Watch this video: https://www.youtube.com/watch?v=QXiFM2U1YFw Epiboly starts at 10 seconds
|Pixel Dimension||10 um X 10 um|
|Pattern Size||1 mm X 1 mm|
|Flow Rate||1 um/min|
Zebrafish early development involves the flow of tissue over a yolk in a process called epiboly. How biology is able to realize epiboly is fascinating.
There are two processes involved: circumferential contraction and retrograde flow-friction(1). The enveloping cell layer (EVL) is connected to the yolk syncytial layer (YSL) at its margin. Inside the YSL are a network of actomyosin rings that create a band around the circumference of the yolk. This band tightens and pulls the EVL over the yolk. Behrndt et al. proved that it is the contraction of the actomyosin ring band that creates this force by cutting the band and watching the band recoil(2). However, contraction of the actomyosin rings can only pull the EVL over the yolk after the EVL is past the equator of the yolk. Therefore, there is one more mechanism that creates a force to pull the EVL over the yolk. Actomyosin rings also flow from the bottom of the yolk towards the EVL inside the YSL. This retrograde flow creates a friction, pulling the EVL over the yolk. Behrndt et al. discovered this mechanism by visualizing epiboly of a cylindrical embryo. Because the circumferential contraction requires a sphere, only retrograde flow-friction generates sufficient force to pull the EVL over the yolk in this set up.
Biological flows driven by actomyosin contraction are also seen in embryonic chick wing bud, dorsal closure in drosophila, cytokinesis, and in a few other processes(3-7).
The flow of a tissue also requires the integrity of the tissue to be maintained. The mechanism by which tension anisotropy of the EVL is homogenized is tension-oriented cell division(8). Otherwise, Compinho et al. show that cells within the tissue would fuse.
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- J. Mertz, Optical sectioning microscopy with planar or structured illumination. Nat. Methods. 8, 811–819 (2011).
- M. S. Hutson, Y. Tokutake, M. S. Chang, J. W. Bloor, Forces for morphogenesis investigated with laser microsurgery and quantitative modeling. Science (2003).
- F. A. Barr, U. Gruneberg, Cytokinesis: Placing and Making the Final Cut. Cell. 131, 847–860 (2007).
- A. Jacinto, S. Woolner, P. Martin, Dynamic analysis of dorsal closure in Drosophila: from genetics to cell biology. Developmental Cell (2002).
- J. M. Sawyer et al., Apical constriction: A cell shape change that can drive morphogenesis. Dev. Biol. 341, 5–19 (2010).
- J. Brock, K. Midwinter, J. Lewis, P. Martin, Healing of incisional wounds in the embryonic chick wing bud: characterization of the actin purse-string and demonstration of a requirement for Rho activation. The Journal of cell biology (1996).
- P. Campinho et al., Tension-oriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly. Nature Cell Biology. 15, 1405–1414 (2013).