Cardiac fiber orientation and the left-right asymmetry determining mechanism.

The invariant nature of body situs within and across vertebrate species implies that a highly conserved pathway controls the specification of the left-right (L/R) axis. Situs-specific morphogenesis begins at the end of this pathway and leads to normal organ arrangement, also known as situs solitus. Occasionally, individuals have a complete, mirror image reversal of this asymmetry, called situs inversus totalis (SIT). In these individuals, gross anatomy is mirror imaged. However, the helical myofiber pattern within the left ventricle (LV) wall is only partially mirror imaged: apical and superficial basal fiber orientation are as in normal persons, whereas the deeper basal layers have an inverted fiber orientation. Because of this bivalent fiber orientation pattern, LV deformation in humans with SIT is mirror imaged only near the base, but near the apex it is as in normal subjects. Apparently, the embryonic L/R controlling genetic pathway does determine situs-specific gross anatomy morphogenesis, but it is not the only factor regulating fiber architecture within the LV wall.