Fig. 5

Figure 5. Slow muscle position and mymk expression correlate with fusion

(A) Fluorescent in situ hybridization (FISH) showing mymk expression (magenta) in a 22-somite stage embryo expressing prdm1a::GFP, co-stained with DAPI for labeling nuclei (cyan) and GFP (yellow, stained for anti-GFP) to mark slow fiber population. Scale bars, 20 μm.

(B) FISH expression profile of mymk along the ML axis in different somite stages from 3 embryos, normalized by maximum mymk FISH intensity for each embryo and average DAPI intensity at each z stack. Position 0 μm corresponds to the notochord/myotome boundary.

(C) mymk expression domain (gray region, with mean ± SD shown by hexagons and error bars, respectively) and localization of fusion events (black dots). Correlation between the FISH data and the fusion events obtained by live imaging is described in STAR Methods. Distance as in (B).

(D) (Di) Scheme showing the definition of fast myocyte fusion timing. (Dii) Position of slow and fast myocytes during fusion in DV-AP axes immediately after somite segmentation (t = 0 min) and at the end of imaging (t = 891 min). (Diii) Based on (Dii), the tracks of slow and fast myocytes from somite segmentation to fusion in the AP axis. (Div)–(Dvii) as (Dii) and (Diii) but in ML-AP axes (Div and Dv) and ML-DV axes (Dvi and Dvii). Position 0 μm in each axis corresponds to the segment center, determined by average cell position.

(E) Distance between each fast myocyte to the nearest slow muscle fiber, where t = 0 is determined by fusion time of each cell. 356 fast myocytes and 118 slow myocytes from 9 myotome segments from 3 embryos. Single-cell tracks shown in gray.

(F) Close-up images of fast myocyte fusion. Fusion event highlighted by arrow. Slow myocyte (expressing prdm1a::GFP) outlined by white dashed line.

(G) Example tracks in DV-ML plane, showing two fusing cells and path of nearest slow fiber.