(51) The Mouse Embryo is Susceptible to Mitotic Slippage

The mouse embryo is susceptible to mitotic slippage

Helia Motamedi¹ and Greg FitzHarris^1,2

1. Centre de Recherche du Centre Hospitalier de l’Université de Montréal, Montréal, Canada, H2X 0A9

2. Département d’Obstétrique-Gynécologie, Université de Montréal, Montréal, Canada, H3T 1J4 

Abstract Text: Erroneous chromosome segregation results in cells with an incorrect number of chromosomes, termed aneuploidy, which is common in preimplantation embryos. In somatic cells, a safeguard called the spindle assembly checkpoint (SAC) prevents the cell from completing mitosis in circumstances that would risk the onset of aneuploidy. However, prolonged SAC activation can cause exit from mitosis despite complete failure of chromosome segregation, in a scenario called mitotic slippage. Indirect observations allude that mitotic slippage might play a role in complex aneuploidies in human embryos. Here, to determine whether mitotic slippage can occur in a mammalian embryo, we induced prolonged M-phase arrest by treating 2-cell mouse embryos separately with two different SAC-activating agents, nocodazole and monastrol, and performed live time-lapse imaging. M-phase arrested embryos underwent actin-dependent cortical ‘bulging’ activity ~11-16h after mitosis entry, transiently taking the appearance of a ‘fragmented’ embryo. However, embryos eventually returned to an ostensibly-normal-looking 2-cell embryo, and nuclei reformed after ~13h and 18h in nocodazole and monastrol respectively, confirming slippage. Interestingly, unlike in C. elegans, we found propensity to mitotic slippage to be driven by developmental stage but not cell-size. Micro-injection of H2B-RFP and live 3D time-lapse imaging at different developmental stages revealed that early-stage embryos were more prone to mitotic slippage (2-cell; 76%; 4-cell: 75%; 8-cell: 47%). However, micromanipulation experiments suggest that this difference between developmental stages is not attributable to cell size. Furthermore, we found post-slippage development to be dependent on M-phase duration. Taken together, our data allude that slippage could cause multinucleation in early embryos affecting embryo health and provides a possible explanation for high levels of embryo fragmentation observed in the clinic.