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WIREs Dev Biol
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Internalizing the vegetal cell mass before and during amphibian gastrulation: vegetal rotation and related movements

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Abstract The movement of the prospective mesoderm and endoderm to the interior of the amphibian embryo starts in the vegetal cell mass well before the onset of overt gastrulation. By an animally directed movement of cells, the vegetal mass constricts its outer part and expands its inner region including the blastocoel floor, in a process of pregastrulation emboly. Further internalization of the vegetal region has been studied in the Xenopus embryo. At the onset of gastrulation, vegetal rotation sets in at the periphery of the vegetal cell mass, first dorsally and then spreading laterally and ventrally. It consists of an intense inward surging of cells due to active cell rearrangements that can be observed in explants of the vegetal cell mass. In its course, the blastocoel floor expands further and becomes apposed to the blastocoel roof. The boundary between apposed floor and roof forms Brachet's cleft. Another effect of vegetal rotation is the downward and inward movement of the mesodermal marginal zone, constituting the first phase of involution. Together, the upward and outward movement of the peripheral vegetal mass and the downward and inward translocation of the marginal zone lead to an apparent rotation of the whole peripheral region of the gastrula. Vegetal rotation continues to contribute to endoderm internalization to near the end of gastrulation. WIREs Dev Biol 2012, 1:301–306. doi: 10.1002/wdev.26 This article is categorized under: Early Embryonic Development > Gastrulation and Neurulation

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Regions of the early (stage 10+) Xenopus gastrula. (a) Overview. Mesodermal and endodermal regions are color coded; green and blue, peripheral and central part of vegetal cell mass. BCR, blastocoel roof; BC, blastocoel cavity; BPL, blastopore lip. Brachet's cleft between arrowheads. Animal‐vegetal and dorso‐ventral axes indicated (bottom left). (b) Higher magnification of dorsal peripheral vegetal cell mass. Boundary to prechordal mesoderm (PCM) indicated by dashed line. Arrowheads, cellular protrusions.

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Major cell movements before and during Xenopus gastrulation. (a–e) Late blastula to mid‐gastrula stages. Movements of regions indicated by arrows (blue, ectoderm; black, mesoderm; red, vegetal cell mass). Red‐dashed arrows, pregastrulation emboly; red‐bold arrows, vegetal rotation; thin red arrows and double arrows: blastocoel floor spreading as a consequence of pregastrulation emboly and vegetal rotation. (f) Slice from the vegetal half of the embryo explanted at stage 10 (top) and after execution of vegetal rotation in vitro (corresponding to embryonic stage 10.5; bottom).

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