Fig. 1

Expression of six1a in the fast muscle precursors initiates before myog expression. Dorsal view of flat mount embryos after WISH for myod at 6 s (A), for myog at 6 s (B), 7 s (C), 15 s (M), for six1a at 15 s (H) and after two-color WISH for six1a (purple) and mespa (red) at 6 s (D) and 9 s (E), and for myog (purple) and mespa (red) at 9 s (F). In all Figures the developmental stage was confirmed by counting somites using DIC (Nomarski) optics. Marking of somites relatively to mespa bands in all Figures was done according to Durbin et al., (2000) unless otherwise stated. Insets in D–F, H, and M show the posterior part of the embryo. Cryostat sections of 10 s embryo after WISH for six1a at the anterior, middle and posterior trunk levels, correspondingly (G, G′, and G″). Lateral view of flat mount embryos after WISH for six1a (I–L, and L′) and myog (N, O, and O′). Whole embryo at 18 s and 20 h (I, J) and posterior body at 22 h and 24 h (K, L). Regions marked by brackets in panels L and O are shown at higher magnification in panels L′ and O′. Bracket in panel J marks weak expression in the tail. Anterior is to the left unless otherwise stated. Dashed lines indicate somite boundaries. Black arrowheads in panels A, C, F indicate expression in fast muscle precursors. Asterisks in panel H mark enhanced six1a expression in the dorsal and rostral part of anterior somites. Note the thin band of six1a expression adjacent to the major mespa band (white arrowhead in panels D and E). Abbreviations: ac, adaxial cells; m, myotome; nc, notochord; nt, neural tube; ov, otic vesicle; pll, posterior lateral line ganglion; psm, presomitic mesoderm. Scale bar, 100 μm.