Simple epithelia are only one cell layer thick. They are characteristic of organs and organ systems primarily concerned with transport, absorption, and secretion, such as the intestine, the vascular system, the digestive glands and other exocrine glands, and the kidney. Stratified epithelia have more than one layer and are typical of surfaces that are subject to frictional stress, such as skin, oral mucosa and esophagus, and vagina.
Figure 1, exocrine pancreas, monkey, H&E X450.
This shows three epithelial forms. In the circle is a well-oriented acinus, a functional group of secretory cells, each of which is pyramidal in shape. The secretory cells form a spherical or tubular structure. The free surface of the cells and the lumen are located in the center of the circle. The lumen is not evident here but is evident in a similar cell arrangement in Figure 4 (see circle). Because the height of the cells (the distance from the edge of the circle to the lu-
Figure 2, kidney, human, H&E x450.
This section shows cross-sectioned tubules of several types. Those that are labeled with the arrows provide another example of a simple cuboidal epithelium. The arrows point to
Figure 3, colon, human, H&E X350.
The simple columnar lining epithelium of the colon shown here consists of a single layer of absorptive cells and mucus-secreting cells (goblet cells). The latter can be recognized by the light staining "goblet" (arrows) that contains the cell's secre-
Figure 4, trachea, monkey, H&E x450.
In addition to the tall columnar cells (CC) in this columnar epithelium, there is a definite layer of basal cells (BC). The columnar cells, which contain elongate nuclei and possess cilia (C), extend from the surface to the basement membrane (clearly visible in the trachea as a thick, acellular, homogeneous region that is part of the connective tissue (CT)). The basal cells are interspersed between the columnar cells. Be-
Figure 5, epididymis, human, H&E X450.
This is another example of pseudostratified columnar epithelium. Again, two layers of nuclei are evident, those of basal cells (BC) and those of columnar cells (CC). As in the previous example, however, although not evident, the columnar cells rest on the basement membrane; thus, the
Figure 6, vagina, human, H&E x225.
This is the stratified squamous epithelium of the vaginal wall. The deeper cells, particularly those of the basal layer, are small, with little cytoplasm, and thus the nuclei men) is greater than the width, the epithelium is simple columnar. The second epithelial type is represented by a small, longitudinally sectioned duct (arrows) extending across the field. It is composed of flattened cells (note the nuclear shape), and on this basis, the epithelium is simple squamous. Finally, there is a larger cross-sectioned duct (asterisk) into which the smaller duct enters. The nuclei of this larger duct tend to be round, and the cells tend to be square in profile. Thus, these duct cells are a simple cuboidal epithelium.
the lateral cell boundaries; note that cell width approximates cell height. The cross-sectioned structures marked with asterisks are another type of tubule; they are smaller in diameter but are also composed of a simple cuboidal epithelium.
tory product. The epithelium lines the lumen of the colon and extends down into the connective tissue to form the intestinal glands (GL). Both cell types are tall with their nuclei located at the base of the cell. The connective tissue (CT) contains numerous cells, many of which are lymphocytes and plasma cells.
cause all of the cells rest on the basement membrane, they are regarded as a single layer, as opposed to two discrete layers, one over the other. Because the epithelium appears to be stratified but is not, it is called pseudostratified columnar epithelium. The circle in the micrograph delineates a tracheal gland similar to the acinus in Figure 1 (circle). Note that the lumen of the gland is clearly visible and the cell boundaries are also evident. The gland epithelium is simple columnar.
epithelium is pseudostratified. Note that where the epithelium is vertically oriented, on the right of the micrograph, there appear to be more nuclei, and the epithelium is thicker. This is a result of a tangential plane of section. As a rule, always examine the thinnest area of an epithelium to visualize its true organization.
appear closely packed. As the cells become larger, they tend to flatten out, forming disk-like squames. Because the surface cells retain this shape, the epithelium is called stratified squamous.
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