Fig. 1. The Drosophila dorsal vessel consists of a posterior heart and an anterior aorta, which can be distinguished on the basis of morphological and molecular criteria. (A,C,E,G) Low-magnification views of stage 16 embryos stained with various markers. (B,D,F,H) High-magnification views of same embryos. (A,B) MEF2 protein accumulation in the muscle cells of the embryo. The dorsal vessel consisted of two parallel rows of cells at the dorsal midline. Note that the distance between the two rows of MEF2-positive nuclei is greater in the heart (Ht) compared to the aorta (Ao). The pattern of Hand transcripts (C,D) and muscle MHC protein (E,F) in the cardial cells of the dorsal vessel can also be used to distinguish heart from aorta, and to visualize the heart lumen. Numbered body segments are indicated. (G,H) Tina-1 transcripts in the dorsal vessel accumulated only in the heart. Scale bars: 50 µm for A,C,E,G; 25 µm for B,D,F,H. All pictures are dorsal views oriented with anterior towards the left. In C,E,G, the location of the heart is bracketed.