2/6/2024 0 Comments Blastopore closure high salt![]() Most invertebrates and anamniotes internalise the mesoderm as a continuous epithelial layer by bending the blastoderm inwards (invagination), as in sea urchins ( McClay et al., 2020) and Drosophila melanogaster ( Gheisari et al., 2020), or by rolling through a slit-shaped opening (involution) as in Xenopus ( Winklbauer, 2020). The mode of internalisation largely determines morphogenesis during gastrulation. Gastrulation is a morphogenetic programme that needs to accomplish three objectives: the generation of the main body axis, the patterning of the germ layers and the internalisation of the mesodermal and endodermal precursors. ![]() Through the spatiotemporal coordination of these cell behaviours, epithelial layers change shape, bend, flow, grow and shrink during morphogenesis ( Collinet and Lecuit, 2021), giving rise to the tissue movements that segregate the germ layers previously specified by specific gene regulatory networks (GRNs). Gastrulation arises from coordinating a set of characteristic epithelial cell behaviours: division, shape change, division, as well as cell rearrangement and/or abandoning the layer via ingression. Gastrulation is a morphogenetic programme, during which the embryo transforms from a single one-dimensional epithelial cell layer (the blastula) into an organised, three-dimensional gastrula consisting of the germ layers (ectoderm, mesoderm and endoderm in triploblastic species). However, early development typically results in the generation of an epithelial cell layer composed of pluripotent progenitors, which execute gastrulation. The early steps of development after fertilisation vary significantly throughout the animal kingdom. Using the insights obtained from these experiments we discuss the effects of the increase in yolk volume on the morphology of gastrulation and provide new insights into two crucial innovations during amniote gastrulation: the transition from a ring-shaped mesoderm domain in anamniotes to a crescent-shaped domain in amniotes, and the evolution of the reptilian blastoporal plate/canal into the avian primitive streak. Here, we review the mechanisms that underlie the plasticity of vertebrate gastrulation both when experimentally manipulated and during evolution. Recent experimental results demonstrate that it is possible to generate different alternative gastrulation modes in single organisms, such as in early cnidarian, arthropod and vertebrate embryos. Surprisingly, this fundamental and early process does not appear to be rigidly constrained by evolutionary pressures instead, the morphology of gastrulation is highly variable throughout the animal kingdom. During gastrulation, early embryos specify and reorganise the topology of their germ layers.
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