Examining cytokinesis in reductive cell divisions

胞质分裂是细胞分裂的最后阶段，在此过程中，单个细胞在最近分离的两组姐妹染色单体周围分裂为两个子细胞。胚胎是一个球形微生物，在4-5天的过程中从单个细胞发育成~100个细胞的囊胚，在此期间，随着每次细胞分裂，细胞的大小逐渐减半，并产生前两个细胞谱系。因此，它是一个非常容易处理的系统，用于在复杂的多细胞环境中检查细胞质分裂的各个方面，这些方面在其他哺乳动物系统中是无法获得的。在这里，我们将使用活的和固定的细胞成像方法、遗传操作和微操作，对小鼠早期胚胎的细胞质分裂进行第一次机械分析。我们将专注于两个独立但相互关联的子项目。首先，我们将确定皱纹速度是由细胞大小还是细胞分化决定的。我们的初步数据表明，与线虫形成鲜明对比的是，在线虫中，沟槽进入沟槽的速度随细胞大小而变化，而在小鼠中，沟槽进入并不随细胞大小而变化。我们将使用各种操作来彻底测试这一概念。相反，我们发现胚胎内的皱纹速度在16细胞阶段有所不同，并有理由相信这是由于该阶段对外部细胞的几种形态发生影响之一，包括改变细胞形状、细胞-细胞黏附、细胞极性的出现以及转录水平的细胞命运决定。我们将探索早期发育中的沟进，并确定这些发展事件中的哪一种影响沟进机制。第二，我们将确定细胞分裂延迟脱落的原因和后果。我们已经证实了一个经典但未经探索的观察结果，即小鼠早期胚胎的脱落大大延迟，导致姐妹细胞之间形成“细胞质桥梁”，通过这种桥梁可以共享大分子。我们将研究这些稳定的细胞质桥梁是如何形成的，重点是TEX14，这是一种负责在生殖细胞中形成稳定桥梁的分子，我们发现它也在胚胎中表达。我们将讨论早期胚胎中的细胞质桥允许姐妹细胞同步细胞周期的可能性，从而影响细胞分裂的时机和细胞分化事件。我们希望发现，小鼠早期胚胎作为部分合胞体存在，用于调节早期细胞周期和细胞命运决定。总体而言，我们希望这个项目能为细胞质分裂的研究建立一个有价值的模型，并提供一些关于细胞质分裂与哺乳动物细胞大小和谱系分化相关的最早的信息。

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.