Epithelium

Epithelium, also called epithelial tissue, is a thin, continuous layer of cells that provides protection and has very little extracellular matrix. It is one of the four fundamental types of animal tissue, alongside connective, muscle, and nervous tissue. This tissue forms the epidermis (the skin’s outer layer), coats the outer surfaces of many internal organs, lines the inner surfaces of body cavities, and covers the inner surfaces of blood vessels.

Epithelial cells come in three main shapes: squamous (flat and scale-like), columnar (tall and column-shaped), and cuboidal (cube-shaped). These cells can be arranged in a single layer, known as simple epithelium (simple squamous, simple columnar, or simple cuboidal), or in multiple layers, called stratified or compound epithelium (stratified squamous, stratified columnar, or stratified cuboidal). In some tissues, a single layer of columnar cells may appear stratified because the nuclei are at different heights; this is called pseudostratified epithelium. All glands are composed of epithelial cells. Epithelial functions include diffusion, filtration, secretion, selective absorption, germination, and transcellular transport. Compound epithelium primarily serves a protective role.

Epithelial layers lack blood vessels (they are avascular) and have no lymph supply, though they are supplied by nerves. They receive nutrients through diffusion from the underlying connective tissue, passing across the basement membrane. Cell junctions are especially common in epithelial tissues.

**Classification**

**Simple epithelium** consists of a single layer of cells, each in direct contact with the basement membrane that separates it from the connective tissue below. It is typically found where absorption and filtration occur, as its thinness facilitates these processes. Epithelial tissues are classified by the number of cell layers and by cell shape. The basic shapes are squamous, cuboidal, and columnar.

By layer, epithelium is either simple (one cell thick) or stratified (two or more cells thick). Both types can be made up of any cell shape. When tall simple columnar cells are viewed in cross-section, their nuclei may appear at different heights, making them look stratified; this is called pseudostratified columnar epithelium. Transitional epithelium contains cells that can change shape from squamous to cuboidal depending on the tension on the tissue.

**Stratified epithelium** is multilayered, unlike simple epithelium. It is found where body linings must withstand mechanical or chemical damage, so that layers can be worn away without exposing deeper tissues. Cells flatten as they move toward the apical surface, though the deepest layers may be squamous, cuboidal, or columnar. Stratified epithelia can have various specializations.

**Structure**

Epithelial cells can take shapes ranging from polyhedral to scutoidal to punakoidal. They are tightly packed into a continuous sheet with almost no intercellular spaces. All epithelia are separated from underlying tissues by an extracellular fibrous basement membrane. The lining of the mouth, lung alveoli, and kidney tubules are made of epithelial tissue. The lining of blood and lymphatic vessels is a specialized form called endothelium.

**Location**

Epithelium lines both the outside of the body (skin) and the inside cavities and lumina. The outermost layer of human skin is composed of dead, stratified squamous, keratinized epithelial cells. Tissues lining the inside of the mouth, esophagus, vagina, and part of the rectum are made of nonkeratinized stratified squamous epithelium. Other surfaces separating body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Epithelial cells also line the lungs, gastrointestinal tract, reproductive and urinary tracts, and form exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. A specialized form, endothelium, lines the inner surfaces of blood vessels and the heart (vascular endothelium) and lymphatic vessels (lymphatic endothelium). Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum. In arthropods, the integument consists of a single layer of epithelial ectoderm from which the cuticle (an outer covering of chitin) arises.

**Basement membrane**

The basal surface of epithelial tissue rests on a basement membrane, while the free (apical) surface faces body fluid or the outside. The basement membrane acts as a scaffold for epithelial growth and regeneration after injury. Epithelial tissue has a nerve supply but no blood supply, so it must be nourished by substances diffusing from blood vessels in the underlying tissue. The basement membrane is a selectively permeable membrane that determines which substances enter the epithelium. The basal lamina is made of laminin (glycoproteins) secreted by epithelial cells. The reticular lamina beneath it is made of collagen proteins secreted by connective tissue.

**Cell junctions**

Cell junctions are especially abundant in epithelial tissues. They regulate interactions between neighboring cells.