FIGURE 22.19 • Photomicrograph of human testis. a. This H&E–stained specimen shows the site that includes the mediastinum of the testis. On the right are seminiferous tubules, and on the left are the anastomosing channels of the rete testis. The arrow indicates termination of a straight tubule that is lined only by Sertoli cells. It is at this site that the tubule content enters the rete testis and the channels are then lined by a simple cuboidal epithelium. 70. b. This higher magnification from a slightly deeper section of the same specimen shows the rete testis (left), a cross section of a seminiferous tubule (upper right), and a terminating straight tubule (asterisk) where it is entering the rete testis. Note the abrupt change in the epithelial lining at this site. As noted, the lining epithelium of the rete testis is simple cuboidal. 275.
extend from the luminal surface of the principal cells (Plate 88, page 822). The stereocilia vary in height from 25 m in the head to approximately 10 m in the tail.  Basal cells that represent small, round cells resting on the basal lamina. They are the stem cells of the duct epithelium.
In addition, migrating lymphocytes called halo cells are often found within the epithelium. Under normal conditions, the epithelium of the epididymis represents the most proximal level of the excurrent duct system in which lymphocytes are present.
Epididymal cells function in both absorption and secretion.
Most of the fluid that is not reabsorbed by the efferent ductules is reabsorbed in the proximal portion of the epididymis. The epithelial cells also phagocytose any residual bodies not removed by the Sertoli cells as well as sperm that degenerate in the duct. The apical cytoplasm of the principal cells contains numerous invaginations at the bases of the stereocilia, along with coated vesicles, multivesicular bodies, and lysosomes (Fig. 22.24).
The principal cells secrete glycerophosphocholine, sialic acid, and glycoproteins, which, in addition to the glycocalyx and steroids, aid in the maturation of the sperm. They have numerous cisternae of rER surrounding the basally located nucleus and a remarkably large supranuclear Golgi apparatus. Profiles of sER and rER are also present in the apical cytoplasm.
The smooth muscle coat of the duct of the epididymis gradually increases in thickness to become three-layered in the tail.
In the head of the epididymis and most of the body, the smooth muscle coat consists of a thin layer of circular smooth muscle resembling that of the efferent ductules. In the tail, inner and outer longitudinal layers are added. These three layers are then continuous with the three smooth muscle layers of the ductus deferens, the next component of the excurrent duct system (Plate 89, page 824).
Differences in smooth muscle function parallel these morphologic differences. In the head and body of the epididymis, spontaneous, rhythmic peristaltic contractions serve to move the sperm along the duct. Few peristaltic contractions occur in the tail of the epididymis, which serves as the principal reservoir for mature sperm. These sperm are forced into the ductus deferens by intense contractions of the three smooth muscle layers after appropriate neural stimulation associated with ejaculation.