DNA synthesis in developing two-cell mouse embryos

Mated CF1 (Carworth) female mice were sacrificed at 2 hr intervals between 29 and 43 hr after human chorionic gonadotrophin (HCG) administration. One- and two-cell eggs were incubated in [³H]thymidine for 1 hr. Labeled two-cell embryos were first observed at 31 hr and reached a maximum number at 35 hr. The S period is approximately 6 hr in duration. Although both blastomeres were labeled in most cases, embryos with only one labeled blastomere were more numerous at later times. In vitro labeling was corroborated by injecting [³H]thymidine directly into the isthmic portion of the oviduct. Embryos usually complete the second cleavage division 18–20 hr after onset of DNA synthesis. The cell cycle at the two-cell stage is thus characterized by a G₁ of close to 1 hr, a 6 hr S, and a G₂ of about 12 hr.Embryos developing in vitro frequently fail to progress beyond the two-cell stage. The block is not due to absence of DNA synthesis since these embryos were found to incorporate [³H]thymidine.