  Wounds in the embryo show a remarkable ability to heal quickly without
leaving a scar. Previous studies have found that an actomyosin ring ("purse
string") forms around the wound perimeter and contracts to close the wound over
the course of several dozens of minutes. Here, we report experiments that
reveal an even faster mechanism which remarkably closes wounds by more than 50%
within the first 30 seconds. Circular and elliptical wounds (~100 um in size)
were made in the blastoderm of early chick embryos and allowed to heal, with
wound area and shape characterized as functions of time. The closure rate
displayed a biphasic behavior, with rapid constriction lasting about a minute,
followed by a period of more gradual closure to complete healing. Fluorescent
staining suggests that both healing phases are driven by actomyosin
contraction, with relatively rapid contraction of fibers at cell borders within
a relatively thick ring of tissue (several cells wide) around the wound
followed by slower contraction of a thin supracellular actomyosin ring along
the margin, consistent with a purse string mechanism. Finite-element modeling
showed that this idea is biophysically plausible, with relatively isotropic
contraction within the thick ring giving way to tangential contraction in the
thin ring. In addition, consistent with experimental results, simulated
elliptical wounds heal with little change in aspect ratio, and decreased
membrane tension can cause these wounds to open briefly before going on to
heal. These results provide new insight into the healing mechanism in embryonic
epithelia.
