Epithelium is the simplest animal tissue that functions as a protective tissue in the animal body. Epithelial cells are tightly packed and contain very little or no matrix. It covers most of the organs and cavities within the body. It also forms a barrier to keep different body systems separate. The skin and lining of buccal cavity, blood vessels, alveoli, and kidney tubules are made of epithelial tissue. Epithelial cells lie on a delicate non-cellular basement membrane that contains a special form of matrix protein known as collagen. Epithelium may be simple (composed of a single layer of cells) or stratified (made up of several layers of cells).

Epithelial tissue is classified into following four types depending upon the shape and function of its cells:

(a) Squamous epithelium.

(b) Cuboidal epithelium.

(c) Columnar/glandular epithelium.

(d) Ciliated epithelium.

The main functions of epithelial tissue are as follows:

(i) It forms the outer layer of skin and protects the underlying cells from drying, injury, and chemical effects.

(ii) It forms the lining of mouth and alimentary canal and protects these organs.

(iii) It helps in elimination of waste products.

(iv) It helps in absorption of water and nutrients.