Chromosomes of mouse primary spermatocytes undergo meiotic divisions after incorporation into homologous immature oocytes.

The primary spermatocytes used were male germ cells at prophase I. The present study was undertaken to see whether bivalent chromosomes of mouse primary spermatocytes can undergo meiotic divisions within maturing oocytes and participate in subsequent embryonic development. Primary spermatocytes (pachytene to diplotene) freshly collected from the testes of mature males were electrofused with immature oocytes shortly before or after germinal vesicle breakdown. After culture in MEM-alpha medium for 15 h, most (> 90%) of the oocytes containing spermatocyte chromosomes underwent maturation and arrested at metaphase II (MII). Among 23 MII oocytes examined, 17 (74%) had one group of chromosomes and one polar body, indicating that male chromosomes had intermingled with those of the females and completed the first meiotic division. Chromosome analyses of these MII oocytes demonstrated their diploidy. The metaphase chromosomes were transferred to enucleated MII oocytes freshly recovered from superovulated mice. After artificial activation, the reconstructed MII oocytes resumed meiosis and developed to the morula/blastocyst stage. However, no pups were born following embryo transfer into recipient females. These findings indicate that the chromosomes of primary spermatocytes undergo meiotic divisions in maturing oocytes and participate in the formation of diploid embryos.