Cell division: first embryonic mitosis

"Prior to fertilization, C. elegans oocytes are arrested in meiotic prophase with nuclei containing two copies of the diploid genome packaged into recombined bivalent chromosomes. The two rounds of meiotic chromosome segregation that generate the haploid oocyte pronucleus are completed in the zygote after the oocytes are fertilized. During each meiotic division, chromosome segregation is accomplished by a small acentriolar meiotic spindle that forms in the embryo anterior. During anaphase of meiosis I and again in meiosis II, the meiotic spindle associates with the cortex in an end-on fashion, and a highly asymmetric cytokinesis-like event extrudes a polar body (Figure 2; Albertson and Thomson, 1993; Clark-Maguire and Mains, 1994; Yang et al., 2003). In addition to the haploid pronucleus, the sperm brings a pair of centrioles into the oocyte, which lacks centrioles due to their degradation during oogenesis. As meiosis completes, the haploid oocyte and sperm-derived pronuclei, located at opposite ends of the embryo increase in size, becoming visible by DIC microscopy. After entering the oocyte, the sperm-derived centriole pair recruits pericentriolar material and acquires the ability to nucleate microtubules (O'Connell, 2000; Pelletier et al., 2004). Subsequently, the two sperm-derived centrioles separate, forming two centrosomes positioned on either side of the paternal pronucleus. Coincident with chromosome condensation during mitotic prophase, the pronuclei migrate towards each other. After the pronuclei meet, the nuclear-centrosome complex moves to the center of the embryo and rotates to align with the long axis of the embryo (Albertson, 1984; Hyman and White, 1987). The miotitc spindle begins to move towards the embryo posterior during metaphase (Labbe et al., 2004; Oegema et al., 2001), and asymmetric elongation during anaphase contributes to its posterior displacement (Albertson, 1984; Grill et al., 2001). Since the cleavage furrow bisects the mitotic spindle, this displacement results in an asymmetric first cleavage (For more on the mechanisms that generate this asymmetry see Asymmetric cell division and axis formation in the embryo). " From "Cell division" by Karen Oegema, WormBook

在受精之前，秀丽隐杆线虫的卵母细胞处于减数分裂前期，其细胞核中含有两套二倍体基因组，这些基因组被包装成重组的联会染色体。在卵母细胞受精后，在受精卵中完成两轮减数分裂染色体分离，以生成单倍体卵母细胞原核。在每个减数分裂过程中，染色体分离是通过在胚胎前端形成的小型无中心体减数纺锤体来实现的。在减数第一次分裂的中期以及减数第二次分裂中，减数纺锤体以端对端的方式与细胞质膜相联接，并发生一种高度不对称的类似胞质分裂的事件，从而排出一个极体（图2；Albertson and Thomson, 1993；Clark-Maguire and Mains, 1994；Yang et al., 2003）。除了单倍体原核外，精子还携带一对中心粒进入卵母细胞，由于在卵生成过程中中心粒的降解，卵母细胞本身缺乏中心粒。随着减数分裂的完成，单倍体卵母细胞和精子来源的原核位于胚胎的两端，其体积逐渐增大，通过相差干涉显微镜可见。精子来源的中心粒对进入卵母细胞后，招募周围中心粒物质并获得了形成微管的能力（O'Connell, 2000；Pelletier et al., 2004）。随后，两个精子来源的中心粒分离，形成位于父方原核两侧的两个中心体。与有丝分裂前期染色体凝聚同时发生的是，原核向彼此迁移。原核相遇后，核-中心体复合物移至胚胎中心并旋转以与胚胎的长轴对齐（Albertson, 1984；Hyman and White, 1987）。在有丝分裂中期，纺锤体开始向胚胎后端移动（Labbe et al., 2004；Oegema et al., 2001），并在后期发生的不对称伸长有助于其向后的位移（Albertson, 1984；Grill et al., 2001）。由于分裂裂隙将纺锤体一分为二，这种位移导致了不对称的第一分裂（关于产生这种不对称性的机制，请参见胚胎中的不对称细胞分裂和轴线形成）。