EPITHELIAL TISSUE

 TYPES OT ISSUES 

         

ANIMAL TISSUE

Tissue - In multicellular animals, a group of similar cells along with intracellular substances perform a specific function. Such an organization is called tissue.

Animal Tissues 

The structure of the cells vary according to their function. Therefore, the tissue are different and are broadly classified into four types :

• Epithelial 

• Connective 

• Muscular 

• Neural

EPITHELIAL 

An Epithelium is a tissue composed of one or more layers of cells covering the external surface of the body and viscera (Internal organs), or lining the cavities in the body and viscera.

Origin - Epithelia arises from all the three primary germ layers : Ectoderm, Mesoderm and Endoderm, of the embryo. For Example, the epidermis arises from the ectoderm, coelomic epithelium from the mesoderm,and epithelial lining of the alimentary canal from the endoderm.

Structure - Epithelial tissue consist of variously shaped cells closely arranged in one or more layers, forming continuous sheets. There is a little intercellular Material between the cells. The cells are held together by intercellular junction. The epithelia rest on a thin, noncellular basement membrane. The epithelial generally lack blood vessels, but still have a good power of repair after injury. They receive useful materials from the blood flowing in the blood vessels of the connective tissue across the basement Membrane and transfer waste matters to the blood in the same way. Nerve endings may penetrates the epithelium. 

Basement membrane - The basement membrane consists of two layers : outer thin basal lamina secreted by the epithelial cells, and inner thick fibrous or reticular lamina contributed by the underlying connective tissue. Both are composed of collagen fibrils associated with glycoprotein.  The basement Membrane provides elastic support, and acts as a selective diffusion barrier. 

Junction between Epithelial cells - The common intracellular junction include tight junction, gap junction, desmosome, interdigitation and intracellular bridge.

Cell surface specialization - The free surface of the epithelial cell may bears special structures, such as microvilli,stereocilia, cilia and flagella, for specific purposes. The difference between microvilli, Stereocilia and Cilia are tabulated. 

Functions.

Epithelia serve a variety of functions -

• Protection - Epithelia cover the surface and line the cavities. Therefore, they are sometimes called surface tissue. They protect the underlying or overlying tissue from -

• Mechanical injury

• Drying up 

• Infection 

• Harmful chemicals 

• Formation of exoskeleton - Surface epithelia produce many exoskeleton structures, such as scales, feather, hair, claws, nails, hoofs and horns, in tetrapod vertebrates. 

• Secretion- Epithelial lining the cavities give rise to glands that provide valuable secretions such as mucus,gastric juice.

• Absorption - Epithelial lining of the intestine absorbs digested food. 

• Excretion - Epithelial lining of uriniferous tubules in the kidneys eliminate nitrogenous waste material as urine. 

• Respiration- Epithelium of the lungs alveoli brings about exchange of oxygen and carbon dioxide between blood and air. 

• Sensation - Sensory epithelium in sense organs, e.g, nasal chambers, picks up stimuli from the environment and send nerve impulses to the central nervous system. 

• Conduction - Ciliated epithelium moves fluid, mucus and other materials in the organs it lines. 

• Reproduction - Germinal epithelium of the seminiferous tubules and ovaries produce spermatozoa and ova respectively. 

• Regeneration - The ability of epithelia to regenerate quickly helps in the healing of wounds. 

• Contractility - Myoepithelium, by its contraction,expels secretions ( milk, saliva, sweat)   from the respective gland. 

classification 

Epithelia are classified on the basis of cell layers and shape of cells.  There are two main types of animal epithelia : 

• Simple 

• Compound 

SIMPLE EPITHELIUM 

The simple epithelia consist of a single layer of cells resting on a basement membrane. They cover a moist surface, where there is little wear and tear by friction. They are further of five types according to the form and structure of their cells : Squamous, cuboidal, columnar, ciliated and pseudostratified. 

Simple Squamous Epithelium 

Structure - This epithelium consists of thin, flat, disc-like cells closely fitted like the tiles in a floor. Hence, it is also called pavement epithelium. The cell outlines are roughly polygonal in surface view. The nucleus is also flattened like the cell. It often lies at the center of the cell and cause a slight bulge. 

Location - The squamous epithelium lines the Bowman's capsule of the urinary tubules and the alveoli of the urinary tubules and the alveoli of the lungs.

Simple squamous epithelium also lines the blood vessels and the coelom. At these places, it is known as endothelium and mesothelium respectively. The cell outlines of endothelium and mesothelium are wavy. These epithelium are termed tessellated epithelia. 

Function - Protection, excretion, gas exchange, and coelomic fluid. 

SIMPLE CUBOIDAL EPITHELIUM 

STRUCTURE - This epithelium consists of cells that are about as tall as wide. The cell outlines are polygonal in surface view. The nucleus is rounded and lies at the center of the cell. 

Location - The cuboidal epithelium lines the small salivary and pancreatic duct.  Hence it is non secretory. It also lines the sweat, salivary and thyroid glands,where it is secretory. It also lines the proximal part of the urinary tubules. Here the cuboidal cell have numerous microvilli at their free ends. The latter give brush like appearance, and the epithelium is called brush bordered cuboidal epithelium. The villi increase the surface area of the free ends of the cells for proper reabsorption of materials. The cuboidal epithelium also covers the ovaries and lines the sperm - producing tubules of the testes. At these place it is known as germinal epithelium because it produces new cells for the formation of gametes. 

FUNCTION - Simple cuboidal epithelium performs the functions of protection, secretions, absorption and gamete formation. 

SIMPLE COLUMNAR EPITHELIUM 

STRUCTURE- The epithelium consists of cells that are much taller than wide. The cell outlines are polygonal in surface view. The nucleus is elongated along the long axis of the cell and may have a variable position. 

Location - The columnar epithelium lines the stomach, intestine and gallbladder. It also forms the gastric glands, intestinal glands and pancreatic lobules. Here it has a secretory role and is called glandular epithelium. Certain cells of the columnar epithelium lining the stomach and intestinal secrete a slimy substance, the mucus, that forms a fine coating over the epithelium. These cells are called globalt cells because they look like a goblet when with a vacuum full of mucus. The epithelium containing mucus - secreting cells, along with the underlying supporting connective tissue, is called mucous membrane, or mucosa. Other columnar cells of the intestinal lining are specialized for the absorption of water and digested food. They bear microvilli at the free ends to increase the surface area. The columnar epithelium of the small intestine is known as brush - border columnar epithelium. 

FUNCTION- simple columnar epithelium provides protection, secretion, and absorption. 

SIMPLE CILIATED EPITHELIUM 

STRUCTURE - The epithelium consists of cells that bear fine, vibratile cytoplasmic processes, the cilia, on the free surface.  A cilium arises from a minus basal granule lying below the cell membrane. Mucus - Secreting goblet cells almost invariably occur at intervals among the ciliated cells.  Mucus spreads over the epithelium as a thin layer. Cillia, by lashing action, move mucus and other substances over the epithelium. The ciliated epithelium is of 2 types :

• CUBOIDAL CILIATED EPITHELIUM - It consists of cuboidal cells that bear cilia on the free surface. It lines certain parts of urinary tubules of the kidney. It keeps the urine moving. 

• COLUMNAR CILIATED EPITHELIUM - It consists of columnar cells which bear cilia on the free surface. It lines the nasal passages, oviduct (Fallopian tubes), terminal bronchioles, Ventricles of the brain, and central canal of the spinal cord of the embryo. 

FUNCTIONS -  It provides protection as well as movement of mucus, urine, eggs and cerebrospinal fluid in a particular direction.

PSEUDOSTRATIFIED EPITHELIUM 

STRUCTURE - This epithelium is one -cell thick but appears 2-layered because all the cells do not reach the free surface, but all cells are attached to the basement membrane, hence the name pseudostratified. The tall cells have oval nuclei that lie at different levels. The short cells have rounded nuclei. Mucus - Secreting goblet cells occur here and there in the epithelium. The Pseudostraified is of two types : Columnar and ciliated. 

• PSEUDOSTRATIFIED COLUMNAR EPITHELIUM 

This epithelium has columnar cells without cilia. It lines the large ducts of certain glands such as parotid salivary glands and the urethra of the human male. It is also a component of olfactory Mucosa.

• PSEUDOSTRATIFIED CILIATED COLUMNAR EPITHELIUM -

This Epithelium has columnar cells. The tall cells bear cilia at the free surface, the short cells without cilia. The pseudostratified columnar ciliated epithelium lines the trachea and large bronchi.The movement of its cilia push the mucus laden with dust particles and bacteria toward the larynx. 

FUNCTIONS - It provides protection, movement of secretions from glands, urine and semen in urethra, and mucus loaded with dust particles and bacteria in trachea. 

ECHINDERMATA

 PHYLUM- ECHINODERMATA 

The phylum echinodermata includes the starfish, brittle star, sand dollars, sea urchin, sea cucumber, and sea lilies. They possess the following characters :-

1. Body from :- The body form has various shapes - star-like, cylindrical, melon-like,disk like, or flower like. It is unsegmented.

2. Symmetry :- The symmetry is radial in the adults and bilateral in the larvae. The radial symmetry of the adult is pentamerous, body parts are arranged in five or multiples of fives. The radial symmetry has been secondarily derived from the bilateral ancestral form, and the echinoderms are in no way related to the radiate phyla, namely, porifera and cnidaria. The echinoderms have a much higher level of structure than the other radiate groups. 

3. Germ layer :- The echinoderms are triploblastic animals. 

4. Level of organisation :- The echinoderms have organ system level of organisation. All systems have radial arrangement. 

5. Head :- The body lacks head in all echinoderms. 

6. Appendages :- Peculiar tube feet (podia) are developed for locomotion. The Tube feet generally protrude out through special radial areas called ambulacra. They are extended and retracted by variation in hydraulic pressure of fluid in them and by contractions of their muscles. 

7. Body wall :- Epidermis is single -layered and ciliated. It overlord a thick dermis containing mesodermal endoskeleton of calcareous plates. The latter often have spines, hence echinodermata muscles are smooth and lie below the dermis. 

Minute pincer-like structures, called pedicellariae, project through the skin in many debris and organisms by opening and closing suddenly. 

              

8. Body cavity :- A true coelom lined by ciliated mesothelium is present. It is enterocoelous, and contains a fluid with free amoeboid cells called coelomocytes. The coelomic fluid plays an important role In food and gas transport. 

A part of the larval coelom is modified into a unique water filled ambulacral, or water vascular system with tube feet to help locomotion. A perforated Plate, termed madreporite, permits entry of water into the ambulacral system. 

9. Digestive Tract :- The digestive tract is usually complete. It is complete in brittle stars.

10. Respiration :- Respiration occurs by gills called dermal branchiae, or papulae in starfishes, by genital warts bursae in brittle stars, by peristomial gills in sea urchins, by cloacal trees in sea - cucumber,  and by tube -feet in all. 

11. Circulatory :- The circulatory system is greatly reduced and is of open type. It is called the haemal system. Blood often lacks a respiratory pigment. There is no heart or pumping vessel. The circulatory system has little role in gas transport. 

12. Excretory system :- There are no excretory organs. Nitrogenous Waste is ammonia (ammonotelism). It diffuses out by gills, bursae, respiratory trees and tube feet.

13. Nervous system :- The nervous system includes a nerve ring and radial nerve cords.  There is no brain. Sense organs are simple. 

14. Reproduction :- Saxes are separate. Males and females are extremely alike. Copulation does not occur. Fertilisation is generally external. Some forms reproduce asexually by self diffusion. 

15. Development :- Life history includes a ciliated, bilaterally symmetrical larva, that undergoes conspicuousness in brittle stars, echinopluteus in sea urchins, auricularia in sea - cucumber and crinoidea in feather star. 

Unique features 

The echinoderms have the following characters :-

• Bilateral symmetry in the larvae and radial symmetry in the adult. 

• Demarcation of the body surface into alternating ambulacral and interambulacral areas. 

• Mesodermal endoskeleton of calcareous plates, usually vascular system for aid in locomotion. 

• Modification of a part of the coelom into a water vascular system for aid in locomotion. 

• Characteristic tube feet for locomotion. 

• Peculiar pedicellariae for cleaning the body surface. 

CLASSIFICATION 

The phylum EChinodermata is divided into five classes :

• Asteroidea 

• Ophiuroidea 

• Echinoidea 

• Holothuroidea 

• Crinoidea

Class 1. Asteroidea 

The astrodea have following characters :-

• Body is flat and star like 

• Arms are not sharply marked off from the central disc 

• Spines are short. 

• Oral surface bears mouth and ambulacral grooves. 

• Aboral surface has annus and madreporite. 

• Pedicellariae, when present, are small 

• Viscera extends into the arms. 

• Larva is bipinnaria or brachiolaria. 

Example :- Asterias - The Starfish

Class 2 Ophiuroidea 

The ophiuroidea have following characters :-

• Body is flat and star-like .

• Spines are short. 

• Arms are sharply marked off from the central disc. 

• Oral surface has mouth and madreporite. 

• Annus and ambulacral grooves are absent. 

• Pedicellariae are absent. 

• Larva is pluteus. 

      Example :- ophiothrix - the brittle star, astropecten - the basket star. 

Class 3. Echinoidea 

The echinoidea have following characters :

• Body is globular or disc-like. 

• There Are no arms. 

• Spines may be small or large. 

• Oral surface bears the mouth. The mouth is fitted with a characteristic biting and chewing toothed apparatus called Aristotle's lantern. 

• Aboral surface carries annus and madreporite. 

• Ambulacral grooves are absent. 

• Pedicellariae are present. 

• Larva is pluteus. 

Example :- Echinus - the sea urchin, 

Class 4. Holothuroidea 

The holothuroidea have following characters :

• Body is long and cylindrical 

• There are no annus. 

• Spines are absent. 

• Oral end bears the mouth surrounded by a ring of branched and untouched tentacles. 

• Aboral end carries an annus. 

• Madreporite is internal. 

• Ambulacral grooves are absent. 

• Larva is auricularia. 

Example - cucumaria - the sea cucumber, Holothuria-the sea cucumber. 

Class 5. Crinoidea 

The crinoidea have following characters :-

• Body is fixed by cirri. 

• Arms are branched. 

• Spines are absent. 

• Oral surface is upper and bears the mouth as well as the annus. 

• Ambulacral grooves are ciliated. 

• There is no madreporite. 

• Viscera extends into the arms.

• Pedicellariae are absent. 

• Larva is doliolaria. 

• Example - Antedon - The feather star. 

THANK YOU EVERYONE 

BLOOD

 BLOOD

Blood is a special connective tissue consisting of a fluid matrix, plasma and formed elements. 

PHYSICAL APPEARANCE 

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Blood is an opaque, mobile fluid connective tissue, mesodermal in origin. It is somehow sticky, and slightly heavier than water bulk for bulk (specific gravity 1.06).It has saltish taste and a mild alkaline reaction PH is 7.4.Its osmotic pressure at 37 degrees celsius is about 7.6 atmosphere. It is bright red when oxygenated and purple when deoxygenated. 

COMPOSITION 

BLOOD consists of a watery fluid called plasma containing certain floating bodies termed formed elements. The plasma and formed volume of the blood respectively. 

PLASMA:- The plasma is a straw coloured, viscous fluid constituting nearly 55 percent of the blood. It is a complex mixture which is in dynamic equilibrium with the intracellular fluid bathing the cells and the intracellular fluid present within the cells. 

It constantly takes up and loose materials as it flows through the capillaries, yet it has a constant chemical composition. It consists of about 90% water 1% inorganic salts in true solution and 7 or 8% proteins in colloidal state. The plasma is formed by food materials, waste products, dissolved gases, regulatory substances, anticoagulant cholesterol and antibodies. These substances do not form an integral part of the plasma as they enter and leave at intervals. They are being carried by the plasma from one place to another in the body.

• Proteins:- The plasma contains a number of proteins : Serum albumin, globulins, prothrombin and fibrinogen. The plasma proteins serve many functions :

• They serve as acid -base buffers, they maintain PH of the blood by neutralizing acid and base. 

• Albumins and globulins maintain osmotic pressure of the plasma so that the letter may retain water. Fall in the level of plasma proteins causes excessive filtering of water from the blood into the tissues. This may produce oedema (swelling) of hands and feet in persons taking protein -deficient diet. 

• Plasma proteins transport certain material in combination with them. Thyroxine is bound to albumin or a specific globulin, insulin is combined with globulin, fatty acids are joined to albumen for transport in the plasma.

• Some globulin called immunoglobulins (Ig), form protective proteins, termed antibodies, in response to the entry of foreign agents, the antigens,into the body. The antibodies inactivate the antigens 

• Prothrombin and fibrinogen play a role in blood clotting. 

INORGANIC SALTS

• The inorganic salts occur in the plasma as ions. Sodium and chloride are the principal cation and anion of the plasma. The anions bicarbonate and phosphate, and the cation potassium,magnesium,calcium,

Iron and manganese occur in smaller amounts. The inorganic salt present in the plasma as dissolved ions are often called blood (plasma) electrolytes. 

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FOOD MATERIALS 

• The food materials present in the plasma are glucose, amino acid, fatty acids and triglycerides. Their amount depends upon the digestion of food in the alimentary canal. Normally an adult person has 80 to 100mg.of glucose per 100 ml of blood 12 hours after a meal. 

• If blood sugar exceeds 180mg., glucose is excreted in the urine, causing the disease diabetes mellitus or hyperglycemia. 

• Fall in blood sugar is called hypoglycemia. 

waste products :- The waste products found in the plasma are urea, uric acid, ammonia and creatinine. These are removed by the kidneys. Their excess causes a toxic effect called uraemia. 

Dissolved Gases :- small amounts of oxygen, carbon dioxide and nitrogen are dissolved in the plasma. 

Regulatory Substances :- These include hormones, vitamins and enzymes. 

Anticoagulant :- A natural strong anticoagulant present in the plasma is a heteropolysaccharide named antiprothrombin, or heparin. It checks clotting of blood in uninjured blood vessels by preventing the conversion of prothrombin into thrombin. It is produced in the liver. 

Cholesterol :- Liver synthesizes cholesterol and releases it into the blood. It is also absorbed into the blood from the food, such as egg, digested in the intestine. It provides material to the tissue cells for the synthesis of membrane lipids, vitamin D,  steroid hormones and bile salt. Cholesterol normally ranges from 50 to 180mg. Per 100 ml. Of blood. Rise in the level of cholesterol in the blood may cause heart trouble. 

RED BLOOD CORPUSCLES (RBC) 

The red blood corpuscles are the most numerous formed elements of the blood. They are the most abundant cells in the human body. The unique feature of the RBCs is the presence of a red, oxygen - carrying pigment,  the haemoglobin, in their cytoplasm. 

• Shape :- The shape of RBCs varies in different vertebrate classes. In fishes, amphibians, reptiles and birds, they are oval, biconvex and nucleated. In mammals, they are circular, biconcave, denuclated discs. Their central part is thinner than the Margin. This shape provides. flexibility and results in a 20to 30% increase in surface area as compared to a sphere. This favours quick diffusion of gases. 

• Size :- Human RBCs are smaller than the white corpuscles. 

• Number :- The RBCs are far more numerous than the WBCs. A normal healthy adult man and woman have about 5.5 and  4.5 million RBCs per cubic millimeter of blood respectively. This is called the total RBCs count. Anemia may be caused by loss of blood (haemorrhage), destruction of RBCs (haemolysis).

• The RBCs count increase during exercise to meet the increased demand of oxygen and at high altitudes to cope with the low oxygen content of the air. 

• An abnormal rise in RBCs count is called polycythemia. 

• Decrease in the number of red blood corpuscles, termed erythrocytopenia, cause oxygen shortage in the blood and tissue. The oxygen shortage stimulates the kidneys cells to secrete a hormone,called erythropoietin,into the blood. 

• This hormone, in turn, stimulates the bone marrow to increase the production of red blood corpuscles. Addition of red corpuscles increases the oxygen -carrying capacity of the blood. As the blood's oxygen level becomes normal, secretion of erythropoietin stops and the production of red blood corpuscles return to normal. 

• Colour :- The RBCs look yellowish when seen singly and red viewed in bulk. They impared red colour to the blood. The colour is due to the solution of iron containing pigment, haemoglobin in them. 

HAEMOGLOBIN 

Hemoglobin is a conjugated protein. It consists of a basic protein globin joined to a non protein group heme, hence the name haemoglobin. Heme is an iron - porphyrin ring. 

A hemoglobin molecule is a complex of 4 heme molecules joined with 4 globin molecules. There is about 15mg. Of haemoglobin in 100ml. of blood. In the lungs,due to high partial pressure of oxygen, haemoglobin takes up oxygen and changes to bright red oxyhaemoglobin. The letter carries 4 oxygen molecules loosely joined to 4Fe ++ ions. In the tissue, due to low partial pressure of oxygen, haemoglobin splits into oxygen and deoxyhaemoglobin. In this way, the RBCs carry oxygen from the lungs to the tissues. 

RBCs also carry carbon dioxide from the tissue to the lungs for elimination. It is transported in two forms: mainly in combination with the water of RBC, forming bicarbonate ions. 

Structure :- A red blood corpuscle is bound by an elastic and semi permeable plasma membrane. This enables it to squeeze through capillaries having a diameter less than their own.It loose plasticity in sickle cell anemia. In this disorder, the RBCs block the capillaries leading to grave consequences. An erythrocyte contains homogeneous cytoplasm which lose the nucleus, endoplasmic reticulum, mitochondria, ribosome and centrioles during the development of the corpuscle.This gives double advantage. The corpuscles have more space to hold haemoglobin. It's oxygen consumption is very low due to lack of organelles so that it can supply more oxygen carried by haemoglobin to the tissue cells. Red blood corpuscles cannot reproduce or carry out cellular metabolism due to lack of organelles. Besides haemoglobin, a red corpuscles also contains several inorganic ions, including those of sodium, potassium, calcium, magnesium, chloride and phosphate. The adult red blood corpuscles of mammals are described as nucleated (denuclated) as when young, they have a nucleus that later disappeared. 

Formation :- Formation of red corpuscles Is called erythropoiesis. It occurs in the liver and spleen in the fetus and in red bone marrow after birth. Protein and iron are components of haemoglobin, and vitamin B12 and folic acid stimulate erythropoiesis. Excess RBCs are stored in the spleen. 

Life span and disposal :- Human RBCs remain functional in the blood for about 120 days. The worn out RBCs are destroyed by phagocytosis in the blood itself and in the spleen and liver in particular. Their iron is returned to the red bone marrow for reuse in the synthesis of fresh haemoglobin. Their pigment is degraded to yellowish pigment bilirubin which is excreted in bile. The pale yellow color of the plasma is mainly due to bilirubin. If bilirubin Is not excreted fully, the skin and mucous membranes of the person become yellowish. This disorder is called jaundice. 

Special property :- In resting (drawn) and slow flowing blood, the RBCs form piles called rouleaux by adhering together due to surface tension. 

WHITE BLOOD CORPUSCLES 

The white blood corpuscles (formed elements) lack haemoglobin.

• Shape :- The WBCs are round or irregular cells.They can change their Shape and are capable of amoeboid movement. This enables them to squeeze out of capillaries into the tissues. This process is called diapedesis. 

• Size :- The WBCs are mostly larger than the red corpuscles. They range in size from 12 to 20 micrometers. 

• Number :-The WBCs are far fewer than the RBCs. Their number varies from 5,000 to 10,000per cubic millimeter of blood. This number is the total count of WBCs. It may increase or decrease abnormally in certain conditions. Rice in WBCs count is called leukocytosis. It is a physiological response to fight infections(e.g pneumonia), fall in WBCs count is termed leukopenia. It occurs in conditions such as Folic Acid deficiency, infection of AIDS virus. WBCs count is useful in diagnosing disease. 

• Colors :- The WBC are colourless. 

• Structure :- The leukocytes are nucleated cells. Their cytoplasm contain mitochondria, hotline apparatus and centrioles besides other organelles. 

• Formation :- Formation of leukocytes is called leukopoiesis. It occurs in lymph nodes, spleen, thymus and red bone marrow. 

• Life span and disposal :- The leukocytes survive for a few (3-4) days only in the blood. 

The WBCs are two types:-

• Granular leukocytes

• Agranular leukocytes 

Agranulocytes These leukocytes lack granules in the cytoplasm and have non lobed, rounded or oval nucleus. Agranulocytes are called mononuclear cells. They have two subtype :-

• Monocytes 

• Lymphocytes 

The monocytes arise in the bone marrow. The B and T lymphocytes are produced in the bone marrow and thymus respectively. And mature in spleen and lymph nodes. Formation of granulocytes is termed granulopoiesis. 

• Monocytes :- these are the largest of all types of leukocytes; they have a large Subrounded or been shaped nucleus and a good amount of cytoplasm. They are very motile, are phagocytic and scavenger in action and engulf bacteria and cellular debris at injured sites. Generally they charge into macrophages after entering tissue spaces. 

Lymphocytes :-These are about the size of the red corpuscles. They have a very large, rounded nucleus and scanty cytoplasm. They are non motile and non phagocytic. They secrete antibodies to destroy microbes and their toxin, reject grafts and kill tumour cells. They also help in healing of injuries. The lymphocytes may differentiate into 2 main types:

• B lymphocytes 

• T lymphocytes 

Granulocytes :- These leukocytes contain granules in the cytoplasm and have lobed nucleus. They are produced in the red bone marrow. Their formation is called granulopoiesis. They have 3 subtype :

• Basophils 

• Eosinophiles 

• Neutrophils 

Basophiles :- These take up basic stains such as methylene blue. They are fairly large and have nearly S-shaped nuclei and a few coarse granules. Granules contain histamine. The basophils release histamine and heparin by exocytosis into the blood. 

Eosinophils :- These stain with acidic dyes such as eosin. They are also fairly large and have bilobed nucleus and abundant coarse granules. The eosinophiles have antihistamine properties. Their number increases in people with allergic conditions such as asthma or hay fever. They also help in dissolving blood clots.

Neutrophiles :-These stain equally well with both acidic and basic dyes. They are quite large and have many nucleus and abundant fine, azurophilic granules. The eosinophils are phagocytic in action. They engulf microbes. They are chemotactically attracted to bacteria peptidases. 

PLATELETS

The platelets (formed elements) also lack haemoglobin.

Shape :- The platelets are rounded or oval, disk like bodies but quickly become stellate in extracted blood. 

Size -: The platelets are the smallest formed element of the blood. 

Number :- The platelets are fewer than the red corpuscles and more than the white blood corpuscles in Number. There are about 250,000 platelets in a cubic millimeter of blood. Increase and decrease in the number of platelets is known as thrombocytosis and thrombocytopenia respectively. 

Formation :- The platelets are formed in the red bone marrow. The formation is known as thrombopoiesis. 

Structure :- The platelets are flat, non nucleated fragments of giant cells called megakaryocytes of bone marrow, rather than true cells. They are bound by a membrane and contain a few organelles and secretory granules in the cytoplasm. They have at the center a group of basophilic granules, which give the appearance of a nucleus. At the site of injury, the platelets release platelet factor or thromboplastin that help in blood clotting. 

Life span and disposal :- The platelets survive for 3-7 days only. They are disposed of by phagocytosis in the blood itself.