Somatic cells and the G2 to M-phase transition in sheep oocytes.

New protein synthesis is essential in sheep oocytes for the progression of the meiotic cycle to metaphase. Inhibition of transcription by alpha-amanitin, a specific inhibitor of RNA polymerase II, arrests meiosis in the G2 phase in cumulus enclosed oocytes but denuded oocytes progress to M-phase. Denuded oocytes injected with alpha-amanitin also undergo maturation. Cumulus-enclosed and denuded oocytes were also cultured on transcriptionally inactive follicle shells to examine the influence of transcription in the granulosa cell compartment on the progression of meiosis. No effect of alpha-amanitin was detected on the maturation rate of both cumulus enclosed and denuded oocytes. We conclude that transcription is required in the cumulus cells to allow translation of stored messages within the oocytes coding for cell-cycle proteins.