Epithelial tissue is usually separated from underlying tissues by an extracellular fibrous basement membrane.

The permeability of epithelial cells regulates the exchange of materials between the body and the external environment, as well as between different parts of the body. It helps maintain homeostasis by controlling the passage of substances such as nutrients, gases, ...

Epithelial tissue cells are tightly packed and form a continuous sheet. They have minimal intercellular spaces and are held together by small amounts of cementing material.

Examples of organs and structures composed of epithelial tissue include the skin, the lining of the mouth, blood vessels, lung alveoli, and kidney tubules.

The primary function of epithelial tissue is to provide covering or lining to organs and cavities, forming barriers that protect underlying tissues and regulate the exchange of materials between different body compartments and the external environment.