I-1: Spermatogonial Ontogeny in Mice

Background:Spermatogenesis is maintained by a pool of the spermatogonia. The group formed by As, Apr and Aal is known as undifferentiated type a spermatogonia (Aund). The Aal spermatogonia will differentiate, with no mitotic division, to the differentiating type A1 spermatogonia. The type A1 spermatogonia undergo 6 successive mitotic divisions to yield A2, A3, A4, intermediate-type (In), and type B spermatogonia. The type B spermatogonia will divide giving rise to spermatocytes, starting the meiotic process. Despite the knowledge of spermatogonial biology in adult mice, spermatogonial development in immature animals has not been fully characterized. Thus, the aim of the present study was to evaluate the ontogeny of the morphological development of the spermatogonial lineage in C57BL/6 mouse testis, using high resolution light microscopy. Materials and Methods: To determinate the spermatogonial morphology, chronology, and absolute number, under high resolution light microscopy analyses, testes from mice at 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 17, 20, 28, 37 and 70 days post partum (pp) were collected and fixed in glutaraldehyde, embedded in araldite, sectioned in 1-μm thickness and stained in toluidine blue-borate. Results: Although we have observed different subclasses of gonocytes, which could be recognized by their morphology, number, and position in the seminiferous cords as gonocytes I, II or III, the total number of gonocytes does not increase after birth. The mitotic index data showed that the gonocytes proliferate immediately after birth but their number did not change due to a balance between mitosis and apoptosis, which was also increased during this period. The morphology of spermatogonia in immature mice was similar to that of adult spermatogonia, although their nuclear diameter was slightly smaller. The A1 spermatogonia are first observed on day 2 pp, and only 24 hours later differentiating type A3 and A4 spermatogonia were observed in the seminiferous cords. This result indicated a shortening of the spermatogonial phase for immature mice of about ~2.5 days when compared to adults, and suggests that gonocytes and/or A1 spermatogonia could directly become A4 spermatogonia, skipping of the developmental sequence of type A spermatogonia. These type A4 spermatogonia are functional as they develop into type B spermatogonia by day 5 pp. At day 8 pp, while differentiation to spermatocytes begins, the numbers of Aund spermatogonia reach their maximal numbers which are maintained through adulthood. Conclusion: The capacity to identify the different spermatogonial subtypes by their morphological characteristics in a normal ontogenetic process could be used as a tool to better understand spermatogonial biology during early postnatal development and provide a simple and reliable method to evaluate spermatogonial development in immature life under adverse circumstances, such as experimental or pathological conditions.