Epithelial surfaces are plentiful in the human body. The entire body has an epithelial covering called skin. The respiratory tract, gastrointestinal tract, and urinary tract all have epithelial linings. Any glandular, exocrine secretion must pass through an epithelial-lined duct.
Epithelia form a mechanical barrier on surfaces exposed to the external environment. Epithelia can be specialized to perform additional functions such as removing inhaled debris with cilia in the respiratory tract, or absorbing nutrients from the gastrointestinal tract, or secreting mucin and fluid to provide lubrication and transport in ducts.
There are several major types of epithelia:
Stratified squamous epithelium: the cells originate from a layer of cells along a basal lamina and form a multilayered structure. Cells toward the surface have smaller nuclei and greater amounts of cytoplasm with keratin. Mucosal surfaces without much wear and tear are not covered with a layer of keratin (non-keratizining squamous epithelium), as on the cervix. On surfaces receiving greater wear and tear, there is a thick layer of acellular keratin (keratinizing squamous epithelium), as on skin.
Columnar epithelium: there are tall cells along a basal lamina. They typically line glandular lumena or ducts. Columnar cells often produce mucin and may be called a mucinous epithelium. An example is the surface lining of the colon. They may have cilia or microvilli along the lumenal border. The cell nuclei are typically located toward the basal lamina. A variation of this pattern is a cuboidal epithelium where the cells are about as tall as they are wide, but there are gradations between cuboidal and columnar epithelium.
Pseudostratified columnar epithelium: not all the cells along the basal lamina reach the lumenal surface, though all contact the basal lamina. This type of epithelium lines the respiratory tract from the nasopharynx down into the bronchioles of the lung. Cilia are present.
Transitional epithelium: the cells are multilayered. The cells are connected in a way that allows stretching and expansion. The overlying cell layer next to the lumen ("umbrella cells") can spread thinly. This epithelium lines the urinary tract from the renal calyces down to the urethra. An example is the ureter.
Cuboidal epithelium: the cells are box-like to rounded. Cuboidal epithelium typically lines ducts draining glands. The small ducts of sweat glands (including breast, a modified sweat gland) can pile up to a stratified cuboidal appearance in larger ducts. Cuboidal epithelium is also found in renal tubules. The mesothelium that lines many body cavities, such as the pleural space, has a simple cuboidal appearance, but mesothelial cells are of mesenchymal origin and thus, technically, are connective tissue.
Glands are composed of collections of specialized epithelial cells that secrete one or more substances. Glands can be categorized as:
Exocrine glands: secretion occurs into a duct which drains to a lumen or a surface. Types of exocrine glands include mucinous glands in which the product secreted has a high concentration of mucinous material and serous glands in which the secretion is watery and often contains proteins such as enzymes. If the secretory product leaves the cell by exocytosis, then merocrine secretion occurs. If a portion of secretory cell cytoplasm forms the secretion, then apocrine secretion has occurred.
Endocrine glands: secretion occurs directly into the bloodstream. The secretion product acts as a hormone, a substance with an action on tissues located at a distance from the endocrine gland. There are also single scattered cells in tissues which have a neuroendocrine function.
Gland structure can be simple or complex. The goblet cells of the gastrointestinal tract are unicellular mucinous glands. Multicellular glands can be tubular, such as those in the gastric mucosa or colon, or acinar, such as those around a central lumen in the pancreas. A large gland can be divided into lobes and lobules, such as in the parotid glands or the breast. Myoepithelial cells, such as those around breast lobules, aid in contraction for secretion.
Specialized Epithelial Components
Basement membrane: every epithelium lies on a basement membrane that separates the epithelial cells from the underlying connective tissue. Integrins in the epithelial cells extend into the basement membrane and attach to laminin which in turn attaches to type 4 collagen and to fibronectin in the portion of the basement membrane known as the basal lamina. There are anchoring fibrils of type 7 collagen. Thus the basal lamina acts as an anchoring point for the epithelium. The basal lamina forms a meshwork that acts as a filter, both physically and electrically, to trap or exclude molecules. A lamina reticularis of the basement membrane interfaces with the underlying connective tissue.
Keratin: this substance is produced by squamous epithelium and provides protection. It is essentially the layered remains of dead cells. The keratin layer is continuously being desquamated and replaced. Hair and nails are additional forms of keratin.
Microvilli: these tiny structures form a brush border that increases the absorbtive area on the surface of the cell. Microvilli have a central core of actin filaments anchored by villin and a terminal web with spectrin.
Cilia: these are motile structures that have a characteristic structure of microtubules with 9 doublets surrounding two central singlets to form an axoneme. Dynein arms with ATPase activity attach to the microtubules to power movement.
Terminal bars at the apical region of epithelial cells represent junctional complexes with several components. "Tight junctions" or zonula occludens form an impenetrable barrier between cells. The zonula adherens has extracellular cadherins which join cytoskeletal elements through transmembrane proteins. The desmosomes have attachment proteins that link to intermediate filaments of cytokeratin. There are also gap junctions that provide a means for selective communication by passage of small molecules between cells.