) ends in an expansion which is intended by one of three renal papillae.

Structure of Uriniferous Tubule

Each kidney is composed of one to three million uriniferous tubules. Each tubule consists of two parts which are embryologically distinct from each other. These are as follows:

The excretory part, called the nephron, which elaborates urine. Nephron is the functional unit of the kidney, and comprises:

a. Renal corpuscle or malpighian corpuscle (for filtration of substances from the plasma) made up of glomerulus (Latin ball) a tuft of capillaries and Bowman's capsule.

b. Renal tubule (for selective resorption of substances from the glomerular filtrate) made up of the proximal convoluted tubule, loop of Henle with its descending and ascending limbs, and the distal convoluted tubule.

The collecting part begins as a junctional tubule from the distal convoluted tubule. Many tubules unite together to form the ducts of Bellini which open into the minor calyces through the renal papillae. Juxtaglomerular apparatus is formed at the vascular pole glomerulus which is intimately related to its own ascending limb of the Henle's loop near the distal convoluted tubule. The apparatus consists of:

a. Macula densa, formed become altered cells of the distal convoluted tubule.

b. Juxtaglomerular cells, formed by the epithelioid cells in the media of the afferent arteriole.

c. Some agranular cells between macula densa and the glomerulus proper.

VASCULAR SEGMENTS

A coronal section through the kidney showing the naked eye structure including the blood supply of the kidney

The renal artery gives 5 segmental branches, 4 from its anterior division and one from its posterior division.

The segments are apical, upper, middle and lower on anterior aspect. On posterior aspect segments seen are posterior and parts of apical and lower segments.

Blood Supply of Kidney

The blood supply of kidney shown belowt:

Abdominal aorta at L2 level - renal artery-5 segmental arteries-each segmental artery-lobar artery-interlobar artery-arcuate artery-interlobular artery-afferent arteriol-glomerulus-efferent arteriols-peritubular plexus-interlobular vein-arcuate vein-interlobar vein-lobar vein-segmental vein-5 segmental veins-renal vein- (IVC) inferior vena cava

Lymphatic Drainage

The lymphatics of the kidney drain into the lateral aortic nodes located at the level of origin of the renal arteries (L2).

Nerve Supply

The kidney is supplied by the renal plexus, an off shoot of the coeliac plexus. It contains sympathetic (T10-L1) fibres which are chiefly vasomotor. The afferent nerves of the kidney belong to segments T10 to T12.

EXPOSURE OF THE KIDNEY FROM BEHIND

In exposing the kidney from behind, the following layers have to be reflected one by one:

1 Skin

2 Superficial fascia

3 Posterior layer of thoracolumbar (lumbar) fascia with latissimus dorsi and serratus posterior inferior

4 Erector spinae, which can be removed for convenience

5 Middle layer of thoracolumbar fascia

6 Quadratus lumborum

7 Anterior layer of thoracolumbar lumbar fascia in which the related nerves are embedded.

HISTOLOGY

Histology of the kidney

The cortex of kidney shows cut sections of glomeruli, many sections of proximal convoluted tubule, some sections of distal convoluted tubule and a few collecting ducts.

Section through the pyramid of the medulla shows light staining collecting ducts, sections of loop of Henle, thick and thin segments of descending and ascending limbs, capillaries and connective tissue.

Nine Regions Of Abdomen ( Anatomy )

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INTRODUCTION Abdominal cavity is the largest cavity. It encloses the peritoneal cavity between its parietal and visceral layers. Parietal layer clings to the wall of parieties while visceral layer is intimately adherent to viscera concerned. So their vascular supply and nerve supply are same as the parieties and viscera respectively. There are very lengthy organs in the abdominal cavity. These had to be disciplined with limited movements for proper functioning of the gut in particular and the body in general. Infections involving the parietal peritoneum impart protective "board-like rigidity" to the abdominal wall. Referred pain from the viscera to a dístant area is due to somatic and sympathetic nerves reaching the same spinal segment.

NINE REGIONS OF ABDOMEN

For the purpose of describing the location of viscera, the abdomen is divided into nine regions by four imaginary planes, two horizontal and two vertical. The horizontal planes are the transpyloric and transtuber- cular planes. The vertical planes are the right lateral and the left lateral planes. The pelvic cavity lies below and posterior to the abdominal cavity. The transpyloric plane of Addison passes midway between the suprasternal notch and the pubic symphysis. It lies roughly a hand's breadth below the xiphisternal joint. Anteriorly, it passes through the tips of the ninth costal cartilage; and posteriorly through the body of vertebra L1 near its lower border.

Organs present on this plane are pylorus of stomach beginning of duodenum, neck of pancreas and hila of the kidneys. The transtubercular plane passes through the tubercles of the iliac crest and the body of vertebra L5 near its upper border. The right and left lateral planes correspond to the midclavicular or mammary lines. Each of these vertical planes passes through the midinguinal point and crosses the tip of the ninth costal cartilage. The nine regions marked out in this way are arranged in three vertical zones, median, right and left. From above downwards, the median regions are epigastric, umbilical and hypogastric. The right and left regions, in the same order, are hypochondriac, lumbar and iliac. Position of many organs is mentioned in Table 18.1. Liver chiefly occupies the right hypochondrium. Stomach and spleen occupy the left hypochondrium. Duodenum lies in relation to posterior abdominal wall. Coils of jejunum and ileum fill up the umbilical, lumbar and iliac regions. Large intestine lies at the periphery of abdominal cavity, caecum, ascending colon on right side, descen- ding colon on left side and transverse colon across the cavity. The pelvic cavity lies posteroinferiorly between pubic symphysis anteriorly and concavity of sacrum including coccyx posteriorly.

PERITONEUM

The peritoneum (Greek stretched over) is a large serous membrane lining the abdominal cavity. Histologically, it is composed of an outer layer of fibrous tissue, which gives strength to the membrane and an inner layer of mesothelial cells which secrete a serous fluid which lubricates the surface, thus allowing free movements of viscera.

The peritoneum is in the form of a closed sac which is invaginated by a number of viscera. As a result, the peritoneum is divided into:

a. An outer or parietal layer.

b. An inner or visceral layer;

c. Folds of peritoneum by which the viscera are suspended.

The peritoneum which is a simple cavity, before being invaginated by viscera becomes highly compli- cated.

Parietal Peritoneum

1 It lines the inner surface of the abdominal and pelvic walls and the lower surface of the diaphragm. It is loosely attached to the walls by extraperitoneal connective tissue and can, therefore, be easily stripped.

2 Embryologically, it is derived from the somato- pleuric layer of the lateral plate mesoderm.

3 Its blood supply and nerve supply are the same as those of the overlying body wall. Because of the somatic innervation, parietal peritoneum is pain sensitive.

Visceral Peritoneum

1 It lines the outer surface of the viscera, to which it is firmly adherent and cannot be stripped. In fact it forms a part and parcel of the viscera.

2 Embryologically, it is derived from the splanchno- pleuric layer of the lateral plate mesoderm.

3 Its blood supply and nerve supply are the same as those of the underlying viscera. Because of the autonomic innervation, visceral peritoneum evokes pain when viscera is stretched, ischaemic or distended.

Ureter Anatomy

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Ureter

Introduction

The ureters are a pair of narrow, thick-walled muscular tubes which convey urine from the kidneys to the urinary bladder.

They lie deep to the peritoneum, closely applied to the posterior abdominal wall in the upper part, and to the lateral pelvic wall in the lower part.

DIMENSIONS

Each ureter is about 25 cm (10 inches) long, of which the upper half (5 inches) lies in the abdomen, and the lower half (5 inches) in the pelvis. It measures about 3 mm in diameter, but it is slightly constricted at five places.

Course

General course of the ureter in the pelvis

The ureter begins within the renal sinus as a funnel- shaped dilatation, called the renal pelvis. The pelvis issues from the hilus of the kidney, descends along its medial margin, or partly behind it. Gradually it narrows till the lower end of the kidney where it becomes the ureter proper.

The ureter passes downwards and slightly medially on the tips of transverse processes and the psoas major muscle, and enters the pelvis by crossing in front of the termination of the common iliac artery.

In the lesser or true pelvis, the ureter at first runs downwards, and slightly backwards and laterally. following the anterior margin of the greater sciatic notch. Opposite the ischial spine, it turns forwards and medially to reach the base of the urinary bladder.

Normal Constrictions

Constrictions in the course of ureter

The ureter is slightly constricted at five places.

1 At the pelviureteric junction.

2 At the brim of the lesser pelvis.

3 Point of crossing of ureter by ductus 'deferens or broad ligament of uterus.

4 During its oblique passage through the bladder wall.

5 At its opening in lateral angle of trigone.

Relations

Renal Pelvis

In the renal sinus, branches of renal vessels lie both in front and behind it.

Outside the Kidney

Anteriorly

On the right side, there are the renal vessels and the second part of the duodenum. On the left side, there are the renal vessels, the pancreas, the peritoneum and the jejunum.

Posteriorly

Psoas major muscle.

Abdominal part of ureter

Anteriorly

On the right side, the ureter is related to:

Anterior relations of the abdominal part of the right ureter

1 Third part of the duodenum

2 Peritoneum

3 Right colic vessels

4 lleocolic vessels

5 Gonadal vessels

6 Root of the mesentery

7 Terminal part of the ileum .

On the left side, the ureter is related to:

Anterior relations of the abdominal part of the left ureter

1 Peritoneum

2 Gonadal artery

3 Left colic vessels

4 Sigmoid colon

5 Sigmoid mesocolon.

Posteriorly

Relations of the ureter to the transverse processes of lumbar vertebrae and the ischial spine

The ureter lies on

1 Psoas major

2 Tips of transverse processes

3 Genitofemoral nerve.

Medially

On the right side there is the inferior vena cava; on the left side, there is the left gonadal vein, and still further medially, the inferior mesenteric vein.

Pelvic Part of Ureter

Relations of the pelvic part of the ureter

In its downward course the relations are as follows.

Posteriorly

1 Internal iliac artery

2 Commencement of the anterior trunk of the internal iliac artery

3 Internal iliac vein

4 Lumbosacral trunk

5 Sacroiliac joint.

Laterally

1 Fascia covering the obturator internus

2 Superior vesical artery

3 Obturator nerve

4 Obturator artery

5 Obturator vein

6 Inferior vesical vein

7 Middle rectal artery

8 In the female, it forms the posterior boundary of the ovarian fossa.

In its downward and forward course, the relations of ureter are different in males and females.

In Males

Posterior view of the male urinary bladder showing the relations of the ureter to the vas deferens and the seminal vesical

1 Ductus deferens crosses the ureter superiorly from lateral to medial side.

2 Seminal vesicle lies below and behind the ureter.

3 Vesical veins surround the terminal part of ureter.

In Females

Anterior view of the uterus and vagina showing the relations of the ureter to the uterine artery, the cervix of the uterus, and the vagina

1 The ureter lies in the extraperitoneal connective tissue in the lower and medial part of the broad ligament of the uterus.

2 Literine artery lies first above and in front of the ureter for a distance of about 2.5 cm and then crosses it superiorly from lateral to medial side.

3 The ureter lies about 2 cm lateral to the supravaginal portion of the cervix. It runs slightly above the lateral for six of the tugina.

4 The terminal portion of the ureter lies anterior to the tugina.

Intravosical Part

The intravesical oblique course of the ureter has a valvular action, and prevents regurgitation of urine from the bladder to the ureter. The ureteric openings lie about 5 cm apart in a distended bladder, and only 2.5 cm apart in an empty bladder.

Blood supply

The ureter is supplied by three sets of long arteries:

1 The upper part receives branches from the renal artery. It may also receive branches from the gonadal, or colic vessels.

2 The middle part receives branches from the aorta. It may also receive branches from the gonadal, or iliac vessels.

3 The pelvic part is supplied by branches from the vesical, middle rectal, or uterine vessels. The arteries to the ureter lie closely attached to peritoneum. They divide into ascending and descending branches which first form a plexus on the surface of the ureter, and then supply it.

The upper and middle parts receive branches from medial side, while the pelvic part gets its arterial supply from lateral side.

Nerve Supply

The ureter is supplied by sympathetic from T10 to L1 segments and parasympathetic from S2 to S4 nerves. They reach the ureter through the renal, aortic and hypogastric plexuses. All the nerves appear to be sensory in function.

HISTOLOGY

Histology of the ureter

Ureter is composed of:

1 The innermost mucous membrane.

2 Middle layer of well developed smooth muscle layer.

3 Outer tunica adventitia.

The epithelial lining is of transitional epithelium. Muscle coat in upper two - thirds has inner longitudinal and outer circular fibers. The lower one - the third comprises an additional outer longitudinal layer. Connective tissue forms the outer layer.

Cell Junctions Physiology

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Different cell junctions

DEFINITION

Cell junction is the connection between neighboring cells or the contact between the cell and extracellular matrix. It is also called membrane junction. Connection between two cells is called intercellular junction. Tight junction, gap junction, adherence junc- tion and desmosome are intercellular junctions. Contact between the cell and extracellular matrix are focal adherence and hemidesmosome.

CLASSIFICATION

Cell junctions are classified into three types:

1. Occluding junctions.

2. Communicating junctions.

3. Anchoring junctions.

OCCLUDING JUNCTIONS

Cell junctions which prevent intercellular exchange of substances are called occluding junctions, i.e. these junctions prevent the movement of ions and molecules from one cell to another cell. Tight junctions belong to this category.

TIGHT JUNCTION

Tight junction is the intercellular occluding junction that prevents the passage of large molecules. It is also called zonula occludens. It is the region where the cell mem- branes of the adjacent cells fuse together firmly. This type of junction is present in the apical margins of epi- thelial and endothelial cells in intestinal mucosa, wall of renal tubule, capillary wall and choroid plexus.

Structure of Tight Junction

Tight junction is made up of a ridge which has two halves. One half of the ridge is from one cell and an- other half is from the other cell. Both halves of the ridge fuse with each other very tightly and occupy the space between the two cells. Each half of the ridge consists of tight junction strands.

Proteins of Tight Junction

1. Tight junction membrane proteins or integral mem- brane proteins, such as occludin, claudin and junc- tional adhesion molecules (JAMS).

2. Scaffold (framework or platform) proteins or peri- pheral membrane proteins or cytoplasmic plaque proteins such as cingulin, symplekin and ZO-1, 2,3. Tight junction membrane protein molecules are anchored in the strands of the ridge and attach with their counterparts of neighboring cell, so that both the cells are held together. The scaffold (platform) proteins are attached with the tight junction membrane proteins and strengthen the anchoring in the ridges.

Functions of Tight Junction

1. Strength and stability

Tight junction holds the neighboring cells of the tissues firmly and thus provides strength and stability to the tissues.

2. Selective permeability (gate function)

Tight junction forms a selective barrier for small mole- cules and a total barrier for large molecules. In the epithelial and endothelial cells, tight junction is the most apical intercellular junction, which functions as selective (semipermeable) diffusion barriers between the neighboring cells. This function is called barrier or gate function. Barrier function of tight junction regulates the interchange of ions, water and varieties of macromol- ecules between the cells. The magnitude of this function varies in different tissues. In some epithelial cells, few substances pass through the tight junction (by diffusion or active transport). In other cells, no substance passes through the tight junction.

3. Fencing function

Tight junction prevents the lateral movement of proteins (integral membrane proteins) and lipids in cell membrane and thus acts as a fence. The fencing function maintains the different composition of proteins and lipids between the apical and basolateral plasma membrane domains. Because of this function, the tight junction is sometimes referred as impermeable junction.

4. Maintenance of cell polarity

Fencing function of the tight junction maintains the cell polarity by keeping the proteins in the apical region of the cell membrane.

5. Blood-brain barrier

Tight junction in the brain capillaries forms the blood- brain barrier (BBB), which prevents the entrance of many substances from capillary blood into brain tissues. Only lipid-soluble substances like drugs and steroid hormones can pass through the blood-brain barrier.

APPLIED PHYSIOLOGY

Diseases caused by mutation of genes encoding proteins of tight junction are listed below:

(Tight junction abnormally)

Cause

1. Mutation of encoding proteins in the genes of tight junction

2. Bacterial and viral infections

Consequences

1. Hereditary deafness

2. Ichthyosis (scaly skin)

3. Sclerosing cholangitis (inflammation of bile duct causing obstruction)

4. Hereditary hypomagnesemia (low level of magnesium in the blood)

5. Synovial sarcoma (soft tissue cancer)

COMMUNICATING JUNCTIONS

Cell junctions which permit the intercellular exchange of substances are called communicating junctions, i.e. these junctions permit the movement of ions and mol- ecules from one cell to another cell. Gap junction and chemical synapse are the communicating junctions.

GAP JUNCTION

Gap junction

Gap junction is the intercellular junction that allows pas- sage of ions and smaller molecules between the cells. It is also called nexus, It is present in heart, basal part of epithelial cells of intestinal mucosa, etc.

Structure of Gap Junction

Membranes of the two adjacent cells lie very close to each other and the intercellular space is reduced from the usual size of 2,5 to 3 nm, Cytoplasm of the two cells is connected by the channels formed by the membranes of both cells. So, the molecules move from one cell to another cell directly through these channels, without hav- ing contact with extracellular fluid (ECF).

Each channel consists of two halves. Each half belongs to one of the two adjacent cells. Each half of the channel is surrounded by 6 subunits of proteins which are called connexins or connexons.

Functions of Gap Junction

1. Diameter of the channel in the gap junction is about 1.5 to 3 nm. So, the channel permits the passage of glucose, amino acids, ions and other substances, which have a molecular weight less than 1,000.

2. It helps in the exchange of chemical messengers between the cells.

3. It helps in rapid propagation of action potential from one cell to another cell.

Regulation of the Diameter of Channels in Gap Junction

In the gap junctions, diameter of each channel is reg- ulated by the intracellular calcium ions. When the concentration of intracellular calcium ion increases, the protein subunits of connexin surrounding the channel come close to each other by sliding. Thus, the diametor of the channel decreases. The diameter of the channel is also regulated by pH, electrical potential, hormones or neurotransmitters.

CHEMICAL SYNAPSE

Chemical synapse is the junction between a nerve fiber and a muscle fiber or between two nerve fibers through which the signals are transmitted by the release of chem- ical transmitters.

APPLIED PHYSIOLOGY

Mutation in the genes encoding the connexins causes diseases, which are given below:

Cause

Mutation of gap junction proteins, particularly connexins (Gap junction abnormally)

Consequences

1. Deafness

2. Keratoderma (thickening of skin on palms and soles)

3. Cataract (opacity of lens in eye)

4. Peripheral neuropathy (damage to the nerves of peripheral nervous system)

5. Charcot-Marie-Tooth disease (a form of neuropathy)

6. Heterotaxia (abnormal arrangement of organs or parts of the body in relation to left-right symmetry)

ANCHORING JUNCTIONS

Anchoring junctions are the junctions that provide strength to the cells by acting like mechanical attach- ments, i.e. these junctions provide firm structural attachment between two cells or between a cell and the extracellular matrix. Anchoring junctions are responsible for the structural integrity of the tissues and are present in the tissues like heart muscle and epidermis of skin, which are subjected to severe mechanical stress.

The firm attachment between two cells or between a cell and the extracellular matrix is provided by either actin filaments or the intermediate filaments. Depend- ing upon this, anchoring junctions are classified into four types:

1. Actin filament attachment:

i. Adherens junction (cell to cell).

ii. Focal adhesion (cell to matrix).

2. Intermediate filament attachment:

iii. Desmosome (cell to cell).

iv. Hemidesmosome (cell to matrix).

ADHERENS JUNCTION

Adherens junction is the cell to cell junction, which connects the actin filaments of one cell to those of another cell. In some places like epithelial linings, this junction forms a continuous adhesion (zonula adherens) just below the tight junctions. In adherens junction, the membranes of the adjacent cells are held together by some transmembrane proteins called cadherins.

Adherens junction provides strong mechanical attachments of the adjacent cells. Adherens junction is pre- sent in the intercalated disks between the branches of cardiac muscles. During the contractions and relaxation of heart, the cardiac muscle fibers are held together tightly by means of this junction. The ad- herens junction present in epidermis helps the skin to withstand the mechanical stress.

FOCAL ADHESION

Focal adhesion is the cell to matrix junction, which connects the actin filaments of the cell to the extracel- lular matrix. In epithelia of various organs, this junction connects the cells with their basal lamina. The trans- membrane proteins, which hold the cell membrane and the matrix are called integrins.

DESMOSOME

Desmosome is a cell to cell junction, where intermedi- ate filaments connect two adjacent cells. Desmosome is also called macula adherens. The membranes of two adjacent cells, which oppose each other, are thickened and become spot-like patches. Intermediate filaments are attached with the thickened patches. Some of these filaments are parallel to the membrane and others are arranged in radiating fashion. Desmosomes function like tight junctions. The transmembrane proteins involved in desmosome are mainly cadherins.

HEMIDESMOSOME

Hemidesmosome is a cell to matrix junction, which con- nects the intermediate filaments of the cell to the ex- tracellular matrix. This type of cell junction is like half desmosome and the thickening of membrane of only one cell occurs. So, this is known as hemidesmosome or half desmosome. Mostly, the hemidesmosome connects the cells with their basal lamina.

CELL ADHESION MOLECULES

DEFINITION

Cell adhesion molecules (CAMS) or cell adhesion proteins are the protein molecules situated on the cell surface.

FUNCTIONS OF CELL ADHESION MOLECULES

1. CAMS are responsible for the binding of cells to their neighbors or to basal lamina (or basal mem- brane).

2. CAMS form the important structures of intercellular connections and are responsible for structural organization of tissues.

TYPES OF CELL ADHESION MOLECULES

Cell adhesion molecules are classified into four types:

1. Cadherins

Cadherins form the molecular limbs between neighbor- ing cells. These CAMS form adherens junction and des- mosome.

2. Integrins

Integrins form the focal adhesion and hemidesmosome.

Check out cell also

Source K Sembulingam's Physiology

Large Intestine Anatomy

2020-07-30T09:59:00.000+05:30

Large Intestine

Features

The large intestine extends from the ileocaecal junction to the anus. It is about blind 1.5 m long, and is divided into the caecum (Latin blind pouch), the ascending colon, right colic flexure, the transverse colon, left colic flexure, the descending colon, the sigmoid colon, the rectum and the anal canal. In the angle between the caecum and the terminal part of the ileum, there is a narrow diverticulum called the vermiform appendix (Latin attachment). The general structure of large intestine is considered first followed by its parts one by one. The structure of the large intestine is adapted for storage of matter reaching it from the small intestines, and for absorption of fluid and solutes from it. The epithelium is absorptive (columnar), but villi are absent. Adequate lubrication for passage of its contents is provided by numerous goblet cells scattered in the crypts as well as on the surface of the mucous membrane. The presence of numerous solitary lymphatic follicles provides protection against bacteria present in the lumen of the intestine.

Relevant Features

The relevant features of the large intestine are as follows:

1 The large intestine is wider in calibre than the smal intestine. The calibre is greatest at its commencement. and gradually diminishes towards the rectum where it is dilated to form the rectal ampulla just above the anal canal.

2 The greater part of the large intestine is fixed, except for the appendix, the transverse colon and the sigmoid colon.

3 The longitudinal muscle coat forms only a thin layer in this part of the gut. The greater part of it forms three ribbon-like bands, called the taeniae coli. Proximally the taeniae converge at the base of the appendix, and distally they spread out on the terminal part of the sigmoid colon to become continuous with the longitudinal muscle coat of the rectum. In the caecum, the ascending colon, the descending colon and sigmoid colon the positions of taeniae are anterior or taenia libera; posteromedial or taenia mesocolica and posterolateral or taenia omentalis but in the transverse colon the corresponding positions of taenia are inferior, posterior and superior. One taenia, taenia libera, is placed anteriorly in the caecum, ascending, descending and sigmoid colon, but is placed inferiorly in the transverse colon. Second taenia, taenia mesocolica is present on the posteromedial surface of caecum, ascending, descending and sigmoid colon, but is placed posteriorly on transverse colon at the site of attachment of the transverse mesocolon. Third taenia, taenia omentalis, is situated postero- laterally in caecum, ascending, descending and sigmoid colon, but is situated on the anterosuperior surface four of greater omentum meet the transverse colon. This change in position is due to twist in transverse colon.

4 Since the taeniae are shorter than the circular muscle coat, the colon is puckered and sacculated.

5 Small bags of peritoneum filled with fat, and calle the appendices epiploicne (Greek to flgat on) are scattered over the surface of the large intestine, except tor appendix, the caecum and the rectum. These are most numerous on the sides of the sigmoid colon and on the posterior surface of the transverse colon.

6 The blood supply to the colon is derived from c marginal artery of Drummond. It is formed by co branches of superior and inferior mesenteric arteries. Terminal branches from the marginal artery are distributed to the intestine as long and short vessels, vasa longa and vasa brevia. The long arteries divide into anterior and posterior branches close to the mesocolic taenia to pass between the serous and muscular coats and reach the amesocolic taeniae. They gradually pierce the muscular coat and reach the submucosa. The anastomosis between the two amesocolic taeniae is extremely poor. So longitudinal incisions should be made along this line.

Short branches arise either from the marginal artery or from the long branches, and the majority of them at once sink into the bowel wall at the mesocolic border. The short and long branches together thus provide the mesocolic region of the wall with abundant blood supply. It is only the amesocolic region which has scanty blood supply. Subserous coat of long branches is intimately related to appendices epiploicae, to which they contribute branches. During removal of these appendages care must be taken not to pull on them in order to avoid traction on the subjacent vessel. Bowel wall is weakened where it is pierced by the vessels and at the sites of attachment of appendices epiploicae. Mucosa may herniate in these situations causing diverticulosis, with associated dangers of diverticulitis, fibrosis and stricture.

7 Lymph from the large intestine passes through four sets of lymph nodes.

a. Epicolic lymph nodes, lying on the wall of the gut.

b. Paracolic nodes, on the medial side of the ascending and descending colon and near the mesocolic border of the transverse and sigmoid colon.

c. Intermediate nodes, on the main branches of the vessels.

d. Terminal nodes, on the superior and inferior mesenteric vessels. In carcinoma of the colon, the related paracolic and intermediate lymph nodes have to be removed. Their removal is possible only after the ligature of the main branch of the superior or inferior mesenteric artery along which the involved lymph nodes lie. It is necessary, therefore, to remove a large segment of the bowel than is actually required by the extent of the disease, in order to avoid gangrene as a result of interference with the blood supply. It is always wise to remove the whole portion of the bowel supplied by the ligated vessel.

8 The nerve supply of the large intestine, barring the lower half of the anal canal, is both sympathetic and parasympathetic. The midgut territory receives its sympathetic supply from the coeliac and superior mesenteric ganglia (T11 to L1), and its para- sympathetic supply from the vagus. Both types of nerves are distributed to the gut through the superior mesenteric plexus.

The hindgut territory receives its sympathetic supply from the lumbar sympathetic chain (L1, L2), and its parasympathetic supply from the pelvic splanchnic nerve (nervi erigentes), both via the superior hypogastric and inferior mesenteric plexuses. Some parasym- pathetic fibres reach the colon along the posterior abdominal wall. The ultimate distribution of nerves in the gut is similar to that in the wall of the small intestine. The parasympathetic nerves are motor to the large intestine and inhibitory to the internal anal sphincter. The sympathetic nerves are largely vasomotor, but also motor to the internal anal sphincter, and inhibitory to colon. Pain impulses from the gut up to the descending colon travel through the sympathetic nerves, and from the sigmoid colon and rectum through the pelvic splanchnic nerves.

Functions of Colon

The functions of the colon are as follows:

1 Lubrication of faeces by mucus.

2 Absorption of the water, salts and the other solutes.

3 Bacterial flora of colon synthesises vitamin B.

4 Mucoid secretion of colon is rich in antibodies of IgA group, which protect it from invasion by micro- organisms.

5 The microvilli (apical tufts) of some columnar cells serve a sensory function.

Caecum

Anterior view of ileocaecal region

Features

Caecum is a large blind sac (Latin blind) forming the commencement of the large intestine. It is situated in the right iliac fossa, above the lateral half of inguinal ligament. It communicates superiorly with ascending colon, medially at the level of caecocolic junction with ileum, and posteromedially with the appendix.

Dimensions

It is 6 cm long and 7.5 cm broad. It is one of those organs of the body that have greater width than the length. The other examples are the prostate, pons and pituitary.

Relations

Relations of caecum

Anterior: Coils of intestine and anterior abdominal wall.

Posterior

1 Muscles: Right psoas and iliacus

2 Nerves: Genitofemoral, femoral and lateral cutaneous nerve of thigh (all of the right side).

3 Vessels: Testicular or ovarian.

4 Appendix in the retrocaecal recess.

Vessels and Nerves

The arterial supply of the caecum is derived from the caecal branches of the ileocolic artery. The veins drain into the superior mesenteric vein. The nerve supply is same as that of the midgut (thoracic 11 to lumbar 1; parasympathetic, vagus).

DEVELOPMENT

Development of the caecum

The caecum and appendix develop from the caecal bud arising from the postarterial segment of the midgut loop. The proximal part of the bud dilates to form the caecum. The distal part remains narrow to form the appendix. Thus initially the appendix arises from the apex of the caecum. However, due to rapid growth of the lateral wall of the caecum, the attachment of the appendix shifts medially.

ILEOCAECAL VALVE

The ileocaecal valve

The lower end of the ileum opens on the posteromedial aspect of the caecocolic junction. The ileocaecal opening is guarded by the ileocaecal valve.

Structure

The valve has two lips and two frenula.

1 The upper lip is horizontal and lies at the ileocolic junction.

2 The lower lip is longer and concave, and lies at the ileocaecal junction.

The two frenula are formed by the fusion of the lips at the ends of the aperture. These are the left or anterior and the right or posterior frenula. The left end of the aperture is rounded, and the right end narrow and pointed.

Control and Mechanism

1 The valve is actively closed by sympathetic nerves, which cause tonic contraction of the ileocaecal sphincter.

2 It is mechanically closed by distension of the caecum.

Functions

1 It prevents reflux from caecum to ileum.

2 It regulates the passage of ileal contents into the caecum, and prevents them from passing too quickly.

Vermiform Appendix

Position oc appendix

This is a worm-like diverticulum arising from the posteromedial wall, of the caecum, about 2 cm below the ileocaecal orifice.

Dimensions

The length varies from 2 to 20 cm with an average of 9 cm. It is longer in children than in adults. The diameter is about 5 mm. The lumen is quite narrow and may be obliterated after mid-adult life.

Positions

Position of the appendix according to the clock

The appendix lies in the right iliac fossa. Although the base of the appendix is fixed, the tip can point in any direction, as described below. The positions are often compared to those of the hour hand of a clock.

1 The appendix may pass upwards and to the right. This is paracolic or 11 o'clock position.

2 It may lie behind the caecum or colon, known as retrocaecal or 12 o'clock position. This is the commonest position of appendix, about 65%.

3 The appendix may pass upwards and to the left. It points towards the spleen. This is the splenic or 2 o'clock position. The appendix may lie in front of the ileum (preileal) or behind the ileum (postileal). The preileal type is most dangerous type.

4 It may pass horizontally to the left (as if pointing to the sacral promontory called promontoric or 3 o'clock position.

5 It may descend into the pelvis called pelvic or 4 o'clock position. This is the second most common position about 30%.

6 It may lie below the caecum (subcaecal) and may point towards the inguinal ligament called as midinguinal or 6 o'clock position.

Appendicular Orifice

1 The appendicular orifice is situated on the postero- medial aspect of the caecum 2 cm below the ileocaecal orifice.

2 The appendicular orifice is occasionally guarded by an indistinct semilunar fold of the mucous membrane, known as the valve of Gerlach.

3 The orifice is marked on the surface by a point situated 2 cm below the junction of transtubercular and right lateral planes .

4 McBurney's point is the site of maximum tenderness in appendicitis. The point lies at the junction of lateral one-third and medial two-thirds of line joining the right anterior superior iliac spine to umbilicus.

Lumen of Appendix

It is quite small and may be partially or completely obliterated after mid-adult life.

Peritoneal Relations

The appendix is suspended by a small, triangular fold of peritoneum, called the mesoappendix, or appendicular mesentery. The fold passes upwards behind the ileum, and is attached to the left layer of the mesentery.

Blood Supply

Arterial supply of caecum and appendix

The appendicular artery is a branch of the lower division of the ileocolic artery. It runs behind the terminal part of the ileum and enters the mesoappendix at a short distance from its base. Here it gives a recurrent branch which anastomoses with a branch of the posterior caecal artery. The main artery runs towards the tip of the appendix lying at first near to and then in the free border of the mesoappendix. The terminal part of the artery lies actually on the wall of the appendix.

Blood from the appendix is drained by the appendicular, ileocolic and superior mesenteric veins, to the portal vein.

Nerve Supply

Sympathetic nerves are derived from thoracic nine and ten segments through the coeliac plexus. Parasym- pathetic nerves are derived from the vagus. Referred pain of appendix is felt at umbilicus, similar to that of small intestine and testis.

Lymphatic Drainage

Most of the lymphatics pass directly to the ileocolic nodes, but a few of them pass indirectly through the appendicular nodes situated in the mesoappendix.

HISTOLOGY

The lumen of appendix is very narrow. There are no villi. The epithelium invaginates to form crypts of Lieberkühn. Muscularis mucosae is ill defined. Submucosa reveals many lymphoid masses. That is why it is called the abdominal tonsil. Muscularis externa comprises two layers. Outermost is the serous layer.

ASCENDING COLON

Ascending colon is about 12.5 cm long and extends from the caecum to the inferior surface of the right lobe of the liver. Here it bends to the left to form the right colic flexure. It is covered by peritoneum on three sides.

Anteriorly, it is related to the coils of small intestine, the right edge of the greater omentum, and the anterior abdominal wall. Posteriorly, it is related to the iliacus, the quadratus lumborum, the transversus abdominis, the lateral cutaneous, ilioinguinal, and iliohypogastric nerves and the right kidney.

RIGHT COLIC FLEXURE (HEPATIC FLEXURE)

Right colic flexure lies at the junction of the ascending colon and transverse colon. Here the colon bends forwards, downwards and to the left. The flexure lies on the lower part of right kidney. Anterosuperiorly, it is related to the colic impression on inferior surface of the right lobe of liver.

TRANSVERSE COLON

Relation of transverse mesocolon to anterior border of pancreas

Transverse colon is about 50 cm long and extend across the abdomen from the right colic flexure to the left colic flexure. Actually, it is not transverse, but hangs low as a loop to a variable extent. It is suspended by the transverse mesocolon attached to the anterior border of pancreas, and has a wide range of mobility.

Anteriorly, it is related to the greater omentum and to the anterior abdominal wall. Posteriorly, it is related to the second part of the duodenum, the head of the pancreas, and to coils of small intestine.

LEFT COLIC FLEXURE (SPLENIC FLEXURE)

Left colic flexure lies at the junction of the transverse colon and the descending colon. Here the colon bends downwards, and backwards. The flexure lies on the lower part of the left kidney and diaphragm, behind the stomach, and below the anterior end of the spleen. The flexure is attached to the eleventh rib (in the midaxillary line) by a horizontal fold of peritoneum, called the phrenicocolic ligament. This ligament supports the spleen and forms a partial upper limit of the left paracolic gutter.

DESCENDING COLON

Descending colon is about 25 cm long and extends from the left colic flexure to the sigmoid colon. It runs vertically up to the iliac crest, and then inclines medially on the iliacus and psoas major to reach the pelvic brim, where it is continuous with the sigmoid colon. The descending colon is narrower than the ascending colon.

Anteriorly, it is related to the coils of small intestine.

Posteriorly, it is related to the transversus abdominis, the quadratus lumborum, the iliacus and the iliohypogastric, ilioinguinal, lateral cutaneous, femoral and genitofemoral nerves; the gonadal and external iliac vessels. psoas muscles;

SIGMOID COLON (PELVIC COLON)

Sigmoid colon is about 37.5 cm long, and extends from the pelvic brim to the third piece of the sacrum, where it becomes the rectum. It forms a sinuous loop, and hangs down in pelvis over the bladder and uterus. Occasionally, it is very short, and takes a straight course. It is suspended by the sigmoid mesocolon and is covered by coils of small intestine.

SOURCE BD CHAURASIA'S Anatomy

Organelles Of Cell (Physiology)

2020-07-30T09:58:00.000+05:30

Organelles Of Cell

CYTOPLASM

Cytoplasm of the cell is the jelly-like material formed by 80% of water. It contains a clear liquid portion called cytosol and various particles of different shape and size. These particles are proteins, carbohydrates, lpids or electrolytes in nature. Cytoplasm also contains many organelles with distinct structure and function.

Cytoplasm is made up of two zones:

1. Ectoplasm: Peripheral part of cytoplasm, situated just beneath the cell membrane.

2. Endoplasm: Inner part of cytoplasm, interposed be- tween the ectoplasm and nucleus.

ORGANELLES IN CYTOPLASM

Cytoplasmic organelles are the cellular structures embed- ded in the cytoplasm. Organelles are considered as small organs of the cell. In Latin the word organelle means small organ. Some organelles are bound by limiting membrane and others do not have limiting membrane. Each organelle is having a definite structure and specific functions.

ORGANELLES WITH LIMITING MEMBRANE

ENDOPLASMIC RETICULUM

Endoplasmic Reticulum

Endoplasmic reticulum is a network of tubular and micro- somal vesicular structures which are interconnected with one another. It is covered by a limiting membrane which is formed by proteins and bilayered lipids. The lumen of endoplasmic reticulum contains a fluid medium called endoplasmic matrix. The diameter of the lumen is about 400 to 700 À. The endoplasmic reticulum forms the link between nucleus and cell membrane by connecting the cell membrane with the nuclear membrane.

Types of Endoplasmic Reticulum

Endoplasmic reticulum is of two types, namely rough en- doplasmic reticulum and smooth endoplasmic reticulum. Both the types are interconnected and continuous with one another. Depending upon the activities of the cells, the rough endoplasmic reticulum changes to smooth endoplasmic reticulum and vice versa.

Rough Endoplasmic Reticulum

It is the endoplasmic reticulum with rough, bumpy or bead-like appearance. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic reticulum. Rough endoplasmic reticulum is vesicular or tubular in structure.

Functions of Rough Endoplasmic Reticulum

1. Synthesis of proteins

Rough endoplasmic reticulum is concerned with the synthesis of proteins in the cell. It is involved with the synthesis of mainly those proteins which are secreted from the cells such as insulin from B-cells of islets of Langerhans in pancreas and antibodies from B lymphocytes.

Ribosomes arrange the amino acids into small units of proteins and transport them into the rough endoplas- mic reticulum. Here, the carbohydrates are added to the protein units forming the glycosylated proteins or gly- coproteins, which are arranged in the form of reticular vesicles. These vesicles are transported mainly to Golgi apparatus for further modification and processing. Few vesicles are transported to other cytoplasmic organelles.

2. Degradation of worn-out organelles

Rough endoplasmic reticulum also plays an important role in the degradation of worn-out cytoplasmic organelles like mitochondria. It wraps itself around the wornout organelles and forms a vacuole which is often called the autophagosome. Autophagosome is digested by lysosomal enzymes (see below for details).

Smooth Endoplasmic Reticulum

It is the endoplasmic reticulum with smooth appearance. Ribosomes are not attached to this. Hence, it is also called agranular endoplasmic reticulum. It is formed by many interconnected tubules. So, it is also called tubular endoplasmic reticulum.

Functions of Smooth Endoplasmic Reticulum

1. Synthesis of non-protein substance

Smooth endoplasmic reticulum is responsible for synthesis of non-protein substances such as cholesterol and steroid. This type of endoplasmic reticulum is abundant in cells that are involved in the synthesis of lipids, phospholipids, lipoprotein substances, steroid hormones, sebum, etc. In most of the other cells, smooth endoplasmic reticulum is less extensive than the rough endoplasmic reticulum.

2. Role in cellular metabolism

Outer surface of smooth endoplasmic reticulum contains many enzymes, which are involved in various metabolic processes of the cell.

3. Storage and metabolism of calcium

Smooth endoplasmic reticulum is the major site of storage and metabolism of calcium. In skeletal muscle fibers, it releases calcium which is necessary to trigger the muscle contraction.

4. Catabolism and detoxification

catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer-oroducing sub- Smooth endoplasmic reticulum is also concerned with stances) in the liver.

GOLGI APPARATUS

Golgi Apparatus

Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle, involved in the processing of proteins. It is present in all the cells except red blood cells. It is named after the discoverer Camillo Golgi. Usu- ally, each cell has one Golgi apparatus. Some of the cells may have more than one Golgi apparatus. Each Golgi apparatus consists of 5 to 8 flattened membranous sacs called the cisternae. Golgi apparatus is situated near the nucleus. It has two ends or faces, namely cis face and trans face. The cis face is positioned near the endoplasmic reticulum. Re- ticular vesicles from endoplasmic reticulum enter the Golgi apparatus through cis face. The trans face is situated near the cell membrane. The processed substances make their exit from Golgi apparatus through trans face.

Functions of Golgi Apparatus

Major functions of Golgi apparatus are processing, pack- ing, labeling and delivery of proteins and other molecules ike lipids to different parts of the cell.

1. Processing of materials

Vesicles containing glycoproteins and lipids are trans- ported into Golgi apparatus. Here, the glycoproteins and lipids are modified and processed.

2. Packaging of materials

All the processed materials are packed in the form of secretory granules, secretory vesicles and lysosomes, which are transported either out of the cell or to another part of the cell. Because of this, Golgi apparatus is called the 'post office of the cell'.

3. Labeling and delivery of materials

Finally, the Golgi apparatus sorts out the processed and packed materials and labels them (such as phosphate group), depending upon the chemical content for delivery (distribution) to their proper destinations. Hence, the Golgi apparatus is called 'shipping department of the cell

Integrated function os the endoplasmic reticulum and Golgi Apparatus

LYSOSOMES

Lysosomes are the membrane-bound vesicular orga- nelles found throughout the cytoplasm. The lysosomes are formed by Golgi apparatus. The enzymes synthe- sized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi ap- paratus. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes. Among the organelles of the cytoplasm, the lys- osomes have the thickest covering membrane. The membrane is formed by a bilayered lipid material. It has many small granules which contain hydrolytic enzymes.

Types of Lysosomes

Lysosomes are of two types:

1. Primary lysosome

Primary lysosome is pinched off from Golgi appa- ratus. It is inactive in spite of having hydrolytic en- zymes.

2. Secondary lysosome

Secondary lysosome is the active lysosome. It is formed by the fusion of a primary lysosome with phagosome or endosome (see below).

Functions of Lysosomes

Lysosomes are often called 'garbage system' of the cell because of their degradation activity. About 50 different hydrolytic enzymes, known as acid hydroxylases are present in the lysosomes, through which lysosomes execute their functions.

Important lysosomal enzymes

1. Proteases, which hydrolyze the proteins into amino acids.

2. Lipases, which hydrolyze the lipids into fatty acids and glycerides.

3. Amylases, which hydrolyze the polysaccharides into glucose.

4. Nucleases, which hydrolyze the nucleic acids into mononucleotides.

Mechanism of lysosomal function

Lysosomal functions involve two mechanisms:

1. Heterophagy: Digestion of extracellular materials on- gulfed by the coll via endocytosis.

2. Autophagy: Digestion and degradation of intracellular materials such as worn-out cytoplasmic organelles

Specific Functions of Lysosomes

1. Degradation of macromolecules

Macromolecules are engulfed by the cell by means of endocytosis. The macromolecules such as bacteria, engulfed by the cell via phagocytosis are called phagosomes or vacuoles. The other macromolecules taken inside via pinocytosis or receptor-mediated endo- cytosis are called endosomes. The primary lysosome fuses with the phagosome or endosome to form the secondary lysosome. Secondary lysosome becomes acidic and the lysosomal enzymes are activated. Bacteria and the other macromolecules are digested and degraded by these enzymes. Second- ary lysosome containing these degraded waste products moves through cyloplasm and fuses with cell membrane. Now the waste products are eliminated by exocytosis.

2. Degradation of worn-out organelles

The rough endoplasmic reticulum wraps itself around the worn-out organelles such as mitochondria and form autophagosomes. One primary lysosome fuse with one autophagosome and form the secondary lysosome. Enzymes in the secondary lysosome are activated. Now. these enzymes digest the contents of autophagosome.

3. Removal of excess secretory products in the cells

Lysosomes in the cells of the secretory glands remove the excess secretory products by degrading the secre- tory granules.

4. Secretdry function:

Secretory lysosomes Lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in cells of immune system. The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules (which contain the particular secretory

Examples of secretory lysosomes:

a. Lysosomes in the cytotoxic T lymphocytes and natu- ral killer (NK) cells secreto perforin and granzymes. which destroy both viral-infected cells and tumor cells. Perforin is a pore-forming protein that Initiates cell death. Granzymes belong to the family of serine proteases (enzymes that dislodge the peptide bonds sary, of the proteins) and cause the cell death by apoptosis.

b. Secretory lysosomes of melanocytes secrete melanin.

c. Secretory lysosomes of mast cells secrete serotonin, which is a vasoconstrictor substance and inflammatory mediator.

PEROXISOMES

Peroxisomes or microbodies are the membrane limited somes contain some oxidative enzymes such as catalase, urate oxidase and D-amino acid oxidase. inward matrix

Functions of Peroxisomes Peroxisomes:

1. Breakdown the fatty acids by means of a process called beta-oxidation. This is the major function of peroxisomes.

2. Degrade the toxic substances such as hydrogen peroxide and other metabolic products by means of detoxification. A large number of peroxisomes are present in the cells of liver, which is the major organ for detoxification. Hydrogen peroxide is formed from poisons or alcohol, which enter the cell. Whenever hydrogen peroxide is produced in the cell, the per- oxisomes are ruptured and the oxidative enzymes are released. These oxidases destroy hydrogen peroxide and the enzymes which are necessary for the production of hydrogen peroxide.

3. Form the major site of oxygen utilization in the cells.

4. Accelerate gluconeogenesis from fats.

5. Degrade purine to uric acid.

6. Participate in the formation of myelin

. 7. Play a role in the formation of bile acids.

CENTROSOME AND CENTRIOLES

Centrosome is the membrane-bound cellular organelle situated almost in the center of cell, close to nucleus. It consists of two cylindrical structures called centrioles which are made up of proteins, Centrioles are responsible for the movement of chromosomes during cell division.

SECRETORY VESICLES

Secretory vesicles are the organelles with limiting mem- brane and contain the secretory substances. These vesicles are formed in the endoplasmic reticulum and are processed and packed in Golgi apparatus. Secretory vesicles are present throughout the cytoplasm. When necessary, vesicles are ruptured and secretory substances are released into the cytoplasm.

MITOCHONDRION

Structure of mitochondrion

Mitochondrion (plural 'mitochondria') is a membrane- bound cytoplasmic organelle concerned with production of energy. It is a rod-shaped or oval-shaped structure with a diameter of 0.5 to 1 p. It is covered by a bilayered membrane. The outer membrane is smooth and encloses the contents of mitochondrion. This membrane contains various enzymes such as acetyl-CoA synthetase and glycerophosphate acetyltransferase.

The inner membrane is folded in the form of shelf-like inward projections called cristae and it covers the inner matrix space. Cristae contain many enzymes and other protein molecules which are involved in respiration and synthesis of adenosine triphosphate (ATP). Because of these functions, the enzymes and other protein molecules in cristae are collectively known as respiratory chain or electron transport system.

Enzymes and other proteins of respiratory chain

1. Succinic dehydrogenase

2. Dihydronicotinamide adenine dinucleotide (NADH) dehydrogenase

3. Cytochrome oxidase

4. Cytochrome C

5. ATP synthase.

Inner cavity of mitochondrion is filled with matrix which contains many enzymes. Mitochondrion moves freely in the cytoplasm of the cell. It is capable of repro- ducing itself. Mitochondrion contains its own deoxyribonu- cleic acid (DNA), which is responsible for many enzymatic actions. In fact, mitochondrion is the only organelle other than nucleus, which has its own DNA.

Functions of Mitochondrion

1. Production of energy

Mitochondrion is called the 'power house' or 'power plant' of the cell because it produces the energy required for cellular functions. The energy is produced during the oxidation of digested food particles like proteins, carbohydrates and lipids by the oxidative enzymes in cristae. During the oxidative process, water and carbon dioxide are produced with release of energy. The re- leased energy is stored in mitochondria and used later for synthesis of ATP.

2. Sýnthesis of ATP

The components of respiratory chain in mitochondrion are responsible for the synthesis of ATP by utilizing the energy by oxidative phosphorylation. ATP molecules dif- fuse throughout the cell from mitochondrion, Whenever energy is needed for cellular activity, the ATP molecules are broken down.

3. Apoptosis

Cytochrome C and second mitochondria-derived activator of caspases (SMAC)/diablo secreted in mitochondria are involved in apoptosis (see below).

4. Storage of calcium

Endoplasmic reticulum forms the major site of calcium ions storage in the cell. Mitochondrion also stores calcium in its matrix.

5. Detoxification functions

Mitochondria in liver cells are responsible for detoxification of ammonia.

ORGANELLES WITHOUT LIMITING MEMBRANE

RIBOSOMES

Ribosomes are organelles without limiting membrane. These organelles are granular and small dot-like struc- tures with a diameter of 15 nm. Ribosomes are made up of 35% of proteins and 65% of ribonucleic acid (RNA), RNA present in ribosomes is called ribosomal RNA (FRNA). Ribosomes are concerned with protein synthesis in the cell.

Types of Ribosomes

Ribosomes are of two types:

i. Ribosomes that are attached to rough endoplasmic reticulum.

ii. Free ribosomes that are distributed in the cytoplasm.

Functions of Ribosomos

Ribosomes are called 'protein factories' because of their role in the synthesis of proteins. Messenger RNA (MRNA) carries the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turo arrange the amino acids into small units of proteins, Ribosomes attached to rough endoplasmic reticulum are involved in the synthesis of proteins such as the en- zymatic proteins, hormonal proteins, lysosomal proteins and the proteins of the cell membrane. Free ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome and mitochondria,

CYTOSKELETON

Cytoskeleton of cell is a cellular organelle formed by complex network of structures in various sizes present throughout the cytoplasm. It determines the shape of the cell and gives support to the cell. It is also essential for the cellular movements and the response of the cell to external stimuli. Cytoskeleton consists of three major protein com- ponents:

1. Microtubule.

2. Intermediate filaments.

3. Microfilaments.

1. Microtubules

Microtubule

Microtubules are the straight, hollow and tubular struc- tures of the cytoskeleton. These organelles without the limiting membrane are arranged in different bundles. Each tubule has a diameter of 20 to 30 nm. Length of microtubule varies and it may be 1,000 times more than the thickness. Structurally, the microtubules are formed by bundles of globular protein called tubulin. Tubulin has two subunits, namely a(Alpha)-subunit and B(Beta)-subunit.

Functions of microtubules

Microtubules may function alone or join with other pro- teins to form more complex structures like cilia, flagella or centrioles and perform various functions.

i. Determine the shape of the cell.

ii. Give structural strength to the cell.

iii. Act like conveyor belts which allow the movement of granules, vesicles, protein molecules and some organelles like mitochondria to different parts of the cell.

iv. Form the spindle fibers which separate the chromo- somes during mitosis.

v. Are responsible for the movement of centrioles and the complex cellular structures like cilia.

2. Intermediate Filaments

Intermediate filament

Intermediate filaments are the structures of cytoskeleton that form a network around nucleus and extend to the periphery of the cell. Diameter of each filament is about 10 nm. The intermediate filaments are formed by rope- like polymers, which are made up of fibrous proteins.

Subclasses of intermediate filaments

Intermediate filaments are divided into five subclasses:

i. Keratins (in epithelial cells).

ii. Glial filaments (in astrocytes).

iii. Neurofilaments (in nerve cells).

iv. Vimentin (in many types of cells).

v. Desmin (in muscle fibers).

Functions of intermediate filaments

Intermediate filaments:

i. Help to maintain the shape of the cell.

ii. Connect the adjacent cells through desmosomes.

3. Microfilaments

Microfilament of ectoplasm

Microfilaments are long and fine thread-like structures with a diameter of about 3 to 6 nm. These filaments are made up of non-tubular contractile proteins called actin and myosin. Actin is more abundant than myosin. Microfilaments are present throughout the cyto- plasm. The microfilaments present in ectoplasm contain only actin molecules and those present in endoplasm contain both actin and myosin molecules.

Functions of microfilaments Microfilaments:

i. Give structural strength to the cell.

ii. Provide resistance to the cell against the pulling forces.

iii. Are responsible for cellular movements like contrac- tion, gliding and cytokinesis (partition of cytoplasm during cell division).

NUCLEUS

Nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 to 22 p and occupies about 10% of total volume of the cell. Nucleus is present in all the cells in the body except the red blood cells. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Presence of nucleus is necessary for cell division.

Most cells have only one nucleus (uninucleated cells). Few types of cells like skeletal muscle cells have many nuclei (multinucleated cells). Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in some cells.

STRUCTURE OF NUCLEUS

Nucleus is covered by a membrane called nuclear membrane and contains many components. Major components of nucleus are nucleoplasm, chromatin and nucleolus.

Nuclear Membrane

Nuclear membrane is double layered and porous in nature. This allows the nucleoplasm to communicate with the cytoplasm. The outer layer of nuclear membrane is continuous with the membrane of endoplasmic reticulum. is continuous with the lumen of endoplasmic reticulum.

Pores of the nuclear membrane are guarded (lined) by protein molecules. Diameter of the pores is about 80 to 100 nm. It is decreased to about 7 to 9 nm because of the attachment of protein molecules to the periphery of pores, Exchange of materials between nucleoplasm and cytoplasm occurs through these pores.

Nucleoplasm

Nucleoplasm is a highly viscous fluid that forms the ground substance of nucleus. It is similar to cytoplasm present outside the nucleus. Nucleoplasm surrounds chromatin and nucleolus. It contains dense fibrillar network of proteins called the nuclear matrix and many substances such as nucleotides and enzymes. The nuclear matrix forms the structural framework for organizing chromatin. The soluble liquid part of nucleoplasm is known as nuclear hyaloplasm.

Chromatin

Chromatin is a thread-like material made up of large molecules of deoxyribonucleic acid (DNA). The DNA molecules are compactly packed with the help of a spe- cialized basic protein called histone. So, chromatin is referred as DNA-histone complex. It forms the major bulk of nuclear material. DNA is a double helix which wraps around central core of eight histone molecules to form the fundamental packing unit of chromatin called nucleosome. Nucle- osomes are packed together tightly with the help of a histone molecule to form a chromatin fiber. Just before cell division, the chromatin condenses to form chromosome.

Chromosomes

Chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the hereditary charac- teristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules. Each DNA contains many genes. Normally, the chromosomes are not visible in the nucleus under microscope. Only during cell division, the chromosomes are visible under microscope. This is because DNA becomes more tightly packed just before cell division, which makes the chromosome visible during cell division.

Diploid cells and haploid cells

All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited from mother and one from father. The cells with 23 pairs of chromosomes are called

diploid cells. The reproductive cells called gametes . sex cells contain only 23 single chromosomes, These cells are called haploid cells.

Sex chromosomes and autosomes

Among the 23 pairs of chromosomes, one pair is con. cerned with determination of sex of the person. Theso chromosomes are called sex chromosomes. Remaining 22 pair of chromosomes that are not concerned with sey determination are named as autosomes. Among the sex chromosomes, one is called X chro- mosome and another one is called Y chromosomes. The cells of females have two X chromosomes and cells of males have one X chromosome and one Y chromosome.

Nucleolus

Nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one or more nucleoli. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. The RNA is synthesized by five different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form the subunits of ribosomes. All the subunits formed in the nucleolus are transported to cytoplasm through the pores of nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes, which play an essential role in the formation of proteins.

FUNCTIONS OF NUCLEUS

Nucleus is considered as brain of the cells. Major func- tions of nucleus are the control of cellular activities and storage of hereditary material. So, it is also called control center. Several processes are involved in the nuclear functions.

DEOXYRIBONUCLEIC ACID: DNA

Deoxyribonucleic acid (DNA) is a nucleic acid that carries the genetic information to the offspring of an organism. DNA forms the chemical basis of hereditary characters. It contains the instruction for the synthesis of proteins in the ribosomes. Gene is a part of a DNA molecule. DNA is present in the nucleus (chromosome) and mitochondria of the cell. The DNA present in the nucleus is responsible for the formation of RNA. RNA regulates the synthesis of proteins by ribosomes. DNA in mitochondria is called non-chromosomal DNA.

STRUCTURE OF DNA

Structure of DNA (A) Double helical structure of DNA (B) Magnified view of the components of DNA

DNA is a double-stranded complex nucleic acid. It is formed by deoxyribose, phosphoric acid and four types of bases. Each DNA molecule consists of two polynu- cleotide chains, which are twisted around one another in the form of a double helix. The two chains are formed by the sugar deoxyribose and phosphate. These two substances form the backbone of DNA molecule. Both chains of DNA are connected with each other by some organic bases..

Each chain of DNA molecule consists of many nucleotides. Each nucleotide is formed by:

1. Deoxyribose: Sugar.

2. Phosphate.

3. One of the following organic (nitrogenous) bases:

Purines:

i. Adenine (A)

ii. Guanine (G).

Pyrimidines:

i. Thymine (T)

ii. Cytosine (C).

Strands of DNA are arranged in such a way that both are bound by specific pairs of bases. The adenine of one strand binds specifically with thymine of opposite strand. Similarly, the cytosine of one strand binds with guanine of the other strand. DNA forms the component of chromosomes, which carries the hereditary information. The hereditary infor- mation that is encoded in DNA is called genome. Each DNA molecule is divided into discrete units called genes.

Gene Physiology

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Gene is the basic hereditary unit of the cell. It is a portion of DNA molecule that contains the message or code for the synthesis of a specific protein from amino acids. It is like a book that contains the information necessary for protein synthesis.

In the nucleotide of DNA, three of the successive base pairs are together called a triplet or a codon. Each codon codes or forms code word (information) for one amino acid. There are 20 amino acids and there is separate code for each amino acid. For example, the triplet CCA is the code for glycine and GGC is the code for proline. Thus, each gene forms the code word for a particular protein to be synthesized in ribosome (outside the nucleus) from amino acids.

GENETIC DISORDERS

Genetic disorder is a disease or condition that occurs due to absence of a gene or a defective gene or by a chromosomal abnormality.

Causes of Genetic Disorders

Genetic disorders occur due to two causes.

1. Genetic variation

Genetic variation means the presence of a gene that dif- fers from normal gene.

2. Genetic mutation

Generally, mutation means an alteration or a change in nature, form, or quality. Genetic mutation refers to change of the DNA sequence within a gene or chromosome of an organism, which results in the creation of a new character,

Classification of Genetic Disorders

Genetic disorders are classified into four types:

1. Single gene disorders.

2. Multifactorial genetic disorders.

3. Chromosomal disorders.

4. Mitochondrial DNA disorders.

1. Single gene disorders

Single gene disorders or mendelian, or monogenic disorders occur because of variation or mutation in one single gene. Examples include sickle cell anemia and Mitochondrial Huntington's disease.

2. Multifactorial genetic disorders

Multifactorial genetic disorders or polygenic disorders are caused by combination of environmental factors and mutations in multiple genes. Examples are coronary heart disease, Alzheimer's disease, arthritis and diabetes,

3. Chromosomal disorders

Chromosomal disorder is a genetic disorder caused by abnormalities in chromosome. It may be due to any change in structure (structural abnormality) or number (numerical abnormality) of chromosomes. It is also called chromosomal abnormality, anomaly or aberration. It often results in genetic disorders, which involve physical or mental abnormalities. Chromosomal disorder is caused by numerical abnormality or structural abnormality.

Chromosomal disorder is classified into two types:

i. Structural abnormality (alteration) of chromosomes which leads to disorders like chromosome instabil- ity syndromes.

ii. Numerical abnormality of chromosomes, which is of two types:

a. Monosomy due to absence of one chromosome from normal diploid number. Example is Turner syndrome.

b. Trisomy due to the presence of one extra chro- mosome along with normal pair of chromosomes in the cells. Example is Down syndrome.

4. Mitochondrial DNA disorders

Mitochondrial DNA disorders are the genetic disorders caused by the mutations in the DNA of mitochondria (non-chromosomal DNA). Examples are Kearns-Sayre syndrome (neuromuscular disorder characterized by myopathy, cardiomyopathy and paralysis of ocular muscles) and Leber's hereditary optic neuropathy (disease characterized by degeneration of retina and loss of vision).

RIBONUCLEIC ACID

Ribonucleic acid (RNA) is a nucleic acid that contains a long chain of nucleotide units. It is similar to DNA, but contains ribose instead of deoxyribose. Various functions coded in the genes are carried out in the cytoplasm of the cell by RNA. RNAS is formed from DNA.

STRUCTURE OF RNA

Each RNA molecule consists of a single strand of poly- nucleotide unlike the double-stranded DNA.

Each nucleotide in RNA is formed by:

1. Ribose: Sugar.

2. Phosphate.

3. One of the following organic bases:

Purines:

i. Adenine (A)

ii. Guanine (G).

Pyrimidines:

i. Uracil (U)

ii. Cytosine (C).

Uracil replaces the thymine of DNA and it has similar structure of thymine.

TYPES OF RNA

RNA is of three types. Each type of RNA plays a specific role in protein synthesis. The three types of RNA are given below.

1. Messenger RNA (mRNA)

Messenger RNA carries the genetic code of the amino acid sequence for synthesis of protein from the DNA to the cytoplasm.

2. Transfer RNA (tRNA)

Transfer RNA is responsible for decoding the genetic message present in mRNA.

3. Ribosomal RNA (rRNA)

Ribosomal RNA is present within the ribosome and forms a part of the structure of ribosome. It is responsible for the assembly of protein from amino acids in the ribosome.

GENE EXPRESSION

Gene expression is the process by which the informa- tion (code word) encoded in the gene is converted into functional gene product or document of instruction (RNA) that is used for protein synthesis.

Gene expression involves two steps:

1. Transcription.

2. Translation.

TRANSCRIPTION OF GENETIC CODE

Transcription means copying. It indicates the copying of genetic code from DNA to RNA. The proteins synthesized in the ribosomes which are present in cytoplasm. However, the synthesis of different proteins depends upon the information (sequence of codon) encoded in the genes of the DNA which is present in the nucleus. Since DNA is a macromolecule, it cannot pass through the pores of the nuclear membrane and enter the cytoplasm. But, the information from DNA must be sent to ribosome. So, the gene has to be transcribed (copied) into MRNA which is developed from DNA.

Thus, the first stage in the protein synthesis is tran- scription of genetic code, which occurs within the nucleus. It involves the formation of mRNA and simultaneous copying or transfer of information from DNA to MRNA. The MRNA enters the cytoplasm from the nucleus and activates the ribosome resulting in protein synthesis. The formation of mRNA from DNA is facilitated by the enzyme RNA polymerase.

TRANSLATION OF GENETIC CODE

Translation is the process by which protein synthesis Occurs in the ribosome of the cell under the direction of genetic instruction carried by MRNA from DNA. Or, it is the process by which the MRNA is read by ribosome to produce a protein. This involves the role of other two types of RNA, namely tRNA and rRNA.

The MRNA moves out of nucleus into the cytoplasm. Now, a group of ribosomes called polysome gets at- tached to MRNA. The sequence of codons in MRNA are exposed and recognized by the complementary sequence of base in tRNA The complementary sequence of base Is called anticodon. According to the sequence of bases in anticodon, different amino acids are transported from the cytoplasm into the ribosome by tRNA that acts as a carrier. With the help of rRNA, the protein molecules are assembled from amino acids. The protein synthesis occurs in the ribosomes which are attached to rough endoplasmic reticulum.

GROWTH FACTORS

Growth factors are proteins which act as cell signaling molecules like cytokines and hormones. These factors bind with specific surface receptors of the target cell and stimulate proliferation, differentiation and/or maturation of these cells. Often, the term growth factor is interchangeably used with the term cytokine. But growth factors are distinct from cytokines. Growth factors act on the cells of the growing tissues. But cytokines are concerned with the cells of immune system and hematopoietic cells. Many growth factors are identified.

CELL DEATH

Cell death occurs by three distinct processes:

1. Autophagy.

2. Apoptosis.

3. Necrosis.

AUTOPHAGY

Autophagy is a normal physiological process in the body by which cells are destroyed. The word AUTOPHAGY is derived from Greek (auto = self, phagy = eating). It is also by which cells are destroyed. The word autophagy is de- called autophagocytosis. Autophagy involves formation autophagosome which contains worn-out cytoplasmic organelles. Autophago- some fuses with lysosome. Refer 'Functions of Lysosome in this chapter. Then the enzymes of the lysosome cause destruction of the organelles by protein degradation. The constituents of the organelles are reused for formation of new cells. Thus, it maintains a balance between break down of damaged organelles and formation new cellular organelles. Autophagy is always considered as a nonapoptotic programmed cell death.

APOPTOSIS

Apoptosis is defined as the natural or programmed death of the cell under genetic control. Originally, apoptosis refers to the process by which the leaves fall from trees in autumn (In Greek, apoptosis means 'falling leaves'). Apoptosis is referred as 'cell suicide' because the cells activate an intracellular death program and kill themselves in a controlled way. This type of programmed cell death is a normal phe- nomenon and it is essential for normal development of the body. In contrast to necrosis, apoptosis usually does not produce inflammatory reactions in the neighboring tissues.

Functional Significance of Apoptosis

The purpose of apoptosis is to remove unwanted cells without causing any stress or damage to the neighbor- ing cells.

Functional significance of apoptosis:

1. Plays a vital role in cellular homeostasis. About 10 million cells are produced every day in human body by mitosis. An equal number of cells die by apopto- sis. This helps in cellular homeostasis.

2. Useful for removal of a cell that is damaged beyond repair by a virus or a toxin.

3. An essential event during the development and in adult stage.

Examples of apoptosis

i. A large number of neurons are produced during the development of central nervous system. But up to 50% of the neurons are removed by apop- tosis during the formation of synapses between neurons.

ii. Apoptosis is responsible for the removal of tissues of webs between fingers and toes during developmental stage in fetus.

iii. It is necessary for regression and disappearance of duct systems during sex differentiation in fetus.

iv. The cell that loses the contact with neighboring cells or basal lamina in the epithelial tissue dies by apop- tosis. This is essential for the death of old enterocytes. v. It plays an important role in the cyclic sloughing of the inner layer of endometrium, resulting in menstru ation.

vi. Apoptosis removes the autoaggressive T cells and prevents autoimmune diseases.

Activation of Apoptosis

Apoptosis is activated by either withdrawal of posi tive signals (survival factors) or arrival of hegative signals.

Withdrawal of positive signals

Positive signals are the signals which are necessary for the Jong-time survival of most of the cells. The positive signals are continuously produced by other cells or some chemi- cal stimulants. Best examples of chemical stimulants are:

i. Nerve growth factors (for neurons).

ii. Interleukin-2 (for cells like lymphocytes).

Absence or withdrawal of the positive signals acti- vates apoptosis.

Arrival of negative signals

Negative signals are the external or internal stimuli which initiate apoptosis. External stimuli are applied by stimulus molecules, death-receptor ligands. Internal stimuli are arising from inside the cell (see below).

Death-receptor ligands

Death-receptor ligands are the substances which bind with specific cell membrane receptors and initiate process of apoptosis. Common death-receptor ligands are of three types.

1. Tumor necrosis factor alpha (TNF-a) TNF-a is a cytokine secreted by may cells including activated T-lymphocytes and natural killer cells.

2. Fas ligand (FASL) Fas ligand (first apoptosis signal ligand) is a trans- membrane protein belonging to tumor necrosis factor superfamily. It is found on activated T-lymphocytes and natural killer cells. It is also called Apo-1L.

3. Trail ligand Trail ligand (TNF - related apoptosis including ligand) belongs tumor necrosis factor ligand family secreted by many types of normal tissue cells, It is also called Apo-2L.

Death-receptors

death receptors are: Death-receptors are the cell membrane receptors which receive the death-receptor ligands. Well characterized

1. TNF receptor-1 (TNFR1)

2. Fas receptors

3. TNF-related apoptosis inducing ligand (TRAIL) re- ceptors called DR4 and DR5.

Initiation of Apoptosis

Apoptosis is initiated by two pathways, the extrinsic pathway and intrinsic pathway. In extrinsic pathway, the signal (stimulus) for apoptosis is received from outside the cell. In intrinsic pathway, the signal arises from within the cell itself. Both extrinsic and intrinsic pathways lead to an ex- ecution process during with the cell death occurs.

Extrinsic Pathway of Apoptosis

Extrinsic pathway of apoptosis is initiated stimulation of death receptors of target cells by signal molecules, death ligands. Generally, the death receptors have an extracel- lular domain and an intracellular (cytoplasmic) domain. Extracellular domain is responsible for binding of ligands and interaction with those ligands. The intracellular do- main is called death domain. When the ligand binds with receptor, the death do- main allows recruitment of many proteins to the receptor and formation of a protein complex called death inducing signaling complex (DISC). Now caspase (see below) is recruited to DISC and gets activated resulting in execu- tion process.

Intrinsic Pathway of Apoptosis

Intrinsic pathway is also called of apoptosis is initiated by internal stimuli arising from following conditions:

1. Cellular stress.

2. Increase in the concentration of intracellular oxi- dants.

3. Viral infection.

4. Damage of DNA.

Internal stimuli activate the proapoptotic BCL-2 fam- ily proteins which in turn inactivate antiapoptotic BCL-2 proteins. The interaction between proapoptotic and antia- poptotic proteins causes destabilization of mitochondrial membrane and release of apoptotic factors which induces execution process. BCL-2 (B-cell lymphoma 2) family proteins are the cell death regulators located near mitochondria.

Execution Process of Apoptosis

Both extrinsic and intrinsic pathways end in the common execution process. Mitochondria plays and important role during this process.

Role of mitochondria in apoptosis

External or internal stimuli initiate apoptosis by activat- ing the proteases called caspases (cysteine-dependent aspartate-specific proteases). Normally, caspases are suppressed by the inhibitor protein called apoptosis inhibiting factor (AIF). When the cells receive apoptotic stimulus, mito- chondria release two protein materials. First one is Cytochrome C and the second protein is called second mitochondria-derived activator of caspases (SMAC) or its homologue diablo.

SMAC/diablo inactivates AIF so that the inhibitor is inhibited. During this process SMAC/diablo and AIF ag- gregate to form apoptosome. This activates caspases. Cytochrome C also facilitates caspase activation. Cell shows sequence of characteristic morphological changes during apoptosis, viz.:

i. Activated caspases digest the proteins of cytoskel- eton and the cell shrinks and becomes round.

ii. Because of shrinkage, the cell losses the contact with neighboring cells or surrounding matrix.

iii. Chromatin in the nucleus undergoes degradation and condensation.

iv. Nuclear membrane becomes discontinuous and the DNA inside nucleus is cleaved into small fragments.

v. Following the degradation of DNA, the nucleus breaks into many discrete nucleosomal units, which are also called chromatin bodies.

vi. Cell membrane breaks and shows bubbled appear- ance.

vii. Finally, the cell breaks into several fragments con- taining intracellular materials including chromatin bodies and organelles of the cell. Such cellular frag- ments are called vesicles or apoptotic bodies,

viii. Apoptotic bodies are engulfed by phagocytes and dendritic cells.

Abnormal Apoptosis

Apoptosis within normal limits is beneficial for the body. However, too much or too little apoptosis leads to abnormal conditions.

Common abnormalities due to too much apoptosis

a. Ischemic-related injuries.

b. Autoimmune diseases like:

i. Hemolytic anemia.

ii. Thrombocytopenia.

iii. Acquired immunodeficiency syndrome (AIDS).

c. Neurodegenerative diseases like Alzheimer's disease.

Common abnormalities due to too little apoptosis

1. Cancer.

2. Autoimmune lymphoproliferative syndrome (ALPS).

CANCER

Cancer is defined as a disease caused by uncontrolled division of abnormal cells in the body.

Apoptosis and Cancer

Generally, when something goes abnormal in a cell, that cell is immediately destroyed by apoptosis. This also prevents development of cancer. However, some of the cells particularly tumor cells evade apoptosis. The cells escaped from apoptosis become cancer cells and con- tinuously proliferate despite their abnormalities.

Free Radicals and Cancer

Free radicals or oxidants are the bactericidal agents se- creted by many cells in the body. Free radicals are also called Reactive oxygen species (ROS). Free radicals are the most potent bactericidal agents. So. even the bacteria which cannot be digested by lyso- somal enzymes are degraded by these radicals. At the same time free radicals are able to cause cancer and other diseases such as heart diseases, neu- rological diseases and autoimmune diseases. Free radicals cause oxidative injury in the cells leading to damage of proteins and DNA resulting in develop- ment of cancer.

Role of Antioxidants

Antioxidants are the substances which protect the cells from damage caused by free radicals. These substances interact with free radicals and stop cellular damage. Hence, the antioxidants are called "free radical scavengers".

Examples of antioxidants

1. Beta-carotene.

2. Vitamin A.

3. Vitamin C.

4. Vitamin E.

NECROSIS

Necrosis (means 'dead' in Greek) is the uncontrolled and unprogrammed death of cells due to unexpected and accidental damage. It is also called 'cell murder' because the cell is killed by extracellular or external events. After necrosis, the harmful chemical substances released from the dead cells cause damage and inflammation of neighboring tissues.

Causes for Necrosis

Common causes of necrosis are injury, infection, inflam- mation, infarction and cancer, Necrosis is induced by both physical and chemical events such as heat, radiation, trauma, hypoxia due to lack of blood flow and exposure toxins.

Necrotic Process

Necrosis results in lethal disruption of cell structure and activity. The cell undergoes a series of characteristic changes during necrotic process, viz.:

1. Cell swells causing damage of the cell membrane and appearance of many holes in the membrane.

2. Intracellular contents leak out into the surrounding environment.

3. Intracellular environment is altered.

4. Simultaneously, a large amount of calcium ions is released by the damaged mitochondria and other organelles.

5. Presence of calcium ions drastically affects the orga- nization and activities of proteins in the intracellular components.

6. Calcium ions also induce release of toxic materials that activate the lysosomal enzymes.

7. Lysosomal enzymes cause degradation of cellular components and the cell is totally disassembled re- sulting in death.

8. Products broken down from the disassembled cell are ingested by neighboring cells.

Reaction of Neighboring Tissues after Necrosis

Tissues surrounding the necrotic cells react to the breakdown products of the dead cells, particularly the derivatives of membrane phospholipids like the arachi- donic acid. Along with other materials, arachidonic acid causes the following inflammatory reactions in the sur- rounding tissues:

1. Dilatation of capillaries in the region and thereby in- creasing local blood flow.

2. Increase in the temperature leading to reddening of the tissues.

3. Release of histamine from these tissues which in. duces pain in the affected area.

4. Migration of leukocytes and macrophages from blood to the affected area because of increased capillary permeability.

5. Movement of water from blood into the tissues caus- ing local edema,

6. Engulfing and digestion of cellular debris and foreign materials like bacteria by the leukocytes and macro- phages.

7. Activation of immune system resulting in the removal of foreign materials.

8. Formation of pus by the dead leukocytes during this process.

9. Finally, tissue growth in the area and wound healing.

CELL ADAPTATION

Cell adaptation refers to the changes taking place in a cell in response to environmental changes.

Normal functioning of the cell is always threatened by various factors such as stress, chemical agents, dis- eases and environmental hazards. Yet, the cell survives and continues the function by means of adaptation. Only during extreme conditions, the cell fails to withstand the hazardous factors which results in destruction and death of the cell.

Cellular adaptation occurs by any of the following mechanisms:

1. Atrophy.

2. Hypertrophy.

3. Hyperplasia.

4. Dysplasia.

5. Metaplasia.

6. Neoplasm.

ATROPHY

Atrophy means decrease in size of a cell. Atrophy of a greater number of cells results in decreased size or wast- ing of the concermed tissue, organ or part of the body.

Causes of Atrophy

Atrophy is due to one or more number of causes such

I. Poor nourishment.

ii. Decreased blood supply.

iii. Lack of workload or exercise.

iv. Loss of control by nerves or homones.

v. Intrinsic disease of the tissue or organ.

Types of Atrophy

Atrophy is of two types, physiological atrophy and patho- logical atrophy. Examples of physiological atrophy are the atrophy of thymus in childhood and tonsils in adoles- cence. The pathological atrophy is common in skeletal muscle, cardiac muscle, sex organs and brain.

HYPERTROPHY

Hypertrophy is the increase in the size of a cell. Hypertro- phy of many cells results in enlargement or overgrowth of an organ or a part of the body. Hypertrophy is of three types.

1. Physiological Hypertrophy

Physiological hypertrophy is the increase in size due to increased workload or exercise. The common physiologi. cal hypertrophy includes.

i. Muscular hypertrophy

Muscular hypertrophy refers to increase in bulk of skel- etal muscles that occurs in response to strength training exercise.

ii. Ventricular hypertrophy

Ventricular hypertrophy means increase in size of ven- tricular muscles of the heart which is advantageous only if it occurs in response to exercise.

2. Pathological Hypertrophy

Increase in cell size in response to pathological changes is called pathological hypertrophy. Example is the ven- tricular hypertrophy that occurs due to pathological condi- tions such as high blood pressure, where the workload of ventricles increases.

3. Compensatory Hypertrophy

Compensatory hypertrophy is the increase in size of the cells of an organ that occurs in order to compensate the loss or dysfunction of another organ of same type. Ex. amples are the hypertrophy of one kidney when the other kidney stops functioning and the increase in muscular strength of an arm when the other arm is dysfunctional or lost.

HYPERPLASIA

Hyperplasia is the increase in number of cells due to increased cell division (mitosis). It is also defined as abnormal or unusual proliferation (multiplication) of cells due to constant cell division. Hyperplasia results in gross enlargement of the organ. Hyperplasia involves constant cell division of the normal cells only. Hyperplasia is of three types.

1. Physiological Hyperplasia

Physiological hyperplasia is the momentary adaptive response to routine physiological changes in the body. For example, during the proliferative phase of each menstrual cycle, the endometrial cels in uterus increase in number.

2. Compensatory Hyperplasia

Compensatory hyperplasia is the increase in number of cells in order to replace the damaged cells of an organ or the cells removed from the organ. Compensatory hyperplasia helps the tissues and organs in regeneration. It is common in liver. After the surgical removal of the damaged part of liver, there is increase in the number of liver cells resulting in regeneration.

Compensatory hyperplasia is also common in epithelial cells of intestine and epidermis.

3. Pathological Hyperplasia

Pathological hyperplasia is the increase in number of cells due to abnormal increase in hormone secretion. It is also called hormonal hyperplasia. For example, in gigantism, hypersecretion of growth hormone induces hyperplasia that results in overgrowth of the body.

DYSPLASIA

Dysplasia is the condition characterized by the abnor- mal change in size, shape and organization of the cell. Dysplasia is not considered as true adaptation and it is suggested as related to hyperplasia. It is common in epithelial cells of cervix and respiratory tract.

METAPLASIA

Metaplasia is the condition that involves replacement of one type of cell with another type of cell. It is of two types.

1. Physiological Metaplasia

Replacement of cells in normal conditions is called physiological metaplasia. Examples are transformation of cartilage into bone and transformation of monocytes into macrophages.

2. Pathological Metaplasia

Pathological metaplasia is the irreversible replacement example, chronic smoking results in transformation of normal mucus secreting ciliated columnar epithelial cells into non-ciliated squamous epithelial cells, which are of cells due to constant exposure to harmful stimuli. For ncapable of secreting mucus. These transformed cells may become cancerous cells if the stimulus (smoking) is prolonged.

INEOPLASM OR TUMOR

Neoplams or tumor is the abnormal growth of any tissue or organ in the body. Neoplasm may be benign (non- cancerous) or malignant (cancerous)

CELL DEGENERATION

Cell degeneration is a process characterized by damage of the cells at cytoplasmic level, without affecting the nu- cleus. Degeneration may result in functional impairment or deterioration of a tissue or an organ. It is common in metabolically active organs like liver, heart and kidney. Degenerative changes are reversible in most of the cells.

Common Causes for Cell Degeneration

1. Atrophy, hypertrophy, hyperplasia and/or dysplasia of cell.

2. Fluid accumulation in the cell.

3. Fat infiltration into the cell.

4. Calcification of cellular organelles.

CELL AGING

Cell aging is the gradual structural and functional changes in the cells that occur Over the passage of time. It is now suggested that cell aging is due to damage of cellular substances like DNA, RNA, proteins and lipids, etc. when the cell becomes old. When more cellular substances are damaged, the cellular function decreases. This causes deterioration of tissues, organs or parts of the body. Finally, the health of the body starts declining and this leads to death. So, the cell aging determines the health and life span of the body.

STEM CELLS

Stem cells are the primary cells capable of reforming themselves through mitotic division and differentiating into specialized cells. These cells serve as repair system of the body and are present in all multicellular organisms.

SOURCE OF STEM CELLS

Stem cells are derived from, embryo, umbilical cord blood and adult.

1. Embryonic Stem Cells

Embryonic stem cells are derived from the inner cell mass of a blastocyst which is an early stage of embryo. It takes about 4 to 5 days after fertilization to reach the blastocyst stage and it has about 30 to 50 cells.

Embryonic stem cells have two important qualities:

i. Self-renewal capacity.

ii. Pluripotent nature, i.e. these cells are capable of differentiating into all types of cells in ectodermal. endodermal and mesodermal layers.

Because of these two qualities, the embryonic stem cells can be used therapeutically for regeneration or replacement of diseased or destroyed tissues. In fact, embryonic pluripotent stem cells are now cultured and Jot of research is going on to explore the possibility of using these cells in curing the disorders like diabetes mellitus by cell replacement technique. But ethical issues arise because the embryo has to be destroyed to collect the stem cells.

2. Stem Cells from Umbilical Cord Blood

Stem cells in umbilical cord blood are collected from the placenta or umbilical cord. Use of these stem cells for research and therapeutic purposes does not create any ethical issue because it does not endanger the life of the fetus or newborn. Because of vitality and easy availability. the umbilical cord blood stem cells are becoming a potent resource for transplant therapies. Nowadays, these stem cells are used to treat about 70 diseases and are used in many transplants worldwide.

3. Adult Stem Cells

Embryonic stem cells do not disappear after birth. But remain in the body as adult stem cells and play a role in repair of damaged tissues. However, their number becomes less. Adult stem cells are the undifferentiated multipotent progenitor cells found in growing children and adults. These are also known as somatic stem cells and are found everywhere in the body. These cells are capable of dividing and reforming the dying cells and regenerating the damaged tissues. So, these stem cells can also be used for research and therapeutic purposes. Adult stem cells are collected from bone marrow.

Two types of stem cells are present in bone marrow.

i. Hematopoietic stem cells, which give rise to blood cells.

ii. Bone marrow stromal cells, which can differentiate into cardiac and skeletal muscle cells.

ADVANTAGES OF STEM CELLS

Aduit stem cells from bone marrow are used in bone mai row transplant to treat leukemia and other blood disorders for 30 years. Recently, it is known that these stem cells can develop into nerve cells, liver cells, skeletal muscle cells and cardiac muscle cells. Recent discoveries also reveal that the stem cells are present in several tissues which include blood, blood vessels, skeletal muscle, liver, skin and brain. It is also found that these cells are capable of differentiating into multiple cell types. So, the cell-based therapy using stem cells may be possible to treat many diseases such as heart diseases, diabetes, Parkinson's disease, Alzheimer's disease, spi- nal cord injury, stroke and rheumatoid arthritis

Source K Sembulingam's physiology

Liver Anatomy

2020-07-30T09:55:00.002+05:30

Liver marked as red organ

Features

The liver is A large and solid gland situated in the right upper quadrant of the abdominal cavity. In the living subject, the liver is reddish brown in colour, soft in consistency, and very friable. It weighs about 1600 g in males and about 1300 g in females.

Location

Location of liver

The liver occupies the whole of the right hypo- chondrium, the greater part of the epigastrium, and extends into the left hypochondrium reaching up to the left lateral line. From the above it will be obvious that most of the liver is covered by ribs and costal cartilages, except in the upper part of the epigastrium where it is in contact with the anterior abdominal wall. The liver is the largest gland in the body. It secretes bile and performs various other metabolic functions. The liver is also called the 'hepar' from which we have the adjective 'hepatic' applied to many structures connected with the organ.

EXTERNAL FEATURES

Liver seen from the front

The liver is wedge-shaped. It resembles a four-sided nyramid laid on one side.

Five Surfaces

It has five surfaces. These are:

1 Anterior,

2Posterior,

3 Superior,

4 Inferior, and

5 Right.

Out of these the inferior surface is well defined because it is demarcated, anteriorly, by a sharp inferior border. The other suffaces are more or less continuous with each other and are imperfectly separated from one another by ill-defined, rounded borders.

One Prominent Border

The inferior border is sharp anteriorly where it separates the anterior surface from the inferior surface. It is somewhat rounded laterally where it separates the right surface from the inferior surface. The sharp anterior part is marked by:

a. An interlobar notch or the notch for the ligamentum teres

b. A cystic notch for the fundus of the gallbladder.

In the epigastrium, the inferior border extends from the left 8th costal cartilage to the right 9th costal cartilage.

Two Lobes

The liver is divided into right and left lobes by the attachment of the falciform ligament anteriorly and superiorly; by the fissure for the ligamentum teres inferiorly; and by the fissure for the ligamentum venosum posteriorly.

The right lobe is much larger than the left lobe, and forms five sixth of the liver. It contributes to all the five surfaces of the liver, and presents the caudate and quadrate lobes

The caudate lobe is situated on the posterior surface. It is bounded on the right by the groove for the inferior vena cava, on the left by the fissure for the ligamentum venosum, and inferiorly by the porta hepatis. Above it is continuous with the superior surface. Below and to the right, just behind the porta hepatis, it is connected to the right lobe of the liver by the caudate process. Below and to the left it presents a small rounded elevation called the papillary process.

The quadrate lobe is situated on the inferior surface, and is rectangular in shape. It is bounded anteriorly by the inferior border, posteriorly by the porta hepatis, on the right by the fossa for the gallbladder, and on the left by the fissure for the ligamentum teres.

The porta hepatis is a deep, transverse fissure about 5 cm long, situated on the infernor surface of the right tobe of the tiver, In res between the caudate lobe above and the quadrate lobe below and in front. The portal pein, the hepatic artery and the hepatic plexus of nertes enter the liver through the porta hepatis, while the right and left hepatic ducts and a few lymphatics leave it.The relations within, the porta hepatis are from behind forwards are the portal vein, the hepatic artery and the hepatic ducts. The lips of the porta hepatis provide attachment to the lesser omentum. The left lobe of the liver is much smaller than the right lobe and forms only one-sixth of the liver. It is flattened from above downwards. Near the fissure for the ligamentum venosum, its inferior surface presents a rounded elevation, called the omental tuberosity or tuler omentale.

Relations

Relations of the inferior surface of the liver

Peritoneal Relations

Most of the liver is covered by peritoneum. The areas not covered by peritoneum are as follows.

1 A triangular bare area, on the posterior surface of the right lobe, limited by the superior and inferior layers of the coronary ligament and by the right triangular ligament.

2 The groove for the inferior vena cava, on the posterior surface of the right lobe of the liver, between the caudate lobe and the bare area.

3 The fossa for the gallbladder which lies on the inferior surface of the right lobe to the right of the quadrate lobe.

4 The area of attachment of lesser omentum and the fissure for attachment of ligamentum venOsum.

Visceral Relations

Anterior surface

The anterior surface is triangular and slightly convex: It is related to the xiphoid process and to the anterior abdominal wall in the median plane, and to diaphragm on each side. The diaphragm separates this surface from the pleura above the level of a line drawn from the xiphisternal joint to the 10th rib in the midaxillary line; and from the lung above the level of a line from the same joint to the 8th rib. The falciform ligament is attached to this surface a little to the right of the median plane.

Posterior surface

The posterior surface is triangular. Its middle shows a deep concavity for the vertebtal column. Other relations are as follows.

1 The bare area is related to the diaphragm; and to the right suprarenal gland near the lower end of the groove for the inferior vena cava.

2 The groove for the inferior vena cava lodges the upper part of the vessel, and its floor is pierced by the hepatic veins.

3 The caudate lobe lies in the superior recess of the lesser sac. It is related to the crura of the diaphragm above the aortic opening, to the right inferior phrenic artery, and to the coeliac trunk.

4 The fissure for the ligamentum venosum is very deep and extends to the front of the caudate lobe. It contains two layers of the lesser omentum. The ligamentum venosum lies on its floor. The ligamentum venosum is a remnant of the ductus venosus of foetal life; it is connected below to the left branch of the portal vein, and above to the inferior vena cava.

5 The posterior surface of the left lobe is marked by the oesophageal impression.

Superior surface

The superior surface is quadrilateral and shows a concavity in the middle. This is the cardiac impression On each side of the impression, the surface is convex to fit the dome of the diaphragm. The diaphragm separates this surface from the pericardium and the heart in the middle; and from pleura and lung on each side.

Inferior surface

The inferior surface is quadrilateral and is directed downwards, backwards and to the left. It is marked by impressions for neighbouring viscera as follows.

1 On the inferior surface of the left lobe there is a large concave gasTIC impression. The left lobe also bears a raised area that comes in contact with the lesser omentum: it is called the tuber omentale.

2 The fissure for the ligamentum teres passes from the inferior border to the left end of the porta hepatis. The ligamentum teres represents the obliterated left umbilical vein.

3 The quadrate lobe is related to the lesser omentum, the pylorus, and the first part of the duodenum. When the stomach is empty the quadrate lobe is related to the first part of the duodenum and to a part of the transverse colon. 4 The fossa for the gallbladder lies to the right of the quadrate lobe.

5 To the right of this fossa the inferior surface of the right lobe bears the colic impression for the hepatic flexure of the colon, the renal impression for the right kidney, and the duodenal impression for the second part of the duodenum.

Right surface

The right surface is quadrilateral and convex. It is related to the diaphragm opposite the 7th to 11th ribs in the midaxillary line. It is separated by the diaphragm from the pleura up to the 10th rib, and from the lung up to the 8th rib. Thus, the upper one-third of the urface is related to the diaphragm, the pleura and the lung, the middle one-third, to the diaphragm and the Costodiaphragmatic he-third to the diaphragm alone recess of the pleura ; and the lower The liver receives 20% of its blood supply through the hepatic artery, and 80% through the portal vein. Before entering the liver, both the hepatic artery and the portal vein divide into right and left branches. Within the liver, they redivide to form segmental vessels which further divide to form interlobular vessels which run in the portal canals. Further ramifications of the interlobular branches open into the hepatic sinusoids. Thus the hepatic arterial blood mixes with the portal venous blood in the sinusoids. There are no anastomoses between adjoining hepatic arterial territories and hence each branch is an end artery.

Venous Drainage

Hepatic sinusoids drain into interlobular veins, which join to form sublobular veins. These in turn unite to form the hepatic veins which drain directly into the inferior vena cava. These veins provide great support to the liver, besides the intra-abdominal pressure. The hepatic veins are arranged in two groups: upper and lower. The upper group consists of three large veins: right, left and middle, which emerge through the upper part of the groove for the inferior vena cava, and directly into the vena cava. These veins keep the liver suspended. The lower group consists of a variable number of small veins from the right lobe and the caudate lobe which emerge through the lower part of the caval Microscopically, the tributaries of hepatic veins, ie central veins are seen as separate channels from those of the portal radicles . open groove pue open into the vena cava.

Lymphatic Drainage

The superficial lymphatics of the liver run on the surface of the organ beneath the peritoneum, and terminate in caval, hepatic, paracardial and coeliac lymph nodes. Some vessels from the coronary ligament may directly join the thoracic duct. The deep lymphatics end partly in the nodes around the end of the inferior vena cava, and partly in the hepatic nodes.

Nerve Supply

The liver receives its nerve supply from the hepatic plexus which contains both sympathetic and parasympathetic or vagal fibres. Nerves also reach the liver through its various peritoneal ligaments.

HEPATIC SEGMENTS

The segments of liver

On the basis of the intrahepatic distribution of the hepatic artery, the portal vein and the biliary ducts, the liver can be divided into the right and left functional lobes. These do not correspond to the anatomical lobes ated by a plane passing on the anterosuperior surface along a line joining the cystic notch to the groove for the inferior vena cava. On the inferior surface, the plane passes through the fossa for the gallbladder; and on the posterior surface it passes through the middle of the of the liver. The physiological lobes are sepa caudate lobe. The right lobe is subdivided into anterior and posterior segments, and the left lobe into medial and lateral segments. Thus there are four segments in the liver.

a. Right anterior (V and VIII),

b. Right posterior (VI and VII),

c. Left lateral (II and III) and

d. Left medial (I and IV).

The hepatic segments are of surgical importance The hepatic veins tend to be intersegmental in their course

FUNCTIONS

Liver is an indispensable gland of the body.

1. Metabolism of carbohydrates, fats and

2. Synthesis of bile and prothrombin

3.Excretion of drugs, toxins, poisons, cholesterol, bile pigments and heavy metals

4.Protective by conjugation, destruction, phagocytosis, antibody formation and excretion

5. Storage of glycogen, iron, fat , vitamin A and D.

HISTOLOGY

Histology of liver

Liver is covered by Glisson's capsule. In the are hexagonal lobules with portal radicles at 3-5 corners. Each radicle contains bile ductule, branch each of portal vein and hepatic artery. Central vein lies in the centre and all around the central vein are the hepatocytes in form of laminae. On one side of the lamina is the sinusoid and on the other side is a bıle canaliculus. Portal lobule seen in human is triangular in shape with three central veins at the sides and portal tract in the centre. The liver acinus is defined as the liver parenchyma around a preterminal branch of hepatic arteriole between two adjacent central veins. The liver acinus is the functional unit of liver. Blood reaches the acinus via branches of portal vein and hepatic artery to open into the sinusoids to reach the central vein On the other hand, the flow of bile is along bile canaliculi, bile ductules and the interlobular bile ducts Hepatocytes in zone I close to preterminal branch are better supplied by oxygen, nutrients and toxins The liver cells in zone III close to central veins are relatively hypoxic while cells in zone II are intermediate in oxygen supply.Histology of the liver can be studied by liver biopsy which is done from right lateral surface.

Source BD Chaurasia's Anatomy

Pancreas Anatomy

2020-07-30T09:55:00.000+05:30

Pancreas marked as red organ

Features

The pancreas is a gland that partly exocrine and partly endocrine. The exocrine part secretes the digestive pancreatic juice; and the endocrine part secretes hormones, e.g. insulin. It is soft, lobulated and elongated organ.

Location

Location and parts of pancreas

The pancreas lies more or less transversely across the posterior abdominal wall, at the level of first and second lumbar vertebrae.

Size and Shape

It is J-shaped or retort-shaped, set obliquely. The bowl of the retort represents its head, and the stem of the retort, its neck, body and tail. It is about 15-20 cm long, 2.5-3.8 cm broad and 1.2-1.8 cm thick and weighs about 90 g.The pancreas is divided (from right to left) into head, neck, body and the tail. The head is enlarged and lies within the concavity of the duodenum. The tail reaches the hilum of the spleen. The entire organ lies posterior to the stomach separated from it by the lesser sac.

HEAD OF THE PANCREAS

Head is the enlarged flattened right end of pancreas, situated within the "C-shaped" curve of the duodenum.

External Features

The head has three borders-superior, inferior and right lateral; two surfaces-anterior and posterior; and one process, called the uncinate process, which projects From the lower and left part of the head towards the left.

Relations

Relations of head of pancreas (a) Anterior (b) Posterior

Three Borders

The superior border is overlapped by the first part of the duodenum and is related to the superior pan- creaticoduodenal artery. The inferior border is related to the third part of the duodenum and to the inferior pancreaticoduodenal artery. The right lateral border is related to the second part of the duodenum, the terminal part of the bile duct and the anastomosis between the two pancreaticoduodenal arteries.

Two Surfaces

The anterior surface is related, from above downwards,

1 The first part of duodenum.

2 Transverse colon.

3 Jejunum which is separated from it by peritoneum.

The posterior surface is related to:

1 Inferior vena cava

2Terminal parts of the renal veins.

3 Right crus of the diaphragm.

4Bile duct which runs downwards and to the right and is often embedded in the substance of pancreas.

Uncinate Process

It is related anteriorly to the superior mesenteric vessels, and posteriorly to the aorta.

NECK OF THE PANCREAS

This is the slightly constricted part of the pancreas between its head and body. It is directed forwards, upwards and to the left. It has two surfaces, anterior and posterior.

Relations

The anterior surface is related to:

(1) The peritoneum covering the posterior wall of the lesser sac, and

(2) the pylorus.

The posterior surface is related to the termination of the superior mesenteric vein and the beginning or the portal vein.

BODY OF THE PANCREAS

The body of the pancreas is elongated. It extends from its neck to the tail. It passes towards the left with a slight upward and backward inclination.

External Features

It is triangular on cross-section, and has three borders (anterior, superior and inferior). A part of the body projects upwards beyond the rest of the superior border, a little to the left of the neck. This projection is known as the tuber omentale.

Relations

Anterior and inferior relations of the body of pancreas

Posterior relations of the body of pancreas

Three Borders

The anterior border provides attachment to the root of the transverse mesocolon. The superior border is related to coeliac trunk over the tuber omentale, the hepatic artery to the right, and the splenic artery to the left. The inferior border is related to the superior mesenteric vessels at its right end.

Three Surfaces

The anterior surface is concave and is directed forwards and upwards. It is covered by peritoneum, and is related to the lesser sac and to the stomach. The posterior surface is devoid of peritoneum, and is related to:

a. The aorta with the origin of the superior mesenteric artery.

b. Left crus of the diaphragm.

c. Left suprarenal gland.

d. Left kidney.

e. Left renal vessels.

f. Splenic vein.

The inferior surface is covered by peritoneum, and is related to the duodenojejunal flexure, coils of jejunum and the left colic flexure.

TAIL OF THE PANCREAS

This is the left end of the pancreas. It lies in the lienorenal ligament together with the splenic vessels. It comes into contact with the lower part of the gastric surface of the spleen.

DUCTS OF THE PANCREAS

The pancreatic ducts

The exocrine pancreas is drained by two ducts, main and accessory.

1 The main pancreatic duct of Wirsung, 3 mm in diameter lies near the posterior surface of the pancreas and is recognised easily by its white colour. It begins at the tail; runs towards the right through the body; and bends at the neck to run downwards, backwards and to the right in the head.

2 It receives many small tributaries which join it at acute anglés to its long axis showing a 'V' shape pattern fórming what has been described as a herring bone pattern.

3 Within the head of the pancreas the pancreatic duct is related to the bile duct which lies on its right side. The two ducts enter the wall of the second part of the duodenum, and join to form the hepatopancreatic ampulla of Vater which opens by a narrow mouth on the summit of the major duodenal papilla, 8 to 10 cm distal to the pylorus.

4 The accessory pancreatic duct of Santorini begins in the lower part of the head, crosses the front of the main duct with which it communicates and opens into the duodenum at the minor duodenal papilla. The papilla of accessory pancreatic duct is situated 6 to 8 cm distal to the pylorus. The opening of the accessory duct lies cranial and ventral to that of the main duct. The two ducts remind the double origin of pancreas from the ventral and dorsal pancreatic buds.

Arterial Supply

Arterial supply of the pancreas

The pancreas is supplied:

1 Mainly by pancreatic branches of the splenic artery,

2 The superior pancreaticoduodenal artery, and

3 The inferior pancreaticoduodenal artery.

Like the duodenum the pancreas develops at the junction of the foregut and midgut, and is supplied by branches derived from both the coeliac and superior mesenteric arteries.

Venous Drainage

Venous drainage of the pancreas

Veins drain into splenic, superior mesenteric and portal veins.

Lymphatic Drainage

Lymphatics follow the arteries and drain into the pancreaticosplenic, coeliac and superior mesenteric groups of lymph nodes.

Nerve Supply

The vagus or parasympathetic and splanchnic sympathetic nerves supply the pancreas through the plexuses around its arteries. Sympathetic nerves are vasomotor. Parasympathetic nerves control pancreatic secretion. Secretion is also influenced by the hormone cholecystokinin produced by cells in the duodenal epithelium. The pancreatic juice contains various enzymes that help in the digestion of proteins, carbohydrates and fats.

Functions

1 Digestive: Pancreatic juice contains many digestive enzymes of which the important ones are as follows: Trypsin breaks down proteins to lower peptides. Amylase hydrolyses starch and glycogen to disaccharides. Lipase breaks down fat into fatty acids and glycerol.

2 Endocrine: Carbohydrates are the immediate source energy. Insulin helps in utilizations of the cells. Deficiency of insulin results in hyper- glycaemia. The disease is called diabetes mellitus. There appears to be poverty in plenty.

3 Pancreatic juice: It provides appropriate alkaline medium (pH 8) for the activity of the pancreatic enzymes.

HISTOLOGY

Histology of pancreas

1 The exocrine part is a serous gland, made up of tubular acini lined by pyramidal cells with basal round nuclei, containing zymogen granules. It secretes the digestive pancreatic juice.

2 The endocrine part of pancreas is made up of microscopic elements called the pancreatic islets of Langerhans. These are small isolated masses of cells distributed throughout the pancreas. They are most numerous in the tail. The islets have various types of cells the most important of which are the beta cells which are granular and basophilic, forming about 80% of the cell population. They produce insulin. Other types of cells are alpha cells with subtype A1 and A2. These are granular and acidophilic and form about 20% of the cell population. A1 cells belong to enterochromaffin group and secrete pancreatic gastrin and serotonin. A2 cells secrete glucagon.

Source BD CHAURASIA'S Anatomy

Spleen anatomy

2020-07-30T09:54:00.000+05:30

SPLEEN

INTRODUCTION

Spleen (Greek splen and Latin Lien) is a lymphatic organ connected to the blood vascular system. It acts as a filter for blood and plays an important role in the immune responses of the body.

Location

Location of spleen (a) in relation to the nine regions of abdomen (b) in relation to the fundus of stomach and the diaphragm

The spleen (Latin low spirits) is a wedge-shaped organ lying mainly in the left hypochondrium, and partly in the epigastrium. It is wedged in between the fundus of the stomach and the diaphragm. The spleen is tetrahedral in shape.

Dimensions

The spleen is soft, highly vascular and dark purple in colour. The size and weight of the spleen are markedly variable. On an average the spleen is 1 inch or 2.5 cm thick, 3 inches or 7.5 cm broad, 5 inches or 12.5 cm long,7 ounces in weight, and is related to 9th to 11th rihe The odd numbers are 1, 3, 5 are 7, 9, 11. Normally, the spleen is not palpable.

Position (Axis of Spleen)

Position of spleen in relation to left 9th - 11th ribs

The spleen lies obliquely along the long axis of the 10th rib. Thus it is directed downwards, forwards and laterally, making an angle of about 45 degrees with the horizontal plane.

External Features

The spleen has two ends, three borders, two surfaces, two angles and hilum.

Two Ends

• The anterior or lateral end is expanded and is more like a border. It is directed downwards and forwards, and reaches the midaxillary line.

• The posterior or medial end is rounded. It is directed upwards, backwards and medially, and rests on the upper pole of the left kidney.

Three Borders

• The superior border is characteristically notched near the anterior end.

• The inferior border is rounded.

• The intermediate border is also rounded and is directed to the right.

Two Surfaces

• The diaphragmatic surface is convex and smooth.

• The visceral surface is concave and irregular.

Two Angles

Anterobasal angle: It is the junction of superior border with lateral or anterior end. It is the most forwand border Anterobasal angle: It is the junction of superior projecting part of spleen. When spleen is enlarged, this is felt first, so this is called as "clinical angle of spleen. Posterobasal angle: Junction of inferior border with lateral or anterior end of spleen.

Hillum

Hilum lies between superior and intermediate borders. It is pierced by branches and tributaries of splenic vessels.

Relations

Peritoneal Relations

Peritoneal ligaments attached to the spleen and common sites of accessory spleen

The spleen is surrounded by peritoneum, and is suspended by following ligaments.

1 The gastrosplenic ligament extends from the hilum of the spleen to the greater curvature of the stomach. It contains the short gastric vessels and associated lymphatics and sympathetic nerves.

2 The lienorenal ligament extends from the hilum of the spleen to the anterior surface of the left kidney. It contains the tail of the pancreas, the splenic vessels, and associated pancreaticosplenic lymph nodes, lymphatics and sympathetic nerves.

3 The phrenicocolic ligament is not attached to the spleen, but supports its anterior end. It is a horizontal fold of peritoneum extending from the splenic flexure of colon to the diaphragm, opposite the 11th rib in the midaxillary line. It limits the upper end of the left paracolic gutter.

Visceral Relalions

Visceral relations of spleen

Visceral surface

The visceral surface is related to the fundus of the stomach, the anterior surface of the left kidney, the splenic flexure of the colon and the tail of the pancreas. The gastric impression, for the fundus of the stomach, lies between the superior and intermediate borders. It is the largest and most concave impression on the spleen. The renal impression, for the left kidney, lies between the inferior and intermediate borders. The colic impressions, for the splenic flexure of the colon, occupies a triangular area adjoining the anterior end of the spleen. Its lower part is related to the phrenicocolic ligament. The pancreatic impression, for the tail of the pancreas, lies between the hilum and the colic impression. The hilum lies on the inferomedial part of the gastric impression along the long axis of the spleen. It transmits the splenic vessels and nerves, and provides attachment to the gastrosplenic and lienorenal ligaments.

Diaphragmatic Surface

The diaphragmatic surface is related to the diaphragm which separates the spleen from the costodiaphrag- matic recess of pleura, lung and 9th, 10th and 11th ribs of the left side.

Blood Supply Of Spleen

Blood supply of spleen

Arterlal Supply

The spleen is supplied by the splenic artery which is the largest branch of the coeltac trunk. The artery is tortuous in its course to allow for movements of the spleen. It passes through the lienorenal ligament to reach the hilum of the spleen where it divides into fu or more branches. These branches enter the spleen supply it. Within the spleen it divides repeatedly to form successfully the straight vessels called penicilli, which further divide into ellipsoids and arterial capillaries Further course of the blood is controversial. According to closed theory of splenic circulation, the capillaries are continuous with the venous sinusoids that lie in the red pulp; the sinusoids join together to form veins. However, according to open theory of splenic circulation, the capillaries end by opening into the red pulp from where the blood enters the sinusoids through their walls. Still others believe in a compromise theory, where the circulation is open in distended spleen and closed in contracted spleen. The splenic circulation is adapted for the mechanism of separation and storage of the red blood cells. On the basis of its blood supply, the spleen is said to have superior and inferior vascular segments. The two segments are separated by an avascular plane. Each segment may be subdivided into one to two disc-like middle segments and a cap-like pole segment. Apart from its terminal branches, the splenic artery gives off:

a. Numerous branches to the pancreas,

b. 5 to 7 short gastric branches, and

c. The left gastroepiploic artery.

Venous Drainage

The splenic vein is formed at the hilum of the spleen. It runs a straight course behind the pancreas It joins the superior mesenteric vein behind the neck of the nancreas to form the portaf vein. Its Iibutaries are the hort gastric. Teft gastroepiploic, pancreatic and inferior mesenteric veins.

Lymphatic Drainage

Splenic tissue proper has no lymphatics. A few Iymphatics arise from the connective tissue of the capsule including trabeculae and drain into the pancreaticosplenic lymph nodes situated along the splenic artery.

Nerve Supply

Sympathetic fibres are-derived from the coeliac plexus. They are vasomotor in nature. They atso supply some smooth muscle present in the capsule.

Functions of the Spleen

1 Phagocytosis: The spleen is an important component of the reticuloendothelial system. The splenic phagocytes include:

a. The reticular cells and free macrophages of the red pulp.

b. Modified reticular cells of the ellipsoids.

c. Free macrophages and endothelial cells of the venous sinusoids, and

d Surface reticular cells of the lymphatic follicle. The phagocytes present in the organ remove cell debris and old and effete RBCS, other blood cells and microorganisms, and thus filter the blood. Phagocytosis of circulating antigens initiates humoral and cellular immune responses

2 Haemopoiesis: The spleen is an important haemo- poietic organ during foetal life. Lymphopoiesis continues throughout life. The lymphocytes manufactured in it take part in immune responses of the body In the adult spleen, haemopoiesis can restart in ertain diseases, like chronic myeloid leukaemia and myelosclerosis.

3 Immune responses: Under antigenic stimulation, there occurs increased lymphopoiesis for cellular responses, and increased formation of plasma cells for the humoral responses.

4 Storage of RBCS: Red blood cells can be stored in the spleen and released into the circulation when needed. This function is better marked in animals than in man.

HISTOLOGY

Histology of spleen

Histologically, spleen is made up of the following four component parts.

1 Supporting fibroelastic tissue, forming the capsule, coarse trabeculae and a fine reticulum. In human, the smooth muscle cells in the capsule and trabeculae are few, and the contraction and distension of spleen are attributed to constriction or relaxation of the blood vessels, which regulate the blood flow in the organ.

2 White pulp consisting of lymphatic nodules arranged around an arteriole called malpighian corpuscle.

3 Red pulp is formed by the collection of cells in the interstices of reticulum, in between the sinusoids. The cell population includes:

a. All types of lymphocytes (small, medium and large)

b. All three types of blood cells (RBC, WBC and platelets)

c. The fixed and free macrophages. Lymphocytes are freely transformed into plasma cells which can produce large amounts of antibodies the immunoglobulins.

4 Vascular system transverses the organ and permeates it thoroughly.

Source BD CHAURASIA'S Anatomy

Human Anatomy

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Definition:- Human Anatomy is the branch of science which explains the structure and functions of human body.

Sub Divisions Of Anatomy

1.Gross Anatomy:- Study of parts of body by naked eye in the disection of cadebus (dead body).

2.Regional Anatomy:- The study of different regions of body (bake, upper, thoracic, abdominal region etc)

3.Systemic Anatomy:- The study of different systems of body ( skeleton, muscular etc)

4.Histology:- Study of different organs at tissue level

5.Embryology:- Study of development and changes of embryo until birth.

6.Surgical Anatomy:- Study in relation to the different surgeries.

7.Sectional Anatomy:- Study of various parts of body in coronal, horizontal and surgical sections.

8.Living Anatomy:- Study of parts of the living human being.

9.Splanchnology:- The study of visceral organs of body.

10.Neuro Anatomy:- Study of nervous system.

Systems of body

1.Integumentary system:- Consist of skin, hairs, nails, sweat glands, oil glands.

2.Skeleton System:- Consist of bones, joints and cartelegous.

3.Muscular System:- Consist of muscles and composed of skeleton muscles.

4.Cardiovascular System:- Consist of blood, heart and blood vessel.

5.Digestive System:- Consist of mouth, pharynx, oesophagus, stomach,small and large intestine, anus,

salivary glands, liver, gall bladder and pancreas.

6.Respiratory System:- Consist of lungs, pharynx, larynx, trachea and bronchial tubes.

7.Urinary Bladder:- Consist of kideys, uterus, utinary bladder and urethra.

8.Reproductive System:- Consist of testis, epididymis, vas deferens, prostate, seminal vesicle,

ejaculatory duct and penis ( in males ) uterus, vagina, uterine tubes and ovaries ( in females ).

9.Lymphatic System:- Consist of lymph vessels, spleen, thymus, lymph nodes and tonsils.

10.Endocrine System:- Consist of pineal gland, hypothalamus, pituitary gland, thymus, thyroid gland, parathyroid gland, adrenal gland, pancreas, ovaries and testis.

11.Nervous System:- Consist of brain, spinal chord, nerves, eyes and ears.

PLAINS OF BODY
Bodies often sectioned along the plain to study the internal anatomy. There are three plains/sections of human body.

1.Saggital section/ Longitudinal Plain
Divides the structure into right and left equal parts.

2.Coronal Section/ Frontal Section
Passes right angles to the surgical plain and divides the body into anterior and posterior parts.

3.Transverse Plain/ Horizontal Plain
Passes right angles to the both surgical and coronal plains. Divides the structure into superior and inferior parts.

BASIC ANATOMICAL TERMS

1.Anterior:- Towards the front. For example: Sternum is anterior to the vertebral column.

2.Posterior:- Towards the back. For Example: Vertebral column is posterior to the sternum.

3.Superior:- Towards the top. For Example: Nose is superor to mouth.

4.inferior:- Towards the bottom. For Example: Mouth is inferior to nose.

5.Caudal:- Towards the bottom. For Example: Neck is caudal to the skull

6.Cranial:- Towards the top. For Example: Skull is cranial to neck.

7.Medial:- Towards the bottom. For Example: Nose is medial to ears.

8.Lateral:- Towards the sideways or away from the mid line. For Example:- Ears are lateral to nose.

9.Superfacial:- Situated on surface. For Example:- Skin is superfacial to bones.

10.Dwe:- Situated towards inside. For Example:- Bones are dwe to the skin.

11.Proximal:- Situated closer to the origin of the body (organ)
. For Example:- Elbow is proximal to wrist.

12.Distal:- Situated further from the origin of body. For Example:- Wrist is distal to the elbow.

ANATOMICAL POSITION

Face facing forward

Standing erect posture

Arms down at side

Palms facing forward

Thumbs pointing to sides

Feet pointing forward