Epithelia - Some Key Points

You should note the following key points.

NYU Virtual Microscope 117.68

1. Epithelia are sheets of cells that line external and internal body surfaces.
2. Epithelia present a barrier to the free movement of substances from the external environment (external world, or lumen of hollow organs) to the internal environment.
3. Epithelia operate specific transport mechanisms for some substances such that they are selectively transported either from the external to internal environments or vice versa
4. All epithelia are attached to a 'carpet' of glycoproteins, proteins, glycosaminoglycans and proteoglycans called the basement membrane.
5. In addition to acting as an attachment platform, the basement membrane also
(a) regulates the behaviour and structure of the cells attached to it
(b) prevents the migration of epithelial cells into the interstitium
(c) acts as an ultrafiltration device regulating what molecules can cross into and out of the epithelial compartment
(d) is the structural interface between the epithelial sheet and the underlying connective tissue.
6. All the cells in an epithelium are attached to one another by intercellular junctions, and the number and type of these junctions determine the structural stability and permeability of the epithelial sheet as a whole.
7. All epithelial cells are linked to one another by gap junctions, non-structural junctions which permit the free diffusion of small molecules between all of the cells in an epithelial sheet. This means all the cells in an epithelium are metabolically coupled to one another.
8. All epithelia undergo turnover at characteristic rates. This implies all epithelia have a reserve stem cell population from which new epithelial cells are derived by mitosis, and that the rate of cell division is tightly coupled to the rate of cell loss.

From a traditional histology perspective the following points should be noted:

9. The basement membrane is not always visible without special staining techniques.
10. Epithelia are classified as either;
(a) simple, a single layer of cells all of which are attached to the basement membrane or
(b) stratified, two or more layers of cells in which only the innermost layer of cells is attached to the basement membrane.
Pseudostratified is an additional classification in which there is a single layer of cells all attached to the basement membrane, but the cells are different sizes and shapes and so gives rise ot the appearacne of being stratified.
11. In addition to the above classification, epithelia are also classified according to the shape of the OUTERMOST cell layers (the inner cell layers are most frequently round or polygonal). The possible shapes are
(a) cuboidal - cells approx. same height and width, generally round nucleus occupying the centre of the cell
(b) columnar - cells are taller than they are broad, generally oval nuclei, which may be displaced toward the bottom or top of the cell
(c) squamous - cells are flat and paving stone like, generally flattened oval nuclei which may cause the apical cell surface to bulge outward
12. If the cells in the epithelium display certain specializations (e.g. cilia) there are also noted (i.e simple cuboidal ciliated epithelium)
13. Epithelia often do not look exactly like their classification because of the method of tissue preparation or the angle at which the tissue was sectioned. Recognizing specific epithelia is a skill that comes with practice. You are generally not expected to identify unknown epithelia, so you will learn to match the actual appearance of a given epithelium with the appearance it should have.

Finally, on the basis of the above, what kind of epithelium is shown in the image accompanying this post?

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