CHROMATIN CONDENSATION ACTIVITY AND CORTICAL ACTIVITY DURING THE FIRST 3 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 occurrenc e of two cytoplasmic activities: the cortical activity and the chromat in 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 defo rmations) was observed only during the first cleavage cycle. In AHs th e surface activity began at the same time as in NHs and disappeared be fore the time of the cleavage division of nucleated halves. Anucleate fragments of blastomeres from 2- and 4-cell embryos did not exhibit an y cortical activity. In the absence of the native nucleus the chromati n condensation activity (assayed by premature chromatin condensation o f interphase thymocyte nuclei introduced into cytoplasts by cell fusio n) 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 ob tained from 2-cell and 4-cell stage blastomeres did not reveal condens ation activity. (C) 1995 Wiley-Liss, Inc.