Development of bovine oocytes reconstructed with a nucleus from growing stage oocytes after fertilization in vitro

The developmental capacity of reconstructed bovine oocytes that contained nuclei from growing stage oocytes, 70-119 microm in diameter, was assessed after fertilization in vitro. Nuclei from growing stage oocytes of adult ovaries were transferred to enucleated, fully grown germinal vesicle (GV) stage oocytes. After culture in vitro, the reconstructed oocytes matured, forming the first polar body and MII plate. To supply the ability to form pronuclei, the resultant MII plate was transferred to enucleated MII oocytes, which were obtained by in vitro culture of cumulus-oocyte complexes. After fertilization in vitro, 11-15% of the reconstructed oocytes developed to morulae and blastocysts. To assess the ability to develop to term, a total of 27 late morulae and blastocysts were transferred to 19 recipient cows. Of the three cows that subsequently became pregnant, one recipient, who received two embryos derived from reconstructed oocytes with a nucleus from oocytes 100 to 109 microm in diameter, continued the pregnancy to Day 278 of gestation. This pregnancy, however, was unexpectedly a triplet pregnancy that included a set of identical twins and resulted in the premature birth of the calves, followed by death from lack of post-parturient treatment. These results show that bovine oocyte genomes are capable of supporting term development before the oocytes grow to their full size, which suggests that growing stage oocytes can be directly used as a source of maternal genomes.