DEVELOPMENT OF NORMAL MICE FROM METAPHASE-I OOCYTES FERTILIZED WITH PRIMARY SPERMATOCYTES

Primary spermatocytes are the male germ cells before meiosis I. To exa mine whether these 4n diploid cells are genetically competent to ferti lize oocytes and support full embryo development, we introduced the nu clei of pachytene/diplotene spermatocytes into oocytes that were arres ted in prophase I (germinal vesicle stage), metaphase I, or metaphase II (Met II). Both the paternal and maternal chromosomes then were allo wed to undergo meiosis synchro nously until Met II. In Be first and se cond groups, the paternal and maternal chromosomes had intermingled to form a large Met II plate, which was then transferred into a fresh en ucleated Met II oocyte, In the third group, the paternal Met II chromo somes were obtained by transferring spermatocyte nuclei into Met II oo cytes twice. After activation of the Met II oocytes that were produced , those microfertilized at metaphase I showed the best developmental a bility in vitro, and three of these embryos developed into full-term o ffspring after embryo transfer. Two pups (one male and one female) wer e proven to be fertile. This finding provides direct evidence that the nuclei of male germ cells acquire the ability to fertilize oocytes be fore the first meiotic division.