Institute of Developmental Biology, University of Cologne, Gyrhofstr. 17, D-50923 Cologne, Germany.
Cold Spring Harb Perspect Biol. 2009 Aug;1(2):a001891. doi: 10.1101/cshperspect.a001891.
The orthogonal axes of Drosophila are established during oogenesis through a hierarchical series of symmetry-breaking steps, most of which can be traced back to asymmetries inherent in the architecture of the ovary. Oogenesis begins with the formation of a germline cyst of 16 cells connected by ring canals. Two of these 16 cells have four ring canals, whereas the others have fewer. The first symmetry-breaking step is the selection of one of these two cells to become the oocyte. Subsequently, the germline cyst becomes surrounded by somatic follicle cells to generate individual egg chambers. The second symmetry-breaking step is the posterior positioning of the oocyte within the egg chamber, a process mediated by adhesive interactions with a special group of somatic cells. Posterior oocyte positioning is accompanied by a par gene-dependent repolarization of the microtubule network, which establishes the posterior cortex of the oocyte. The next two steps of symmetry breaking occur during midoogenesis after the volume of the oocyte has increased about 10-fold. First, a signal from the oocyte specifies posterior follicle cells, polarizing a symmetric prepattern present within the follicular epithelium. Second, the posterior follicle cells send a signal back to the oocyte, which leads to a second repolarization of the oocyte microtubule network and the asymmetric migration of the oocyte nucleus. This process again requires the par genes. The repolarization of the microtubule network results in the transport of bicoid and oskar mRNAs, the anterior and posterior determinants, respectively, of the embryonic axis, to opposite poles of the oocyte. The asymmetric positioning of the oocyte nucleus defines a cortical region of the oocyte where gurken mRNA is localized, thus breaking the dorsal-ventral symmetry of the egg and embryo.
果蝇的正交轴是在卵子发生过程中通过一系列层次分明的对称破缺步骤建立的，其中大多数步骤可以追溯到卵巢结构中固有的不对称性。卵子发生始于由 16 个细胞组成的生殖细胞囊的形成，这些细胞通过环管连接。这 16 个细胞中的两个具有四个环管，而其他细胞则具有较少的环管。第一个对称破缺步骤是选择这两个细胞中的一个成为卵母细胞。随后，生殖细胞囊被体细胞滤泡细胞包围，以产生单个卵室。第二个对称破缺步骤是卵母细胞在卵室内的后定位，这一过程是通过与一组特殊的体细胞的粘附相互作用来介导的。后定位伴随着微管网络的依赖 par 基因的重新极化，从而建立了卵母细胞的后皮层。对称破缺的接下来两个步骤发生在卵母细胞体积增加约 10 倍后的中卵子发生期间。首先，来自卵母细胞的信号指定后滤泡细胞，使存在于滤泡上皮内的对称预图案极化。其次，后滤泡细胞向卵母细胞发送信号，导致卵母细胞微管网络的第二次重新极化和卵母细胞核的不对称迁移。这个过程再次需要 par 基因。微管网络的重新极化导致 bicoid 和 oskar mRNA 的运输，分别是胚胎轴的前和后决定因素，到卵母细胞的相对极点。卵母细胞核的不对称定位定义了卵母细胞的一个皮质区域，其中 gurken mRNA 被定位，从而打破了卵和胚胎的背腹对称。
