Epithelia: Functions of the Two Major Types of Epithelia
What are epithelia?
Epithelia are continuous sheets of cells (one or more layers thick) that cover the exterior surfaces of the body, line internal closed cavities and body tubes that communicate with the outside environment (the alimentary, respiratory and genitourinary tracts), make up the secretory portions of glands and their ducts, and are found in the sensory receptive regons of certain sensory organs (e.g. ear & nose).
What do they do?
Epithelia cover and lining surfaces (e.g. skin), are involved in absorption (e.g. the intestine), secretion (e.g. glands), can be sensory (e.g. neuroepithelium) or contractile (e.g. myoepithelial cells).
What are their characteristics?
Epithelia are a continuous sheet of cells covering the surfaces of the body. They stand on a basement membrane which provides a site of attachment for the epithelium, and acts as a selective filtration barrier. There is very little intercellular material. Epithelia do not have their own blood supply (avascular), they rely on diffusion for exchange of oxygen and metabolites. Epithelia contain specialised cell-cell junctions that bind adjacent cells to each other. They also contain communicating junctions (gap junctions) that allow allow communication between adjacent cells. Click here to find out about cell-cell junctions. Cells in epithelia show a polarity along the axis between the external and internal environment, and their apical (free) surfaces often exhibit modifications.
There are two major types of epithelia:
Covering epithelia and Glandular epithelia
(Note: epithelia is the plural form of epithelium!)
Covering Epithelia are important for:
Selective diffusion - for example transfer of gases, nutrients and waste products between the blood and surrounding tissues. These epithelia are usually simple squamous type, to provide the smallest barrier to diffusion. Click here to see some simple squamous epithelia.
Absorption/secretion - for example absorption of nutrients from the gut, and secretion of enzymes for digestion. These epithelia are usually columnar, as the cells contain lots of endoplasmic reticulum, and golgi, for secretion, and they often have microvilli, to increase the surface area of the apical surface for absorption. Click here to see some examples of columnar epithelia.
Physical protection - for example, the skin protects underlying tissues from sunlight, heat, cold, abrasion, etc. These epithelia are always stratified (many layers), and can sometimes be keratinised on its apical surface, as in skin. The keratin helps to waterproof the skin. Click here to find out about stratified epithelia.
Containment - i.e. a barrier between the inside and outside of the gut etc.
Glands are an organised collection of secretory epithelial cells. Most glands are formed during development by proliferation of epithelial cells so that they project into the underlying connective tissue. Some glands retain their continuity with the surface via a duct and are known as EXOCRINE GLANDS. Other glands lose this direct continuity with the surface when their ducts degenerate during development. These glands are known as ENDOCRINE glands.
Thus the two types of glands are called : Exocrine and Endocrine.
Can be Unicellular - Goblet cells, or Multicellular
All exocrine glands are glands with ducts. The secretions are delivered into ducts and end up on the epithelial surface. For example, the sweat glands are exocrine glands.
There are several different basic types of exocrine glands. Click here to find out what types of exocrine glands there are.
Glands without ducts. The secretions are released directly into the blood stream.
Glands are dealt with in more detail in the Glandular Tissues topic.
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