Cytoplasmic fragmentation in activated eggs occurs in the cytokinetic phase of the cell cycle, in lieu of normal cytokinesis, and in response to cytoskeletal disorder.

The timing of cytoplasmic fragmentation in relation to the cell cycle was studied in mature oocytes and early cleavage stages using mouse oocytes and embryos as experimental models. The central approach was to remove the nuclear apparatus, in whole or in part, from non-activated and activated oocytes and early embryos, and follow their response during subsequent culture in vitro. Oocytes arrested in metaphase of the second meiotic division did not fragment following complete removal of the meiotic apparatus, provided they were not subsequently activated. Exposure of spindle-chromosome-complex-depleted oocytes to activation conditions immediately after enucleation led to fragmentation, although not until control embryos entered first mitosis. Delaying activation until 24 h post-enucleation led to earlier fragmentation. Enucleation of normally fertilized or artificially activated oocytes after emission of the second polar body also led to fragmentation coinciding with the first mitosis in nucleated control embryos. However, if artificially activated oocytes were prevented from completing second meiosis, by exposure to cytochalasin, and then enucleated, this universal wave of fragmentation was preceded in some cytoplasts by limited fragmentation after just a few hours in culture, and coinciding with completion of meiosis II in nucleated oocytes. Fragmentation also occurred in the second mitotic cell cycle, but it was limited to blastomeres of fertilized oocytes that were enucleated in late interphase. These results indicate that fragmentation in oocytes and early embryos, though seemingly uncoordinated, is a precisely timed event that occurs only in mitotically active cells, during the cytokinetic phase of the cell cycle, in lieu of normal cytokinesis, and in response to altered cytoskeletal organization.