Organization of the Seminiferous Epithelium and the Cycle, and Morphometric Description of Spermatogonial Subtypes (Rodents and Primates)

Spermatogenesis encompasses three main cell types: spermatogonia that proliferate, spermatocytes that carry out the process of meiosis, and haploid spermatids that develop into sperm. Differentiating spermatogonia are formed at species-specific intervals, and in each tubule cross-section 4 or 5 generations of spermatogenic cells can be observed. As the timing of the development of the germ cells is always similar, specific associations of germ cell types are seen in tubule cross-sections. Each epithelial area looks similar with regular intervals and goes through the cell associations in a specific order. This is called the cycle of the seminiferous epithelium and the cell associations are called stages (usually 12).

In non-primate mammals, the spermatogonial stem cells (SSCs) are single cells (As spermatogonia) that divide 2–3 times per epithelial cycle and render either two new singles (self-renewal) or the daughter cells stay together and form a pair. The niche for these SSCs likely is that part of the tubule basal lamina that borders on interstitial venules and arterioles. The pairs divide further into chains of spermatogonia that differentiate during epithelial stage VIII. The organization of the epithelium heavily depends on a peak in retinoic acid levels during stages VIII-IX that both induces spermatogonia to differentiate and preleptotene spermatocytes to enter meiotic prophase. Surprisingly, germ cells can develop normally while in abnormal epithelial stages, the strict epithelial organization is not required for qualitatively normal spermatogenesis.

The present knowledge on primate SSCs and spermatogonial multiplication is discussed but does not yet allow clear conclusions.