Analysis of Two Aspects of Left-Right Patterning of the Vertebrate Heart: Heart Tube Position and He

Although the phenotypes of situs inversus totalis, isolated malposition of the heart (e.g., isolated dextrocardia), and heterotaxy syndrome have been familiar to physicians for decades Fig. 1 ), the question of how left-right positional information within an embryo is assigned at a molecular level has only become the subject of intense scrutiny in the last decade. In order to fully examine the implications of recently reported experiments, we must first understand which of two aspects of left-right asymmetry they address.   Fig. 1.  Varieties of visceral organ lateralization. Top left: normal. The point of the heart (cardiac ventricular apex) lies in the left hemithorax; the heart’s base-to-apex direction is denoted by the straight bold line. The liver is right-sided. Modified from ref. 15 . Top right: situs inversus totalis. Heart (arrows), liver, and stomach sidedness are all mirror image of normal. The vertical bar denotes the midline. Courtesy of P. Weinberg. Bottom left: isolated dextrocardia. The heart’s base-to-apex orientation (straight bold line) is to the right; however, all other organs (including the liver and gut) are normally positioned. Bottom right: heterotaxy syndrome. The sidedness of each visceral organ, including the heart (arrows) and the liver, is unpredictable with respect to the midline (vertical bar) and with respect to other organs. Courtesy of P. Weinberg.