[Genetic control of cardiac tube formation in Drosophila]

In Drosophila, the heart is composed of a simple linear tube constituted of 52 pairs of myoendothelial cells which differentiate during embryogenesis to build up a functional mature organ. The cardiac tube is a contractile organ with autonomous muscular activity which functions as a hemolymph pump in an open circulatory circuit. The cardiac tube is organized in metamers which contain six pairs of cardioblasts per segment. Within each metamer the cardioblasts express a combination of genetic markers underlying their functional diversity. For example, the two most posterior cardiac cells in segments A5 to A7 differentiate into ostiae which allow the inflow of hemolymph in the tube. An additional axial information along the anteroposterior axis orchestrates the subdivision of the cardiac tube into an "aorta" in the anterior region and a "heart" in the posterior region which behave as distinct functional entities. The major pacemaker activity is located in the most caudal part of the heart. This analysis has being made possible by the identification and the utilization of specific morphological and genetic markers and an in vivo observation of cardiac function in the embryo. Functional organogenesis of the cardiac tube is accurately controlled by genetic programs that have been in part identified. Hox genes are responsible for the axial subdivision of the tube into functional modules. They activate, in their specific domains of expression, target genes effectors of the terminal differentiation. On the other hand, part of the information required for segmental information is provided by Hedgehog, a morphogen secreted by dorsal ectoderm, whose activity triggers the ostiae formation in the heart domain.