Animal Tissues

A tissue may be defined as a group of one or more types of cells having a similar origin and specialized for a specific function or functions along with the intercellular material.

Branch of biology dealing with the study of tissue is called histology. The term ‘tissue’ was introduced by Bichat and also known as �ther of histology’. Mayer coined the term ‘histology’ and the founder of histology is Marcello Malpighi. Following types of tissue is found in animals :

Epithelial Tissue

An epithelium is a tissue composed of one or more layers of cells that cover the body surface and lines its various cavities. It serves for protection, secretion and excretion. The word 𠆎pithelium’ (G. epi = upon, thele = nipple) was introduced by Ruysch. They are located on the outer surfaces of organs, including the skin. They form the linings of tracts, cavities and vessels. Epithelial tissue evolved first in animal kingdom. It originate from all the three primary germ layers.

Structure

Cells are arranged in one or more layers, cells are compactly arranged and there is no inter cellular matrix between them. Neighbouring cells are held together by intercellular junctional complexes like desmosomes, tight junctions, interdigitations etc. The cells of lowermost layers always rest on a non living basement membrane or basal lamina. Basement membrane is made up of no cell product of epithelial tissue. It is formed of mucopolysaccharides, glycoprotein and collagen or reticular fibres. Blood vessels are absent in the epithelial tissues. However, nerve endings may penetrate the epithelium. It posses very high capacity of renewal (mitotic cell division). The following types of modifications and junctions are found in the plasma membrane of adjacent epithelial cells to keep the cells together.

Microvilli : It is simple and minute cytoplasmic processes arising from free exposed surfaces of the cell. They absorb material. e.g. Intestine.

Stereocilia : It is non-motile cytoplasmic processes. e.g. Epididymis, vas deference.

Kinocilia : It is contractile motile fibrous processes arising from basal granules. e.g. Oviduct, Fallopian tube.

Tight junctions (Zona occludens) : At certain places the plasma membranes of adjacent cells are tightly packed or even fused together. e.g. Brain.

Desmosomes : Desmosome is present in epithelial tissue. They consist of thickened area and several fine tonofibrils extending from򠺬h plasma membrane into cytoplasm of respective cells. Macula adherens is a kind of desmosome. e.g. Vagina, Urinary bladder.

Gap junction : At place, the adjacent cells form ion-rich gap junctions for intercellular communication and chemical exchange. These junctions probably do not provide physical support.

Interdigitations : These are interwoven finger-like processes of plasma membranes of adjacent cells.

Intercellular bridges : These are minute projections that arise from adjacent cell membranes. The intercellular bridges make contact with one another.

Functions

Epithelial tissues have a wide spread distribution throughout the body and serve several important functions -

(1) Generalized protection is the most important function of membranous epithelium. It is the relatively tough and impermeable epithelial covering of the skin that protects the body from mechanical and chemical injury and also from invading bacteria and other disease causing micro-organisms.

(2) Epithelial structures specialized for sensory functions are found in the skin, nose, eye and ear.

(3) Glandular epithelium is specialized for secretory activity, secretory products include hormones, mucous, digestive juices and sweat.

(4) The epithelium lining of the gut and respiratory tracts allows the absorption of nutrients from the gut.

(5) It is the specialized epithelial lining of kidney tubules that makes the excretion and concentration of excretory products in the urine.

(6) Ciliated epithelium moves fluid, mucous and other materials in the organs it lines.

(7) Germinal epithelium of the seminiferous tubules and ovaries produces spermatozoa and ova respectively.

(8) The ability of epithelia to regenerate quickly helps in the healing of wounds.

(9) Pigmented epithelium of retina darkens the cavity of eyeball.

(10) The epithelia check the absorption of harmful or unnecessary materials.

(11) Epithelium of alveoli of the lungs brings about exchange of gases between blood and air.

(12) Epithelium also produce exoskeletal structures such as scales, feathers, hair, nail, claws, horns and hoofs.          

Types of epithelial tissue

Mainly based on the location and functions of tissue it is following types -   

(1) Simple epithelium : It is simple in structure and basically formed by single layer cells.

(i) Simple squamous epithelium : It is consists of only one layer of flat, scale like cells, usually polygonal cells which are closely fitted together like the tiles of a mosaic. It is also known as pavement epithelium. e.g., It forms lining of blood vessels, lymph vessel, heart, peritoneum, pleura, Bowman’s capsule, thin segment of loop of Henle and lung alveoli.

(ii) Simple cuboidal epithelium : The simple cuboidal epithelium is composed of one layer of cuboidal shaped cells resting on a basement membrane. The nuclei are situated centrally. e.g. the cuboidal epithelium is present in the small salivary and pancreatic ducts, thyroid vesicles, parts of membranous labyrinth, PCT, DCT, ovaries, seminiferous tubules of testes, ciliary bodies, choroid, iris of eyes, thin bronchioles and sweat gland of mammalian skin.

(iii) Simple columnar epithelium : It consists of a single layer cells, many of which have modified structure. Three common modifications are goblet, cilia and microvilli. Simple columnar epithelium is present in the stomach and intestine. e.g. located inner lining of gall bladder and bile duct. It also occurs in the gastric gland, intestinal glands, pancreatic lobules.

(iv) Simple ciliated epithelium : It bears numerous delicate hair like outgrowths called cilia arising from basal granules help to create a current to transport the materials. The ciliated epithelium is of two types :

(a) Ciliated columnar epithelium : It lines respiratory tract (Lower end of bronchi),ꃺllopian tubes (oviducts), ventricles of brain (ependyma), central canal of spinal cord, tympanic cavity.

(b) Ciliated cuboidal epithelium : It occurs in certain parts of nephrons of the kidneys.

(v) Pseudostratified epithelium : It is always consist of single layer of irregularly shaped columnar cells, touches the basement membrane. The long cells have oval nuclei however,

Short cells have rounded nuclei although epithelium is one cells thick, but it gives the appearance of a stratified epithelium, hence it is called pseudostratified epithelium. Mucous secreting goblet cells are numerous and cilia are present. It is of two types -

(a) Pseudostratified columnar ciliated epithelium : It is found in the lining of trachea and bronchi (Upper).

(b) Pseudostratified columnar epithelium : It is found in certain segments of human male urethra and parotid salivary gland, vasa deferentia and epididymis.

(c) Stratified squamous epithelium : The cells in the deepest layer are columnar or cuboidal with oval nuclei. It is called germinative layer. The cells of this layer divide by mitosis to form new cells.

(2) Compound epithelium : It is complexed in structure and basically formed by two or more than two layers of cells.

(i) Stratified squamous keratinised epithelium : Stratified squamous epithelium is characterized by multiple layers of cells with typical flattened squamous cells at the free or outer surface of the sheet. The presence of keratin in these cells contributes to the protective qualities of skin covering the body surface. Keratin is dead and waterproof so it protects the underlying tissues from abrasion and infection e.g. epidermis of the skin of land vertebrates.

(ii) Stratified squamous non keratinised epithelium : Itsਏree surface is moist, and the outer epithelial cells, unlike those found in the skin, do not contain keratin. This type of epithelium serves a protective function. It is found lining the oral cavity (buccal cavity), pharynx, oesophagus, anal canal, lowerpart of urethra, vocal cords, vagina, cervix (lower part of uterus) and cornea of eyes. 

(iii) Stratified cuboidal epithelium : It is consists of two or more rows of low cuboidal-shaped cells which are arranged randomly over a basement membrane. It is found in the sweat gland ducts, larger salivary and pancreatic ducts.

(iv) Stratified columnar epithelium : It is protective epithelium has multiple layers of columnar cells, only the most superficial cells are truly columnar in appearance. Epithelium of this type is rare. It is found in male urethra and in the mucous layer near the anus. It also lines mammary gland ducts and epiglottis.

(v) Stratified columnar ciliated epithelium : It lines the larynx and upper part of the soft palate.

(3) Specialized epithelium : This type of epithelium are specialized to perform specific activity hence, specialized in structure also. They are as follows -        

(i) Transitional epithelium (Urothelium) : It is often consists ten or more layers thick. It lacks germinative layer, basement membrane. Stratified transitional epithelium is typically found in the body areas such as the wall of urinary bladder, ureter and renal pelvis. It is located in all the hollow viscera subjected to stress and protects organ wall from tearing.

(ii) Neurosensory epithelium : Olfactory mucosa, called Schneiderian membrane, lining of internal nares, retina of eyes and epithelial covering of tongue containing taste buds are examples of neurosensory epithelia. The sensory cells bear, at their free ends, slender “sensory hairs” to receive specific stimuli. Basely, these cells are connected, by means of synapses, with fine fibrils of sensory nerves.

(iii) Pigmented epithelium : The epithelial cells of the basal layer of retina contain pigment. Hence, this layer is often referred to as a pigmented epithelium. e.g. - Pigmented layer of retina, iris and skin.

(iv) Germinal epithelium : Specialized cuboidal cells capable of producing gametes as found in gonads. Germinal epithelium produces gametes e.g., ova (Female gametes) and sperms (Male gametes)

Glands

Glandular epithelium are specialized for secretory activity. A cell, tissue or organ which secretes a useful chemical material is known as gland. Glands are made up of cuboidal epithelial cells which are more secretory. All glands arise as folding of epithelia. The golgi body in gland cells are larger and more secretory. Most of the glands of body are merocrine types. It originate from all three germinal layers. (ecto, meso and endoderm). Liver is the largest gland of the body and lined by glandular epithelium.

Types of glands

(1) Unicellular gland : It਌onsist of unicellular gland cells which are called as goblet cells or chalice cells. They secrete mucous and found in mucosa of intestine and stomach. Mucous lubricates the food for easy peristalsis. Their life span is about 2-3 days.

(2) Multicellular gland : It consist of many cells and are generally located in underlying connective tissue e.g. gastric and intestinal glands.

(3) Exocrine gland : These are those glands which discharge their secretory products into ducts. It is also called ducted glands or glands of external secretion.਎.g. Salivary glands, Mammary glands and Tear glands.

(4) Endocrine gland : It is often called ductless gland, because they discharge their secretory products (hormones) directly into the blood. e.g. Pituitary gland, thyroid, parathyroid and adrenal glands.

(5) Heterocrine gland : These are those glands which are partly endocrine and partly exocrine in function. e.g. Pancreas.

Structural classification of exocrine glands

Multicellular exocrine glands are classified by structure, using the shape of their ducts and the complexity (branching) of their ducts system as distinguishing characteristics. Shape include tubular and alveolar (Sac like). Simple exocrine glands e.g. intestinal glands, mammalian sweat glands, cutaneous glands of frog etc. have only one duct leading to surface. Compound਎xocrine glands have two or more ducts e.g. liver, salivary glands etc.

Classification of glands on the basis of their mode of secretion

(1) Apocrine gland : Apocrine glands collect their secretory products near the apex or tip, of the cell and then release it into a duct by pinching off the distended end. This process results in some loss of cytoplasm and damage to the cell. e.g. Mammary glands. (Modified sweat gland)

(2) Holocrine gland : Holocrine glands collect their secretory products inside the cell and then rupture completely to release it. These cells self destruct to complete their functions. e.g. Sebaceous glands. In case of rabbit sebaceous glands are found in dermis of skin. Pineal body and thymus can also be considered as holocrine gland.

(3) Merocrine gland : Merocrine glands (Eccrine or Epicrine glands) discharge their secretory product directly through the cell or plasma membrane, without injury to the cell wall and without loss of cytoplasm. e.g. Sweat glands, exocrine region of vertebrate pancreas, salivary glands and intestinal glands etc.

Classification of glands on the basis of nature of product

(1) Mucous gland : Secret slimy mucous e.g. goblet cells, palatine gland, gland of uterus, some gastric gland and gland of colon.

(2) Serous gland : Produce watery secretion. e.g. pancreas, parotid, salivary gland, sweet gland and intestinal gland.

(3) Seromucous gland : Secrete mixed liquid. e.g. Most gastric gland, sublingual, submaxillary salivary gland, pancreas.

(4) Cytogenic gland : They produce cells e.g. Testis and ovary.

Connective tissue

It connects and supports all the other tissues, the intercellular element predominating. The cellular element is usually scanty. In function this tissue may be mechanical, nutritive and defensive. It is a tissue made up of matrix (abundant intercellular substance or ground substance) and living cells that connects and support different tissues. All connective tissues in the body are formed by mesoderm.

Structure

There are large intercellular spaces between the cells. Intercellular spaces are filled with large amount of extracellular materials formed of insoluble protein fibres lying in an amorphous, transparent ground substance called matrix. Ageing of an animal body is associated with deterioration in its connective tissues.

Functions

(1) Their chief function is to bind other tissues together in the organs.

(2) Certain connective tissues such as adipose tissues store fat.

(3) Skeletal connective tissues like bones and cartilages provide the body with a supporting skeletal frame work.

(4) Fluid connective tissues such as blood and lymph transport various materials in the body.

(5) Plasma cells synthesize antibodies, viz., macrophages. Lymphocytes ingest cell debris, harmful bacteria and foreign matter. Thus these cells of connective tissues are protective in function.

(6) The jelly-like ground substance of connective tissues acts as shock absorber around some organs such as eye balls and kidneys.

(7) The bone marrow produces blood cells.

(8) Areolar tissue acts as packing material in various organs.

(9) Collagen fibres of connective tissue help in repair of injured tissues.

Types of connective tissues

Connective tissue proper possess soft viscous semisolid or semi-fluid matrix. It is divided into following types :

(1) Areolar Tissue : Areolar tissue is loose connective tissue, possess transparent gelatinous, highly vascular and sticky matrix which have variety of cells and fibres. It allows movement of part connected by it (Muscle and their compound). Areolar tissue mainly consist of different types of cells and fibres.

(i) Cells of areolar tissue : It has following types -

Fibroblast : It is most abundant cells, produces fibres, called as fibroblasts in their young active phase and fibrocytes when old and inactive. It synthesize proteins (Collagen, elastin and reticulin). These are undifferentiated mesenchyme stem cells, capable to give rise other cells of connective tissue. Collagen and elastin are formed by fibroblasts.

Histiocytes or Macrophages or Clasmatocytes : These are polymorphic cells. These are amoeboid cells and these are main phagocytes of connective tissue. They are having most active lysosomes and phagocytise dead cells and pathogens. Macrophages remove the dead cells and damaged cells and clean the body so called scavenger cell. All types of macrophages take part in phagocytosis.

Reticular cells : Present only in the reticular tissue and stellate in appearance. Infact they are modified fibroblast producing reticular fibres.

Mast cells : Mast cells were discovered by Paul Echrlich. It is large, irregular ovoid cells found in areolar tissue. and its number increase during allergies. It produces or secretes histamine (vasodilator), serotonin (vasoconstrictor) and heparin (anticoagulant). Histamine dilate the blood vessels in allergic and inflammatory conditions. Heparin checks the clotting of blood inside the blood vessels. Serotonin act as vasoconstrictor to arrest bleeding.

Lymphocytes : These are the smallest, less numerous and spherical or ovoid cells resembling lymphocytes of blood and lymph. These actively move about by pseudopodia. Their function is to form and carry antibodies. That is why, they are seen in large numbers of sites of inflammation.

Plasma cells (Plasmacytes) : These are usually small and rounded, superficially resembling lymphocytes but are sluggishly amoeboid and short-lived (only 2 or 3 days). These are the most potential antibody-forming cells of body presumably, mature lymphocytes (B-lymphocytes form antibody) transform into plasma cells or proliferate to form plasma cells.

Fat or Adipose cells (Adipocytes or Lipocytes) : A few, large and spherical cells occur in areolar tissue, singly or in clusters around small blood vessels. Each cell contains a large globule of fat surrounded by a thin peripheral layer of cytoplasm having a nucleus.

Eosinophils : These cells closely resemble the eosinophilic leucocytes of blood. These probably play a phagocytic role in inflammatory and allergic reactions.

Chromatophores : These are pigment cell present in specialised areas such as skin and eye. They are much branched and packed with pigment granules. They are stellate (Star like) cells, which are phagocytic in nature. They phagocytes melanin producing cells and retain melanin hence they provide colour to the skin and other organs. Melanin is black pigment which protects body from ultraviolet rays of sun.

Mesenchyme cells : These are reserve undifferentiated cell which can be transformed into other types of cells when needed.

(ii) Fibres of areolar tissue : These are made up with protein and non living structures of protein produced by fibroblasts and present in matrix of connective tissue and are of three types -

Collagenous fibres : These are the most abundant fibrous element of areolar and other connective tissues. There are long, unbranched fibres of a soluble and shining collagen protein (tropo collagen). These fibres are more strengthful and provide maximum tensile strength. These are colourless and hyaline, yet called white fibres to distinguish them from yellow elastin fibres. Collagen protein is the most abundant protein of the body constitutes 25% the total body protein. Collagen fibre can be stained by eosin. When collagen fibres are removed from the areolar tissue they become loose and elastic. e.g. Bone, Cartilage, Ligement and tendon.

Yellow elastin fibres : Formed of elastin protein, these fibres are less numerous, thinner, branched, anastomosing, and of a pale yellow colour. These are very elastic and remain streched due to tension in the areolar tissue, when broken in teased preparations, these coil and curl like tense wires. Elastin is probably the most resistant of all body proteins to chemical changes. Thousands of years old ‘mummies’ still have their arteries intact due to well-preserved elastin fibres. They are the orceinophilic i.e. stained by orcein.

Reticulin fibres : These are delicate, freely branching and inelastic fibres of reticulin protein, found interwoven, to form networks. These are very abundant in embryos, new born babies and in healing and regenerating wounds. In areolar tissues of adults, these are mostly replaced by collagen fibres, but remain abundant in lymphoid and blood forming tissues and in the stroma of pancreas, liver etc. They are stained with AgBr and AgNO₃ hence are called Argentophillic or Argyrophillic. On boiling collagen and reticular fibres both convert in glue.            

(2) White fibrous tissue : It is modified form of areolar tissue. Only collagen fibres are present in the matrix and cells are mainly fibroblasts, present at the joints between skull bones and makes them immovable, also found in the dermis of higher mammals. It is of two types -

(i) Tendons : A tendon is non-elastic but flexible tissue consists of parallel bundles of collagenous fibres between which rows of fibroblasts are present. It joins the muscles to bones. It also form chordae tendinae which joins the cusps of atrioventricular valves of heart with the wall of ventricles.           

(ii) Sheath : In a sheath, the bundles of white fibres lie in a criss-cross manner. The fibroblasts are not in rows but are scattered in the areolae. The sheath form protective covering.

(3) Yellow fibrous tissue : The matrix is with numerous and closely packed yellow or elastin fibres which are similar to but thicker than those of areolar connective tissue. It is elastic and flexible. It forms wall of blood vessels, lungs, true vocal chords, trachea, capsule of spleen and bronchioles. It also forms sheet in ligaments. Ligaments is a modified yellow elastic fibrous tissue and connects bone to bone.

(4) Adipose tissue : It is modified form of areolar tissue made up of specialized large spherical fat cells (below the skin) or adipocytes. Adipose tissue chiefly act as 𠇏ood reserves” or fat depots for storage and metabolism of lipids. Besides this, they also act as heat insulators and pressure, pull and push absorbers. Adipocytes are of two types :

(i) Unilocular adipocyte (White adipose tissue) : Common fat of body, having single large fat globule, maintain body temperature, found beneath skin subcutaneous fat panniculus adiposus, blubber of whales and elephants, hump of camel and tail of merino sheep, yellow bone marrow, around kidneys and blood vessels, mesentries, omenta and the fat bodies of frog.

(ii) Multilocular adipocyte (Brown fat) : Each multilocular adipocytes have several small fat globules, contain more number of mitochondria, found in rats and other rodents, polar bear, penguins, seal, walrus, in new born human babies and hibernating mammals (rats and other rodents) on oxidation it yields about 20 times more energy than ordinary fat.

(5) Reticular tissue : It is a modified form of areolar connective tissue characterized by the matrix is fluidy in nature. The matrix contains large number of stellate-shaped reticular cells, each with a number of protoplasmic processes. Reticular tissue is found in spleen, thymus, tonsils, lymph glands, liver, bone-marrow, lamina propria of mucosa of stomach and intestine. The reticular cells act as phagocytes and form a part of defence system of the body.

(6) Myeloid tissue : It is modification of reticular tissue. Its ground substance is plasma. It posses heavy network of reticular fibres. In active form the cells are myeloblasts. It is found in red bone marrow or haemopoitic tissue and fat reserve of yellow bone marrow.

Skeletal tissue

It provide support and surface for attachment of muscle. Skeletal connective tissue form the frame work of body. It provide rigidity to body. These protect the various organ and help in locomotion. It is of three types : Cartilage, Bones, Notochord.

Cartilage

Cartilage is a solid but semi-rigid and flexible connective tissue. Cartilage is a nonvascular connective tissue, consisting of cells embeded in a resilent matrix of chondrin. Chondrin is a protein of cartilage. Regeneration of cartilage can occur from its peri-chondrium. Cartilage is said to be metabolically nearly inactive. In kids the cartilage cells show 2 types of growth.

(1) Appositional or Perichondral or Secondary or Exogenous growth : It is due to deposition of matrix and division of chondrogenic cells of periphery. It leads to growth in thickness.

(2) Endogenous or Interstitial growth : It is due to deposition of matrix and division in inner cells of cartilage. It leads to growth in size.

Types of cartilage : It is of following types - (1) Hyaline cartilage : It is most primitive and glass like cartilage. Its matrix is transparent homogenous and pearly white or bluish green in colour, contain chondrin. It is slightly elastic and also known as articular cartilage because it forms the articular surface of joints. Hyaline cartilage is found in trachea, larynx and bronchi, limb bones (called hyaline cap), sternum, in the hyoid apparatus nasal septum, ribs (sternal parts) larynx (cricoid, thyroid), nasal cartilage (nasal septum).

(2) Fibro cartilage (White fibrous cartilage) : In this cartilage, the small amount of matrix of cartilage is packed with large number of bundles of thick white (collagen) fibres. So it is toughest and less flexible. It is found in intervertebral discs and acts as shock absorber. It is also found in pubic symphysis and helps in parturition (child birth). The intervertebral discs remain contracted when the body is active, but relaxed when the body is at rest. That is why, our body becomes a bit taller during sleep and after death.

(3) Elastic cartilage (Yellow elastic cartilage) : In this cartilage, the matrix is packed with yellow or elastic fibres which run in all directions to form a network. Owing to the presence of yellow fibres, it is very flexible. It gives recoiling power to structures. It is found in mammalian pinna, pharyngotympanic tube, epiglottis, some laryngeal and bronchiolar cartilages.

(4) Calcified cartilage : It is modified hyaline cartilage, It is hard and non elastic due to deposition of calcium salt-hydroxy appetite in matrix. It is found in pubis of old frog, supra-scapula of frog, quadrate cartilage of frog, shark vertebrae, in man ends of long bone, head of humerus and femur. Calcification may also occur as a regular growth process of bone due to age. It reduces elasticity of the cartilage and makes it more rigid.

Bone

Bone is a highly calcified (mineralized), hard and rigid connective tissue. It is the major component of adult vertebrate endoskeleton. Besides its mechanical function of supporting the body architecture and internal organs as a frame work, of protecting delicate organ like brain, heart, etc. of forming to muscles to facilitate movement and locomotion, the bone is also a metabolically dynamic tissue which functions as a homeostatic reservoir of ions of calcium, magnesium, phosphorous, etc. About 97% of total calcium of body occurs in the endoskeleton.

Structure of bone

Periosteum : It is a membrane that forms an envelop around the bone. Periosteum is comprises of two distinct layers. Outer layer consist of thin white fibrous connective tissue. Inner layer consist of osteoblasts, osteoblasts are spider like bone cells, also known as bone forming cells, because they produces new bone materials.

Matrix : Matrix is composed of protein called ossein. The matrix forms thin plates called lamellae. Lamellae are of three types. Haversian lamellae (occur around Haversian canal) concentric or circumferential lamellae (inner to periosteum and outer to endosteum) and interstitial lamellae (between Haversian system). In the lamellae minute bone cells osteocytesਊre present.

Endosteum : It is present outer to the bone marrow cavity. Endosteum is a membrane which lines the marrow cavity. It is comprises of two distinct layers, one is of fibrous connective tissue and another is osteoblasts.

Bone marrow : Bone marrow is a specialized type of soft, diffuse connective tissue called “Myeloid tissue”. It takes part in production of blood cells hence known as haemopoietic tissue. It is composed of adipose tissue, areolar tissue and blood. It is of two types -

(1) Red bone marrow : Red in colour due to presence of lot of blood vessels. In foetal life and at birth present in entire skeleton. After 5^th year red bone marrow replaced by yellow bone marrow, at 20-25 years red bone marrow present at ribs, sternum, clavicles, vertebrae, scapula, pelvis, epiphysis of humerus and femur. Produces RBCs, WBC, platelets, granular, leucocytes like basophils eosinophils and neutrophils.

(2) Yellow bone marrow : Yellow in colour and has much fatty tissue (adipose tissue), present in shaft of long਋ones. Produces blood cells in emergency i.e. at the time of excessive loss of blood, yellow bone marrow may be replaced by red bone marrow in anaemia.

Haversian system : A haversian canal, its lamellae and osteocytes form a haversian system. Haversian canals are found in bone matrix of long bone, like humerus of mammals. Haversian canals contain artery and veins, osteoblasts in areolar tissue, nerves and lymph. It is also called osteon.

Types of bone cells : Four types of cells are found in bone :

(1) Osteoprogenitor cell : Develops into osteoblast cell due to mitotic cell division.

(2) Osteoblast : Bone forming cells found in all bone surfaces. It is small cells synthesize and secrete osteoid, an important part of ground substance. Process of osteoblast is called canaliculi.

(3) Osteocyte : Mature, nondividing osteoblast surrounded by matrix, lying within lacunae.

(4) Osteoclast : Bone destroying cells take part in reabsorption of bones, contain large amount of acid phosphatase enzyme.

Types of bone  

On the basis of their texture : The bones are divided into two categories spongy or cancellous or tubecular bones and compact or periosteal bones.

On the basis of origin of bone : Ossification or osteogenesis is the process of bone formation. A bones are classified into four categories - Cartilaginous, Dermal, Sesamoid and Visceral bones

On the basis of treatment : These are of two types - Dried bone and Decalcified bone

Functions of bone

(1) Support : Bones form the framework of the body and contribute to the shape, alignment and positioning of the body.

(2) Protection : Bony 𠇋oxes” protect the delicate structures they enclose, 

(3) Movement : Bones with their joints constitute levers that move as muscle contract. 

(4) Mineral storage : Bones are the major reservoir for calcium, phosphorus and other minerals.

(5) Haematopoiesis : Blood cell formation is carried out by myeloid tissue.

Notochord

       It is found in all chordate, It is replaced by vertebral column in vertebrate. Notochord is rod like structure. Notochord is made up of chordal cells.

Vascular tissues

It is a mobile connective tissue derived from mesoderm which consists of fibre-free fluid matrix and specialised living cells that are not formed in situ, can neither divide nor secrete matrix. Vascular tissue regularly circulates in the body, takes part in transport of material and performs such activities as scavenging healing of wounds and defence against pathogens. Vascular tissue is of two types, blood and lymph,

Blood

In chordates, and in annelids amongst the non chordates, the blood is a red and opaque fluid of salty taste and peculiar smell. It is a little heavier than water. The study of blood is called haematology. It is red coloured liquid connective tissue which originates from the mesoderm. It reaches into the various organs through the blood vessels and transports various chemical substances between different tissues. During embryonic state, the blood is mainly formed in the liver but little blood is also formed in the spleen and ribs. In adults, the blood is formed in the red bone marrow. The blood formation is called as haemopoiesis.

Viscosity - 4.7, p^H -7.4

Specific gravity - 10.4 - 1.07

Volume - 5-6 litre/70 Kg or 1/13^th part of total body weight

Plasma

It constitutes about 5% of body weight. It represents matrix of blood. Plasma is slightly alkaline and transparent. It forms 55-60% by volume of blood. Plasma contains : Water (91-92%), Solid (8-9%). Plasma solid part consists of organic (7%) and inorganic (1%) substances which are as follows :

Organic constituents of plasma : Some are its own constituents, while others are those which are transported by it. All these are divisible into following categories :

(1) Plasma proteins : Protein constitute about 7% part of plasma and remain in it as colloid particles. These mainly include albumins, globulins, prothrombin and fibrinogen.

Globulins are mainly formed by plasma cells in lymphoid organs. Other plasma proteins are mainly formed in liver. These render the plasma viscous, and maintain its osmotic pressure (7.5 atmospheric) and pH. Prothrombin and Fibrinogen are essential for blood clotting. Albumins are mainly responsible for maintaining osmotic pressure in plasma and for osmoregulation in cells and tissue fluids. Globulins help in osmoregulation and transport of proteins and other substances, but most globulins are immunoglobulins, which act as antibodies, destroying harmful bacteria, virus and toxins in blood and tissue fluids. Some proteins, acting as enzymes, also occur in the plasma.

(2) Digested nutrients : These include glucose, fats, fatty acids, phospholipids, cholesterol, nucleosides, amino acids, vitamins etc. These are the supplied by the blood to all cells of body.

(3) Excretory substances : These chiefly include ammonia collected by blood from body cells and urea, uric acid, creatine, creatinine etc., collected mainly from the liver and transported to kidneys for excretion.

(4) Hormones : These are secreted and released in blood by endocrine glands.

(5) Dissolved gases : Each 100 ml. of water of blood plasma contains about 0.29 ml of O₂, 5 ml. of CO₂ and 0.5 ml of nitrogen dissolved in it.

(6) Defence compounds : Certain immunoglobulins or antibodies and some other substances, such as lysozyme (a polysaccharide) and properdin (a large protein) always occur in the plasma. These serve to destroy bacteria, viruses and toxic substances that may enter into the blood from outside, or from body tissues.

(7) Anticoagulant : Mast cells of connective tissues continuously release, in blood plasma, a conjugated polysaccharide, named heparin. The latter serves to prevent coagulation of blood while it is flowing in intact blood vessels.

Inorganic constituents of plasma : Chloride and bicarbonate salts of sodium are the main inorganic constituents. Traces of other salts, like phosphates, bicarbonates, sulphates and iodides of calcium, magnesium and potassium are also found. All salts constitute about 1% of plasma. These remain as ions (electrolytes) and maintain the alkalinity of plasma. A balanced quantity of salt ions in the plasma is essential for proper functioning of nervous system, muscles and other tissues.

Blood corpuscles

Blood corpuscles form 40-50% of the blood and are of three types viz. Red blood corpuscles, white blood corpuscles and platelets.

(1) Red blood corpuscles (RBCs or Erythrocytes) : These occur only in vertebrates and are the most abundant (99%) of blood corpuscles, imparting the characteristic red colour to the blood. The shape, size and structure of RBCs vary in different types of vertebrates, but their function is the same in all, namely to transport respiratory gases, especially the oxygen (O₂).

RBCs of frog : Amphibian RBCs are largest amongst the vertebrates. Those of Amphiuma and Proteus are largest amongst amphibians about 82 $\mu$m. These are flattened and oval, disclike, but slightly biconvex due to a large oval and centrally-placed nucleus.

RBCs of mammals : Mammals have smallest RBCs amongst the vertebrates. Those of Musk deer are smallest amongst the mammals. Whereas the RBCs of other vertebrates are oval and nucleated, those of mammals are roughly circular (except those of the family camellidae - camels, llamas, dromedaries - which are oval in shape) and non-nucleated.

RBCs of human : They are about 7.4$\mu$m in diameter and its thickness is 1 to 1.5$\mu$m. It is pale yellow in colour but appear to be red in group. Surface area of all RBCs of a person totals about 1500 to 2000 times the surface area of the body itself.

Structure of RBCs : Each RBC is bounded by a dynamic, enzyme-containing plasma membrane. In a human RBC, about 26.5 crore molecules of haemoglobin are packed in the intracellular framework. Water constitutes about 60% of RBC. The rest is solid. Haemoglobin forms about 34% of wet and 90% of dry weight of an RBC. Thus, 100 ml of normal human blood contains about 15 gm of haemoglobin on an average. An apparatus named haemoglobinometer is used to determine the haemoglobin contents of blood.

Structure of haemoglobin : Haemoglobin is a purple coloured iron (in the form of Fe⁺²) containing respiratory pigment of RBCs. It consists of two parts haem (5%) and globin (95%). It is conjugated protein and made up of 4 globin chains with each attached to haem molecule by Co-ordinate bond. Globin is formed of 4 polypeptide chains 2 chain with 141 amino acids and 2 chain with 146 amino acid each. Each RBC contains. One-gram haemoglobin binds 1.34 ml oxygen. Amount of Hb is measured with the help of haemometer. A male has a greater amount of haemoglobin than a female. The amount of haemoglobin in normal man and woman is 14-16 gm/100 ml and 12-14 gm/100 ml respectively, while in children is slightly higher about 16.5 gm/100 ml of blood and foetus with 23 gm/ 100 ml.

Number of RBC : The number of RBC are counted by instrument haemocytometer. The total number of RBC per cubic mm of blood is called RBC count.

Life span of RBC : The life span of red blood corpuscles circulating in the blood stream varies in different animals. RBC have longest life span in blood.

Function of RBCs : The major function of erythrocytes is to receive O₂ of respiratory surfaces and then transport and readily deliver it to all cells of body. This important function is performed by haemoglobin which has a great ability to combine loosely and reversibly with O₂ and is, hence, called “respiratory pigment”. Haemoglobin, in annelids, is dissolved in the plasma because of absence of red blood corpuscles. In mollusc and some arthropods, etc., a different respiratory pigment, haemocyanin is found dissolved in the plasma. This pigment is bluish due to presence of copper in place of iron.

Haemolysis : Due to bursting of plasma membrane of RBCs. Its haemoglobin comes out. This process is called haemolysis. Some fat solvent and snake venom cause haemolysis. When RBCs are placed in hypotonic solution haemolysis take place. When human RBCs are placed in pure water or distilled water they will swell and burst. Some times in haemolysis, the RBCs lose their contents by diffusion and hence maintain their emptied forms intact. These are then called “shadows” or “ghosts” of RBCs.

Rouleaux formation :  If a drop of fresh blood is placed on a slide under coverslip. RBCs adhere together by their concave surfaces like stacks or pile coins. This is called Rouleaux formation. It occurs probably due to forces of surface tension. It may also occur temporarily in blood vessels wherever circulation becomes unduly slow for some time.

ESR : It is called erythrocyte sedimentation rate. This test is measured by “Wintrobe’s tube” and “Western blotting” method. It is the rate of sinking/settling down of RBC in the plasma to form rouleaux. Man has lower ESR as compared to women and it is lowest in new born. Normal value of ESR in male is about 5 mm and in female 10 mm in first hour. A rise in ESR indicates the presence of infective/ destructive/ inflammatory diseases.

(2) White blood corpuscles (WBCs) or Leucocytes : They are nucleated, colourless and complete cells. They are bigger than RBC but their number is less. WBC shown least constancy in shape. The number of WBC is 5,000 to 10,000 per cubic mm. They are formed in red bone marrow, spleen, thymus and lymph nodes from myelocytes and the process is called as myelecoeisis. The life of WBC is of 15 hours to 2 days. The WBC are destroyed outside the blood vessels and the process by which the come out is called as diapedesis. An increase in the number of white blood corpuscles is called leucocytosis. More than 20,000 per cubic mm. indicates some disease. A decrease below 5000/Cu.mm is called leucopenia as in typhoid fever. The leucocytes are divided into two main varieties.

(i) Granular leucocytes : These cells develop in the red bone marrow from the same parent cells as the erythroblasts, i.e., myeloblast in the red bone marrow. These are granular leucocytes of roughly spherical shape, 10$\mu$ to 15$\mu$ in diameter, actively amoeboid and containing a large number of stainable granules. Their nucleus is irregular and divided into 2 to 5 interconnected lobes. Hence, these are also called polymorphonuclear leucocytes.

(a) Neutrophils are the most abundantand most active type of WBCs. Nucleus has 3-5lobes. They are phagocytic.

(b) Eosinophils are phagocytic with bilobed nucleus. High eosinophil count indicates allergic conditions and parasitic infestations.

(c) Basophis are nonphagocytic with 2-3 lobes of nucleus. They are also involved in allergic reactions.

(ii) Agranular leucocytes : They have a few non-specific or no granules in the cytoplasm and the nucleus is spherical to kidney shaped. They comprise about 25-30 % of all leucocyte and have two varieties.

(a) Lymphocytes Protect from pathogens and are involved in the production of antibodies.

(b) Monocytes are the largest corpuscles and are phagocytic.

(3) Blood platelets : These are protoplasmic disc that are found in mammalian blood (lower vertebrates have spindle-shaped cells named thrombocytes). Platelets arise as detached tips of protoplasmic processes extending from the cytoplasm of giant cells, megakaryocytes of red bone marrow. The shape is oval to round, often stellate. There are approximately 300,000 platelets in a cubic millimetre of blood. Platelets are non-nucleated. Life span is about 5-9 days.

Coagulation or Clotting of blood

Process of formation of blood clot is also known as blood coagulation. Normal time of blood clotting is 3 to 8 minutes. Blood clotting is checked in blood vessels by presence of anticoagulant. When an injury is caused to a blood vessel bleeding starts which is stopped by a process called blood coagulation or clotting. This process can be described under four major stages.

Stage I is concerned with the formation of thromboplastin released from damaged tissue or platelets. Thromboplastin helps in the formation of the enzyme thrombokinase.

Stage II involves the conversion of prothrombin into thrombin with the help of thromokinase and calcium ions.

Stage III involves the conversion of a soluble protein fibrinogen in plasma to insoluble network of fibrous material called fibrin by the action of thrombin.

Stage IV is the formation of red solid mass called blood clot by trapping of blood cells particularly RBCs by fibrin network.

Functions of blood : On basis of the above account, the general functions of blood can be briefly enumerated as follows :

(1) Blood is the fluid medium which transports different materials between various parts.

(2) The leucocytes of blood play the important role of defense by inactivating and destroying harmful toxins and invaders like bacteria, viruses, fungi and animal parasites.

(3) Blood leucocytes phagocytes and destroy cell debris and inert foreign particles in blood and tissues. Thus, these act as “scavengers” to clean the body’s internal environment.

(4) Blood maintains the normal temperature of body. It prevents a sharp rise or fall in temperature which may be caused in any tissue due to abnormal rate of metabolism.

(5) By coagulating at an injury, and by stimulating repairing of damaged tissues, the blood helps in rapid healing of wounds and injuries.

(6) Blood helps in the maintenance of a proper internal environment in the body by regulating the amount of salts, acids, bases and water, etc. in the tissue fluids.  

Lymph

Lymph can be defined as blood minus RBCs but more WBCs. Lymph is chiefly made of plasma plus leucocytes. Most important centre for the formation of lymph is interstitial space. Interstitial fluid, intercellular fluid, tissue fluid and lymph all are same in composition. Exchange of materials between blood and tissue fluid occurs through blood capillaries.

Functions of lymph : The basic function of lymph is to bring back, into the vascular circulation, the cell debris, large colloid particles and the part of the blood plasma that had diffused out from arterial capillaries into the tissue fluid but has failed to return back into venous capillaries. The white corpuscles of the lymph are the same as those of the blood and have the same functions of defense and of assistance in tissue repair and healing. In intestinal wall, lymph capillaries, called lacteals, are specially meant for absorption of fats.

Muscular tissues

Contractility and motility (movement) are fundamental properties of protoplasm. That is why, all cells possess potential motility. Contraction for motility in the cells results essentially from the interaction of two contractile proteins, actin and myosin. These tissues are obviously responsible for movements of organs and locomotion of the body in response to stimuli. These develop from embryonic mesoderm except for those of the iris and ciliary body of eyes, which are ectodermal in origin. About 40% to 50% of our body mass is of muscles. The muscle cells are always elongated, slender and spindle-shaped, fibre-like cells, These are, therefore called muscle fibres. These possess large numbers of myofibrils formed of actin and myosin. Muscle cells lose capacity to divide, multiply and regenerate to a great extent. Study of muscle is called myology. Types of muscle are following -

Striated or striped muscles

Most muscles of body are striated. These generally bring about voluntary movements under conscious control of brain and, hence, called voluntary muscles. Most of these are inserted at both ends upon bones in different parts of the body depend upon these muscles. Hence, these are also called skeletal muscles. Movements of limbs and the body solely depend upon these muscles. Hence these are also called somatic muscles. These are also called phasic type of muscles, because contraction in these is rapid, but brief and fatigue occurs quickly.

Fine structure of striated muscle fibres : Striated muscle fibres shows transverse striation in the form of regular alternate dark A (anisotropic) and light I (isotropic) bands. The 𠆊’ band contains about 120Å thick and 1.8 $\mu$ long “myosin filaments”. The I band contains about 60Å thick and 1.0 $\mu$ long �tin filament” which are twice as many as myosin filaments. Each I band is divided into two equal halves by a thin, fibrous and transverse zig-zag partition, called ‘Z’ band (‘ Z’ disc) or Krause’s membrane. Each segment of a fibril between two adjacent ‘Z’ bands is called a sarcomere. It is 2.3 $\mu$ long in uncontracted mammalian striated fibres. A slender transverse line, the ‘M’ or Hansen’s line is visible in middle of each 𠆊’ band. The major, middle region of 𠆊’ band is comparatively lighter, but its terminal parts appear darker. The middle lighter region is called ‘H’ zone. Due to the geomatric bonding pattern, the end of each myosin filament is, thus, encircled by the ends of six actin filaments (hexagon), while the end of each actin filaments is encircled by the ends of three myosin filaments (trigon).

Ultrastructure of myofilaments : At the molecular level, each myosin filament is composed of about 500 thread-like myosin molecules. Three different kinds of proteins participate in the composition of actin filaments. The major part of an actin filament is a coiled double helical strand whose each arm is a linear polymer of small and globular molecules (monomers) actin protein. Another coiled double helical, but thiner, strand runs along the whole length of actin strand. Each arm of this strand is a polymer of fibre-like molecules of tropomyosin protein. The third protein is troponin.

Working of striated muscles : H.E. Huxley and A.F. Huxley in 1954 proposed a theory to explain the process of muscular contraction. This theory is known as ‘sliding filament theory’. It was observed that when a fibril contracts, its 𠆊’ bands remain intact, while the ‘I’ bands progressively shorten and eventually disappear when the fibril has shortened to about 65% of its resting length. At this stage. ‘H’ zones also disappear because the actin filaments of both sides in each sarcomere reach, and may even overlap each other at the “M” line, and the ‘Z’ lines now touch the ends of myosin filaments. It was further observed that if a fibre is mechanically streched, the zones of overlap between thick and thin filaments are shorter than in resting condition, resulting in wider ‘H’ zones. These observations led Huxley to propose that shortening of the fibrils in contraction is brought about by sliding movement of actin filaments over myosin filaments towards “M” line by means of rapidly forming and breaking cross bridges or rachets at the spurs of myosin filaments. Thus, the sarcomere were recognised as the ‘ultimate units of contraction’.

Smooth muscles

These are called smooth, plain nonstriated involuntary or unstriped muscles due to absence of striations. These occur in the walls of hollow internal organs (alimentary canal, gall bladder, bile ducts, respiratory tracts, uterus, urinogenital ducts, urinary bladder, blood vessels, etc.), in capsules of lymph glands, spleen etc., in iris and ciliary body of eyes, skin dermis, penis and other accessory genitalia, etc. There is no connection of these muscles with bones. Smooth muscles of skin dermis, called arrector pilli muscles, are associated with hair roots, and are responsible for flesh (erection of hairs). Those of penis form a muscular network which helps in its erection and limping.

Structure : Smooth muscle fibre is unbranched goose-spindle shaped, uninucleated and has no sarcolemma. Contraction is slow, involuntary under the control of ANS. Functionally smooth muscles are of two types -

(1) Single-unit smooth muscle : Single unit smooth muscle fibres are composed of muscle fibres closely joined together, contract as a single unit. e.g., urinary bladder, gastrointestinal tract, small arteries and small veins.

(2) Multi-unit smooth muscles : Are composed of more independent muscle fibres, contract as separate units e.g. - hair root muscle, muscles on the wall of large blood vessels, ciliary muscles, muscles of iris and bronchi.

Cardiac muscles

Heart wall (also the wall of large veins just where these enter into the heart) is made up of cardiac muscles and, hence, called myocardium. Structurally, these muscles resemble striated muscles but, functioning independently of the conscious control of brain, these are involuntary like the smooth muscles. Cardiac muscle cells of fibres are comparatively shorter and thicker, cylindrical, mostly uninucleate with a central nucleus, somewhat branched and covered by a sarcolemma.

Characteristics of a muscle

Antagonistic muscles : The striated muscles occur in antagonistic pairs one pulls a bone in one direction, while the other pulls it back in reverse direction to its normal position. For example, the biceps muscle, extending from shoulder to radius, bends or flexes the arm at the elbow, whereas the triceps extending from ulna to the shoulder, straightens the arm. Thus, biceps is a flexor and triceps an extensor for bending the arm.

Single twitch : When a muscle receives a single excitation impulse, it respond by a sudden partial contraction (twitch) lasting for about 0.5 second in man. Each twitch is followed by a refractory period during which the muscle does not respond to next stimulus. The refractory period is, however, so short (0.002 second) that the muscle can respond to the second stimulus while still in contraction phase in response to the first stimulus.

Tetanus : Generally, whole muscles contract, not in a single twitch, but in sustained contractions evoked by a series of nerve impulses reaching them in rapid succession. Such a sustained contraction is called tetanus. Described above should not be confused with the disease of “tetanus” (lock jaw) caused by tetanus bacillus. This disease is characterised by abnormal muscular contractions. Nor it should be confused with “tetany” which is muscular spasm occurring due to deficiency of parathyroid hormone.

Muscle tone or “Tonus” : Even at rest the striated muscles normally remain in a state of mild sustained partial contraction to maintain the body posture. This is called muscle tone. It is a mild state of tetanus.

Paralysis : When supply of motor impulses to a muscle is completely cut off due to destruction, either of the control centres in brain, or of the concerned motor nerves, or due to blocking of myoneural junctions by the use of certain drugs, the muscle function is completely impaired. This is called paralysis of the muscle.

Muscle fatigue : A muscle that has contracted many times at short intervals, exhausts its store of ATP and glycogen and accumulates lactic acid. Hence its contractility gradually decreases and finally stops.

Oxygen debt : During active work or exercise, the rate of oxygen supply by the lungs falls short of the requirement of the muscles. Hence, lactic acid accumulates in the muscles and the breathing gradually becomes hard to enhance O₂ intake by the lungs. This is called oxygen debt.

Involuntary action of skeletal muscles : Muscles are capable of utilizing, in their mechanical work, only about 20% to 40% of energy liberated from glucose. The unutilized energy is lost as “heat” dissipated into the environment. This heat helps in maintenance of body temperature. “Shivering with cold” in winter is caused by a quick involuntary reaction of striated muscles.

Rigor mortis : Rigidity that develops in the muscles after death is known as rigor mortis. It is due to permanent irreversible contraction, establishment of permanent link between actin and myosin and also fall in the concentration of ATP molecules.

Cori’s cycle : Lactic acid is transported by blood to liver and there it is converted to glycogen through Cori’s cycle.

Contraction period : Time taken in sliding of filament is called contraction time. (10 to 100 milli second).

Relaxation time : It is time taken in relaxation of fibre i.e. active transport of calcium from sarcoplasm to cisternae. (10 to 100 milli second)

Refractory period : It is time in a muscle or nerve fibre when they are non responding to second stimulus. Infact in this period there is temporary loss of excitability. Refractory period for skeletal and cardiac muscle is 5 and 300 milli second respectively.

Hypertrophy and Atrophy of muscles : Muscles which are put to excessive work become thick and strong. This is called their hypertrophy. Conversely, if certain muscles are not used for a long period, those become thin and weak. This is called their atrophy (disuse atrophy). Cardiac muscle have a poor regenrative power.

Nervous tissue

A most complex tissue in the body, composed of densely packed interconnected nerve cells called neurons (as many as 10¹⁰ in the human brain). It specialized in communication between the various parts of the body and in integration of their activities. Nervous tissue is ectodermal (from neural plate) in origin. It forms the nervous system of the body which controls and coordinates the body functions. There is no intercellular matrix between neurons. These have permanently lost the power of division as have no centriole and have minimum power of regeneration. So these cannot be cultured in vitro. Irritability is the main function of nervous tissue.

Composition of nervous tissue : Nervous tissue is formed of four types of cells :

(1) Neurons (nerve cells)     (2) Neuroglia

(3) Ependymal cells                (4) Neuro-secretory cells

Neurons

A neuron is a nerve cell with all its branches. Neuron is formed from neuroblast. It is the structural and functional unit of nervous system. It is the longest cell of the body.

(1) Cyton : It is also called perikaryon or soma or cell body. Its granular cytoplasm is called neuroplasm which has following structures :

(i) A large, spherical, centrally placed nucleus with a single nucleolus.

(ii) Numerous fine threads called neurofibrils for the conduction of nerve impulses.

(iii) A number of small, basophilic granules called Nissl’s granules formed of rough endoplasmic reticulum with ribosomes and are sites of protein synthesis.

(iv) Neuroplasm has large number of mitochondria to provide high energy for impulse conduction.

(v) Neuroplasm may have melanophores with melanin pigment and lipochromes with orange or yellow pigment.

(vi) A mature neuron has no centriole, so it cannot divide.

(vii) A “Barr body” is often seen abutting against the inner surface of nuclear membrane of cytons in females. This has been proved to be a transformed ‘X’ chromosome.

(viii) Certain neurons having flask-shaped cytons and called purkinje cells, occur in the cerebellum of the brain.

(2) Neuron processes : The processes of neurons, called neurites, extend varying distances from the cyton and are of two types - dendrites or dendrons and an axon or axis cylinder (neuraxon).

(i) Dendron : These are several short, tapering much branched processes. The dendrites contain neurofibrils, neurotubules, Nissl’s granules and mitochondria. They conduct nerve impulse towards the cell body.

(ii) Axon : This is a single very long, cylindrical process of uniform diameter. It arises from a conical projection, the axon hillock, of the cyton. The axon contains neurofibrils and neurotubules but lacks Nissl’s granules. Axon is usually branched only terminally into slender branches called telodendria. The latter have knobbed ends called endbulbs or axon terminals or buttons or synaptic knobs or end plates. The synaptic knobs contain mitochondria and secretory vesicles.

Types of neurons : Neurons are divided into different categories on different basis.

(1) On the basis of functions : Neurons are divided into three categories :

Sensory (afferent) neurons : These are found in sense organs. Their dendrons receive the nerve impulse from the nerve process of the receptor cell while their axon forms the synapse with dendron of the next neuron. These may be naked or encapsulated e.g. olfactoreceptors and gustatoreceptors.

Internuncial neurons : These are located in the dorsal horn of the spinal cord. These are called association neurons (when their axon synapses with the dendron of motor neuron of same side) or commissural neuron (when their axon synapses with the dendron of motor neuron of opposite side).

Motor (efferent) neurons : These are always present in the ventral horn of the spinal cord. Their axon ends into the muscle fibres or glands cells. These conduct the nerve impulses to the effector organs which respond to the stimuli.

(2) On the basis of number of nerve processes : Neurons are of three types -

Unipolar neurons : In these neurons, only one nerve process arises from the cyton which acts as axon but there is no dendron. These are found only in early embryos. The unipolar neuron of the adult gives rise to a single nerve process, which immediately divides into a dendron and an axon. Such unipolar neurons are called pseudo-unipolar neurons. These are found in the dorsal root ganglia of spinal nerves and in the roots of V, IX and X cranial nerves.

Bipolar neurons : In these neurons, the cyton gives rise to two nerve processes out of which one acts as an axon while other acts as a dendron. These are found in the olfactory epithelium of nasal chamber and retina of eye. These may be isopolar or heteropolar (dendrons being irregularly branched). Ganglia of VIII cranial nerve.

Multipolar neurons : In these neurons, the cyton gives rise to several nerve processes out of which one acts as an axon while remaining nerve processes act as dendrons. These are found in the central nervous system and the ganglia of autonomic nervous system of adult.

Neuroglia or Glia cells

Neuroglia consists of the supporting and packing cells found in the brain, spinal chord and ganglia. These are non nervous cells. These are ten times more numerous than neurons. In some parts of body the neuroglial cells are called by certain other name such as muller cells in retina, pituicytes in posterior pituitary gland and satellite cells in ganglia.

Types : The neuroglia cells are of three types -

(1) Astrocytes : These are large sized and star-shaped cells with numerous processes which form blood brain barrier therefore only located in brain.

(2) Oligodendrocytes : These have a few branched processes which resemble the dendrons of the neurons. Present myelin sheath around CNS Axon, example schwann cells. 

(3) Microglial cells : These are small sized and spindle-shaped. The microglia cells act as the defensive phagocytes in central nervous system. They arise from the monocytes.

Functions

(1) These are capable of division and help in wear and tear of the central nervous system.

(2) These insulate the adjoining neurons and prevent the lateral transmission of impulses.

(3) These provide nutrition to the neurons.

(4) These act as phagocytes and eat up the microbes.

(5) These help in memory processes.

(6) They acts as Blood brain barrier (BBB) i.e. they inhibit contact between neuron and blood, along with endothelium of capillary. The exchange of material between blood and neuron is always through these neuroglial cells i.e., they are mediator.

Ependymal cells

These are cuboidal and ciliated epithelial cells which lines the cavities of brain (ventricles) and spinal chord (central canal). These form an epithelium called ependyma. Ependymal cell for circulation of CSF.

Neurosecretory cells

These are special type of neurons of the hypothalamus of brain. These are endocrine in function and secrete neurohormones which are carried by the blood of hypophyseal portal system to anterior lobe of pituitary gland and stimulate the secretion of their trophic hormones e.g., TSH, STH, FSH, LH, ACTH, etc.

Nerve fibres

Axon or dendron of a nerve cell covered with one or two sheath is termed as nerve fibre. The nerve fibres are of two types - medullated or myelinated and non medullated or non myelinated regarding their structure.

(1) Medullated nerve fibres : A medullated nerve fibre typically consists of a central core, the axis cylinder, or neuraxis, surrounded by two sheaths : inner thick medullary sheath and outer thin neurilemma.

(i) Axis cylinder : The axis cylinder is simply the axon or dendron of a nerve cell. It contains longitudinal neurofibrils and mitochondria in its neuroplasm, called axoplasm, limited by cell membrane termed axolemma. It is the axolemma that conducts the nerve impulses.

(ii) Medullary sheath : The medullary sheath is composed of a shinning, white, fatty substance called myelin. This sheath perhaps serves as an insulating layer, preventing loss of energy of the nerve impulse during its passage along the fibre. The medullary sheath is continous around the fibres in the central nervous system, but in the fibres of the peripheral nerves it is absent at certain points known as the Node of Ranvier. The part of a nerve fibre between two successive nodes is termed the internode.

(iii) Neurilemma : The neurilemma consists of tubular sheath cells (Schwann’s cells) placed end to end. The neurilemma is continuous over the Nodes of Ranvier. The function of the Schwann’s cells is to produce the myelin sheath around the neuraxis. The medullated nerve fibres within the brain and spinal chord lack neurilemma. Instead, they have an incomplete covering of neuroglia cells, which probably produce the myelin sheath. Neurilemma present around the peripheral nerve fibres enables them to regenrate after injury. Nerve fibres in the brain and spinal chord do not regenrate after injury due to lack of neurilemma. The medullated nerve fibres occur in the white matter of the brain and spinal chord and in the cranial and spinal nerves.

(2) Non medullated nerve fibres : A non medullated nerve fibre consists of an axis cylinder enclosed by neurilemma and connective tissue. These fibres appears grey in colour in the fresh state. The non-medullated nerve fibres occur in the autonomic nerves.

Nerves

The nerves are thread like structures extending between the central nervous system and the receptor of effector organs of the body. These conduct the nerve impulses to and from the central nervous system.

Each nerve is formed of several bundles of nerve fibres, called fasciculi. Each nerve fibre of the bundle is covered by a thin sheath of connective tissue called endoneurium, while each fasciculus is enclosed by another sheath of white fibrous connective tissue called perineurium. All the fasciculi are held together by the connective tissue and are enclosed by a thick coat of white fibrous connective tissue called epineurium. On average, a nerve contains about twice as many unmyelinated fibres as myelinated fibres.

Types of nerves : The nerves are of three types according to the nature of the nerve fibres they are composed of -

(1) Sensory or afferent nerves : The nerves with sensory fibres are called sensory nerves. Example - Olfactory, optic and auditory nerves (I, II, VIII).

(2) Motor or efferent nerves : The nerves having efferent fibres are termed motor nerves. Example - Oculomotor, Pathetic and abducens nerves (III, IV, VI).

(3) Mixed nerves : Some nerves have both afferent and efferent fibres. These are known as mixed nerves. Example - Trigeminal, facial, glossopharyngeal and vagus nerves (V, VII, IX, X).