This chapter discusses our current knowledge on the major segregation events that lead to the individualization of the building blocks of vertebrate organisms, starting with the segregation between “outer” and “inner” cells, the separation of the germ layers and the maintenance of their boundaries during gastrulation, and finally the emergence of the primary axial structure, the notochord. The amphibian embryo is used as the prototypical model, to which fish and mouse development are compared. This comparison highlights a striking conservation of the basic processes. It suggests that simple principles may account for the formation of divergent structures. One of them is based on the non-adhesive nature of the apical domain of epithelial cells, exploited to segregate superficial and deep cell populations as a result of asymmetric division. The other principle involves differential expression of contact cues, such as ephrins and protocadherins, to build up high tension along adhesive interfaces, which efficiently creates sharp boundaries.

本章讨论了我们目前对导致脊椎动物生物体构建块个体化的主要分离事件的了解，从“外部”和“内部”细胞之间的分离、胚层的分离和它们的边界的维护开始。在原肠胚形成过程中，最终出现了主要的轴向结构，脊索。两栖动物胚胎被用作原型模型，将鱼和小鼠的发育与其进行比较。这种比较突出了基本过程的惊人守恒。它表明简单的原理可以解释不同结构的形成。其中之一是基于上皮细胞顶端域的非粘附性，由于不对称分裂，被利用来分离表层和深层细胞群。