Possible involvement of a cell cycle control system dependent on nuclear activities in establishment of the cell division interval in early Xenopus embryos

In early Xenopus embryos, continuous exposure of embryos to aphidicolin and inactivation of the nucleus by ultraviolet-irradiation induce prolongation of the cell division interval. The extent of prolongation of the cell divi sion interval appears to depend on how heavily DNA replication is suppresse d by the treatments. Embryos showing significantly prolonged cell division intervals tend to fail normal cell divisions, often forming abortive furrow s. There appears to be a critical point for the extent of prolongation of t he cell division interval at 30% that divides the success and failure of no rmal cell division. This percentage, 30%, coincides with that for the prolo ngation of the interval of oscillatory activities seen in enucleated eggs. The presence of an intact nucleus that can undergo DNA replication rescues normal cell divisions with normal intervals and a normal furrow. Histologic ally, the nucleus in embryos showing more than 30% prolongation of cell div ision intervals appears to fail DNA replication and remain unduplicated wit h a round morphology like the interphase nucleus. A number of unduplicated asters lacking chromosomes are found in these embryos. We conclude from the se results that early Xenopus embryos, like late embryos, have a cell cycle control system that is affected by DNA replication and is involved in esta blishing the cell division interval.