MECHANICS OF CARDIAC LOOPING

During the early stages of embryonic development, the heart is a smoot h-walled, muscle-wrapped tube that bends and rotates in a vital, but p oorly understood, morphogenetic process called looping. Since looping involves biomechanical forces, this paper examines two mechanically ba sed hypotheses for the bending component of cardiac looping. The first hypothesis is that an initial tension in or near the dorsal mesocardi um (DM), a longitudinal structure along the outside of the ventricle, drives the deformation. To relieve the bending stresses in the tube, t he myocytes change shape passively, and then they deform actively to c ontinue the process to completion of a full loop. In the second hypoth esis, contraction of circumferentially arranged actin macrofilaments p roduces circumferential compression and longitudinal expansion (due to incompressibility) of the myocytes. The DM locally constrains the lon gitudinal deformation, forcing the tube to bend. The feasibility of th ese hypotheses was evaluated using theoretical models and published ex perimental results. The models, which consist of beams composed of two layers representing the DM and the ventricular myocardium, show that the hypotheses are consistent with most of the known data, but further studies are necessary. In this regard, the models provide a conceptua l framework for designing experiments to investigate the mechanics of looping. (C) 1995 Wiley-Liss, Inc.