CELLULAR INTERACTIONS DURING HEART MORPHOGENESIS IN THE DROSOPHILA EMBRYO

The formation of the dorsal vessel or heart in a Drosophila melanogast er embryo can be divided into three main steps: i) the determination s tep allows individualization of heart precursor cells from the dorsal mesoderm. They are arranged in clusters of seven to nine cells, locate d in each of the eleven segments of the trunk. Preliminary observation s suggest that the gene Notch could participate in the choice of fate that the cardioblasts and the pericardial cells will adopt within the cardiogenic region. In the same line, a new gene, whose expression, as revealed by a P-lacZ insertion, is initiated at gastrulation in the d eveloping mesoderm and becomes restricted within the mesoderm to the m yogenic lineages, could participate in the determination of the cardio blasts identity; ii) once the cardioblasts have separated from the dor sal mesoderm, they reorganize to form an epithelial monolayer. The gen e coding for the alpha-subunit of the transduction protein Go, which i s expressed in the cardioblasts shortly before this step, could be inv olved in this process. Indeed, mutants in the Goer: gene are affected in the formation of the cardiac endothelium; and iii) the last step co nsists of the migration of the cardiac epithelium towards the dorsal m idline of the embryo to form the dorsal vessel by apposition of the tw o layers of cardioblasts. We show that an extracellular matrix compone nt is specifically expressed at the surface of the dorsal vessel and c ould participate in the interaction between the dorsalmost ectodermal cells and the heart during this migration step.