Regulation of the appearance of division asynchrony and microtubule-dependent chromosome cycles in Xenopus laevis embryos.

Divisions of animal-cap blastomeres dissociated from Xenopus laevis embryos are synchronous mostly up to 12th cleavage or the 13th cell cycle, but become asynchronous afterward, during the midblastula transition (MBT), and at the same time, chromosome cycles become microtubule-dependent and are arrested in mitosis if treated with nocodazole. To investigate causes for these changes in cell-cycle control, we observed division synchrony in animal-cap blastomeres dissociated from embryos whose nucleocytoplasmic ratio (N/C) had been altered by constriction of zygotes or by delaying nucleation into zygote halves and compared their mitotic indices in the presence and absence of nocodazole. Thus, we found that asynchronous divisions always commenced when N/C reached the value of 128 to 256 times that of an animal blastomere of the 32-cell embryo, corresponding to the 12th and 13th cycles of a normal embryo, while the number of synchronous cycles became variable, ranging from 9 to 14, depending on the initial N/C. Treatment with alpha-amanitin or cycloheximide did not alter the number of synchronous cycles. However, the time at which the mitotic index of nocodazole-treated blastomeres first exceeded that of control remained constant, at 3 to 5 hr after 5th cleavage, regardless of the initial N/C. Thus, chromosome cycles of blastomeres first became sensitive to nocodazole at a variable N/C, ranging from 8 to 1024 times that of an animal blastomere of the 32-cell embryo. The timing of the appearance of nocodazole sensitivity was unaffected by alpha-amanitin treatment, whereas it was markedly delayed following cycloheximide treatment. These results suggest that the commencement of division asynchrony is N/C-dependent, whereas the development of microtubule-dependent cell cycles is age-dependent, most likely being programmed by the translation of stored mRNAs.