Ransick A
Department of Zoology, University of Texas, Austin 78712.
Dev Biol. 1991 Jan;143(1):185-98. doi: 10.1016/0012-1606(91)90065-b.
Asexual spheroids of the genus Volvox contain only two cell types: flagellated somatic cells and immotile asexual reproductive cells known as gonidia. During each round of embryogenesis in Volvox obversus, eight large gonidial precursors are produced at the anterior extremity of the embryo. These cells arise as a consequence of polarized, asymmetric divisions of the anteriormost blastomeres at the fourth through nine cleavage cycles, while all other blastomeres cleave symmetrically to yield somatic cell precursors. Blastomeres isolated from embryos at any point between the 2-cell and the 32-cell stage cleaved in the normal pattern and produced the same complement and spatial distribution of cell types as they would have in an intact embryo. This result indicates that intrinsic features control the cleavage patterns and developmental potentials of blastomeres, and rules out any significant role for cell-cell interactions in gonidial specification. When substantial quantities of anterolateral cytoplasm were deleted from uncleaved gonidia or 4-cell stage blastomeres, the cell fragments frequently regulated and embryos were produced with the expected number of asymmetrically cleaving cells and gonidial precursors at their anterior ends. However, when anterior cytoplasm was deleted from 8-cell stage blastomeres, the depleted cells frequently failed to cleave asymmetrically and produced no gonidial precursors. Furthermore, when compression was used to reorient cleavage planes at the fourth division cycle, so that anterior cytoplasm was transmitted to more than the normal number of cells, those cells receiving a significant amount of such cytoplasm cleaved asymmetrically to produce supernumerary gonidial precursors. Together, these last two experiments indicate that blastomeres in the V. obversus embryo acquire (at least by the end of the third cleavage cycle) a polarized organization in which anterior cytoplasm plays a causal role in the process of reproductive-cell specification.
有鞭毛的体细胞和称为生殖细胞的无运动能力的无性生殖细胞。在反向团藻的每一轮胚胎发生过程中，胚胎前端会产生八个大型生殖细胞前体。这些细胞是第四至九个分裂周期中最前端的卵裂球进行极化、不对称分裂的结果，而所有其他卵裂球则对称分裂以产生体细胞前体。在2细胞期至32细胞期之间的任何时间从胚胎中分离出的卵裂球都以正常模式分裂，并产生与完整胚胎中相同的细胞类型互补和空间分布。这一结果表明，内在特征控制着卵裂球的分裂模式和发育潜能，并排除了细胞间相互作用在生殖细胞特化中的任何重要作用。当从未分裂的生殖细胞或4细胞期卵裂球中大量去除前外侧细胞质时，细胞碎片经常进行调节，胚胎产生的不对称分裂细胞和前端生殖细胞前体数量符合预期。然而，当从8细胞期卵裂球中去除前部细胞质时，耗尽的细胞经常无法进行不对称分裂，也不产生生殖细胞前体。此外，当在第四次分裂周期使用压缩来重新定向分裂平面，以使前部细胞质传递到比正常数量更多的细胞时，那些接收大量此类细胞质的细胞会不对称分裂以产生额外的生殖细胞前体。这最后两个实验共同表明，反向团藻胚胎中的卵裂球（至少在第三次分裂周期结束时）获得了一种极化组织，其中前部细胞质在生殖细胞特化过程中起因果作用。
