Department of Systems Biology, Harvard Medical School, Boston, MA 02115, USA.
Curr Biol. 2010 Nov 23;20(22):2040-5. doi: 10.1016/j.cub.2010.10.024. Epub 2010 Nov 4.
Current models for cleavage plane determination propose that metaphase spindles are positioned and oriented by interactions of their astral microtubules with the cellular cortex, followed by cleavage in the plane of the metaphase plate [1, 2]. We show that in early frog and fish embryos, where cells are unusually large, astral microtubules in metaphase are too short to position and orient the spindle. Rather, the preceding interphase aster centers and orients a pair of centrosomes prior to nuclear envelope breakdown, and the spindle assembles between these prepositioned centrosomes. Interphase asters center and orient centrosomes with dynein-mediated pulling forces. These forces act before astral microtubules contact the cortex; thus, dynein must pull from sites in the cytoplasm, not the cell cortex as is usually proposed for smaller cells. Aster shape is determined by interactions of the expanding periphery with the cell cortex or with an interaction zone that forms between sister-asters in telophase. We propose a model to explain cleavage plane geometry in which the length of astral microtubules is limited by interaction with these boundaries, causing length asymmetries. Dynein anchored in the cytoplasm then generates length-dependent pulling forces, which move and orient centrosomes.
目前的卵裂面确定模型提出，星体微管通过与细胞皮层的相互作用来定位和定向中期纺锤体，然后在中期板的平面上进行分裂[1,2]。我们表明，在早期的青蛙和鱼类胚胎中，细胞异常大，中期的星体微管太短，无法定位和定向纺锤体。相反，在核膜破裂之前，前期星体中心和定向一对中心体，纺锤体在这些预先定位的中心体之间组装。前期星体中心和定向中心体通过动力蛋白介导的拉力。这些力在星体微管与皮层接触之前起作用；因此，动力蛋白必须从细胞质中的位点而不是细胞皮层拉动，这与通常为较小细胞提出的观点相反。星体的形状由扩展的外围与细胞皮层或在末期姐妹星体之间形成的相互作用区之间的相互作用决定。我们提出了一个模型来解释卵裂面几何形状，其中星体微管的长度受与这些边界的相互作用限制，导致长度不对称。然后，锚定在细胞质中的动力蛋白产生长度依赖性拉力，这些拉力使中心体移动和定向。
