Cytogenetic Analysis of Cattle Oocytes Remaining Uncleaved after in vitro Fertilization

A cytogenetic study was performed on uncleaved cattle oocytes after in vitro fertilization. To assess the contribution of chromosome anomalies to failure of bovine in vitro fertilization, 1044 uncleaved oocytes were investigated cytogenetically after in vitro fertilization. Uncleaved oocytes were selected after 72 h of culture post IVF and fixed for chromosomal analysis. A total of 744 oocytes were fixed on slides. Successful chromosome analysis was carried out on 472 oocytes; the remaining 272 oocytes could not be evaluated. Eight (1.1%) were immature oocytes in the germinal vesicle stage. Another 44 (5.9%) in anaphase and 52 (7.0%) in Telophase I were considered immature oocytes. About 27.4% of oocytes had a normal haploid chromosome complement; 48 (6.4%) were hypohaploid; 20 (2.7%) were hyperhaploid and 4 (0.5%) were diploid. Ninety-two (12.4%) oocytes showed prematurely condensed chromosomes of the G -phase from sperm, as well as a set of 1 maternal anaphase I or metaphase II chromosomes. The sperm nuclei showed a remarkable variation in the degree of condensation. In conclusion, oocyte immaturity and premature condensation of sperm head chromosome caused considerable fertilization failure. The high rate of chromosomal abnormalities in the uncleaved oocytes represents natural selection that occurs in IVF when oocytes/zygotes with chromosome abnormalities fail to cleave.