Strain difference in the timing of meiosis resumption in mouse oocytes: involvement of a cytoplasmic factor(s) acting presumably upstream of the dephosphorylation of p34cdc2 kinase.

Oocytes from eight inbred strains of mice were screened for the timing of germinal vesicle breakdown (GVB) in vitro. This characteristic varied between strains, reaching most extreme values in oocytes from AKR and BALB/c mice (3.1 and 1.6 h after release from dibutyryl cAMP block, respectively; p < 0.0001). The difference between AKR and BALB/c mice was confirmed in experiments in which GVB was induced in vivo by stimulation with exogenous gonadotrophins. Analysis of the rate of GVB in hybrids obtained after fusion of nuclear and cytoplasmic fragments of oocytes from both strains suggests that the factor responsible for the difference between AKR and BALB/c mice is located in the cytoplasm of the prophase oocytes. Finally, in oocytes from both strains stimulated to resume meiotic maturation with okadaic acid, an inhibitor of protein phosphatases types 1 and 2A, the rate of GVB was the same (2.2h and 2.3h for AKR and BALB/c, respectively; p=0.48). This suggests that the difference between strains is not related to the amount or quality of the pre-MPF (Maturation Promoting Factor) stored in the prophase oocyte, but to the factor(s) acting upstream of the dephosphorylation of p34cdc2 kinase in the pathway leading to pre-MPF activation.