The ultrastructure of Drosophila heart cells.

The functionality of the Drosophila heart or dorsal vessel is achieved by contributions from several tissues. The heart tube itself is composed of different types of cardiomyocytes that form an anterior aorta and a posterior heart chamber, inflow tracts and intracardiac valves. Herein we present an in-depth ultrastructural analysis of all cell types present in the Drosophila heart at different developmental stages. We demonstrate that the lumen-forming cardiomyocytes reveal a complex subcellular architecture that changes during development. We show that ostial cells, for which it was previously shown that they are specified during embryogenesis, start to differentiate at the end of embryogenesis displaying opening structures that allow inflow of hemolymph. Furthermore we found, that intracardiac valve cells differentiate during larval development and become enlarged during the 3. instar larval stages by the formation of cellular cytoplasmic free cavities. Moreover we were able to demonstrate, that the alary muscles are not directly connected to the heart tube but by extracellular matrix fibers at any stage of development. Our present work will provide a reference for future investigations on normal heart development and for analyses of mutant phenotypes that are caused by defects on the subcellular level.