Keller R, Cooper M S, Danilchik M, Tibbetts P, Wilson P A
Department of Zoology, University of California, Berkeley 94720.
J Exp Zool. 1989 Aug;251(2):134-54. doi: 10.1002/jez.1402510204.
Morphometric data from scanning electron micrographs (SEM) of cells in intact embryos and high-resolution time-lapse recordings of cell behavior in cultured explants were used to analyze the cellular events underlying the morphogenesis of the notochord during gastrulation and neurulation of Xenopus laevis. The notochord becomes longer, narrower, and thicker as it changes its shape and arrangement and as more cells are added at the posterior end. The events of notochord development fall into three phases. In the first phase, occurring in the late gastrula, the cells of the notochord become distinct from those of the somitic mesoderm on either side. Boundaries form between the two tissues, as motile activity at the boundary is replaced by stabilizing lamelliform protrusions in the plane of the boundary. In the second phase, spanning the late gastrula and early neurula, cell intercalation causes the notochord to narrow, thicken, and lengthen. Its cells elongate and align mediolaterally as they rearrange. Both protrusive activity and its effectiveness are biased: the anterioposterior (AP) margins of the cells advance and retract but produce much less translocation than the more active left and right ends. The cell surfaces composing the lateral boundaries of the notochord remain inactive. In the last phase, lasting from the mid- to late neurula stage, the increasingly flattened cells spread at all their interior margins, transforming the notochord into a cylindrical structure resembling a stack of pizza slices. The notochord is also lengthened by the addition of cells to its posterior end from the circumblastoporal ring of mesoderm. Our results show that directional cell movements underlie cell intercalation and raise specific questions about the cell polarity, contact behavior, and mechanics underlying these movements. They also demonstrate that the notochord is built by several distinct but carefully coordinated processes, each working within a well-defined geometric and mechanical environment.
利用来自非洲爪蟾原肠胚期和神经胚期完整胚胎细胞的扫描电子显微镜（SEM）形态测量数据以及培养外植体中细胞行为的高分辨率延时记录，分析了脊索形态发生过程中细胞事件的潜在机制。随着脊索形状和排列的变化以及后端添加更多细胞，脊索变得更长、更窄且更厚。脊索发育过程可分为三个阶段。在第一阶段，发生在原肠胚晚期，脊索细胞与两侧体节中胚层细胞区分开来。两个组织之间形成边界，边界处的运动活性被边界平面内稳定的片状突起所取代。在第二阶段，跨越原肠胚晚期和神经胚早期，细胞插入导致脊索变窄、增厚和延长。细胞在重新排列时纵向伸长并沿中侧方向排列。突出活性及其有效性存在偏向性：细胞的前后（AP）边缘前进和后退，但产生的移位比更活跃的左右两端少得多。构成脊索侧边界的细胞表面保持不活跃。在最后阶段，从神经胚中期持续到晚期，越来越扁平的细胞在其所有内部边缘扩展，将脊索转变为类似一叠披萨切片的圆柱形结构。脊索也通过从中胚层的环胚孔环向其后端添加细胞而延长。我们的结果表明，定向细胞运动是细胞插入的基础，并提出了关于这些运动背后的细胞极性、接触行为和力学的具体问题。它们还表明，脊索是由几个不同但精心协调的过程构建而成的，每个过程都在明确的几何和力学环境中起作用。
