Formation and function of intracardiac valve cells in the Drosophila heart.

Drosophila harbours a simple tubular heart that ensures haemolymph circulation within the body. The heart is built by a few different cell types, including cardiomyocytes that define the luminal heart channel and ostia cells that constitute openings in the heart wall allowing haemolymph to enter the heart chamber. Regulation of flow directionality within a tube, such as blood flow in arteries or insect haemolymph within the heart lumen, requires a dedicated gate, valve or flap-like structure that prevents backflow of fluids. In the Drosophila heart, intracardiac valves provide this directionality of haemolymph streaming, with one valve being present in larvae and three valves in the adult fly. Each valve is built by two specialised cardiomyocytes that exhibit a unique histology. We found that the capacity to open and close the heart lumen relies on a unique myofibrillar setting as well as on the presence of large membranous vesicles. These vesicles are of endocytic origin and probably represent unique organelles of valve cells. Moreover, we characterised the working mode of the cells in real time. Valve cells exhibit a highly flexible shape and, during each heartbeat, oscillating shape changes result in closing and opening of the heart channel. Finally, we identified a set of novel valve cell markers useful for future in-depth analyses of cell differentiation in wild-type and mutant animals.

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.