Epithelial tissues line the outer surfaces of the body, internal passageways, and form the secretory portions of glands. Because they cover body surfaces and line cavities, these tissues serve as the body's primary interface with the external environment. A defining feature of epithelium is cellularity, meaning the cells are packed tightly together with almost no intercellular matrix between them. Every epithelium rests on a thin, extracellular structure called the basement membrane, which anchors the tissue to underlying connective tissue and acts as a selective filter. Furthermore, epithelial sheets are avascular, lacking blood vessels entirely. They rely completely on diffusion of nutrients from adjacent blood vessels in the underlying connective tissue to survive and repair themselves.
Epithelial cells show distinct polarity, meaning they have structurally and functionally different sides. The apical surface faces the body exterior, body cavity, or lumen of an internal organ, often featuring microphilic borders or cilia. Conversely, the basal surface attaches to the underlying basement membrane. To maintain this organized barrier and prevent unregulated leakage of substances between cells, adjacent epithelial cells lock together using specialized cell junctions:
Midwives and nurses identify epithelial tissues using a two-part naming system based on the number of cell layers and the shape of the surface cells.
Cell shapes fall into three main categories:
Because simple epithelia are only one cell thick, they form permeable barriers designed for transport. Simple squamous epithelium lines the air sacs of the lungs and blood vessels, permitting quick diffusion of gases and nutrients. Simple cuboidal epithelium makes up the walls of kidney tubules and small gland ducts, actively secreting and reabsorbing molecules. Simple columnar epithelium lines most of the digestive tract from the stomach to the rectum, where it manages nutrient absorption and secretes protective mucus. A variation called pseudostratified ciliated columnar epithelium appears to have multiple layers because nuclei sit at different heights, though every cell touches the basement membrane. This tissue lines the upper respiratory tract, using beating cilia and mucus to trap and sweep away inhaled dust and pathogens.
Where the body experiences heavy mechanical stress and abrasion, protection takes priority over absorption. Stratified squamous epithelium forms the outer layer of the skin and lines wet openings like the mouth and vagina. In the skin, outer cells fill with the tough, waterproof protein keratin, while internal mucosal surfaces remain non-keratinized. Stratified cuboidal and stratified columnar epithelia are rare, appearing mainly in the large ducts of sweat glands, salivary glands, and parts of the male urethra. Another specialized protective tissue is transitional epithelium, found exclusively in the urinary system. Transitional cells change shape and slide over one another as the bladder fills and stretches, preventing toxic urine from leaking back into the body cavity.
A gland consists of one or more cells that manufacture and release a specific product called a secretion. Glands develop from epithelial tissue and fall into two main groups based on how they release their products: