Further Development 11.3: A Fountain of Vegetal Movement

Amphibians and Fish

How is it that endodermal cells of the vegetal hemisphere exhibit a blastocoel-directed migration, such that a tissue-level rotation is generated? The mechanisms of vegetal rotation are only starting to be elucidated. Wen and Winklbauer (2017) recently employed a live explant and embryo analysis of cell behaviors exhibited in the vegetal hemisphere during Xenopus gastrulation. They detailed a regionally diverse series of cell rearrangements and migratory velocities over the course of a cell’s rotating displacement across the vegetal hemisphere (Figure 1). Endodermal cells appear to use an amoeboid-like behavior of cell migration, during which a wide leading edge forms new junctions with adjacent cells while a narrowing and retracting trailing edge advances the cell forward. In the most vegetal regions, this migration resembles true ingression behaviors as a cell crawls from one cell to the next toward the blastocoel (see Figure 1B). Interestingly, as the cell nears the blastocoel floor, its velocity progressively increases until the cell undergoes a reorientation of its leading and trailing edges toward the dorsal and ventral sides of the explant tissue (creating the fountain-like behavior). Similarly, in the embryo, cells will also reorient toward both the ventral and dorsal sides. Although several factors, among them C-cadherin, fibronectin, and ephrin B1, have all been suggested as being necessary for these migratory events, more information is needed about the guidance and locomotor mechanisms before they are well understood.

**Figure 1** Differential cell migration drives vegetal rotation in *Xenopus* gastrulation. Cell migration patterns were followed in vegetal hemisphere explants. (A) Schematic of the changes in cell behavior observed over a 60-minute period. The reduction in the length of the purple-and-white bar signifies the displacement of cells in the vegetal cell mass (white portion of bar) over time. A combination of cells disappearing from (brown) and emerging at (yellow) the surface is shown among dividing cells (blue) and cells that either reduced (red) or enlarged (green) their surface area. White arrows represent the overall “fountain-like” pathway of cell migration by vegetal cells. (B,C) Different types of migratory patterns are exhibited in different regions of the vegetal explant (boxed areas in A represent areas for representative data in B and C). Cells begin by ingressing through the crowd of cells moving from vegetal to progressively more animal locations (B, blue arrows). Once at the blastocoel roof, cells undergo shape rearrangements and turn to move laterally (C, white arrows). (A after J. W. Wen and R. Winklbauer. 2017. *eLife* 6: e27190/CC BY 4.0.)

Literature Cited

Wen, J. W. and R. Winklbauer. 2017. Ingression-type cell migration drives vegetal endoderm internalisation in the Xenopus gastrula. Elife 6.

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