MPF components and meiotic competence in growing pig oocytes

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Abstract
Growing pig oocytes (≤90 μm in diameter) are unable to resume meiosis in vitro. The objective of our present experiments has been to identify the reasons for meiotic competence in these cells. By comparing histone H1 kinase activity in growing and fully grown oocytes we demonstrate that incompetence is associated with an inability to activate H1 kinase in growing oocytes. Immunoblotting was used to determine whether this kinase inactivity resulted from a lack of either p34cdc2 protein or B‐type cyclin. The results established that each of these cell cycle molecules are present in comparable amounts in both growing and fully grown oocytes. In the third series of studies experiments were carried out in an attempt to induce p34cdc2 activation during growth. Treatment with okadaic acid, an inhibitor of phosphatase 1 and 2A known to stimulate and accelerate the transition into M‐phase of the meiotic cycle in a number of different species, was able to induce p34cdc2 kinase activity and facilitated the G2‐ to M‐phase in growing oocytes. We conclude that although growing oocytes in pigs have sufficient key cell cycle components for the G2 to M transition, they remain incapable of converting these components to active maturation‐promoting factor (MPF) until growth is virtually completed. Inhibition of phosphatase 1 or 2A induces the formation of active MPF during growth by an as yet unidentified pathway.