A mechanism of cardiac loop formation was investigated using a mathematical model,vertex dynamics that was extended to the model applicable to three-dimensionally undulating cell sheets.The initially straight heart tube is transformed into a loop characterized by a helical winding with a handedness during heart development.I succeeded in a computer simulation of the loop formation under a few assumptions.The heart tube consists of polygonal domains of myocardial cells.The polygonal domain has its own polarity that directs the direction of domain growth.Hence,surface of the heart tube expands isotropically.In addition I found that a longitudinal area of non-growing domains is necessary for the loop formation.Next, I considered the isotropic expansion.For the loop formation, a simple isotropic expansion based on the planar cell polarity (PCP) is not enough.In addition to a reference direction by the PCP, the domain has to have a one-side handedness (left or right) with respect to the reference direction.That is, the domain necessarily has an absolute chirality based on PCPs.I would like to propose for the cardiac looping that (1) cells of the developing heart tube grow anisotropically based on the absolute chirality and (2) the tube has a non-growing longitudinal region.