ORIGIN OF HEART ASYMMETRY - RIGHT AND LEFT CONTRIBUTIONS TO EARLY CHICK EMBRYO HEART

The contributions of right and left precardiac mesoderm to the hearts of chick embryos at stages 9+ to 12+ have been determined. Embryos cultured by the New technique were labeled on one side with tritiated thymidine at stages 5-7. The label was applied by means of small pieces of Millipore filter. Twenty-five embryos were studied by serial sections, autoradiography, and graphical reconstructions; 27 embryos were studied by dissection of the epimyocardium, which was transferred to microscopical slides and prepared as squash preparations. These were autoradiographed, and the number of cells originating from each side was determined. At stage 10-, significantly more cells have been contributed to the epimyocardium from the right side than from the left side. This relation is maintained in the cephalic part of the bulboventricular loop at later stages. In the caudal part of the bulboventricular loop, the relation is reversed, with significantly more cells originating from the left side. The asymmetry in cell numbers may be the expression of a primary difference between the developmental potencies of right and left heart primordia through which the laterality of the heart loop formation is determined. The distribution of endocardial labeled cells was found to coincide largely with the distribution of epimyocardial labeled cells. © 1969.