Ability of in vitro maturing bovine oocytes to transform sperm nuclei to metaphase chromosomes

Bovine oocytes at the germinal vesicle stage were inseminated in Brackett & Oliphant's medium with bovine serum albumin, caffeine and heparin. Eight hours after insemination, oocytes were transferred into tissue culture medium-199 containing 10% fetal calf serum and cultured for 5-40 h at 39 degrees C in 5% CO2 in air. The proportions of unpenetrated and penetrated oocytes reaching metaphase II increased as the time of examination increased, reaching 70 and 65% 40 h after transfer, respectively. When oocytes were penetrated by more than four spermatozoa, meiotic maturation was greatly retarded. Sperm nuclei were decondensed in most (81%) penetrated oocytes 5 h after transfer. The decondensed sperm nuclei were recondensed and then transformed to metaphase chromosomes which were morphologically compacted at first but became slightly dispersed later. The formation of the metaphase chromosomes was observed in 86% of penetrated oocytes examined 40 h after transfer, and occurred in all metaphase II oocytes at that time. In oocytes penetrated by more than nine spermatozoa, no such transformation of sperm nuclei was observed. Well-developed male and female pro-nuclei were observed in only three (6%) of 51 oocytes penetrated 40 h after transfer.