Cell-cycle aspects of growth and maturation of mammalian oocytes

Abstract

In this review, recent data concerning growth and maturation of nonmammalian and mammalian female germ cells are compiled with regard to the increased understanding of somatic cells mitotic cycles, from yeast to human tissues. These data allow us to conclude that growing oocytes of nonvertebrates, lower vertebrates, and mammals resemble somatic cells in the G1 phase of the mitotic cycle in their metabolic and cell cycle behavior. Transcriptional and translational activity of growing oocytes and G1 somatic cells is not compatible with the activation of maturation promoting factor (MPF), with chromatin condensation or with nuclear membrane disintegration. Growing oocytes, even when they are in the dictyate stage of the first meiotic division, promptly inactivate MPF introduced into their cytoplasm by fusion or microinjection, just as do somatic interphase cells. In mammals, the LH surge induces "de novo" RNA and protein synthesis in granulosa cells. This metabolic change in granulosa cells abolishes their inhibitory activity, and meiosis in fully grown oocytes in preovulatory follicles is then resumed. Resumption of meiosis requires an activation of pre MPF molecules within oocytes. This can be achieved either without (mouse, rat, and rabbit) or with (pig, sheep, and cow) an active protein synthesis by the oocytes. The species specificity is probably dependent on the presence or absence of cyclin-like and/or mos-like molecules in fully grown oocytes. Both major events during GVBD, chromatin condensation, and nuclear envelope disintegration require protein phosphorylation. Experimentally, these two phosphorylation activities can be separated one from another. The active MPF molecules are amplified autocatalytically in amphibian and starfish oocytes. However, an increase of MPF activity in mouse and pig oocytes, similarly as in Rana pipiens and sturgeon oocytes, requires an active protein synthesis.