Fig. 2. Differentiation of the cardiac tube during embryogenesis. Dorsal confocal images of the embryonic cardiac tube double labeled with (A-G) anti-Prc (red) and anti--Spectrin (green); (H-N) anti-Prc (red) and anti-DMef2 (green); (O) anti-- Spectrin (green) and anti-ß-gal in svp^AE127P-line (red); (P) anti-Tin (green) and anti-ß-gal in svp^AE127P-line (red). (A,H) Whole cardiac tube; (B,D,F,I,K,M) part of the aorta; and (C,E,G,J,L,N) part of the heart. (A,H) At late stage 16, the cardiomyocytes in the cardiac tube display signs of morphological heterogeneity. The cells in the aorta (open arrowheads) and in the heart (arrowheads) with morphological features that distinguish them from the other cardioblasts situated between them are the cells that express svp (O,P). The other cardioblasts are the tin-expressing cells (P). Their nuclei in (H) have an egg-like shape as compared to the nuclei of the other cardioblasts. These morphological features are more pronounced in the heart than in the aorta. Note the larger size of the heart cardioblasts compared to the aorta cardioblasts. (B,C,I,J) At the onset of dorsal closure (stage 13), all cardioblasts along the whole length of the cardiac tube appear similar (arrows). The svp-expressing cardioblasts, however, which will differentiate into the second ostia in the heart, have initiated a slight change in their shape (arrowheads). Only one row of cardioblasts is shown. (D,E,K,L) At late stage 14, the cardiac tube is not yet closed but the cardioblasts in the heart are already larger than in the aorta and the svp- expressing cells (arrowheads) have acquired their distinctive morphology. (F,G,M,N) At stage 16, the difference in size between heart and aorta cardiomyocytes is accentuated. The specific morphology of the svp-expressing cells is clearly visible in the aorta (open arrowheads) as well as in the heart (arrowheads). The asterisks in A,G,H,J,L indicate the alary muscles. In all views, anterior is to the left.

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