CYTOGENETIC ANALYSIS OF DAY-7 BOVINE EMBRYOS BY CYTOPHOTOMETRIC DNA MEASUREMENTS

A Feulgen staining procedure that stains the DNA of individual fixed n uclei stoichiometrically was used to analyse cytophotometrically the i ncidence of total ploidy and mixoploidy in 28 day-7 bovine embryos tha t had been fixed after collection ('non-cultured' embryos). The influe nce of culture on the incidence of chromosome abnormalities was furthe r studied in another group of 24 embryos ('cultured' embryos) by cultu ring them for 24h in Whittingham's medium. Of the total 52 embryos stu died, two appeared to be entirely abnormal: one embryo was completely haploid, whereas the other embryo was completely triploid. Individual hyperdiploid nuclei and hypodiploid nuclei were frequently observed in the otherwise diploid embryos. As haploid polar bodies can still be p resent in morulae and blastocysts (to a maximum of three), only embryo s with more than three hypodiploid nulcei were considered as abnormal. Of the 'non-cultured' embryos, 33.3% had one or more hyperdiploid nuc lei, whereas 51.9% had more than three hypodiploid nuclei. In this lat ter group, 35.7% of the embryos also had hyperdiploid nuclei. The resu lts also showed that day-7; bovine embryos that are completely haploid , completely triploid or mixoploid cannot be detected only by examinin g their morphology. It is concluded that the incidence of, especially, mixoploidy in embryos can be better studied by measuring the DNA cont ent of the individual nuclei of an embryo rather than by analysing chr omosomes, as in the latter method only dividing cells can be analysed. The presence of hyperdiploid and hypodiploid nuclei may indicate the frequent occurrence of mitotic segregation failures during mitosis in bovine embryos. These segregation failures may cause the death of cell s, which in rum may contribute to a reduced embryo viability. This con tention is supported by the results of the 'cultured' embryos that sho w that in this group the mean percentage of hypodiploid nuclei was low er than that in the group of 'non-cultured' embryos, whereas the mean number of nuclei and the mean percentage of degenerated nuclei was app roximately the same in both groups of 'non-cultured' and 'cultured' em bryos.