TESTIS, EPIDIDYMIS, AND VAS DEFERENS
The testicle is encased within a thick, ﬁbrous capsule known as the tunica albuginea. The tunica is covered by the closely adherent, glistening peritoneum (tunica vaginalis). Multiple septa from the capsule divide the interior of the testicle into several dozen pyramid- shaped lobules. The testis shows ethnic variations in size, but is normally 4 cm in length and 3 cm in diameter (18 to 20 mL in volume).
Within each testicle, each lobule contains one or several tortuous seminiferous tubules which, when uncoiled, measure 1 to 2 ft in length. These tubules converge at the testicular hilum (mediastinum testis), where they straighten and anastomose to form the rete testis. The rete testis tubules empty into 8 to 10 efferent ducts (ductuli efferentes) that carry sperm to the caput epididymis. Occasionally a blind-ending efferent duct is observed (vas aberrans). Spermatoceles are thought to be the result of pathologic dilation of the efferent ducts.
Testicular histology reveals evidence of both exocrine (sperm production) and endocrine (androgen production) functions within the organ. In the normal, adult testis, seminiferous tubules are lined with a basement layer of laminated connective tissue containing elastic ﬁbers and ﬂattened myoid cells. On this layer rests the germinal epithelium and sustentacular cells known as Sertoli cells. The intertubular connective tissue contains groups of large polygonal cells termed Leydig cells, whose cytoplasm holds many lipid granules that contain testosterone and other androgens. Characteristics of maleness, including body hair, muscle mass, deepened voice, and sexual function are several androgen dependent functions.
The epididymis is a comma-shaped organ located along the posterolateral surface of the testis. It is a tightly coiled, tortuous duct 3 to 4 m in length, embedded in dense connective tissue. Passage through the epididymis induces many changes to newly formed sperm, including a gain in functional motility and alterations in surface charge, membrane proteins, immunoreactivity, phospholipids, fatty acid content, and adenylate cyclase activity. These changes improve cell membrane structural integrity, increase fertilization ability, and improve motility. Spermatozoa within the testis have very poor or no motility. They become progressively motile and functional only after traversing the epididymis. The transit time of sperm through the epididymis has been estimated at 12 days in humans.
Extensions from the tunical sheath that surrounds the epididymis enter interductal spaces and form septa that divide the duct into histologically characteristic regions: the caput or head, corpus or body, and cauda or tail. The 8 to 10 ductuli efferentes within the caput region coalesce to form a single epididymal duct within the corpus and cauda epididymis. The epididymis is distinguished histologically by its ciliated epithelium that consists of two main cell types: principal cells and basal cells. Principal cells vary in height along the length of the epididymis mainly because of the length of associated stereocilia. Principal cell nuclei are elongated and often possess large clefts and one or two nucleoli. Consistent with absorptive and secretory function, their cellular apices have numerous coated pits. There are far fewer basal cells than principal cells in the epididymis. Tear-shaped basal cells rest on the basal lamina and extend approximately toward the lumen, their apices forming threads between adjacent principal cells. Thought to be derived from macrophages, they are likely the precursors of the principal cells.
The vas deferens originates as a continuation of the cauda epididymal duct. During this transition, the muscular coat of the tubule increases dramatically, the tortuosity of the duct decreases, and epithelial cells lose cilia. The vas continues for about 25 cm and becomes the ampulla of the vas before joining with the seminal vesicle and forming the proximal ejaculatory duct. In cross section, the vas deferens has an outer adventitial connective tissue sheath containing blood vessels and small nerves, a muscular coat that consists of a middle circular layer surrounded by inner and outer longitudinal muscle layers, and an inner mucosal layer with a pseudostratiﬁed epithelial lining. The outer diameter of the vas deferens varies from 1.5 to 3 mm, and the lumen of the unobstructed vas deferens varies from 0.2 to 0.7 mm in diameter, dimensions easily handled using microsurgical approaches to surgical reconstruction after vasectomy or other blockage.