Seasonal differences in equine spermatocytogenesis.

Spermatocytogenesis plays a pivotal role in regulation of spermatogenesis; however, its details remain relatively obscure in nonrodent species. The equine testis contains approximately 100% more spermatogonia in summer than in winter and appears to be a good model to identify the flexible components of spermatocytogenesis that cause seasonal changes in daily sperm production. Testes were taken from horses in the winter (n = 47) and in summer (n 43). Tissues were fixed by glutaraldehydeperfusion and submission in osmium, embedded in Epon or methacrylate, sectioned at 0.5 micromole or 5 micromole, stained with toluidine blue, and observed using bright-field microscopy. The combined total number of A1, A2, A3, and B1 (A plus B1) spermatogonia/testis and the numbers of B2 spermatogonia or early primary spermatocytes were determined by stereology of Epon sections involving testicular volume density and volume of spermatogonial nuclei. In a subset of horses, different spermatogonial subtypes A1, A2, A3, and B1) were counted per 100 Sertoli cells in each of the 8 spermatogenic stages and expressed as percentage of afl A plus B1 spermatogonia. The number of each spermatogonial subtype/testis for the large series of horses was calculated by multiplying the number of A plus B1 spermatogonia/testis (determined for each horse) by the percentage of that given spermatogonial subtype. Season did not significantly affect the number of any given subtype per 100 Sertoli cells in any stage or percentages of different subtypes of spermatogonia. Numbers of A1 (p < 0.05), A2, A3, B1, or B2 spermatogonia (p < 0.01) were greater in the breeding season. Season did not (p > 0.05) influence the yield of B2 spermatogonia per A3 or B2 per B1 spermatogonia; however, the yields of B2/A1 and B2/A2 were greater (p < 0.01) in the breeding season. Seasonal differences in spermatogonial number are due to a combination of a small but significant increase in number of A1 spermatogonia and a significant (p < 0.01 ) increase in yield of spermatogonial divisions early in spermtocytogenesis in the breeding season.