Plate 87 Seminal Vesicle

The seminal vesicles are evaginations from the end of each ductus deferens that form tightly coiled tubes. Although sections through this structure may show many lumina, they are all profiles of a single continuous tubular lumen. The seminal vesicles are lined with a pseudostratified columnar epithelium that closely resembles that of the prostate gland.

The secretion of the seminal vesicles is a whitish yellow viscous material that contains fructose, other simple sugars, amino acids, ascorbic acid, and prostaglandins. Although prostaglandins were first isolated from the prostate gland (hence the name), they are actually synthesized in large amounts in the seminal vesicles. Fructose is the primary nutrient source for the sperm in the semen.

The mucosa rests on a thick layer of smooth muscle that is directly continuous with that of the ductus deferens, from which the seminal vesicle evaginates. The smooth muscle consists of an indistinct inner circular layer and an outer longitudinal layer (compare with the three layers of the ductus epididymis and the ductus deferens, Plate 84), which are difficult to distinguish. Contraction of the smooth muscle coat during ejaculation forces the secretions of the seminal vesicles into the ejaculatory ducts. Beyond the smooth muscle is the connective tissue of the adventitia.

Figure 1, seminal vesicle, human, H&E x30.

This figure shows a cross section of a seminal vesicle. Because of the coiled nature of the vesicle, two almost distinct lumina, lying side by side, appear to be present. They are, however, connected so that, in effect, all of the internal spaces are continuous and what is seen here is actually a two-dimensional configuration reflecting coiling of the tube.

The mucosa of the seminal vesicles is characterized by being extensively folded or ridged. The ridges vary in size and typically branch and interconnect with one another. The larger ridges may form recesses that contain smaller ridges, and when cut obliquely, these appear as mucosal arches that enclose the smaller folds (arrows). When the

Figure 2, seminal vesicle, human, H&E x220.

This higher magnification of the mucosal folds reveals the epithelium (Ep) and the underlying loose connective tissue or lamina propria (LP). The epithelium is described as pseudostratified. It is composed of low columnar or cuboidal cells and small, round basal cells. The latter are randomly interspersed between the larger principal cells, but they are relatively sparse. For this reason, the epithe-

plane of section is normal to the surface, the mucosal ridges appear as "villi." In some areas, particularly the peripheral region of the lumen, the interconnecting folds of the mucosa appear as alveoli. Each of these chambers is, however, simply a pocket-like structure that is open and continuous with the lumen. The mucosa is subtended by a very cellular loose connective tissue (CT) that, in turn, is surrounded by smooth muscle (SM).

The seminal vesicles are paired elongated sacs. Each vesicle consists of a single tube folded and coiled on itself with occasional diverticula in its wall. The upper extremity ends as a cul-de-sac; the lower extremity is constricted into a narrow straight duct that joins and empties into its corresponding ductus deferens.

lium may not be readily recognized as pseudostratified. In some areas, the epithelium appears thick (arrowhead) and, based on the disposition of the nuclei, would seem to be multilayered. This is due to a tangential section of the epithelium and is not a true stratification. The lamina propria of the mucosa is composed of a very cellular connective tissue containing some smooth muscle cells and is rich in elastic fibers.

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