While the reductive divisions of meiosis are conserved in every eukaryotic kingdom of life, the regulation of meiosis in mammals differs dramatically between males and females. The differences between oogenesis, the production of eggs, and spermatogenesis, the production of sperm, are outlined in Table 19.1.

Table 19.1

Sexual dimorphism in mammalian meioses.

Spermatogenesis is the production of sperm from the primordial germ cells. Once the vertebrate PGCs arrive at the genital ridge of a male embryo, they become incorporated into the sex cords. They remain there until maturity, at which time the sex cords hollow out to form the seminiferous tubules, and the epithelium of the tubules differentiates into the Sertoli cells. The initiation of spermatogenesis during puberty is probably regulated by the synthesis of BMP8B by the spermatogenic germ cells, the spermatogonia. When BMP8B reaches a critical concentration, the germ cells begin to differentiate. The differentiating cells produce high levels of BMP8B, which can then further stimulate their differentiation. Mice lacking BMP8B do not initiate spermatogenesis at puberty (Zhao et al. 1996).

The spermatogenic germ cells are bound to the Sertoli cells by N-cadherin molecules on both cell surfaces and by galactosyltransferase molecules on the spermatogenic cells that bind a carbohydrate receptor on the Sertoli cells (Newton et al. 1993; Pratt et al. 1993). The Sertoli cells nourish and protect the developing sperm cells, and spermatogenesis—the developmental pathway from germ cell to mature sperm—occurs in the recesses of the Sertoli cells (Figure 19.17). The processes by which the PGCs generate sperm have been studied in detail in several organisms, but we will focus here on spermatogenesis in mammals.

Figure 19.17

Drawing of a section of the seminiferous tubule, showing the relationship between Sertoli cells and the developing sperm. As cells mature, they progress toward the lumen of the seminiferous tubule. (After Dym 1977.)

After reaching the gonad, the PGCs divide to form type A₁ spermatogonia. These cells are smaller than the PGCs and are characterized by an ovoid nucleus that contains chromatin associated with the nuclear membrane. The A₁ spermatogonia are found adjacent to the outer basement membrane of the sex cords. They are stem cells, and at maturity, they are thought to divide so as to make another type A₁ spermatogonium as well as a second, paler type of cell, the type A₂ spermatogonium. Thus, each type A₁ spermatogonium is a stem cell capable of regenerating itself as well as producing a new cell type. The A₂ spermatogonia divide to produce the A₃ spermatogonia, which then beget the type A₄ spermatogonia. It is possible that each of the type A spermatogonia are stem cells, capable of self-renewal. The A₄ spermatogonium has three options: it can form another A₄ spermatogonium (self-renewal); it can undergo cell death (apoptosis); or it can differentiate into the first committed stem cell type, the intermediate spermatogonium. Intermediate spermatogonia are committed to becoming spermatozoa, and they divide mitotically once to form the type B spermatogonia. These cells are the precursors of the spermatocytes and are the last cells of the line that undergo mitosis. They divide once to generate the primary spermatocytes—the cells that enter meiosis. It is not known what causes the spermatogonia to take the path toward differentiation rather than self-renewal; nor is it known what stimulates the cells to enter meiotic rather than mitotic division (Dym 1994).

Looking at Figures 19.17 and 19.18, we find that during the spermatogonial divisions, cytokinesis is not complete. Rather, the cells form a syncytium whereby each cell communicates with the others via cytoplasmic bridges about 1 μm in diameter (Dym and Fawcett 1971). The successive divisions produce clones of interconnected cells, and because ions and molecules readily pass through these intercellular bridges, each cohort matures synchronously. During this time, the spermatocyte nucleus often transcribes genes whose products will be used later to form the axoneme and acrosome.

Figure 19.18

The formation of syncytial clones of human male germ cells. (After Bloom and Fawcett 1975.)

Each primary spermatocyte undergoes the first meiotic division to yield a pair of secondary spermatocytes, which complete the second division of meiosis. The haploid cells thus formed are called spermatids, and they are still connected to one another through their cytoplasmic bridges. The spermatids that are connected in this manner have haploid nuclei, but are functionally diploid, since a gene product made in one cell can readily diffuse into the cytoplasm of its neighbors (Braun et al. 1989). During the divisions from type A₁ spermatogonium to spermatid, the cells move farther and farther away from the basement membrane of the seminiferous tubule and closer to its lumen (see Figure 19.17). Thus, each type of cell can be found in a particular layer of the tubule. The spermatids are located at the border of the lumen, and here they lose their cytoplasmic connections and differentiate into sperm cells. In humans, the progression from spermatogonial stem cell to mature sperm takes 65 days (Dym 1994).