Mitotic domains in the early embryo of the zebrafish

At the midblastula transition in the zebrafish, three, and only three, spatially separate mitotic domains arise with distinctive cycle lengths and rhythms. As in Drosophila and at about the equivalent stage, the mitotic domains reflect the fate map, but they do so only very crudely: two are extraembryonic and the third forms the entire embryo. The domains appear not to subdivide during gastrulation, when the germ layers form and when cells probably commit to their eventual fates. The domains may signal specification of morphogenesis rather than cell fate, because, shortly after they appear, each assumes a different role during epiboly, the first morphogenetic movement of the embryo. During meroblastic cleavage, and continuing in the early blastula, zebrafish blastomeres divide rapidly and synchronously. At the time of the tenth cleavage, the beginning of the midblastula transition, the cell cycle lengthens, and, as in Xenopus and Drosophila, cycle length comes under nucleocytoplasmic control (D.A.K. and C.B.K., manuscript in preparation). This nucleocytoplasmic control seems to be maintained during cycle lengthening in the next 2 or 3 cycles, comprising a midblastula transition period. We now show that functionally distinct subsets of cells that arise during this period have reproducibly different mitotic cycle lengths.