(Posted:15-Jul-2015)

It covers the body surface and lines the hollow tubular structures. Characteristics of epithelial tissue: 1)The constituent cells are closely packed together and the intracellular substanceis reduced to minimum. 2)The cells rest on basement membrane,which is made mostly of amorphus substance. 3)Epithelial tissue contains no cappilaries,and the nutrition is derived by diffusion from the nearest capillaries. 4)The surface cells are destroyed by wear and tear and are replaced by new cells produced by mitosis. Function: 1)Protection: Skin provides protection of the body from external environment. 2)Absorption: Epithelial lining of the alimentary tract allows absorption of the digested food stuff. 3)Secretion:Goblet cells of the alimentary and respiratory tracts secrets mucus,which provides protection and minimises friction of the passages. 4)Lubrication: There is particularly observed in serous cavities.e.g: peritoneal,pleural or pericardial where a capillary layer of fluid lubricates the interval between parietal and visceral layers and prevents separation of layers. Simple Epithelium:- It consist of a single layer of epithelial cells resting on a basement membrane and is present on those areas where absorption is more but wear and tear is less.Simple epithelium consists of the following types: A)Simple squamous. B)Columner with or without cilia. C)Pseudo-stratified. (A)Simple squamus:- The cells are flat with irregular outline and fit together by their edges to form a continuous,thin membrane. Distribution: *Lining of lung alveoli; *Inner surface of the serous cavity; * Inner lining of the heart,blood vessels and lymph vessels; (B)Columner epithelium: The cells are cylindrical resting on a basement membrane and the nuclei are oval and placed centrally or towards the basement membrane.Some of the columner cells are specialized by possesing brush borders or cilia. Distribution:*Gastrointestinal tract and its gland. *Greater part of the male urethra,vas deference. *Follicles of prostate,bulber urethral and greater vestibular glands. (C) Pseudo-stratified epithelium: In some situations the ciliated columner cells are of different heights,resting on a common basement membrane.All cells do not reace the surface and their nuclei are placed at different levels,resembling false stratifications.Hence this type is known as pseudo stratified columner ciliated epithelium.Mucous membrane of the nasal cavity,trachea and bronchi belongs to this type of epithelium. Compound Epithelium:- It consists of a several layers of cells and appears in those areas where wear and tear is more, and absorption is less. Compound epithelium is of two kinds: 1.Stratified and 2.Transitional epithelium. A)SRTATIFIED SQUAMOUS EPITHELIUM: Tdeepest layer of cells is columner,placed vertically on the basement membrane;succeding layers consists of polyhedral cells;most superficial cells are flattened. Distribution: * Keratinized epithelium-Epithelium of skin. *Non-keratinized epithelium-Tongue,mouth,lower pharynx,oesophagus,lower part of the anal canal,vagina,cornea etc. B)STRATIFIED COLUMNER EPITHELIUM: It consists of superimposed fusiform cells and is found in some parts of the male urethra. 2.Transitional epithelium: It is somewhat similar to the non-keratinised stratified epithelium.But unlike the flattened surface cells of stratified epithelium,the surface cells of transitional epithelium are large and rounded with abundunt cytoplasm and prominent nuclei.The surface cells are connected by tight junctions.The mucus membrane of the urinary bladder and ureters are made of transitional epithelium.