Domingo C, Keller R
Department of Molecular and Cell Biology, University of California, Berkeley 94720, USA.
Development. 1995 Oct;121(10):3311-21. doi: 10.1242/dev.121.10.3311.
We show that notochord-inducing signals are present during Xenopus laevis gastrulation and that they are important for both inducing and organizing cell behavior and differentiation in the notochord. Previous work showed that convergent extension of prospective notochordal and somitic mesoderm occurs by mediolateral cell intercalation to produce a longer, narrower tissue. Mediolateral cell intercalation is driven by bipolar, mediolaterally directed protrusive activity that elongates cells and then pulls them between one another along the mediolateral axis. This cell behavior, and subsequent notochordal cell differentiation, begins anteriorly and spreads posteriorly along the notochordal-somitic boundary, and from this lateral boundary progresses medially towards the center of the notochord field. To examine whether these progressions of cell behaviors and differentiation are induced and organized during gastrulation, we grafted labeled cells from the prospective notochordal, somitic and epidermal regions of the gastrula into the notochordal region and monitored their behavior by low light, fluorescence videomicroscopy. Prospective notochordal, epidermal and somitic cells expressed mediolateral cell intercalation behavior in an anterior-to-posterior and lateral-to-medial order established by the host notochord. Behavioral changes were induced first and most dramatically among cells grafted next to the notochordal-somitic boundary, particularly those in direct contact with the boundary, suggesting that the boundary may provide signals that both induce and organize notochordal cell behaviors. By physically impeding normal convergent extension movements, notochordal cell behaviors and differentiation were restricted to the anteriormost notochordal region and to the lateral notochordal-somitic boundary. These results show that mediolateral cell intercalation behavior and notochordal differentiation can be induced in the gastrula stage, among cells not normally expressing these characteristics, and that these characteristics are induced progressively, most likely by signals emanating from the notochordal-somitic boundary. In addition, they show that morphogenetic movements during gastrulation are necessary for complete notochord formation and that the prospective notochord region is not determined by the onset of gastrulation.
我们发现，非洲爪蟾原肠胚形成过程中存在诱导脊索的信号，这些信号对于诱导和组织脊索中的细胞行为及分化非常重要。先前的研究表明，预期的脊索和体节中胚层的汇聚延伸是通过中侧细胞插入实现的，从而产生更长、更窄的组织。中侧细胞插入由双极的、中侧方向的突出活动驱动，这种活动使细胞伸长，然后沿着中侧轴将它们相互拉近。这种细胞行为以及随后的脊索细胞分化从前部开始，沿着脊索 - 体节边界向后扩散，并从这个外侧边界向内侧朝着脊索区域的中心推进。为了研究这些细胞行为和分化的进程在原肠胚形成过程中是否被诱导和组织，我们将来自原肠胚预期的脊索、体节和表皮区域的标记细胞移植到脊索区域，并通过弱光荧光视频显微镜监测它们的行为。预期的脊索、表皮和体节细胞按照宿主脊索建立的从前到后、从外侧到内侧的顺序表达中侧细胞插入行为。行为变化首先在移植到脊索 - 体节边界旁边的细胞中诱导发生，而且最为显著，特别是那些与边界直接接触的细胞，这表明该边界可能提供诱导和组织脊索细胞行为的信号。通过物理阻碍正常的汇聚延伸运动，脊索细胞行为和分化被限制在最前部的脊索区域以及外侧的脊索 - 体节边界。这些结果表明，在原肠胚阶段，中侧细胞插入行为和脊索分化可以在通常不表达这些特征的细胞中被诱导，并且这些特征是逐步诱导的，最有可能是由脊索 - 体节边界发出的信号所诱导。此外，它们表明原肠胚形成过程中的形态发生运动对于完整的脊索形成是必要的，并且预期的脊索区域不是由原肠胚形成的开始所决定的。
