Logan C Y, McClay D R
Department of Zoology, Duke University, Durham, NC 27708, USA.
Development. 1997 Jun;124(11):2213-23. doi: 10.1242/dev.124.11.2213.
During sea urchin development, a tier-to-tier progression of cell signaling events is thought to segregate the early blastomeres to five different cell lineages by the 60-cell stage (E. H. Davidson, 1989, Development 105, 421-445). For example, the sixth equatorial cleavage produces two tiers of sister cells called 'veg1' and 'veg2,' which were projected by early studies to be allocated to the ectoderm and endoderm, respectively. Recent in vitro studies have proposed that the segregation of veg1 and veg2 cells to distinct fates involves signaling between the veg1 and veg2 tiers (O. Khaner and F. Wilt, 1991, Development 112, 881-890). However, fate-mapping studies on 60-cell stage embryos have not been performed with modern lineage tracers, and cell interactions between veg1 and veg2 cells have not been shown in vivo. Therefore, as an initial step towards examining how archenteron precursors are specified, a clonal analysis of veg1 and veg2 cells was performed using the lipophilic dye, DiI(C16), in the sea urchin species, Lytechinus variegatus. Both veg1 and veg2 descendants form archenteron tissues, revealing that the ectoderm and endoderm are not segregated at the sixth cleavage. Also, this division does not demarcate cell type boundaries within the endoderm, because both veg1 and veg2 descendants make an overlapping range of endodermal cell types. The allocation of veg1 cells to ectoderm and endoderm during cleavage is variable, as revealed by both the failure of veg1 descendants labeled at the eighth equatorial division to segregate predictably to either tissue and the large differences in the numbers of veg1 descendants that contribute to the ectoderm. Furthermore, DiI-labeled mesomeres of 32-cell stage embryos also contribute to the endoderm at a low frequency. These results show that the prospective archenteron is produced by a larger population of cleavage-stage blastomeres than believed previously. The segregation of veg1 cells to the ectoderm and endoderm occurs relatively late during development and is unpredictable, indicating that later cell position is more important than the early cleavage pattern in determining ectodermal and archenteron cell fates.
在海胆发育过程中，细胞信号事件的逐层进展被认为在60细胞期时将早期的卵裂球分离为五种不同的细胞谱系（E. H. 戴维森，1989年，《发育》第105卷，421 - 445页）。例如，第六次赤道卵裂产生了两层姐妹细胞，称为“veg1”和“veg2”，早期研究推测它们分别被分配到外胚层和内胚层。最近的体外研究表明，veg1和veg2细胞向不同命运的分离涉及veg1层和veg2层之间的信号传导（O. 卡纳和F. 威尔特，1991年，《发育》第112卷，881 - 890页）。然而，尚未使用现代谱系示踪剂对60细胞期胚胎进行命运图谱研究，并且veg1和veg2细胞之间的细胞相互作用在体内也未得到证实。因此，作为研究原肠胚前体如何被指定的第一步，我们使用亲脂性染料DiI(C16)对海胆物种华丽斜海胆的veg1和veg2细胞进行了克隆分析。veg1和veg2的后代都形成了原肠组织，这表明外胚层和内胚层在第六次卵裂时并未分离。此外，这次分裂并没有划分内胚层内的细胞类型边界，因为veg1和veg2的后代都产生了一系列重叠的内胚层细胞类型。卵裂期间veg1细胞向外胚层和内胚层的分配是可变的，这一点从第八次赤道分裂时标记的veg1后代未能可预测地分离到任何一种组织以及对形成外胚层的veg1后代数量的巨大差异中可以看出。此外，32细胞期胚胎的DiI标记的中体也以低频率向内胚层贡献细胞。这些结果表明，预期的原肠胚是由比之前认为的更多的卵裂期卵裂球产生的。veg1细胞向外胚层和内胚层的分离在发育过程中相对较晚发生且不可预测，这表明在决定外胚层和原肠胚细胞命运方面，后期的细胞位置比早期的卵裂模式更重要。
