Cyclin synthesis controls the progression of meiotic maturation in mouse oocytes

To study the mechanisms involved in the progression of meiotic maturation i n the mouse, we used oocytes from two strains of mice, CBA/Kw and KE, which differ greatly in the rate at which they undergo meiotic maturation. CBA/K w oocytes extrude the first polar body about 7 hours after breakdown of the germinal vesicle (GVBD), whilst the oocytes from KE mice take approximatel y 3-4 hours longer. In both strains, the kinetics of spindle formation are comparable, While the kinetics of MAP kinase activity are very similar in b oth strains (although slightly faster in CBA/Kw), the rise of cdc2 kinase a ctivity is very rapid in CBA/Kw oocytes and slow and diphasic in KE oocytes , When protein synthesis is inhibited, the activity of the cdc2 kinase star ts to rise but arrests shortly after GVBD with a slightly higher level in C BA/Kw oocytes, which may correspond to the presence of a larger pool of cyc lin B1 in prophase CBA/Kw oocytes, After GVBD, the rate of cyclin B1 synthe sis is higher in CBA/Kw than in KE oocytes, whilst the overall level of pro tein synthesis and the amount of messenger RNA coding for cyclin B1 are ide ntical in oocytes from both strains. The injection of cyclin B1 messenger R NA in KE oocytes increased the H1 kinase activity and sped up first polar b ody extrusion. Finally, analysis of the rate of maturation in hybrids obtai ned after fusion of nuclear and cytoplasmic fragments of oocytes from both strains suggests that both the germinal vesicle and the cytoplasm contain f actor(s) influencing the length of the first meiotic M phase, These results demonstrate that the rate of cyclin B1 synthesis controls the length of th e first meiotic M phase and that a nuclear factor able to speed up cyclin B synthesis is present in CBA/Kw oocytes.