Regulation of the meiotic cycle in oocytes of domestic mammals.

The progression of meiosis from prophase to metaphase II involves modifications to somatic signals, activation of a master regulator system in the cytoplasm and the manipulation of independent components in the nucleus. Studies on amphibian oocytes suggest that the cytoplasmic component, maturation promoting factor (MPF), is a key component of the master regulator system. A similar and ubiquitous role for MPF in mammalian oocytes is indicated by cell fusion studies. However, the nature of the somatic signals responsible for prophase arrest and the sequence of intracellular events necessary for the synthesis and activation of MPF differ markedly between species. The most fundamental of these differences is in the G2- to M-phase transition where, in rodents, phosphorylation changes alone activate the MPF cascade while in domestic mammals both transcription and new protein synthesis is required for MPF production. One of the proteins (47,000 daltons) involved in the MPF cascade has been tentatively identified in ovine oocytes and has been shown to be dependent upon transcription at the initiation of maturation. The synthesis of additional protein, probably with anti-MPF activity, is thereafter required to drive the cycle from metaphase I to anaphase. The final phase of cell cycle-related protein synthesis occurs immediately before the formation of the metaphase II plate. The Ca2+-sensitive cytostatic proteins synthesized at this stage maintain the MII chromatin in a condensed state and prevent the completion of the meiotic cycle until fertilization when the cytostatic proteins disappear and meiosis is completed.