Sperm nuclei entering parthenogenetically activated mouse oocytes before the first mitosis transform into pronuclei. An ultrastructural study.

Background: This report is an extension of previous observations (Maleszewski 1992. Mol. Reprod. Dev., 33:215-221) on the behavior of mouse sperm nuclei incorporated into parthenogenetically activated mouse oocytes prior to the first cleavage division and undergoing transformation during mitosis.

Method: Artificially activated mouse oocytes were inseminated in vitro and an ultrastructural analysis was performed of sperm-derived nuclei present in two parthenogenetic two-cell embryos.

Results: Both chromatin and nuclear envelope of sperm derived-nuclei are structurally identical with those of oocyte-derived nuclei and of the nuclei of blastomeres of normal two-cell embryos.

Conclusions: Cytoplasm of the parthenogenote during the first mitotic division has the ability to transform sperm nucleus into a male pronucleus just like the cytoplasm of a metaphase II oocyte.