Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport, and detection of sensation.
Epithelial layers are avascular, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane. Epithelia can also be organized into clusters of cells that function as exocrine and endocrine glands. Exocrine and endocrine epithelial cells are highly vascular. Cells in epithelium are densely packed together like bricks in a wall, leaving very little intercellular space. The cells can form continuous sheets, which are attached to each other at many locations by tight junctions and desmosomes.
Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control the paracellular transport.
Epithelial tissue that is only one cell thick is known as simple epithelium. If it is two or more cells thick, it is known as stratified epithelium. However, when taller simple epithelial cells (e.g. columnar) are viewed in cross section with several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified" epithelium. There are three principal morphologies associated with epithelial cells (Figure 1): squamous epithelium has cells which are wider than they are tall (flat and scale-like); cuboidal epithelium has cells whose height and width are approximately the same (cube shaped); and columnar epithelium has cells taller than they are wide (column shaped). In addition, the morphology of the cells in transitional epithelium may vary from squamous to cuboidal, depending on the amount of tension on the epithelium.
Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.
The primary functions of epithelial tissues are:
- to protect the tissues that lie beneath it from radiation, desiccation, toxins, and physical trauma;
- the regulation and exchange of chemicals between the underlying tissues and a body cavity;
- the secretion of hormones into the blood vascular system;
- the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts glandular epithelium;
- secretion, selective absorption, protection, transcellular transport, and detection of sensation.