Wei Y, Mikawa T
Department of Cell Biology, Cornell University Medical College, New York, NY 10021, USA.
Development. 2000 Jan;127(1):87-96. doi: 10.1242/dev.127.1.87.
Gastrulation in the amniote begins with the formation of a primitive streak through which precursors of definitive mesoderm and endoderm ingress and migrate to their embryonic destinations. This organizing center for amniote gastrulation is induced by signal(s) from the posterior margin of the blastodisc. The mode of action of these inductive signal(s) remains unresolved, since various origins and developmental pathways of the primitive streak have been proposed. In the present study, the fate of chicken blastodermal cells was traced for the first time in ovo from prestreak stages XI-XII through HH stage 3, when the primitive streak is initially established and prior to the migration of mesoderm. Using replication-defective retrovirus-mediated gene transfer and vital dye labeling, precursor cells of the stage 3 primitive streak were mapped predominantly to a specific region where the embryonic midline crosses the posterior margin of the epiblast. No significant contribution to the early primitive streak was seen from the anterolateral epiblast. Instead, the precursor cells generated daughter cells that underwent a polarized cell division oriented perpendicular to the anteroposterior embryonic axis. The resulting daughter cell population was arranged in a longitudinal array extending the complete length of the primitive streak. Furthermore, expression of cVg1, a posterior margin-derived signal, at the anterior marginal zone induced adjacent epiblast cells, but not those lateral to or distant from the signal, to form an ectopic primitive streak. The cVg1-induced epiblast cells also exhibited polarized cell divisions during ectopic primitive streak formation. These results suggest that blastoderm cells located immediately anterior to the posterior marginal zone, which secretes an inductive signal, undergo spatially directed cytokineses during early primitive streak formation.
羊膜动物的原肠胚形成始于原条的形成，确定的中胚层和内胚层的前体细胞通过原条进入并迁移到它们在胚胎中的目的地。羊膜动物原肠胚形成的这个组织中心是由胚盘后缘的信号诱导产生的。这些诱导信号的作用方式仍未明确，因为已经提出了原条的各种起源和发育途径。在本研究中，首次在鸡胚内追踪了从原条形成前的XI-XII期到HH3期鸡胚盘细胞的命运，此时原条最初形成且中胚层尚未迁移。利用复制缺陷型逆转录病毒介导的基因转移和活体染料标记，将3期原条的前体细胞主要定位到胚胎中线与上胚层后缘交叉的特定区域。未观察到前外侧上胚层对早期原条有显著贡献。相反，前体细胞产生的子细胞进行了垂直于胚胎前后轴的极化细胞分裂。产生的子细胞群体沿纵向排列，延伸至原条的全长。此外，来自后缘的信号cVg1在上胚层前边缘区的表达诱导相邻的上胚层细胞，而不是信号外侧或远离信号的细胞，形成异位原条。在异位原条形成过程中，cVg1诱导的上胚层细胞也表现出极化细胞分裂。这些结果表明，位于分泌诱导信号的后边缘区前方紧邻的胚盘细胞在早期原条形成过程中经历了空间定向的胞质分裂。
