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.
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. Tubules are surrounded by a thin layer of contractile myoid cells.
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.
(The image should be animated, if you watch patiently.)
The head (flattened, about 5 Ám long and 3 Ám wide) chiefly consists of the nucleus (greatly condensed chromatin!). The anterior 2/3 of the nucleus is covered by the acrosome, which contains enzymes important in the process of fertilisation. The posterior parts of the nuclear membrane forms the so-called basal plate.
The neck is short (about 1 Ám) and attached to the basal plate. A transversely oriented centriole is located immediately behind the basal plate. The neck also contains nine segmented columns of fibrous material, which continue as the outer dense fibres into the tail.
The tail is further divided into a middle piece, a principal piece and an end piece. The axonema (the generic name for the arrangement of microtubules in all cilia) begins in the middle piece. It is surrounded by nine outer dense fibres, which are not found in other cilia. In the middle piece (about 5 Ám long), the axonema and dense fibres are surrounded by a sheath of mitochondria. The middle piece is terminated by a dense ring, the annulus. The principal piece is about 45 Ám long. It contains a fibrous sheath, which consists of dorsal and ventral longitudinal columns interconnected by regularly spaced circumferential hoops. The fibrous sheath and the dense fibres do not extend to the tip of the tail. Along the last part (5 Ám) of the tail, called the end piece, the axonema is only surrounded by a small amount of cytoplasm and the plasma membrane.
- Spermatogonia ...
- are the stem cells of the germ cell population.
- divide mitotically to produce primary spermatocytes as well as more spermatogonia.
- are found at the base of the tubular epithelium.
- have relatively large round nuclei and lie adjacent to the basement membrane of the tubular epithelium.
- Primary spermatocytes ...
- are cells in the first stage in meiosis, during which DNA replicates twice.
- divide to produce secondary spermatocytes.
- have relatively large round nuclei.
- are found at mid-levels within the tubular epithelium.
- Secondary spermatocytes ...
- divide one more time, without further DNA replication, to produce spermatids.
- have relatively large round nuclei (like primary spermatocytes).
- are found at mid-levels within the tubular epithelium (like primary spermatocytes).
- Spermatids ...
- are the product of the final meiotic division.
- are found near the lumen of the tubule.
- have small round nuclei.
- undergo an elaborate process of maturation (called spermiogenesis) to become spermatozoa.
- Spermatozoa ...
- are highly specialized, motile cells, each with a single large flagellum.
- form by maturation (i.e., without further cell division) from spermatids.
- have very small, highly condensed, oval to conical nuclei. (The shape of entire sperm cells, with long flagella, is not evident in tissue sections.)
- are found near the lumen of the tubule.
- See your histology textbook for more detailed information on spermatozoa (including important features such as the acrosome, the middle piece, and the axoneme) and the process of spermiogenesis.With fine histological material and exquisite attention to nuclear detail, one may not only distinguish among spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids, and spermatozoa, but also distinguish various stages of cell division within each cell type.
Historical note: Because reproduction was once one of the grandest mysteries of life, the process of gamete production has long received intense attention from biologists. One major discovery was the marvelous dance of chromosomes during meiosis. The profound significance of this process became apparent with the rediscovery of Mendel's laws, when biologists realized that the segregation and independent assortment of "genetic factors" (what we now call genes) paralleled the behavior chromosomes behaved during meiosis. Chromosomes were the visible, physical agents of inheritance!
With such a history, it should come as no surprise that science has accumulated a vast amount of detailed information describing both spermatogenesis and spermiogenesis. Growth of knowledge continues apace, now motivated less by a sense of profound mystery than by an expectation that a sufficiently detailed understanding the process might yield better methods of birth control.
Seminiferous tubules are separated from one another by a delicate connective tissue stroma, or interstitial tissue. This interstitial tissue between the tubules contains clusters of endocrine Leydig cells which secrete testosterone.
Testosterone-secreting Leydig cells occur in clusters within the interstitial tissue (stroma) of the testis.
Leydig cells may be recognized not only by their location within the testicular interstitium but also by their round nuclei and extensive acidophilic cytoplasm.
Leydig cells have an appearance typical of steroid-secreting cells. With electron microscopy they can be seen to contain abundant smooth endoplasmic reticulum and mitochodria with tubular cristae. Leydig cells may contain small eosinophilic cytoplasmic inclusions called Reinke's crystaloids. With age, Leydig cells may accumulate lipofuscin (brown "wear-and-tear" pigment).
Rete Testis and Efferent Ductules
SIUC / School
of Medicine / Anatomy / David
Last updated: 29 April 2009 / dgk