Author

Rudolf Winklbauer

Additional Author(s): Erich W. Damm

Published on SDB CoRe: Apr 2 2012

Morphogenic Movements: Gastrulation
Embryonic Patterning: Compartments/Boundaries
Morphogenesis: Cell Movements; Cell Shape Changes; Cell/Tissue Polarity
Organism: Xenopus
Stage of Development: Embryo

Object Description

This scanning electron micrograph shows an early Xenopus (frog) gastrula that has been sagittally fractured to visualize its anatomy.  (a) Prospective mesodermal and endodermal regions of the embryo are color coded, while the ectoderm in the animal portion of the embryo is not.  Brachet’s cleft (space between the two white arrowheads) separates the internalized mesoderm and the ectoderm. Animal-vegetal and dorso-ventral axes are indicated in the bottom left corner. BC=blastocoel cavity, BCR=blastocoel roof, BPL=blastopore lip. (b) Higher magnification of dorsal peripheral vegetal cell mass (green in a). The boundary to prechordal mesoderm (PCM, pink in a) is indicated by a dashed line. Cellular protrusions (indicated by yellow arrowheads) suggest active cell migration toward the animal pole. WIREs Dev Biol 2012. DOI: 10.1002/wdev.26

Gastrulation Movements

The early gastrula consists of a coherent, multilayered cell mass which surrounds the blastocoel cavity (BC). The blastocoel wall is thickest at the lower, vegetal side of the embryo and thinnest in the upper, animal part, the blastocoel roof (BCR). The basic body plan of multicellular animals comprises an outer ectodermal layer, an inner endoderm, and an intermediary mesoderm. Before gastrulation, these prospective germ layers are located in the blastocoel wall. In a sense, the essence of gastrulation is the movement of the mesoderm and endoderm to the interior of the embryo. This movement has just started on the dorsal (right) side of this early gastrula. An indentation of the surface is caused by the formation of bottle-shaped cells (dark blue) in the outermost epithelial layer of the gastrula, and it defines the blastopore lip (BPL). Interiorly, the leading edge mesendoderm (yellow) has begun to move animally. Behind it, the precordal mesoderm (pink) and the chordamesoderm (red) are in the process of moving inside. Mesoderm internalization (red) is less pronounced at the ventral (left) side. The central (blue) and peripheral (green) vegetal cell mass of prospective endoderm will move inside by the process of vegetal rotation, while at the same time the ectodermal BCR will spread vegetally to cover the whole embryo in a movement termed epiboly.

References

Winklbauer, R. and Damm, E.W. Internalizing the vegetal cell mass before and during amphibian gastrulation: vegetal rotation and related movements. WIREs Dev Biol, 2012, 1:301–306.

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