Together with connective, muscle, and nervous tissue, epithelium is one of the four basic types of tissues in the animal body. It lines the internal cavities and external surfaces of the body. Epithelial cells are tightly joined by cell-cell contacts. Several molecular complexes form these anchoring junctions, such as desmosomes (zonula adherens), tight junctions (zonula occludens), and adherens junctions (zonula adherens). They form bridges between cytoskeletons of contiguous cells, getting cytoplasmic membranes so close between each other that the extracellular space is very narrow or completely occluded. Strong cell-cell junctions allow epithelium to counteract stretching forces that usually happen in places like skin or gut, and are also very important in the more superficial surfaces of the epithelium where it works as a molecular barrier. For example, in the gut epithelium, tight junctions create a barrier between adjacent cells that force external molecules to enter the body through the cytoplasm, but not via intercellular spaces. Epithelium contains very scarce extracellular matrix.
Epithelial cells are organized in one or several layers. The free surface of the epithelium is the apical part and the inner surface is the basal part. Thus, it is a polarized tissue. The apical part has different functions than the inner one. For example, the apical surface may show cell specializations such as cilia, flagella and microvilli, distinct membrane molecules, a layer of keratin, etcetera. The main function of the deepest cells of a multilayered epithelium is to renew the upper layers by continuous proliferation. Epithelium rests on a highly organized sheet of extracellular matrix, the basal lamina, which completely covers its basal surface. Basal lamina is produced by both epithelial cells and underlying connective cells. Since there is no blood vessel network in the epithelium, it must be nourished by substances diffusing from nearby tissues, mainly connective tissues.
Many functions are accomplished by epithelium: protection against mechanical assaults, prevention of desiccation, filtering, selective absorption, transport of substances, contain sensory structures, secretion of covering substances, and many others. Some of these functions are done by apical cell structures or by molecules located in the apical cell membrane.
Epithelium receives different names depending on where it is located. For example, the skin epithelium is referred as epidermis, in the internal surfaces such as abdominal cavities or cardiac cavity it is known as mesothelium, blood vessels are internally lined by epithelium known as endothelium. Furthermore, epithelium can be classified according to the cell layers and to the shape of the cells of the most superficial layer. Epithelium may have apical structures such as microvilli, cilia and flagella, and some epithelia produce structures such as hair, nails and feathers.
The epithelial cells can be traced back to the three germ layers of the embryo: endoderm, mesoderm and ectoderm. For example, endothelium of blood vessels are derived from mesoderm, epidermis from ectoderm, and gut epithelium comes from endoderm.
Some eptithelia contain cells that are specialized in producing and secreting substances to the external surfaces or into internal cavities of the body. They are known as glandular epithelium. The non glandular epithelium is referred as covering epithelium.
There are some very particular epithelia which are not mainly involved in covering or secretion. Neuroepithelium (olfactory and gustatory epithelium). germinal epithelium (it forms seminiferous tubules of testis) and myoepithelial cells (with contractile ability) are among these special epithelia.