Binucleate blastomeres in preimplantation human embryos in vitro: failure of cytokinesis during early cleavage.

The nuclei of disaggregated blastomeres from two hundred preimplantation human embryos were examined between days 2 and 4 after insemination in vitro by vital labelling with a polynucleotide-specific fluorochrome. Although the majority of blastomeres had a single nucleus, binucleate blastomeres containing two nuclei of equal size were common and other blastomeres had fragmented nuclei or were anucleate. Seventeen per cent of normally fertilized embryos at two- to four-cell stage had at least one binucleate blastomere, and this increased to 65% at the nine- to 16-cell stage when individual embryos had between one and six binucleate blastomeres. The proportion of binucleate blastomeres in normally fertilized embryos increased from 5 to 10% over this period, whereas in abnormally fertilized, polyspermic or parthenogenetic, embryos the proportion was significantly higher during early cleavage stages but decreased at the nine- to 16-cell stage when the majority of these embryos arrest (25 and 6%, respectively). The incidence of anucleate blastomeres in normally fertilized embryos was also high, especially in those of poor morphology. In contrast, blastomeres with fragmented nuclei were relatively uncommon and the incidence was variable among classes and stages of development. Estimates of the volume of binucleate blastomeres based on measurement of their diameters and comparison with mononucleate blastomeres at various cleavage stages indicated that these blastomeres arise from a failure of cytokinesis between the second and fourth cleavage divisions. On this basis, assignment of binucleate blastomeres to particular cleavage stages in normally fertilized day 4 embryos suggests that at least some of these blastomeres arising during early cleavage persist without further cell division for up to 48 h. At the cellular level, therefore, blastomeres with either binucleate or abnormal nuclei contribute to cleavage stage arrest in vitro.