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

Abstract

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.