Development of Microtubule‐Dependence of the Chromosome Cycle at the Midblastula Transition in Xenopus laevis Embryos

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Abstract
Animal‐cap cells isolated from Xenopus laevis morulae and blastulae are cultured for 2 to 6 hr in medium containing nocodazole, Colcemid or taxol, at concentrations completely inhibiting cell division. At 20°C, cells from each control embryo undergo synchronous cell cycles up to the 12th, with a period of 32 min, of which 60% represents the chromosome condensation (mitotic or M‐) phase, and the average mitotic index remains near 50%. Cells treated with nocodazole, Colcemid or taxol before 12th cleavage undergo chromosome cycles with a similar period as controls, albeit without chromosome segregation, and the average mitotic index remains near 50%. From the 12th to 15th cycles, control cycles become asynchronous, their period gradually increases 2 to 3 times, and the mitotic index declines to 10%. In cells treated after 12th cleavage with taxol, the mitotic index declines, similarly to control cells. However, in nocodazole‐treated cells, it increases steadily, and exceeds 70% at 2 hr of treatment, but gradually declines to 40% at 6 hr. Therefore, while inhibition of microtubule activities does not significantly alter the timing of chromosome condensation cycles during synchronous cleavage, inhibition of microtubule assembly can prolong M‐phase during asynchronous cleavage after the midblastula transition.