Fig. 5. Ectopic ostium formation occurs upon induction of ectopic heart. (A,B,E) Anti-MHC staining alone. (C,D,F) Anti-Tin staining in black, anti-MHC staining in brown. (A) In wild type, anti-MHC staining revealed the fine structure of the embryonic heart. At each segmental boundary, the heart was broader, and this structure was coincident with two pairs of cells with a unique morphology (bracketed). (B) In the aorta, dorsal vessel shape was uniform. (C) In wild type, the cells forming the broader region of the heart did not express tin and were therefore svp-expressing cells (S, bracketed), which formed the ostia. Expression of tin is indicated by T (arrows). (D) Although svp and tin expression were mutually exclusive in the aorta, svp-expressing cells were not distinguishable based upon the morphology of the vessel. (E) In twi-gal4/+; UAS-abd-A/+ embryos, variations in dorsal vessel diameter similar to that found in the heart were seen in anterior locations (indicated by asterisks). Occasionally, tiny perforations in the heart wall were observed (os, arrowhead), consistent with the interpretation that ectopic ostia were being formed. (F) These ectopic ostia form from the svp-expressing population of cells, as the widening of the dorsal vessel occurred at locations where Tin was absent (asterisks). Segment boundaries are indicated in E and F to demonstrate that the broadening of the heart occurred at ectopic locations. All panels are dorsal views of stage 16 embryos oriented with anterior towards the left. Scale bar: 20 µm.

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