Animal Tissues

Body of a multicellular animal is formed of many cells. Group of cells similar in structure , function and origin form a "Tissue". Branch of Biology that deals with the study of tissues is known as 'Histology'. Animal cells have narrow space around them which contains fluid called as Extra cellular fluid/ Intercellular fluid. ORIGIN OF TISSUES: All tissues originate from the cells of any of the three germ layers - ectoderm, mesoderm and endoderm. TYPES OF TISSUES: 4 major types of tissues are there- Epithelial tissues, Connective tissues, Muscular Tissue and Nervous Tissue. 1.1. Epithelial Tissues : ( Gr. Epi - Upon thele - nipple) Epithelial tissues are the tissues composed of one or more layers of cells covering the external surface of the body and internal organs or lining the cavities in the body and viscera There are no blood vessels supplying nourishment to these tissues but they receive nutrients from underlying connective tissues. Structure: These tissues consist of variously shaped cells arranged closely in one /more layers. There is little intercellular material between the cells and cells are held together by intercellular junctions. Epithelia rests on a thin basement membrane which is made up of Glycoproteins and collagen fibres. Types: On the basis of arrangement and shape of cells, the epithelia are classified into two groups:- Covering & Glandular Epithelia A. COVERING EPITHELIA: These cover the external surface or lining or the cavities. It is of 2 types - Simple & Compound.  (i) Simple Epithelia: They consist of single layer of cells resting on basement membrane. These cover the surface where there is lesser wear & tear. These are further of five types: - (a) Simple Squamous Epithelium : Consist of thin flat, polygonal closely fitted cells like the floor tiles. So it is also known as ' pavement epithelium'. It has round/oval nucleus. Present on inner lining of lung alveoli, blood vesse;s and peritoneum of bodyearity. Function: protection , excretion, gaseous exchange (b) Simple Cuboidal Epithelium : Consists of polygonal cells which appear cuboidal . These are found in proximal tubules of kidneys, salivary glands, pancreatic ducts, thyroid follicles & ovary. Function: Protection, secretion of juices, excretion, absorption. (c) Simple Columnar Epithelium: In this, cells are tall and placed side by side like column. Nucleus is oval . Found in inner surface of intestine, gall bladders, stomach, gastric & intestinal glands. Function : Secretion , absorption (d) Simple Ciliated Epithelium: These consist of cells that bear fine , cytoplasmic process 'cilia' on the free surface. It lines the inner surface of some hollow organs like fallopian tubes, nasal passage, bronchioles etc. Function: Cilia help in movement of particles, free cells or mucus over the epithelial surface (e) Pseudo-Stratified Epithelium : consist of ciliated or non-ciliated epithelium in which columnar cells are arranged in single layer but appear multilayered because some cells are shorter than others. It covers the inner lining of trachea, large bronchi. Functions: Protection, movement of secretion from glands (ii) Compound Epithelium : It consists of many layer of cells which may be different in shape. Deepest layer rests on basement membrane. Layers of cells give a stratified appearance . So, it is also known as 'Stratified Epithelium'. Based upon the shape of cells in outer layer, compound epithelium is mainly of four types:- (a) Stratified Squamous: Cells have different shapes in different layers. Deepest layer have columnar or cuboidal cells with oval nuclei. These divide by mitosis & add new cells to outer layers. It is called 'Germinative layer'. Middle layer cells are polyhedral with round nuclei. It is called ' Intermediate layer' Superficial layers are flat with elongated nuclei. These layers are called Squamous layers. It is of two types - keratinized & non- keratinized. Keratinized squamous epithelium has Keratin in place of cytoplasm in few outer layers. It forms epidermis of skin in land vertebrates. Cells in outer layers ( with keratin) are dead. Non-keratinized squamous epithelium does not have keratin. So cells on outer layer are living . It covers moist surfaces like buccal cavity, pharynx, oesophagus, cornea of eye. It provides moderate protection against abrasion. (b) Stratified Cuboidal Epithelium : Outer layer has cuboidal cells .Basal layer has columnar cells. It forms epidermis of fish , lines the sweat gland ducts, salivary ducts and pancreatic ducts. (c) Stratified Columnar Epithelium : It has columnar cells in outermost & innermost layers. It covers epiglottis , lines the mammary gland ducts and parts of urethra. (d) Stratified Ciliated Columnar Epithelium : Outer layer has ciliated columnar cells. Basal layer has columnar cells. It lines the larynx & upper part of soft palate. Other types of compound epithelia is Transitional Epithelium : Superficial layer has blood, rectangular/oval cells. Middle layer has polygonal or pear shaped cells. Basal layer has cuboidal cells. Cells can change from simple to compound epithelium i.e. they are in transitional phase. It lines inner surface of ureter, urinary bladder etc. and can stretch . It prevents loss of water & helps in expansion of organs by stretching. Other types of epithelia which are neither simple nor compound type are Neuroepithelial cells: These perform sensory function, e.g. cells of taste bud Myoepithelial cells: Branched cells and contain muscle protein ( myosin & actin) . These play role in contraction of muscles. B. GLANDULAR EPITHELIA : It has columnar cells which secrete useful materials like secretions of glands. The epithelia forms glands also. Glands can be of following types:-  (i) Unicellular: These consist of isolated glandular cells. e.g. goblet cells & alimentary canal. (ii) Multicellular: - These glands consists of a cluster of cells. (iii) Simple Gland: -A gland with single unbranched duct is called Simple gland. e.g. Tubular gland in human intestine. The secretory part of the gland consists of epithelial cells arranged as tubes (tubular) or sacs (acini, alveoli) or both. (v) Compound Gland: - A gland with branched system of ducts is called as compound gland. e.g. Gland in Pancreas and sub-mandibular salivary glands. In these tubules/acinus of secretory duct may be coiled or branched and opens in to single duct. (v) Exocrine Glands: -These glands transport their seceretions to the site of action by ducts. e.g. Salivary gland, tear gland, Gastric gland, Intestinal gland. (vi) Endocrine Glands: - (Ductless glands): These discharge their seceretions called 'hormones' into blood which carries them to the site of action. e.g. Pituitary gland, Thyroid gland, Adrenal gland.  (vii) Mixed Gland: -When a gland performs both, exocrine and endocrine functions, it is called Mixed Gland. e.g. Pancreas. (viii) Holocrine Glands: -In these, whole cell fills with seceretion and later breaks down and is shed as secretion. e.g. Sebaceous Glands. (ix) Merocrine Glands: -The secretion leaves the cell through cell membrane and cell contents remain intact. e.g. Pancreas. (x) Apocrine Glands: -In these, only apical portion of the cytoplasm is discharged along with secretory product. e.g. Mammary Glands.  1.2 CONNECTIVE TISSUE: - It is made up of variously shaped, loosely arranged cells lying in the extra cellular matrix. Extra cellular matrix has protein fibres, tissue fluid and amorphous ground substance (glycoaminoglycans+glycoproteins). Connective tissue play a role in defence, tissue repair, fat storage etc. Types: - 8 types of connective tissues are there:-  (i) Areolar Tissue : - (Loose connective Tissue). It is found beneath epithelia of many viscenal organs. It joins different tissues and forms packing between them. Three types of cells are present in this tissue: - (a) Fibroblast: -are main cells, have irregular flat cells with protoplasmic processes. These synthesize proteins collagen and elastin. (b) Macrophages: -These destroy microbes, foreign particles and cells of damaged tissues by phagocytosis. (c) Mast Cells: -large, irregular, ovoid cells. These store inflammation producing substances which are released at the place of injury. Two types of fibres are present in this tissue: - (a) White collagen fibre: - made of collagen protein, occur as bundles, lack elasticity. (b) Yellow elastic fibre: -made of elastin protein, don't occur as bundles, are elastic in nature. Both fibres prevent displacement and injury of tissues and organs. Collagen fibres are formed at the site of injury. (ii) Adipose Tissue: -It is a fat-storing connective tissue-found beneath skin, around kidneys & other places. It contains fat cells 'adipocytes' , fibroblasts, macrophages, collagen fibres & elastic fibres. Function: -synthesizes and stores fat, acts as shock absorber and prevents heat loss by forming insulating layer. (iii) White tissue: - It has few fibroblasts scattered in the dense network of thick collagen fibre bundles. Function: -As it has great strength, it provides strong attachment between various structures. (iv) Tendon: -It is formed of bundles of collagen fibres arranged parallely. Fibroblast cells occur in these. Function: -These connect skeletal muscles with bone. (v) Ligament: - It is made of yellow elastin fibres which forms densely crowded matrix. Fibroblasts lie scattered in these. Function: -It connects the bones at joints and provides movability. (vi) Cartilage: -It is solid, semi-rigid connective tissue. It contains matrix made of Chondrin (consists proteoglycans). Cartilage cells 'Chondriocytes' lie scattered in the matrix in small spaces called 'lacunae' in the clusters of two or three: - Four types of cartilages are there: - (a) Hyaline: -blue in colour, matrix is clear, translucent & lacks fibres. Present in sternum, hyoid & ribs. (b) Fibrous: -contains matrix rich in bundles of collagen fibres & chondrocytes. Found in intervertebral disc. (c) Elastic: -It consists of yellow elastin fibres and is elastic in nature. Seen in pinna, tip of nose, epiglottis. (d) Calcified: -initially like hyaline cartilage. Later it gets hardened due to deposition of calcium salts. e.g. pubis of pectoral gindle of frog. (vii) Bone: -It is solid, rigid and hardest connective tissue. It has matrix (made of protein 'Ossein', sulphates, carbonates and fluoride salts of Ca & Mg), irregular spaces 'lacunae' in the matrix and osteocytes (bone cells) in the lacunae. Compact bone forms dense outer layer of all the bones and is made of parallely & longitudinally arranged column like structures-Haversian system. Each Haversian system has a central canal (Haversian canal) and many lamellae surround it. This canal has blood vessels & nerves. Hollow cavities are called 'Marrow cavities' and are filled with bone marrow. Types: -2 types -Compact & Spongy. Shaft of long bones are compact bones. These have lamellae around canal without gaps. Spongy bones have trabeculae, forming network with small spaces between them. Spaces have red bone marrow, which forms WBC & RBC. (viii) Blood: -It is fluid connective tissue composed of plasma and blood corpuscles. Blood is also called 'Vascular tissue' because it is part of the vascular system of many animals. Plasma-is yellow, slightly alkaline (pH 7.4) , somewhat viscous fluid. It has 92% water & 8% solid. There are three plasma proteins-albumin, globulin, fibrinogen. Solutes present in plasma are glucose protein, amino acids, cholesterol, fatty acids, vitamins, enzymes, hormones, antibodies etc. Inorganic salts present in plasma are ions of Na, Mg, K, Ca, etc. Normal blood-sugar level is 90-120 mg/100 ml blood. Blood Corpuscles are of three types-RBC, WBC and Platelets. RBCs (Erythrocytes) are red in colour due to red pigment haemoglobin. It may be nucleated/non-nucleated. Shape varies, size ranges from 7-8 m in diameter. Adult woman has 4-5 million RBCs/cu mm of blood. Adult man has 5.5 million/cu mm of blood. Haemoglobin (Hb) is a protein made of protein globin and ferrous (Fe2+) containing tetrapyrole porphyrin ring called 'Haeme'. 1 molecule of HB binds to four molecules of oxygen. RBCs have an average life span of about 120 days. WBCs (leucocytes) are colourless & nucleated. Their number in adult human is 7.5  3.5 × 103 per cubic mm of blood. These are of two types - Granulocytes (with granules in cytoplasm) and Agranulocytes (without granules). Platelets: - (Thrombocytes.) There are very small (2-3 m), non-nucleated, oval, biconvex disc like structures which secrete thromboplastin. Their number varies from 0.15 to 0.40 million/cu mm blood. During injury these get clumped at injured spot and release chemicals called 'Platelet factors' . These help in blood coagulaton. Serum is blood without corpuscles & fibrin protein. (ix) Lymph: - is translucent alkaline fluid present between capillaries and tissues. It contains plasma and.leucocytes. Function: -Lymph carries materials from tissues into the blood stream and vice-versa. 1.3 MUSCLE TISSUE: - Muscle tissues are thin and elongated cells made up of diffetentiated cells containing contractile proteins. Muscle cells are called 'Myocytes'. Types: -Muscles are of three types: -Skeletal muscles, Cardiac muscles and Smooth muscles. (a) Skeletal muscles: -These consist of bundles of very long cylindrical, multinucleated cells/ fibres (diameter 10-100 mm). Skeletal muscles are attached to bones by tendons. Each fibre has elastic outer covering 'Sarcolemma', cytoplasm called 'Sarcoplasm' filled and Myofibrils lying in the sarcoplasm. Each muscle fibre shows alternate light and dark bands called I-bands (Isotropic bands) and A-bands (anisotropic bands) respectively under light microscope. Each I-band has a dark membrane (Z-line) at its center. Myofibrils are of 2 types-Thick filaments & Thin filaments. Thick filaments occur in A-band only and are made up of protein 'myosin'. Their both ends are free. Thin filaments occur in I-band butextend to some distance in A-band also. This partial overlapping of thin & thick filaments in A-band gives A-band a dark appearance. Thin filaments are made of protein actin, troponin and tropomysin. Their one end is attached to Z-line and the outer end is free. (b) Cardiac muscle: -These are found in the wall of heart and large veins. Cardiac muscle consists of short, cylindrical fibres joined end to end and interconnected by oblique bridges forming a complex 3-dimensional network. They are mostly uninucleate, contain actin & myosin and show cross-striations. Intercalated discs are seen at places. These are specialized regions of cell membranes at the ends of adjoining cardiac muscle fibres forming dense junctions. Cardiac muscles can contract. (c) Smooth muscle: -These are found in walls of hollow viscera. These are long (20 m-500 m), spindle shaped and have a single oval nucleus surrounded by sarcoplasm. There are fewer mitochondria and no striations. These can not contract voluntarily. Function: -These help in peristalsis. 1.4  NERVOUS TISSUE: -It forms the nervous system of animals. It has 2 types of cells-Nerve cells/Neurons and Glial cells. Neurons have many long processes and transmit nerve impulse. Glials have short processes and support the neurons. Structure of neuron: -It has a cell body (cyton) and two/more cytoplasmic process (neurites) arising from it. Cell body varies in size & form. It has cytoplasm, nucleus and cell membrane. Nissl bodies/granules (rough endoplasmic reticulum with free ribosomes) are also present. Neurites are of 2 types:- Axons and Dendrons/dendrites. Axon is long and carries nerve impulses away from cell body. Dendron conducts nerve impulse towards cell body. Junction between axon of one neuron and dendrite of another neuron is called Synapse. Nerve impules pass between neurons through the synapse with the help of chemicals like Acetylcholine. Types of Neurons: - (a) Unipolar: -have only one axon and no dendron. Found in early embryos. (b) Bipolar: -These have one axon and no dendron. Found in olfactory epithelium and retina of eye. (c) Multipolar: -These have many processes arising from the cell body. Longest one acts as axon and remaining are dendrons. Found in brain and spinal cord. Nerve Fibres: -Extended axons/dendrons of a neuron are called nerve fibres. Each nerve fibre is covered by neurolemma (made of Schwann cells). Nerve fibres are of two types: - Nerve Impulse : -Potential changes propagated along the membrane of the nerve fibre is called 'Nerve Inpulse' It carries message to neurons or muscles or glands.