ISOLATION OF THE MURINE CYCLIN B2 CDNA AND CHARACTERIZATION OF THE LINEAGE AND TEMPORAL SPECIFICITY OF EXPRESSION OF THE B1 AND B2 CYCLINS DURING OOGENESIS, SPERMATOGENESIS AND EARLY EMBRYOGENESIS

A cDNA encoding the murine cyclin 112 (cycB2) was isolated from an adu lt mouse testis cDNA library as part of studies designed to identify c yclins involved in murine germ cell development. This cycB2 cDNA was t hen used to examine the pattern of cycB2 expression during male and fe male germ cell development and in early embryogenesis, and to compare this expression with the previously characterized expression of cycB1. A single 1.7 kb cycB2 transcript was detected by northern blot hybrid ization analysis of total RNA isolated from midgestation embryos and v arious adult tissues. Northern blot and in situ hybridization analyses revealed that cycB2 expression in the testis was most abundant in the germ cells, specifically in pachytene spermatocytes. This is in contr ast to the highest levels of expression of cycB1 being present in earl y spermatids. In situ analysis of the ovary revealed cycB2 transcripts in both germ cells and somatic cells, specifically in the oocytes and granulosa cells of growing and mature follicles. The pattern of cycB1 and cvcB2 expression in ovulated and fertilized eggs was also examine d. While the steady state level of cycB1 and cycB2 signal remained con stant in oocytes and ovulated eggs, signal of both appeared to decreas e following fertilization. In addition, both cycB1 and cycB2 transcrip ts were detected in the cells of the inner cell mass and the trophecto derm of the blastocyst. These results demonstrate that there are linea ge- and developmental-specific differences in the pattern of the B cyc lins in mammalian germ cells, in contrast to the co-expression of B cy clins in the early conceptus.