EMBRYONIC ORIGIN AND DIFFERENTIATION OF THE DROSOPHILA HEART

We have followed the normal development of the different cell types as sociated with the Drosophila dorsal vessel, i.e. cardioblasts, pericar dial cells, alary muscles, lymph gland and ring gland, by using severa l tissue-specific markers and transmission electron microscopy. Precur sors of pericardial cells and cardioblasts split as two longitudinal r ows of cells from the lateral mesoderm of segments T2-A7 (''cardiogeni c region'') during stage 12. The lymph gland and dorsal part of the ri ng gland (corpus allatum) originate from clusters of lateral mesoderma l cells located in T3 and T1/dorsal ridge, respectively. Cardioblast p recursors are strictly segmentally organized; each of T2-A6 gives rise to six cardioblasts. While moving dorsally during the stages leading up to dorsal closure, cardioblast precursors become flattened, polariz ed cells aligned in a regular longitudinal row. At dorsal closure, the leading edges of the cardioblast precursors meet their contralateral counterparts. The lumen of the dorsal vessel is formed when the traili ng edges of the cardioblast precursors of either side bend around and contact each other. The amnioserosa invaginates during dorsal closure and is transiently attached to the cardioblasts; however, it does not contribute to the cells associated with the dorsal vessel and degenera tes during late embryogenesis. We describe ultrastructural characteris tics of cardioblast differentiation and discuss similarities between c ardioblast development and capillary differentiation in vertebrates.