Overview: Epithelia
- Epithelia are tissue sheets made of closely packed cells that cover body surfaces, line internal spaces, and form glands.
- Epithelial tissue can develop from all three embryonic germ layers, so it is not limited to one developmental origin.
- Embryonic germ layers are early cell layers in the embryo that give rise to body tissues.
- Ectoderm, mesoderm, and endoderm are the three embryonic germ layers that can produce epithelial tissues.
- External surfaces of the body are usually covered by epithelia, with exceptions such as tooth surfaces and articular cartilage.
- Internal surfaces are lined by epithelia where body cavities, tubes, ducts, or hollow organs need a controlled boundary.
- Articular cartilage is the smooth cartilage covering joint surfaces, and it is an exception because it is not covered by epithelium.
- Closely packed cells are a key epithelial feature because epithelial cells form a barrier with very little extracellular space.
- Extracellular space is the space outside cells, and epithelia have little of it compared with many connective tissues.
- Epithelial polarity has three main domains: apical, lateral, and basal; the term basolateral combines the lateral and basal domains.
- Polarity means sidedness, and epithelial cells are polar because different parts of the cell face different environments.
- Apical domain is the epithelial cell surface facing a lumen, outside surface, or free space.
- Lateral domain is the epithelial cell surface contacting neighboring epithelial cells.
- Basal domain is the epithelial cell surface attached to the basement membrane.
- Basolateral domain is a combined term for the basal and lateral surfaces when they are considered together.
- Biochemical domains differ because the apical, lateral, and basal membranes can contain different proteins, lipids, and receptors.
- Functional domains differ because each epithelial surface performs different jobs, such as absorption, adhesion, secretion, or barrier control.
- Structural domains differ because surface specializations such as microvilli, junctions, and basal attachments occur in different epithelial regions.
- Basement membrane is a thin extracellular-matrix layer that separates epithelium from underlying connective tissue.
- Basal lamina is the epithelial-side layer of the basement membrane.
- Lamina reticularis is the connective-tissue-side layer of the basement membrane.
- Underlying connective tissue supports epithelia and contains blood vessels that nourish them.
- Blood vessels usually do not enter epithelial sheets, so epithelia depend on nearby connective tissue for nutrients.
- Avascular means lacking blood vessels, and epithelia are avascular even when they sit close to vascular connective tissue.
- Diffusion is movement of molecules from one region to another, and epithelial cells receive nourishment by diffusion through the basal lamina.
- Attachment to the basal lamina helps epithelial cells stay organized, maintain polarity, and receive support from underlying tissue.
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