Epiboly

Epiboly describes one of the five major types of cell movements that occur in the gastrulation stage of embryonic development of some organisms.

While human embryos do not experience epiboly, this movement can be studied in sea urchins, tunicates, amphibians, and most commonly zebrafish.

Epiboly in zebrafish is the first coordinated cell movement, beginning at the dome stage late in the blastula period and continuing throughout gastrulation.

[8] 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.

Ring-like structures of filamentous actin have been observed at the leading edge of the enveloping layer, where it contacts the yolk cell.

[10] 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".

[8] Claudin E is a molecule found in tight junctions that appears to be expressed in the EVL and required for normal zebrafish epiboly, supporting this hypothesis.