By combining their movies with computer simulations, Aynur Kaya and Jerome Solon in Brunner's group discovered that the actin cable doesn't act as a drawstring, but rather as a ratchet. With every force pulse of the amnioserosa cells, the actin cable contracts and stops the epithelial cells from moving back away from the gap when the amnioserosa cells relax. This ratchetlike action means epithelial cells can move in only one direction: over the gap, bringing about dorsal closure. "Essentially, you have a field of cells that creates the driving force," Damian summarises, "and then you need to translate this force into movement by adding ratchets that lock the cells into the state where they should move".
The researchers believe this mechanism could apply not only to dorsal closure and wound healing, but also to many developing tissues, since moving tissue around is central to development.
The pulsing amnioserosa cells (on the left) pull on the smaller neighboring epidermis cells. The result is a stepwise dorsal-ward (towards the left) displacement of the epidermis front. Displacement is sustained by the ratchet-like function of the actin cable that forms at the boundary between amnioserosa cells and epidermis. The movie was recorded with a standard spinning disc confocal microscope using 60x magnification.
(Photo Credit: Damian Brunner/EMBL)
This is a clipping of a Drosophila embryo undergoing dorsal closure of the epidermis. The movie was recorded with a standard spinning disc confocal microscope using 60x magnification. An image stack was recorded every minute.
(Photo Credit: Damian Brunner/EMBL)
The microscope image of the dorsal closure of a fly embryo shows alternating stripes of epithelial cells with aligned microtubule bundles (green) and epithelial cells treated with a microtubule-destroying drug (blue). Labeled in red is the protein actin that lines the border of cells, particularly the amnioserosa cells occupying the eye-shaped opening.
(Photo Credit: Damian Brunner/EMBL)