Epithelium (epi-upon, thella - growing) forms the outer covering of the body and the living of internal organs and cavities.
There are two types of opithelial tissues namely .simple epithelia. and .compound epithelia. based. on the number of layers or strata.
Simple epithelium is composed of a single layer of cells and forms the lining of body cavities, ducts and vessels. It helps in diffusion, absorption, filtration and secretion of substances. On the basis of the shape of the cells, it is further divided into three types:
i) Simple squamous epithelium (Pavement epithellum) It is composed of a single layer of flat and tile-like cells, each with a centrally located .ovoid nucleus.. It is found in endothelium of blood vessels, mesothelium of body cavities (pleura, peritoneum, and pericardium), wall of Bowman.s capsule of nephron, lining of alveoli of lungs etc.
ii) Simple cuboidal epithelium :
It is composed of a single layer of cube-like cells with centrally located spherical nuclei. It is found in germinal epi. thelium, proximal and distal convoluted tubules of nephron. Cuboidal epithelium of proximal convoluted tubule of nephron has .microvilli..
iii) Simple columnar epithelium :
It is composed of a single layer of tall and slender cells with oval nuclei located near the base. It has mucus-secreting .goblet cells. in some places. It is of two types
a) Ciliated columnar epithelium : Columnar epithelial cells have cilia on their free surface. It is mainly present in the inner surface of hollow organs like fallopian tubes, ventricles of brain, central canal of spinal cord, bronchioles, etc.
b) Non-ciliated columnar epithelium : Columnar cells are without cilia. It is found in the lining of stomach and intestine. Microvilli are present in the columnar epithelium of intestine to increase the surface area of absorption.
B) Compound epitheliumi (stratified epithelium): Non-ciliated epithelium It is made up of more than one layer of cells. Its main function is to provide protection against chemical and mechanical stress. It covers the dry surface of the skin as stratified, keratinized, squamous epithelium. It covers the moist surface of buccal cavity, pharynx, oesophagus and vagina as stratified non-keratinized ssquamous epithelium. It forms the inner lining of the larger ducts of salivary glands, sweat glands and pancreatic ducts as stratified cuboidal epithelium. It forms the wall of the urinary bladder as transitional epithelium.
C) Glandular epithelium :
Some of the columnar or cuboidal cells that get specialised for the production of certain secretions, form glandular epithelium. The glands are of two types - unicellular glands consisting of isolated glandular cells such as goblet cells of the gut, and multicellular glands, consisting of clusters of cells such as salivary glands. On the basis of the mode of pouring of their secretions, glands are divided into two types namely exocrine and endocrine glands. Exocrine glands are provided with ducts, secrete mucus, saliva, earwax (cerumen), oil, milk, digestive enzymes and other cell products. In contrast, endocrine glands are ductless and their products are .hormones., which are not sent out via ducts, but are carried to the target organs by blood. Based on the mode of secretion, exocrine glands are further divided into. i, merocrine glands (e.g. : pancreas) which release the secretory granules without the loss of other. cellular material.
II. Apocrine glands (e.g..: mammary glands) in which the apical part of the cell is pinched off along with the secretory product and il. Holocrine glands (e.g. : sebaceous glands), in which the entire cell disintegrates to discharge the contents.
