What is Epithelial Tissue?
Animal tissues are divided into four major classes on the basis of their functions: epithelial tissue, connective tissue, muscle tissue and nervous tissue. Epithelial tissues are sheets of cells that cover body surfaces. They form the linings of tracts, cavities, and vessels, as well as the outer surfaces of organs, including the skin. So, the epithelial tissues are thin tissues that cover all the exposed surfaces of the body. Epithelial Tissue forms the external skin, the inner lining of the mouth, digestive tract, secretory glands, the lining of hollow parts of every organ such as the heart, lungs, eyes, ears, the urogenital tract, as well as the ventricular system of the brain and central canals of the spinal cord.
Epithelial Tissue Cells
The cells of the epithelium are set very close to each other, separated only by very thin films of extracellular material. Neighboring cells are held together by intercellular junctions such as desmosomes, inter-digitations, tight junctions, gap junctions, intercellular bridges and closed fitted fold. Epithelial tissue is always anchored to connective tissue by means of a non-cellular layer, known as basement membrane, composed of polysaccharides. Epithelium is generally devoid of blood vessels.
Epithelial Tissue Cells obtain nutrition by diffusion from blood vessels in underlying tissues. However, delicate nerve fibers frequently penetrate into the intervals between epithelial cells. Epithelium has a great capacity to repair damage. It grows rapidly after injury to repair the defect.
Characteristics of Epithelial Tissue
Epithelial tissue has the following characteristics according with its role as a covering tissue subject to mechanical damage.
i) Cells of epithelial tissue are frequently attached to the basement membrane.
ii) Adjacent cells are joined together by an intercellular cement.
iii) Connecting bridges of cytoplasm may be present between the cells.
iv) A large number of layers of cells may be built to resist abrasion, e.g., epidermis of skin.
v) There is rapid division of epithelial layer to resist abrasion and maintain only a thin layer of cells, e.g., intestines.
vi) Resistance to abrasion may be increased by the presence of keratin, e.g., epidermis of skin.
Functions of Epithelial Tissue
Some major functions attributed to epithelial tissue are as follows.
a) Protection: - Epithelial tissue protects the primary tissues from dehydration, entry of germs, and mechanical and chemical injuries.
b) Secretion: - Epithelial glands secrete various substances like mucus, gastric juices, hormones, etc., that are necessary for the normal metabolic activities of the organism.
c) Absorption: - Epithelial lining of the intestine helps in the absorption of digested food materials.
d) Excretion: - Epithelial lining of uriniferous tubules in the kidney eliminates nitrogenous waste materials.
e) Conduction: - Ciliated epithelia as those of respiratory and genital passages conduct mucus or fluids in the ducts they line.
f) Respiration: - Epithelial coating of the lung alveoli brings about gaseous exchange between and air.
g) Exoskeleton formation: - Surface epithelia produce many useful exoskeleton structures like scales, nails, hoofs, horns, feathers, etc.
h) Sensation: - Sensory epithelium in sense organs sends nerve impulses from the environment to the central nervous system.
i) Reproduction: - Germinal epithelium of sex organs produces gametes i.e. sperms or ova.
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