Tunics and Mediastinum
Each testis is suspended in a outpouching of the peritoneal cavity. This cavity, like the rest of the peritoneum, is lined by a serosa consisting of mesothelial cells supported by a fibrous connective tissue. Around the testis, the parietal peritoneum is named tunica vaginalis while the visceral peritoneum is named tunica albuginea. Both of these tunics consist of fibrous connective tissue with a thin surface of mesothelium.
The thickened posterior portion of the tunica albuginea, called the mediastinum, receives the blood vessels, lymphatics, nerves, and ducts which serve the testis. Fibrous septa extend from the mediastinum into the body of the testis.
Seminiferous Tubules
The bulk of each testis consists of seminiferous tubules embedded in relatively sparse interstitial tissue. Sperm cells are produced by the tubules, while hormones are produced by endocrine cells (Leydig cells) within the interstitium.
Unlike the tubules in a typical exocrine gland, each seminiferous tubule forms a tightly coiled loop, nearly a meter in length, which opens at both ends into the rete testis.
Although such organization into loops is unique, the resulting appearance in tissue section is typical for a tubular gland -- lots of round or oval slices across tubules, with occasional tangential sections of the epithelium.
A few hundred tubules comprise one testis. Thin connective tissue septa, arising in the mediastinum, separate tubules into lobules.
Infertility due to atrophy of the testicular tubules can result from a number of causes, ranging from undescended testes (cryptorchidism) to inflammation (orchitis) to malnutrition or hormonal imbalance. For images, see WebPath and WebPath or Milikowski & Berman's Color Atlas of Basic Histopathology, pp. 426-427.
The tubules are lined by a complex epithelium which is most easily understood as consisting of two very different cell populations, Sertoli cells and germ cells.
Sertoli cells are support cells. These cells form what is basically a simple columnar epithelium. Each Sertoli cell rests on the basement membrane and extends to the lumen. The Sertoli cells create the environment in which germ cells carry out the fundamental reproductive function of gamete production.
The simple columnar nature of the Sertoli epithelium is most evident prior to puberty, before the germ cells begin producing sperm.
In an adult testis, Sertoli cell nuclei are often inconspicuous among the much more numerous nuclei of germ cells in various stages of meiosis and maturation. Nevertheless, the nuclei of Sertoli cell can be readily recognized as those typical of columnar epithelium -- oval, euchromatic, nuclei, usually with prominent nucleoli.
The cytoplasm of Sertoli cells assumes an elaborate shape, enveloping germ cells at various stages in meiosis.
Primary spermatocytes, produced by mitosis from spermatogonia, move away from the base of the epithelium and are sealed off from the basal surface by tight junctions between Sertoli cells. This has the effect of separating haploid germ cells, which are antigenically foreign, from circulating antibodies. As meiosis proceeds toward the epithelial surface, the germ cells remain nestled in pockets in the Sertoli cell cytoplasm. Specific Sertoli cell functions include nutritional support, phagocytosis of residual bodies (shed cytoplasm) from spermatids, and secretion.
Sperm Cell Formation
Male germ cells comprise a unique cell population which continually produces new male gametes, or spermatozoa, in the process called spermatogenesis. Germ cells at all stages of meiosis are found embedded within the epithelium of the seminiferous tubules.
