Anucleate fragments of parthenogenetic eggs and of maturing oocytes contain complementary factors required for development of a male pronucleus.

Eight-h-old anucleate fragments of artificially activated mouse oocytes produced within 30 min after activation were fused with anucleate fragments of ovarian oocytes bisected at the GV stage and matured in vitro for 4-5 h. The resulting cybrids were inseminated soon after fusion and fixed 3-4 h or 14 h later. After 3-4 h of culture most of the sperm nuclei underwent decondensation. Early male pronuclei were observed sporadically. After overnight culture small or fully formed male pronuclei were present in monospermic and dispermic cybrids. The abortive nuclei, designated as stage IIa (earlier described by Borsuk and Tarkowski, 1989) were also observed and their occurrence was strongly correlated with the rate of polyspermy. In unfused (control) anucleate fragments of maturing and activated oocytes inseminated at the same time as the cybrids, male pronuclei were never observed. These results show that the cytoplasms of activated eggs and of maturing oocytes contain complementary factors required for the transformation of the sperm nucleus into the male pronucleus. The former has lost the capability to break down the sperm nucleus envelope but contains factors required for pronuclear growth. The latter is able only to initiate the process of transformation, i.e., to break down the nuclear envelope and decondense the denuded sperm chromatin. However, the activity of the factor responsible for nuclear envelope breakdown is limited because in cybrids penetrated by more than two spermatozoa, transformation of sperm nuclei was usually abnormal.