NUCLEI FROM FERTILIZED MOUSE EMBRYOS HAVE CALCIUM-RELEASING ACTIVITY

During mammalian fertilization, the sperm triggers a series of intrace llular Ca2+ oscillations which initiate oocyte activation and the form ation of pronuclei. Oocyte activation can be induced artificially by a variety of chemical and physical stimuli which elevate intracellular calcium. We show that the transfer of nuclei from 1- and 2-cell-stage fertilized mouse embryos to unfertilized oocytes stimulates the comple tion of meiosis and the formation of pronuclei. Nuclei from embryos th at had developed to the 4-cell stage did not stimulate meiotic resumpt ion. The ability to cause oocyte activation was specific to nuclei tra nsferred from fertilized embryos as nuclei from parthenogenetic embryo s or cytoplasts from fertilized or parthenogenetic embryos did not ind uce activation. Nucleus-induced oocyte activation was associated with the generation of intracellular Ca2+ transients, which were seen after nuclear envelope breakdown of the transferred nuclei. Treatment of th e oocyte with the intracellular Ca2+ chelator, BAPTA, prior to nuclear transfer inhibited intracellular Ca2+ transients and oocyte activatio n. The specific Ca2+-releasing activity of the nucleus was not caused by sperm-induced protein synthesis since similar activity was present in nuclei originating from embryos exposed to cycloheximide throughout fertilization, The specific ability of nuclei from fertilized embryos to stimulate Ca2+ transients and oocyte activation was also found in nuclei from embryos parthenogenetically activated by the injection of a partially purified cytosolic sperm factor. The results suggest that the fertilizing sperm introduces Ca2+-releasing activity which becomes associated with the nucleus of early mammalian embryos.