What is epithelial tissue and it's …

Epithelial tissue is the covering or protective tissue in the animal body. Cells of epithelial tissues are tightly packed and form a continuous sheet. They have only a small amount of cementing material between them. They have very little intercellular spaces. Epithelium covers most organs and cavities within the body and also forms a barrier between different body systems to keep them separate. The skin, the lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules are all made of epithelial tissues. Epithelial tissues may be squamous (simple or stratified), columnar, cuboidal or glandular. The permeability of the cells of various epithelia play an important role in regulating the exchange of materials between the body and the external environment. The structure and function of different types of epithelial tissues are as follows:-

(A) Squamous epithelium can be of two types:-

(a) Simple squamous epithelium: Simple squamous epithelial cells are a simple flat kind of epithelium. They are extremely thin and flat. They form a delicate lining. They are present in the lining of the blood vessels or the alveoli (in lungs) where transportation of substances occurs through a selectively permeable membrane. They are also present in the esophagus and the lining of the mouth are also covered with this type of cells.
(b) Stratified squamous epithelium: Stratified squamous epithelial cells are arranged in a pattern of layers. E.g. Skin epithelial cells are arranged in many layers to prevent wear and tear.
(B) Columnar epithelium: Columnar epithelial cells are present where absorption and secretion occurs as in the inner lining of the intestine. These cells are long or columnar (pillar-like). They facilitate movement across the epithelial barrier. In the respiratory tract, the columnar epithelial tissue has hair-like projections (cilia) on the outer surfaces of epithelial cells. The movement of the cilia pushes the mucus forward and clears it.
(C) Cuboidal epithelium: Cuboidal epithelium is made up of cube-shaped cells which provide mechanical support. They form the lining of the kidney tubules and ducts of salivary glands.

(D) Glandular epithelium: A multicellular gland or glandular epithelium is formed when a portion of the epithelial tissue folds inward and a multicellular gland is formed. An epithelial cell sometimes acquires additional specialisation as a gland cell. Gland cells can secrete substances at the surface of the epithelium.