Epiboly

Epiboly describes one of the five major types of cell movements that occur in the gastrulation stage of embryonic development of some organisms. Epiboly is the spreading and thinning of the ectoderm while the endoderm and mesoderm layers move to the inside of the embryo. When undergoing epiboly, a monolayer of cells must undergo a physical change in shape in order to spread. Alternatively, multiple layers of cells can also undergo epiboly as the position of cells is changed or the cell layers undergo intercalation. While human embryos do not experience epiboly, this movement can be studied in sea urchins, tunicates, amphibians, and most commonly zebrafish.

Epiboly describes one of the five major types of cell movements that occur in the gastrulation stage of embryonic development of some organisms. Epiboly is the spreading and thinning of the ectoderm while the endoderm and mesoderm layers move to the inside of the embryo. When undergoing epiboly, a monolayer of cells must undergo a physical change in shape in order to spread. Alternatively, multiple layers of cells can also undergo epiboly as the position of cells is changed or the cell layers undergo intercalation. While human embryos do not experience epiboly, this movement can be studied in sea urchins, tunicates, amphibians, and most commonly zebrafish.

Zebrafish

General movements

Epiboly in zebrafish is the first coordinated cell movement, beginning at the dome stage late in the blastula period and continuing throughout gastrulation. At this point the zebrafish embryo contains three portions: an epithelial monolayer known as the enveloping layer (EVL), a yolk syncytial layer (YSL) which is a membrane-enclosed group of nuclei that lie on top of the yolk cell, and the deep cells (DEL) of the blastoderm which will eventually form the embryo's three germ layers (ectoderm, mesoderm, and endoderm). The EVL, YSL, and DEL all undergo epiboly. Radial intercalation occurs in the DEL. Interior cells of the blastoderm move towards the outer cells, thus "intercalating" with each other. The blastoderm begins to thin as it spreads toward the vegetal pole of the embryo until it has completely engulfed the yolk cell. The EVL also moves vegetally during epiboly, increasing its surface area as it spreads. Work in the ray-finned fish fundulus has shown that no large rearrangements occur in the EVL; instead, cells at the leading edge of the EVL align and constrict. The YSL also moves towards the vegetal pole, spreading along the surface of the yolk and migrating slightly ahead of the blastomeres. Once epiboly is complete, the DEL, EVL, and YSL have engulfed the yolk cell, forming a closure known as the blastopore.

Molecular mechanisms of epiboly

Signaling

The molecule fibronectin has been found to play a role in radial intercalation. Other signaling pathways that appear to function in epiboly include the Wnt/PCP pathway, PDGF-PI3K pathway, Eph-Ephrin signaling, JAK-STAT signaling, and the MAP kinase cascade.

Other vertebrates

Epibolic movements have been conserved in vertebrates. Though most work on epiboly has been done in fish, there is also a body of work concerning epiboly in the African clawed frog, Xenopus laevis. Comparisons of epiboly in amniotes, teleosts and X. laevis show that the key movement of epiboly in the fish and frog is radial intercalation while in amniotes it would appear to be cell division in the plane of the epithelium. All groups undergo cell shape changes such as the characteristic flattening of cells to increase surface area.

References

External links

Developmental mRNA expression pattern of the zebrafish cdx1b gene

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