Abstract
This study indicated that prolonged exposure of donor cell nuclei to oocyte cytoplasm before activation results in abnormal chromatin morphology, and reduced development to compacted morula/blastocyst stage in vitro. However, after transfer of embryos to recipients, there was no difference in pregnancy rates throughout gestation. Chromatin morphology was evaluated for embryos held 2, 3, 4 and 5 h between fusion and activation. In embryos held 2 h, 15/17 (88.2%) embryos contained condensed chromosomes, while only 12/24 (50.0%) embryos held 3 h exhibited this characteristic. The proportion of embryos with elongated or fragmented chromosomes tended to increase with increased hold time. While 15/19 (78.9%) of embryos held 2 h developed a single pronucleus 6 h after activation, only 8/22 (36.4%) had one pronucleus after a 4-h hold. Embryos held 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 h cleaved at rates of 207/281 (73.7%), 142/166 (85.5%), 655/912 (71.8%), 212/368 (57.6%), 406/667 (60.9%), 362/644 (56.2%) and 120/228 (52.6%) respectively. Further development to compacted morula/blastocyst stage occurred at rates of 78/281 (27.8%), 42/166 (25.3%), 264/912 (28.9%), 79/368 (21.5%), 99/667 (14.8%), 94/644 (14.6%) and 27/228 (11.8%) respectively. Embryos held less than 2.5 h between fusion and activation established pregnancies in 18/66 (27.3%) of recipients, while embryos held over 2.5 h established pregnancies at a rate of 17/57 (29.8%). This study indicates that holding bovine nuclear transfer embryos less than 2.5 h between fusion and activation results in improved nuclear morphology and increased development to compacted morula/blastocyst stage, and results in pregnancy rates equivalent to embryos held over 2.5 h.
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