Cell segregation in gastrulation: the role of cell fate specification

多细胞生物体的发展是由构图和形态发生这两个密切相关的活动驱动的。虽然图案化意味着细胞命运在空间和时间上的产生和组织，但形态发生包括有机体形成的过程。多细胞生物体的形态发生通常涉及其组成细胞的数量、大小、形状和位置的有组织的变化。越来越多的证据表明，细胞命运规范决定了细胞的形态发生潜能，反过来，细胞形态发生又影响着细胞命运规范和维持1。然而，在发育中的生物体中，细胞命运指定和形态发生之间的动态相互作用才刚刚开始被阐明。在大多数多细胞生物的发育过程中，第一个主要的形态发生过程是原肠形成。在原肠分裂过程中，祖细胞分离成不同的细胞群，形成不同的胚层：外胚层、中胚层和内胚层。生殖层的形成和空间组织在两栖类胚胎中得到了最广泛的研究，并涉及到细胞迁移和嵌入之间的复杂相互作用。Townes和Holtfreter从上个世纪中叶开始的开创性研究表明，进入单细胞悬浮液的初级生殖层祖细胞具有显著的自组织成不同生殖层的能力，其相对位置类似于它们的胚胎起源4。生殖层祖细胞的这种自组织能力可以用它们的粘附性的差异来解释，尽管区别的细胞粘附性，而不是其他细胞特性，如细胞皮质张力和细胞迁移，在原肠形成过程中对生殖层形成的具体贡献仍不清楚。此外，尽管解释祖细胞分离和通过差异细胞黏附和/或皮质张力形成生殖层的模型通常假设每种祖细胞类型表现出不变和统一的性质决定其特定的形态发生潜力7-9，但在体外和体内的观察表明，在生殖层形成过程中，祖细胞在细胞命运规格和分化方面经历了高度动态的变化。在这个跨学科的研究方案中，我们将确定胚胚层细胞能够分化为外胚层、中胚层和内胚层细胞的可逆过程与祖细胞-细胞接触形成之间的关系，以驱动原肠胚形成胚层。我们将确定在原肠形成过程中祖细胞命运规范和细胞-细胞接触形成是如何相互影响的，以及它们的相互作用如何影响祖细胞分离和组织结构。为此，我们将遵循简化论的方法，首先使用最少的体外细胞

The development of multicellular organisms is driven by the intimately connected activities of patterning and morphogenesis. While patterning denotes the generation and organization of cell fates in space and time, morphogenesis encompasses the processes by which the organism takes shape. Morphogenesis of multicellular organisms typically involves organized changes in the number, size, shape and position of their constituent cells. There is mounting evidence that cell fate specification determines the morphogenetic potential of cells, and conversely, that cell morphogenesis influences cell fate specification and maintenance1. However, the dynamic interplay between cell fate specification and morphogenesis within a developing organism has only begun to be elucidated. The first major morphogenetic process in the development of most multicellular organisms is gastrulation. In gastrulation, progenitor cells segregate into distinct population of cells that give rise to the different germ layers ¿ ectoderm, mesoderm and endoderm2. The formation and spatial organization of germ layers have been most extensively studied in amphibian embryos and involve a complex interplay between cell migration and intercalation3. Seminal studies by Townes and Holtfreter from the middle of the last century have demonstrated that primary germ layer progenitor cells taken into single cell suspensions have the remarkable capacity to self-organize into distinct germ layers, the relative position of which resembles their embryonic origin4. This self-organizing capacity of germ layer progenitor cells has been explained by differences in their adhesive properties5, although the specific contribution of differential cell adhesion, as opposed to other cell properties such as cell cortex tension and cell migration, to germ layer formation during gastrulation remains unclear6. Moreover, while models that explain progenitor cell segregation and germ layer formation by differential cell adhesion and/or cortex tension commonly assume that each progenitor cell type displays invariant and uniform properties determining its specific morphogenetic potential7-9, observations made in vitro and in vivo suggest that progenitor cells undergo highly dynamic changes in cell fate specification and differentiation during germ layer formation10-12. In this interdisciplinary research proposal, we will determine the relationship between progenitor cell fate specification ¿ the reversible process by which blastoderm cells become capable of differentiating into ectoderm, mesoderm and endoderm cells ¿ and progenitor cell-cell contact formation in driving germ layer formation during gastrulation. We will determine how progenitor cell fate specification and cell-cell contact formation influence each other during gastrulation, and how their interaction influences progenitor cell segregation and tissue organization. To this end, we will follow a reductionist approach and first use minimal in vitro cel