A Changing Pattern of the Cell Cycle during the First Two Cleavage Divisions of the Mouse

The cell cycles of the early cleavage stages of the mouse were analyzed by examining Feulgen-stained ova. The period from ovulation to the completion of second cleavage division was investigated. The ova donors were C57BL/6 × DBA/2 female mice, which were hormonally superovulated. To estimate the durations of DNA synthesis and mitotic phases of the cleavage divisions, the ova were pooled into culture medium, and as a function of time, aliquots were removed from the batch of pooled ova. The ova specimens were Feulgen-stained and classified as the ova nuclei in G₁, S, G₂ or mitosis by use of a cytophotometric technique and then the durations of these phases were determined by probit analysis.
The pronuclear stage had a generation time of 11 hr, with a G₁ phase of 6 hr and a short S phase of 1.7 hr. In contrast the two-cell stage had a generation time of 18 hr, with a G₁ phase of 2 hr and an S phase of 3 hr. The duration of cleavage division also changed; the first cleavage division spanned 3 hr while the second spanned 1 hr.