Two states of active spermatogenesis switch between reproductive and non‐reproductive seasons in the testes of the medaka, Oryzias latipes

Seasonal change in spermatogenesis was examined in the restricted spermatogonium‐type testes of a teleost, Oryzias latipes. Histological observation revealed that the number of each stage of germ cells during most of the non‐reproductive season, from October to January (O–J period) was nearly half of that during the reproductive season, from May to July (M–J period), except for type B spermatogonia (B‐gonia), which was actually equal. As a result, the ratio of primary spermatocytes (P‐cytes) to B‐gonia was remarkably small in the O–J period. Despite the differences between both time periods, the proliferative activity of type A spermatogonia (A‐gonia), B‐gonia, or P‐cytes was at a similar level in both periods. Moreover, in cultured testes treated with bromodeoxyuridine as a cell‐lineage tracer, P‐cytes differentiated to spermatids in 11–15 days in both M–J and O–J periods. These indicate that spermatogenesis is active in each period at a different state. In the spermatogenic testis, A‐gonial proliferation was maintained by human follicle stimulating hormone/luteinizing hormone in culture. Whereas cell death of B‐gonia and/or P‐cytes gradually increased in the M–J period in spite of those cells being constant in population sizes. In transition to the O–J period, A‐gonia and P‐cytes first decreased, which was accompanied by a decrease in proliferative activity of A‐gonia and relative increase of dead cells from B‐gonia and/or P‐cytes against live P‐cytes. These suggest that A‐gonial proliferation and cell death of B‐gonia and/or P‐cytes that is induced coordinately with B‐gonial differentiation are critical for the spermatogenic control.