Cardiac Looping and the Left-Right Axis: Integrating Morphologic, Molecular, and Genetic Analyses of Vertebrate Left-Right Asymmetry

Abstract

Comparative vertebrate anatomy reveals a strikingly diverse complexity of heart structures, ranging from the more familiar four-chambered hearts of mammals and avians to the three-chambered forms of reptiles, amphibians, and fish. Yet, despite variations in anatomic form, the functionality of all vertebrate hearts is the same: to support and maintain separation of pulmonary and systemic blood flow. To this end, one of the highly conserved morphologic features of vertebrate embryonic heart development is the process of cardiac looping. Simply defined, cardiac looping is the rightward bending and rotation of primary heart tube that occurs over several developmental stages and that culminates in the contact of the venous and arterial poles of the heart. The significant outcome of cardiac looping is that the initially adjacent regions of the heart tube become repositioned with respect to one another and with the great vessels such that "remodeling" of the tube into a multichambered organ with distinct inlet and outlet components can occur.