The First Mitotic Division of the Human Embryo is Highly Error-prone

Aneuploidy in human embryos is surprisingly prevalent and increases drastically with maternal age, resulting in miscarriages, infertility and birth defects. Frequent errors during the meiotic divisions cause this aneuploidy, while age-independent errors during the first cleavage divisions of the embryo also contribute. However, the underlying mechanisms are poorly understood, largely because these events have never been visualised in living human embryos. Here, using cell-permeable DNA dyes, we film chromosome segregation during the first and second mitotic cleavage divisions in human embryos from women undergoing assisted reproduction following ovarian stimulation. We show that the first mitotic division takes several hours to complete and is highly variable. Timings of key mitotic events were, however, largely consistent with clinical videos of embryos that gave rise to live births. Multipolar divisions and lagging chromosomes during anaphase were frequent with no maternal age association. In contrast, the second mitosis was shorter and underwent mostly bipolar divisions with no detectable lagging chromosomes. We propose that the first mitotic division in humans is a unique and highly error-prone event, which contributes to fetal aneuploidies.