Transition of the blastomere cell cycle from cell size-independent to size-dependent control at the midblastula stage in Xenopus laevis

Dissociated animal cap blastomeres of Xenopus laevis blastulae were culture d at a lo cv Ca level (1 mu M) from 9th to 18th cell cycle at 22 +/- 1 degr ees C and observed by a time-lapse video recorder. Blastomeres cleaved uneq ually to increase variability in cell size as cell cycles progressed, but s ynchronously at a constant cell cycle time of about 30 min up to the 12th c leavage in diploid cells, and up to the 13th cleavage in haploid cells, reg ardless of their cell sizes. Thereafter, blastomeres cleaved asynchronously at varying cell cycle times in proportion to the inverse square of their r adii. The transition from the cell size-independent to -dependent cell cycl es occurred at the critical cell radius, 37.5 mu m for the diploid and 27.9 mu m for the haploid. While the protein synthesis inhibitor, cycloheximide (CHX) lengthened cell cycle times two- to six-fold, epidermal growth facto r (EGF) had no significant effect on the cell cycle. CHX-treated blastomere s synchronously cleaved at a constant cell cycle time of 60 min up to the 1 2th cleavage. Thereafter, cell cycle times became variable in proportion to the inverse square of radii in the presence of CIM at 0.10-0.14 mu g/ml, b ut to the inverse cube of radii at 0.18 mu g/ml. The critical cell size of CHX-treated blastomeres for the transition from cell size-independent to -d ependent cell cycles remained the same as that of untreated blastomeres. Fr equency distributions of cell cycle times of synchronous cell cycles were m onomodal with the peak at 30 min, except for CHX-treated blastomeres with t he peak at 60 min. in contrast, frequency distributions of asynchronous cel l cycles were polymodal with peaks at multiples of a unit time of 30-35 min . To explain these results, we propose that blastomere cytoplasm has 30-min cycles that repeatedly produce mitosis promoting factor (MPF) in a quantit y proportional to the cell surface area. MPF is neutralized when it titrate s a nuclear inhibitor present in a quantity proportional to the genome size , and sequestered in the nucleus. When the total amount of MPF produced exc eeds the threshold required to titrate all of the inhibitor, mitosis is ini tiated. (C) 2000 Wiley-Liss, Inc.