The cnidarian “planula” larva shows radial symmetry around a polarized, oral-aboral, body axis and comprises two epithelia cell layers, ectodermal and endodermal. This simple body plan is set up during gastrulation, a process which proceeds by a variety of modes amongst the diverse cnidarian species. In the hydrozoan laboratory model Clytia hemisphaerica, gastrulation involves a process termed unipolar cell ingression, in which the endoderm derives from mass ingression of individual cells via a process of epithelial-mesenchymal transition (EMT) around the future oral pole of an epithelial embryo. This contrasts markedly from the gastrulation mode in the anthozoan cnidarian Nematostella vectensis, in which endoderm formation primarily relies on cell sheet invagination. To understand the cellular basis of gastrulation in Clytia we have characterized in detail successive cell morphology changes during planula formation by Scanning and Transmission Electron Microscopy combined with confocal imaging. These changes successively accompany epithelialization of the blastoderm, EMT occurring in the oral domain through the bottle cell formation and ingression, cohesive migration and intercalation of ingressed cells with mesenchymal morphology, and their epithelialization to form the endoderm. From our data, we have reconstructed the cascade of morphogenetic events leading to the formation of planula larva. We also matched the domains of cell morphology changes to the expression of selected regulatory and marker genes expressed during gastrulation. We propose that cell ingression in Clytia not only provides the endoderm, but generates internal forces that shape the embryo in the course of gastrulation. These observations help build a more complete understanding of the cellular basis of morphogenesis and of the evolutionary plasticity of cnidarian gastrulation modes.

刺胞幼虫的幼虫显示出围绕极化的口腔，体轴的径向对称性，并包括两个上皮细胞层，即外胚层和内胚层。这个简单的身体计划是在胃化过程中建立的，该过程以多种方式在各种刺胞动物中进行。在水生动物实验室模型Clytia hemisphaerica中，胃造瘘涉及一个称为单极细胞浸润的过程，其中内胚层通过单个细胞的质量浸润通过上皮胚胎未来口腔极周围的上皮-间质转化（EMT）过程获得。这与花胶线虫的线虫Nematostella vectensis的胃化模式形成鲜明对比。，其中内胚层的形成主要取决于细胞片的内陷。了解Clytia胃泌尿的细胞基础我们已经通过扫描和透射电子显微镜结合共聚焦成像详细描述了扁平形成过程中连续的细胞形态变化。这些变化相继伴随胚盘的上皮化，EMT通过瓶状细胞的形成和进入，在间质形态进入的细胞的内聚迁移和嵌入，以及它们的上皮形成内胚层在口腔区域内发生。根据我们的数据，我们重建了导致扁平幼虫形成的形态发生事件的级联。我们还匹配了细胞形态变化的域，以与在胃泌乳过程中表达的选定调控基因和标记基因的表达相匹配。我们建议细胞进入Clytia不仅提供内胚层，而且还会产生内力，从而在胃形成过程中使胚胎成形。这些观察结果有助于建立对形态发生的细胞基础和刺胞生殖方式的进化可塑性的更完整的了解。