| To investigate changes in cell cycle patterns during development of Xenopus laevis, cell divisions of dissociated animal cap blastomeres of blastulae were observed in vitro at 22 +/- 1°C by means of a time-lapse video recorder.;1. During cleavage, blastomeres divided unequally to give birth to daughter cells of unequal sizes before as well as after the midblastula transition (MBT) which corresponds to the 12th cell cycle. Before MBT, cells divided synchronously at constant intervals (30min). Cell cycle duration remained independent of cell size and ploidy, up to the 12th cleavage in diploid cells, and up to the 13th cleavage in haploid cells.;2. The distribution of cell cycle durations among blastomeres became multimodal when the cells became smaller than the critical size after MBT. Cells divided more frequently at intervals close to multiples of a unit time (30 to 35 min) than intermediate intervals, suggesting "quantizement" of cell cycle durations. Probabilistic analyses of the cell cycle were carried out by constructing alpha- and beta-curves defined by Smith and Martin (1973) from the data obtained from this study.;3. When protein synthesis of blastomeres was partially inhibited in the presence of cycloheximide at low concentrations (0.1--0.18 mug/ml), cell cycle durations lengthened about 2 fold, but the cell cycle synchrony was maintained up to the 11th cycle.;4. Cell cycle patterns were compared between blastomeres in different regions of an embryo, as well as between untreated and growth factor-treated blastomeres. Cell cycle asynchrony appeared in the order of the vegetal, followed by equatorial and animal region. However, no significant difference in the timing of cell cycle asynchrony was found between the dorsal and ventral regions. Treatment with xbFGF, the growth factor known to act on the animal cap cells as a ventral mesoderm inducer, caused immediate cell cycle lengthening in dissociated blastomeres, and asynchronization of cell cycles was delayed until the 15th cell cycle.;5. To explain the results summarized above, the following hypothesis was proposed. Blastomeres of Xenopus embryos undergo cycles of cytoplasmic activity that produces mitosis-promoting factor (MPF) or cyclins periodically at intervals of about 30min. MPF activity arises in the cortex or near the blastomere cell surface when synthesis of cyclin B occurs near the cell surface. On the other hand, MPF is neutralized stoichiometrically by an inhibitor in the nucleus, whose quantity is proportional to the genome size or DNA content of the cell. (Abstract shortened by UMI.). |
