Epiboly describes one of the five major types of cell movements that occur in the gastrulation stage of embryonic development of some organisms.
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.
Molecular mechanisms of epiboly
Cytoskeletal and cell adhesion components
Completion of epiboly requires the coordination of cytoskeletal changes across the embryo. The YSL appears to play a prominent role in this process. Studies on
fundulus demonstrated that the YSL is capable of undergoing epiboly even when the blastoderm has been removed, however, the blastoderm cannot undergo epiboly in the absence of the YSL.
In zebrafish, there is a
microtubule array in the yolk that extends from the animal to the vegetal pole of the embryo, and that contracts as epiboly progresses.
Treating embryos with the microtubule depolymerizing agent
nocodazole completely blocks epiboly of the YSL and partially blocks epiboly of the blastoderm, while treating with the microtubule stabilizing agent
taxol blocks epiboly of all cell layers.
There is also evidence for the importance of
actin-based structures in epiboly. Ring-like structures of filamentous actin have been observed at the leading edge of the enveloping layer, where it contacts the yolk cell.
It is thought that a network of filamentous actin in the yolk might constrict in a
myosin-II dependent manner to close the blastopore at the end of epiboly, via a "purse-string mechanism".
Treating embryos with the actin destabilizer
cytochalasin b results in delayed or arrested epiboly.
There is still debate on the extent to which the DEL and EVL epibolic movements are active movements.[A. Bruce and R. Winklbauer 03-P005 Zebra fish epiboly as a model of vertebrate cell rearrangement, Mechanisms of Development 126 (2009)] The EVL contacts the YSL by means of tight junctions. It is thought that these contacts allow the YSL to "tow" the EVL towards the vegetal pole.
The cell-cell adhesion molecule E-cadherin has been shown to be required for the radial intercalation of the deep cells.
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.
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