Understanding early cardiac morphogenesis, especially the process of cardiac looping, is of fundamental interest for diverse biomedical disciplines. During the past few years, remarkable progress has been made in identifying molecular signaling cascades involved in the control of cardiac looping. Given the rapid accumulation of new data on genetic, molecular, and cellular aspects of early cardiac morphogenesis, and given the widespread interest in cardiac looping, it seems worth reviewing those aspects of the looping process that have received less attention during the past few years. These are terminological problems, the "gross" morphological aspects, and the biomechanical concepts of cardiac looping. With respect to terminology, emphasis is given to the unperceived fact that different viewpoints exist as to which part of the normal sequence of morphogenetic events should be called cardiac looping. In a short-term version, which is preferred by developmental biologists, cardiac looping is also called dextral- or rightward-looping. Dextral-looping comprises only those morphogenetic events leading to the transformation of the originally straight heart tube into a c-shaped loop, whose convexity is normally directed toward the right of the body. Cardioembryologists, however, regard cardiac looping merely as a long-term process that may continue until the subdivisions of the heart tube and vessel primordia have approximately reached their definitive topographical relationship to each other. Among cardioembryologists, therefore, three other definitions are used. Taking into account the existence of four different definitions of the term cardiac looping will prevent some confusion in communications on early cardiac morphogenesis. With respect to the gross morphological aspects, emphasis is given to the following points. First, the straight heart tube does not consist of all future regions of the mature heart but only of the primordia of the apical trabeculated regions of the future right and left ventricles, and possibly a part of the primitive conus (outflow tract). The remaining part of the primitive conus and the primordia of the great arteries (truncus arteriosus), the inflow of both ventricles, the primitive atria, and the sinus venosus only appear during looping at the arterial (truncus arteriosus) and venous pole (other primordia). Second, dextral-looping is not simply a bending of the straight heart tube toward the right of the body, as it has frequently been misinterpreted. It results from three different morphogenetic events: (a) bending of the primitive ventricular region of the straight heart tube toward its original ventral side; (b) rotation or torsion of the bending ventricular region around a craniocaudal axis to the right of the body, so that the original ventral side of the heart tube finally forms the right convex curvature and the original dorsal side forms the left concave curvature of the c-shaped heart loop; (c) displacement of the primitive conus to the right of the body by kinking with respect to the arterial pole. Third, dextral-looping does not bring the subdivisions of the heart tube and vessel primordia approximately into their definitive topographical relationship to each other. This is achieved by the morphogenetic events following dextral-looping. This review seeks to bring together data from the diverse disciplines working on the developing heart. Anat Rec 259:248-262, 2000. (C) 2000 WiIey-Liss, Inc.
