Epithelial tissue is a thin protective layer of cells which covers the surface of the body and lines the various body cavities and internal organs, including the blood vessels. The outermost layer of skin and the lining surfaces of the mouth, nose, lungs, stomach etc.
It may be flat, cuboidal or columnar in shape. The cells of the epithelial tissue fit tightly together leaving no space between them. Its functions are protection, absorption, secretion, sensory perception etc.
Its five structurally distinct categories are as follows:
Squamous epithelium: It is composed of cells which are thin plates of irregular shapes and seen in the outer layer of skin, in the lining of mouth and nasal cavities.
Stratified epithelium: Found in skin and cornea. It is composed of several layers of the same or different kinds of epithelial tissue.
Cuboidal epithelium: It is found in some parts of kidney tubules and in some glandular ducts such as those of salivary glands.
Columnar epithelium: It contains vertically arranged cylindrical or brick like cells. These cells are usually tall in size. It is found in the inner lining of the stomach and intestines.
Ciliated or columnar epithelium: Seen in the lining of the trachea or wind pipe, the columnar epithelium is ciliated. The cilia constantly keep lashing and move the materials which enter these regions.
Glandular epithelium: It contains some large cells which secrete certain chemical substances. Seen in the lining of the stomach and the intestine. At certain places in the body the glandular epithelium is folded inwards to form compact, hollow or tubular glands, for example sweat glands, tear glands or the liver.