Cellular interactions during heart morphogenesis in the Drosophila embryo.

The formation of the dorsal vessel or heart in a Drosophila melanogaster embryo can be divided into three main steps: i) the determination step allows individualization of heart precursor cells from the dorsal mesoderm. They are arranged in clusters of seven to nine cells, located in each of the eleven segments of the trunk. Preliminary observations 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 developing mesoderm and becomes restricted within the mesoderm to the myogenic lineages, could participate in the determination of the cardioblasts identity; ii) once the cardioblasts have separated from the dorsal mesoderm, they reorganize to form an epithelial monolayer. The gene coding for the alpha-subunit of the transduction protein Go, which is expressed in the cardioblasts shortly before this step, could be involved in this process. Indeed, mutants in the Go alpha gene are affected in the formation of the cardiac endothelium; and iii) the last step consists of the migration of the cardiac epithelium towards the dorsal midline of the embryo to form the dorsal vessel by apposition of the two layers of cardioblasts. We show that an extracellular matrix component is specifically expressed at the surface of the dorsal vessel and could participate in the interaction between the dorsalmost ectodermal cells and the heart during this migration step.