An analysis of multinucleated blastomere formation in human embryos

Human embryos were disaggregated into component blastomeres42–72 h after insemination. The blastomeres were scoredfor the number of nuclei present and blastomeres of known nuclearmorphology were returned to individual culture drops for 16–20h, after which they were scored for cleavage and nuclear morphology.In all, 48% of mononucleated blastomeres cleaved during thisperiod, but only 76% of these produced two mononucleated daughterblastomeres; in the remainder, one or more of the blastomereswas abnormally nucleated. During overnight culture, 30% of multinucleatedblastomeres and 30% of anucleate blastomeres cleaved, the majorityproducing abnormally nucleated daughter blastomeres. The majorityof blastomeres which showed no sign of cleavage after overnightculture retained the same nuclear morphology as when originallydisaggregated. However, a small number of mononucleated blastomerescontained two nuclei after culture, indicating that karyokinesismay have taken place in the absence of cytokinesis. Overall,30% of blastomeres with more than one nucleus seemed to ariseby this mechanism, the remainder probably arising by errorsof chromosome segregation and/or packaging at mitosis. In addition,25/111 mononucleated daughter cells arose either after abnormaldivision of mononucleated parent cells or after division ofmultinucleated cells, suggesting that 23% of newly formed mononucleatedcells might be chromosomally abnormal. The results of DNA quantitationindicated that very few (12/131, 9.2%) blastomeres (whetheruni-or multinucleated) had a DNA content outside the 2-4C range.The embryos used for these studies had been cultured in oneof three commonly used in-vitro fertilization (IVF) media: modifiedT6, Earle's balanced salts or Universal IVF medium (a commercialmedium from Medi-Cult). A retrospective analysis was carriedout of the number of embryos containing multinucleated blastomeresat disaggregation and of the total proportion of isolated blastomereswhich were multinucleated in three groups of embryos, each ofwhich had been cultured in one of the IVF media. Both theseparameters were found to vary between cohorts of embryos culturedin the different media. The mechanism(s) by which culture mediumcomposition might affect multinucleation of human blastomeresis discussed, as is the significance of these data for reliablepreimplantation diagnosis of genetic status. aberrant cleavage/acytokinesis/culture medium/multinucleated blastomeres