Function of epithelial tissue.

 The epithelial tissue which forms the covering of the body, the skin and the lining of the cavities which open on to the surface is mainly protective. It prevents injury to the underlying tissues, prevents the loss of fluid from these tissues and also prevents the passage of fluid into the structure which are covered by skin. Micro-organisms cannot pass through healthy skin but they can and do pass through abraded skin.

Secretory. Most of the secreting glands and their ducts are composed of columnar epithelium. Very often the epithelium lining the gland and its duct is continuous with that of the surface in which the glands lie. Simple tubular and simple saccular glands are just involutions from the surface such as the simple tubular glands of the intestine. When these involutions branch, the structure become more complicated, as in the formation of compound tubular glands such as those of the kidney, and compound racemose or saccular glands such as the salivary glands and the pancreas.

   The endocrine glands are also composed of epithelial cells which may be massed together or may line hollow vesicles as occurs in the thyroid gland where the vesicles are lined by columnar epithelial cells, cubical in shape. These cell produce their secretion colloid but there is no duct from these glands and therefore the secretion reach the blood stream either directly or through the lymphatics.