DIVERSIFICATION OF CELL-CYCLE CONTROLS IN DEVELOPING EMBRYOS

During embryogenesis, a genetic program coordinates cell proliferation with morphogenesis and cell differentiation. Recent studies using Dro sophila have shown how, as development proceeds, this program directs different cell types to acquire unique modes of cell cycle regulation. As maternal cell cycle factors are exhausted and replaced by differen tially expressed zygotic factors; an increasing repertoire of gene pro ducts become potential regulators of the cycle. Cyclin B, Cdc25, and C yclin E each act as limiting regulators in Drosophila in specific cell types at particular developmental stages. The genes encoding these an d many other candidate regulators have been cloned from mice, but thei r roles in vivo have yet to be understood.