Mitotic activity during preimplantation development of human embryos.

The rate of cell division of human embryos in vitro was estimated throughout the preimplantation period. Seventy-five human embryos were assessed by performing nuclei counts on whole mount preparations at 24 h intervals between day 2 and day 6 of development. The results indicate that the cell doubling time increases after the first two division cycles. The mean (+/- SEM) doubling time between day 2 and day 6 was 31 +/- 2.51 h. At the end of the designated culture periods, embryos were exposed to a microtubule disrupting agent, which caused the nuclei to arrest in metaphase. Thus, a profile of the mitotic activity at given stages of development was obtained. The numbers of actively dividing cells were frequently lower than anticipated, indicating that nuclei counts cannot be directly correlated with continuing mitotic activity.