The Morphogenesis of Corrected Transposition and Other Anomalies of Cardiac Polarity

ACRUCIAL event during the development of the heart is the coiling of the primitive heart tube. It is this coil that enables the heart to convert from a simple tube with its chambers in tandem into the adult form with two pairs of chambers in parallel. Indeed, topologically there is no other way in which a tube can by simply extending its walls form four interconnecting chambers except by first falling into a coil. It may be a naive view of evolution, but what a remarkable event it was when the heart first increased in length faster than did its enclosing space and, with this seemingly extravagant growth, put itself into the geometric form that made possible much more complicated types of development, including the four-chambered heart encountered in man. The complexity enabled by coiling is diagrammed in figure 1. For an elongating cylinder to form a coil, there must be an eccentric distribution of different growth rates within the tube. It is the thesis of the present report that disturbances in the distribution of this difference in growth rate may account for a special group of cardiac anomalies. These are the anomalies in which the components of the heart have abnormal spatial relationships with each other or with body axes but may themselves be normally or nearly normally developed. They are disturbances in cardiac organization as contrasted with disturbances of cardiac differentiation, and go by many different names: corrected transposition, ventricular inversion, various levocardias and dextrocardias, certain types of single ventricle, etc.