Shimizu T, Ishii R, Takahashi H
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Dev Growth Differ. 1998 Jun;40(3):257-66. doi: 10.1046/j.1440-169x.1998.00001.x.
Unequal cleavage that produces two blastomeres of different size is a cleavage pattern that many animals in a variety of phyla, particularly in Spiralia, adopt during early development. This cleavage pattern is apparently instrumental for asymmetric segregation of developmental potential, but it is also indispensable for normal embryogenesis in many animals. Mechanically, unequal cleavage is achieved by either simple unequal cytokinesis or by forming a polar lobe at the egg's vegetal pole. In the present paper, the mechanisms for unequal cytokinesis involved in the first three cleavages in the oligochaete annelid Tubifex are reviewed. The three unequal cleavages are all brought about by an asymmetrically organized mitotic apparatus (MA). The MA of the first cleavage is monastral in that an aster is present at one pole of a bipolar spindle but not at the other. This monastra form, which arises as a result of the involvement of a single centrosome in the MA assembly, is both necessary and sufficient for unequal first cleavage. The egg cortex during the first mitosis is devoid of the ability to remodel spindle poles. In contrast to the non-cortical mechanisms for the first cleavage, asymmetry in the MA organization at the second and third cleavages depends solely on specialized properties of the cell cortex, to which one spindle pole is physically connected. A cortical attachment site for the second cleavage spindle is generated de novo at the cleavage membrane resulting from the first cleavage; it is an actin-based, cell contact-dependent structure. The cortical microtubule attachment site for the third cleavage, which functions independently of contact with other cells, is not generated at the cleavage membrane resulting from the second cleavage, but is located at the animal pole; it may originate from the second polar body formation and become functional at the 4-cell stage.
不等裂产生两个大小不同的卵裂球，这是许多不同门类的动物，尤其是螺旋动物门中的动物在早期发育过程中采用的一种卵裂模式。这种卵裂模式显然有助于发育潜能的不对称分离，但对许多动物的正常胚胎发生也是不可或缺的。从机制上讲，不等裂可通过简单的不等胞质分裂或在卵的植物极形成一个极叶来实现。在本文中，我们综述了寡毛纲环节动物颤蚓前三次卵裂中涉及不等胞质分裂的机制。这三次不等裂均由不对称组织的有丝分裂器（MA）引起。第一次卵裂的有丝分裂器是单星体的，即双极纺锤体的一极有星体而另一极没有。这种单星体形式是由于单个中心体参与有丝分裂器组装而产生的，对于第一次不等卵裂来说既是必要的也是充分的。第一次有丝分裂期间的卵皮层缺乏重塑纺锤体极的能力。与第一次卵裂的非皮层机制不同，第二次和第三次卵裂时，有丝分裂器组织的不对称仅取决于细胞皮层的特殊性质，一个纺锤体极与该皮层物理相连。第二次卵裂纺锤体的皮层附着位点是在第一次卵裂产生的卵裂膜上重新形成的；它是一种基于肌动蛋白、依赖细胞接触的结构。第三次卵裂的皮层微管附着位点独立于与其他细胞的接触发挥作用，它不是在第二次卵裂产生的卵裂膜上形成的，而是位于动物极；它可能起源于第二极体的形成，并在4细胞阶段开始发挥作用。
