Fig. 7: Model of how first division errors can lead to different euploid/aneuploid embryos.
From: The first mitotic division of human embryos is highly error prone
Top row, a pair of homologous chromosomes (blue) are correctly segregated into the 2-cell embryo. Subsequent divisions (dotted black arrow), if occurring without error, would lead to an 8-cell embryo in which all blastomeres are euploid (blue). Second row; Non-disjunction during the first mitosis would result in trisomic and monosomic blastomeres (purple and red) with two potential outcomes: upper—lagging chromosome becomes a micronucleus (m.m.) that undergoes chromothripsis and potential chromosome loss. Outcome is euploid (blue) and monosomic (red) lineages in 8-cell (and beyond) embryo. Lower—without chromosome loss the outcome is a mosaic embryo with monosomic and trisomic lineages (purple and red). Such aneuploid lineages may be rescued in later divisions through further ‘second hit’ non-disjunction, possibly resulting in euploid blastomeres with a uniparental disomy signature.
