Effect of delaying DNA replication on blastocyst formation in the mouse.

Differentiation in the early mouse embryo involves cellular responses to both spatial and temporal signals. The temporal signals that trigger blastocyst formation, the first differentiative event, are not yet understood, but it has been suggested that the numbers of DNA replications undergone since fertilization might act as the timing mechanism. Preimplantation mouse embryos were treated with aphidicolin, an inhibitor of eukaryotic DNA polymerase alpha, for eight hours during the S phase of the fourth cleavage division. This treatment produced a delay in cell division but the morphologic event of cavitation, which signals the onset of blastocyst formation, was not delayed. Treated embryos actually cavitated a few hours ahead of control embryos at approximately half the cell number. This result indicates that blastocyst formation is not timed by the number of DNA replicative cycles completed since fertilization, but by some other intrinsic cellular clock.