Myocardial cell shape change as a mechanism of embryonic heart looping

Measurements of local area changes of the embryonic myocardial surface indicate that the prospective right side expands during looping. Observations from sectioned specimens showing that the right side of the myocardium also becomes thinner during looping suggest that the increase in area may result from myocardial cell flattening. Light microscopic examination of silver impregnated hearts suggests that individual cells increase in apical surface area as the right side becomes convex. Computer-assisted analyses of scanning electron micrographs show that cells of the prospective convex side increase significantly in apical surface area during looping, although the relative alignment of their major surface axes remains unchanged and random. Myocardial cells of the concave side of the heart have a smaller apical surface area and their major surface axes are aligned circumferentially in relation to the heart. We propose that these regional changes in cell shape and alignment mediate heart looping.