Figure 1. FC and FCM arrangements and fusion profile of individual muscles.

(A–C) Stage 12 (A), 13 (B) and 14 (C) rp298-lacZ embryos were stained with antibodies against ß-gal to label FC/myotube nuclei (green) and Lmd to label FCMs (blue). Three-dimensional renderings of single mesodermal hemisegments at stage 12 (A, 1 grid unit = 5.7 µm), 13 (B, 1 grid unit = 10.9 µm) and 14 (C, 1 grid unit = 14.1 µm) are shown. Each panel shows an external view (left) and a side view rotated 90° clockwise (right). Red arrows point to dorsal, green arrows point to anterior and blue arrows point to external. SM stands for somatic mesoderm and VM stands for visceral mesoderm. (A) At stage 12 the somatic mesoderm folds into two layers so that the FCs (green) are concurrently the most external and internal cells (yellow arrows, A), with the FCMs (blue) in between. (B) At stage 13, the internal FCs and FCMs have moved externally to underlay the overlaying epidermis (not labeled). The FCs (green) appear to rest on top of the FCMs (blue) at this stage and the cells are tightly packed together. (C) By stage 14, the number of rp298-lacZ expressing nuclei (green) have increased due to fusion. The FCMs (blue) have separated from one another. (D) Wild type stage 12–15 embryos were stained with antibodies against Eve (DA1), Runt (DO2) or Slouch (DT1, VT1 and VA2) in combination with phalloidin to assist accurate staging. The number of nuclei for each muscle and stage were counted in 50 hemisegments (A2-4). Graph showing the percentage of fusion events that occur during each stage for each muscle during the course of fusion (7.5–13 hours AEL). The mean number of nuclei observed for each muscle at stage 15 is 100% and a single nucleus is 0%. (E) Schematic showing which muscles were analyzed for wild type fusion profiles (blue). Adapted from (7).