Using immunohistochemistry, the expression of the D-type cyclin proteins was studied in the developing and adult mouse testis. Both during testicular development and in adult testis, cyclin D1 is expressed only in proliferating gonocytes and spermatogonia, indicating a role for cyclin D1 in spermatogonial proliferation, in particular during the G1/S phase transition. Cyclin D2 is first expressed at the start of spermatogenesis when gonocytes produce A1 spermatogonia. In the adult testis, cyclin D2 is expressed in spermatogonia around stage VIII of the seminiferous epithelium when Aal spermatogonia differentiate into A1 spermatogonia and also in spermatocytes and spermatids. To further elucidate the role of cyclin D2 during spermatogenesis, cyclin D2 expression was studied in vitamin A-deficient testis. Cyclin D2 was not expressed in the undifferentiated A spermatogonia in vitamin A-deficient testis but was strongly induced in these cells after the induction of differentiation of most of these cells into A1 spermatogonia by administration of retinoic acid. Overall, cyclin D2 seems to play a role at the crucial differentiation step of undifferentiated spermatogonia into A1 spermatogonia. Cyclin D3 is expressed in both proliferating and quiescent gonocytes during testis development. Cyclin D3 expression was found in terminally differentiated Sertoli cells, in Leydig cells, and in spermatogonia in adult testis. Hence, although cyclin D3 may control G1/S transition in spermatogonia, it probably has a different role in Sertoli and Leydig cells. In conclusion, the three D-type cyclins are differentially expressed during spermatogenesis. In spermatogonia, cyclins D1 and D3 seem to be involved in cell cycle regulation, whereas cyclin D2 likely has a role in spermatogonial differentiation.
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