Chromatin condensation activity and cortical activity during the first three cell cycles of a mouse embryo.

One-cell parthenogenetic haploid embryos and blastomeres of the 2- and 4-cell diploid mouse embryos were observed in vitro for the occurrence of two cytoplasmic activities: the cortical activity and the chromatin condensation activity. For this purpose anucleated halves (AHs) and nucleated halves (NHs) were produced by bisection of one-cell embryos and of blastomeres. The cortical activity (manifested by surface deformations) was observed only during the first cleavage cycle. In AHs the surface activity began at the same time as in NHs and disappeared before the time of the cleavage division of nucleated halves. Anucleate fragments of blastomeres from 2- and 4-cell embryos did not exhibit any cortical activity. In the absence of the native nucleus the chromatin condensation activity (assayed by premature chromatin condensation of interphase thymocyte nuclei introduced into cytoplasts by cell fusion) could also have been detected only in the first cleavage cycle. In AHs this activity appeared at the time when NHs started to cleave and disappeared after the NHs finished the first cleavage division. AHs obtained from 2-cell and 4-cell stage blastomeres did not reveal condensation activity.