Problems of Oocyte Maturation and the Control of Chromosome Cycles*

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Abstract
During oocyte maturation and zygote development chromosomes undergo cyclic changes, alternaing the condensed and decondensed states. In oocytes, zygotes and perhaps in other cells, the chromosome cycle appears to be controlled in same way by common cytoplasmic factors. Among them, maturation‐promoting factor (MPF) plays a particularly important role, although the germinal vesicle substances and cytoplasmic membrane vesicles are indispensable for the chromosomal changes. MPF precursor is stored in fully grown oocytes of most species, but replenishing MPF after its fall during cell cycles requires protein synthesis. During oocyte maturation protein synthesis increases following the activation of MPF, and the synthesized proteins bind with chromosomes that have condensed to a metaphase state. The temporal correlation between the appearance of MPF with chromosome condensation activity and spindle formation observed in various cells suggest a major role played by MPF in the control of chromosome and microtubule assembly cycles. Thus, MPF is a regulator that coordinates the functions of various cell components to advance the chromosome cycle from interphase to metaphase. Therefore, a key to understanding the control of the chromosome cycle lies in knowing factors on which MPF activity is dependent. Although some physiological parameters of the cell are known to affect MPF activity, including Ca ion levels, intracellular pH, protein synthesis activity, cAMP levels, and protein phosphorylation, it seems difficult to assign the control of MPF cycles to any of these parameters. On the contrary, MPF cycles appear to regulate changes in these parameters. Rather, since MPF has the ability to amplify itself by activating its precursor, thus being involved in the MPF‐generating system in the cell, the MPF cycle may be an autonomous process. This notion may be supported by the recent observation of the oscillatory activity of MPF in cytosols extracted from frog eggs. We propose theoretical models to explain the MPF oscillator in the cell.