Temporal Appearance of Seasonal Changes in Numbers of Sertoli Cells, Leydig Cells, and Germ Cells in Stallions1

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Abstract
The temporal appearance of seasonal changes in numbers of Leydig, Sertoli, and germ cells was evaluated to determine if seasonally increased daily spermatozoan production might be preceded by changes in numbers of either of two somatic testicular cells. A significant increase in numbers of spermatogonia and Sertoli cells preceded the significant increase in number of Leydig cells in the approaching breeding season. Seasonal changes in parenchymal weight and in numbers of Sertoli cells, Leydig cells, and germ cells were maximal in May and June. Numbers of A or B spermatogonia in June were 2.4 to 2.5 times the number present in January. During the same time period, numbers of other germ cells, as well as Leydig cells and Sertoli cells, were increased by 1.5 to 1.9 times. The magnitude of change between January and March (first time period that the change was significant) was greater for A spermatogonia (1.7-fold) than for other cell types (1.3-fold to 1.5-fold). Hence, the need to accommodate more spermatogonial progeny might cause increased testicular size and number of somatic cells, including Sertoli cells. Season did not influence the rate of degeneration between A and B spermatogonia. However, in the breeding season, the conversion of B spermatogonia to primary spermatocytes was reduced. The lack of a seasonal difference in the ratio of primary spermatocytes per Sertoli cell was consistent with a limited capacity of individual Sertoli cells to accommodate primary spermatocytes. Hence, number of Sertoli cells might be the limiting factor that modulates the number of B spermatogonia degenerating during mitosis and ultimately modulates spermatozoan production rates. Seasonal stimuli responsible for modulating spermazogenesis in stallions were not mediated directly through number of Leydig cells. Although a significant increase in number of Sertoli cells preceded significant increases in daily spermatozoan production based on elongated spermatids, significant increases in number of Sertoli cells did not precede or exceed the magnitude of change in number of A spermatogonia. In fact, the number of spermatogonia might have influenced nwnbers of these somatic cells.

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