Mouse primary spermatocytes can complete two meiotic divisions within the oocyte cytoplasm.

This study was undertaken to determine whether primary spermatocyte nuclei can complete meiosis after transfer into maturing or mature oocytes and can participate in normal embryogenesis. When injected into maturing mouse oocytes at prometaphase of the first meiotic division, spermatocyte chromosomes became arranged on a first meiotic metaphase (Met-I) spindle. Thus, oocytes contained two sets of Met-I chromosomes. When these oocytes were matured in vitro and artificially activated, pronuclei and polar bodies of both maternal and paternal origin were formed, and zygotes began development. When single spermatocyte nuclei were injected into fully mature oocytes at metaphase of the second meiosis (Met-II), the spermatocyte nuclei transformed into a Met-I configuration, resulting in the formation of oocytes with both maternal (Met-II) and paternal (Met-I) chromosome complements. After activation of these oocytes, half of each chromosome set was separated into polar bodies. Transfer of nuclei from polar bodies of "paternal" origin into other Met-II oocytes resulted in the formation of oocytes with two sets of Met-II chromosomes, one maternal and one paternal in origin. When activated, two pronuclei and two polar bodies were formed and zygotes began development. It is concluded that nuclei of primary spermatocytes are able to undergo two successive meiotic divisions within oocyte cytoplasm. Thus, factors that drive chromosome condensation, organization of metaphase, and chromosome separation at anaphase in oocytes can drive these same maturation processes in primary spermatocyte nuclei. When a total of 258 two-cell embryos were transferred to foster mothers, only two live pups were born to two mothers. One died shortly after birth and the other 3 wk after birth. The reasons for poor embryonic and neonatal development remain to be determined.