Expression of mitotic cyclin B1 is not confined to proliferating cells in the rat testis

Spermatogenesis is a precisely controlled and timed process comprising mitotic divisions of spermatogonia, meiotic divisions of spermatocytes, and the maturation and differentiation of haploid spermatids. Cell proliferation is controlled by genes involved in the regulation of the cell cycle. Among the principal regulatory proteins are cyclins, which are categorized according to their appearance during the cell cycle. B-type cyclins are mitotic cyclins and function at the G2/M transition of the cell cycle. We have investigated the expression and regulation of cyclin B1 during rat spermatogenesis. Rat cyclin B1 was isolated from a testis cDNA library and further used as a probe to detect mRNA expression. Northern blot hybridization of testis mRNA revealed the presence of a single 1.7-kilobase transcript. In situ hybridization showed stage-specific expression during spermatogenesis with highest expression found in late pachytene spermatocytes and early round spermatids. This pattern was confirmed in fractions of isolated germ cells. Immunocytochemistry displayed highest protein levels in round spermatids. Depletion of gonadotropins did not change the quantitative and qualitative expression pattern of cyclin B1. Therefore, the signals triggering the onset of cyclin B1 expression seem not to originate from the pituitary-gonadal endocrine axis and might therefore be paracrine factors originating within the germinal epithelium. Our observations suggest that cyclin B1 plays a hitherto unknown role in spermatid maturation in addition to its known function in dividing cells.