CARDIOVASCULAR DEVELOPMENT IN THE ZEBRAFISH .1. MYOCARDIAL FATE MAP AND HEART TUBE FORMATION

We have analyzed the origin of cardiac progenitors in the zebrafish em bryo by injection of single blastomeres with a lineage tracer dye, and examined the formation of the zebrafish heart tube by serial sectioni ng of immunostained embryos. At the 512-cell stage (early blastula), m ost cardiac progenitors lie in a marginal zone that extends from 90-de grees longitude (midway between the future dorsal and ventral axis) th rough 180-degrees longitude (the future ventral axis) to 270-degrees l ongitude. By focusing on myocardial progenitors located at 90-degrees (and 270-degrees) longitude, we found that a single cell injected in t he early blastula can contribute progeny to both the atrium and ventri cle. A cell injected in the midblastula contributes progeny to either the atrium or ventricle, but not both. This analysis suggests that, at least for.these myocardial progenitors, the atrial and ventricular li neages separate in the midblastula. Precardiac cells involute early du ring gastrulation and turn towards the animal pole with other early in voluting cells. These cardiogenic cells reach the embryonic axis aroun d the 8-somite stage, and there they coalesce to form a pair of myocar dial tubular primordia on either side of the midline. By the 21-somite stage, the tropomyosin-immunoreactive myocardial tubes have moved clo ser to each other, and a distinct group of cells, the endocardial prog enitor cells, sits medially between them. The myocardial tubes then fu se to enclose the endocardial cells and form the definitive heart tube . By 22 hours postfertilization (26-somite stage), the heart tube is c learly beating. The regionalization of cardiac myosin heavy chain expr ession distinguishes the cardiac chambers at this stage, although they are not morphologically delineated until 36 hours. This work shows th at cardiogenic regions can be identified in the early blastula, and th at chamber restriction seems to arise in the midblastula. Additionally , it provides the basis for embryological perturbation at the single c ell level, as well as for the genetic analysis of heart tube formation in the zebrafish.