EPITHELIAL TISSUE | COVERING OF THE BODY | Medicoze

         Epithelium forms a protective covering over the internal and external surfaces of the body.The ectoderm forms the skin,the mesoderm forms the pleura, pericardium and peritoneum and the endoderm forms the endothelial lining of the blood vessels and gut.Most of the glands are epithelial in origin.It can serve as a selective barrier and can be resistant to harmful metabolites, chemicals and bacteria.It is characterized by a minimal amount of intercellular substance and a tendency to form sheets of cells of one or more layers and having a capability of continuous replacement. It is simple,transitional or stratified.

1.)Simple epithelium

This consists of a single layer of cells on a basement Membrane.It is described as squamous,cuboidal or columnar, depending on the shape of its cells. Squamous cells are found lining the alveoli of the lungs, the blood vessels and the serous cavities . Cuboidal cells line the ducts of many glands. Columnar cells are often ciliated and may be modified as mucus-forming goblet cells; they line much of the alimentary, respiratory and reproductive tracts. Mucus,a glycoprotein, accumulates in the cell and is discharged from its free surface.

2.)Transitional epithelium

This contains two or three layers of cells, most of which are attached to the basement membrane and are nucleated. It lines most of the urinary tract, is stretchable and does not desquamate. It contains few glands.

3.)Stratified squamous epithelium

This also has two or more layers of cells. Cells in contact with the basement membrane are columnar cells. The more superficial cells are flattened, and the surface cells have no nuclei and are continually being rubbed away . This form of epithelium covers the exterior of the body, lines both ends of the alimentary tract and is particularly suited to areas exposed to wear and tear. In the upper respiratory tract the differing lengths of the columnar cells gives the appearance of a double layer, and this is known as pseudostratified columnar epithelium, it contains numerous mucous cells.