Fig. 3. Differentiation of the larval ostiae. Dissected third instar larval cardiac tube stained with anti-DMef2 (A,B) or with mCD8GFP (C,D) whose expression is driven in the myogenic lineage by 24B-GAL4. (A,B) The large nuclei labeled by anti-DMef2 are tin-expressing cells nuclei whereas the small nuclei are svp-expressing cell nuclei (Molina and Cripps, 2001). In the aorta, these latter cells (arrows) form a butterfly-like structure in which the striated muscle fibers from each cell converge in a central position (red arrowhead). No sign of opening is apparent in the aorta while, in the heart, the cardiomyocytes with small nuclei form genuine ostiae (large open arrow). (C,D) mCD8GFP labels the membranes of all the cardiomyocytes. The two cells with small nuclei (arrows) in the aorta are flanked on either side by two cells with larger nuclei (arrowheads) and they do not form a functional ostia. Genuine opening is visible in the heart between the first pair of ostiae cells (long open arrow). The aorta is separated from the heart by a non muscular cardiovascular valve (cvv). The double headed arrow points to the very thin cytoplasm in the heart. lu, lumen.