Roles of Ca ions and ooplasmic factors in the resumption of metaphase-arrested meiosis in Rana pipiens oocytes.

Fully grown amphibian oocytes are arrested at the diplotene stage of meiosis. When they undergo meiotic maturation, meiosis resumes and the oocyte chromosomes condense to metaphase. During this period, the oocyte cytoplasm develops 'chromosome condensation activity' (CCA), the ability to induce the formation of metaphase chromosomes from nuclei transplanted into the oocytes. The cytoplasm also produces 'maturation promoting factor' (MPF), the substance that induces meiotic maturation when injected into oocytes. Also, before meiosis is arrested again at the 2nd metaphase, the cytoplasm develops 'cytostatic factor' (CSF), the substance that causes metaphase arrest when injected into zygotes. Since CSF-arrested zygotes have properties similar to those of metaphase-arrested oocytes, including the potential to resume cell cycle activities, CSF appears to be a genuine factor that causes meiotic arrest. Following oocyte activation, meiosis is completed and the chromosomes decondense to form a pronucleus. During this period, the oocyte cytoplasm loses its CCA and develops 'chromosome decondensation activity' (CDA), the ability to decondense the chromatin of injected nuclei. Concomitantly, MPF and CSF disappear. Both MPF and CSF are inactivated by Ca2+ ions, in vitro. The sensitivity of CCA to Ca2+ and the requirement for Ca2+ during the development of CDA have also been demonstrated in vitro by incubating demembranated sperm nuclei in cell-free preparations from unactivated or activated oocytes. Preparations made from unactivated oocytes in the presence of EGTA exhibit CCA, whereas those made in the absence of EGTA, as well as those made from activated eggs, exhibit CDA. Unactivated ooplasmic preparations made using EGTA lose CCA and develop CDA when Ca2+ ions are added to them. However, at low Ca2+ concentrations CCA is sustained and, when unactivated and activated preparations are mixed, is able to overcome CDA. Therefore, it is likely that at low intracellular Ca2+ levels in unactivated oocytes, CSF is stable and CCA predominates over CDA, thus preventing oocyte chromosomes from decondensing. However, when Ca2+ levels are elevated during oocyte activation, CSF disappears and CCA is replaced by CDA. This change in cytoplasmic activities may allow meiosis to resume.