Examining cytokinesis in reductive cell divisions

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Cytokinesis is the final stage of cell division, during which a single cell is divided into two daughter cells around the two sets of recently separated sister chromatids. The embryo is a spherical micro-organism that develops from a single cell into a ~100 cell blastocyst over the course of 4-5 days, during which time cells progressively halve in size with each cell division, and the first two cell lineages arise. It is thus a highly tractable system for examining aspects of cytokinesis in a complex multicellular context that are inaccessible in other mammalian systems. Here we will use live and fixed cell imaging approaches, genetic manipulations, and micro-manipulation, to perform the first mechanistic analysis of cytokinesis in the early mouse embryo. We will focus on separate two separate but inter-related sub-projects. First, we will determine whether furrowing speed is determined by cell size or cell differentiation. Our preliminary data suggests that, in stark contrast to C elegans where the speed of furrow ingression furrowing scales as a function of cell size, in mouse furrow ingression does not scale to cell size. We will use a variety of manipulations to test this notion thoroughly. Rather, we have found that furrowing speeds within the embryo diverge at the 16 cell stage, and have reason to believe this is attributable to one of several morphogenetic influences upon the outer cells at that stage, including changing cell shape, cell-cell adhesion, emergence of cellular polarity, and transcriptional-level cell fate decisions. We will explore furrow ingression during early development, and determine which of these developmental events impacts furrow ingression mechanisms. Second, we will determine the cause and consequence of abscission delay in dividing cells. We have confirmed a classic but unexplored observation that abscission is greatly delayed in the early mouse embryos, causing the formation of `cytoplasmic bridges' between sister cells through which large molecules can be shared. We will examine how these stable cytoplasmic bridges are formed, focussing on TEX14, the molecule responsible for the formation of stable bridges in germ cells, which we have found is also expressed in embryos. We will address the possibility that cytoplasmic bridges in the early embryo allow sister-cells to synchronise cell cycles, and thereby impact cell-division timing and cell-differentiation events. We expect to find that the early mouse embryo exists as a partial syncytium that serves to regulate early cell cycles and cell fate decisions. Overall, we expect this programme to establish the mouse embryo as a valuable model for the study of cytokinesis, and provide some of the very first information relating cytokinesis to cell size and lineage differentiation in a mammalian setting.