Curr Top Dev Biol 2020 Jan 01;136:243-270. doi: 10.1016/bs.ctdb.2019.09.002.

Mesoderm and endoderm internalization in the Xenopus gastrula.

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Mesoderm and endoderm internalization in the Xenopus embryo are based on a number of region-specific morphogenetic processes that co-act in the vegetal half of the gastrula. In the multilayered wall surrounding the blastocoel, the apical layer engages in bottle cell formation and associated invagination and involution movements, and in cell intercalation in the plane of the layer. Of these epithelial-type processes, only bottle cell formation has been analyzed mechanistically. In the deep layers of the blastocoel wall, cell-on-cell migration drives the internalization of mesoderm by various forms of involution and of the endodermal cell mass by vegetal rotation. In the mesoderm, cells migrate in a mesenchymal mode with the aid of locomotory protrusions, whereas cells of the vegetal cell mass resemble free bottle cells that engage in ingression-type amoeboid migration. Cells rearrange by differential migration leading to parallel or orthogonal forms of intercalation and respective types of convergent extension. The interaction of the various apical and deep layer processes gives rise to dorsal multilayer invagination, ventrolateral internal involution, peak involution and orthogonal convergent extension of the dorsal posterior mesoderm, vegetal rotation, and blastopore constriction. It is speculated how these multilayer gastrulation movements could be derived from mechanisms in invertebrate single-epithelium gastrulae.

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