Fig. 1

The chevron architecture of the future myotome emerges early after segmentation from the presomitic mesoderm (PSM). (A) Sketch of a 21-somite zebrafish embryo. Transverse plane to AP axis is shown in red. Two somites, at stages S1 and S3, are highlighted. (B) Sketch of S3-stage somite (t≈90 min postsegmentation) in the transverse plane. Dark blue (red) cells are future slow (fast) muscle cells, respectively. The dark and light blue planes represent the cross-sectional views shown in C and D. The notochord is at the center, with the neural tube (light blue) located more dorsally and ventral tissues below (yellow). (C and C′) Cartoon of somite shape after segmentation: (C) plane lying z=8μm from notochord and underlying tissues and (C′) plane crossing the notochord, neural tube, and ventral tissues. Inset shows shape of somites superimposed on underlying tissues. (D and D′) Confocal images of embryos expressing Lyn-td-Tomato and superimposed contours of (D) somites and PSM (red lines) and (D′) neural tube, notochord, and ventral tissues (blue, green, and yellow lines, respectively) at t=0,100,300 min postsegmentation from PSM for midbody somites (i.e., around the red plane in Fig. 1A). (E) The 3D evolution of somite shape after segmentation from PSM of a representative wild-type embryo. (F) Cross-sectional area and solidity (ratio of somite area to area of its convex hull) of segmented somites for the most medial layer of future fast muscle fibers (as in D) as a function of time after segmentation. Shaded region represents ±1 SD. Average is performed over 11 somites from 6 embryos.