[Cytogenetic analysis of sister chromosome sets in the second polar body and in pronuclei of unicellular mouse embryos. I. Frequency and origin of aneuploidy in embryos heterozygous for the reciprocal chromosome translocation T[14;15]6Ca].

We carried out a cytogenetic study of ovulating oocytes and unicellular embryos, heterozygous by reciprocal chromosomal translocation T[14;15]6Ca. Okadaic acid was used to induce premature condensation of the interphase chromosomes in the embryos, and the number of G1 chromosomes was counted in the second polar body and pronuclei. It was shown that cytogenetic analysis of the sister chromosomal sets adequately determines the frequency of chromosomal segregation errors during oocyte meioses I and II. Trisomy and monosomy were observed in 36.2% embryos, while 2.2% featured tetrasomy or double monosomy. Errors of the first meiotic division caused aneuploidy in 28.5% embryos; trisomy and monosomy resulted from the homologs non-disjunction and chromatid presegregation in 17.6 and 10.9%, respectively. Numeral chromosomal aberrations in 4.1% of the embryos resulted from abnormal chromosomal segregation during oocyte meiosis II, while paternal chromosomal aberrations were found in 5.8% embryos. The main advantage of the proposed method is not only the higher accuracy in estimating the meiotic error frequency, but also the possibility to trace the origin of aneuploidy in mammalian embryos.