Comparison of <i>MyoR</i> and <i>MyoD</i> expression domains during chick first branchial arch development.

<p>(A) Lateral view of a HH20 chick embryo hybridized with a <i>MyoR</i> probe. HH20 (B,C), HH22 (D,E) HH24 (F,G) and HH26 (H,I) embryos were frontally sectioned at the head level. The plane of section is indicated in the panel A. Adjacent sections from each stage were hybridized with DIG-labelled antisense probes for either <i>MyoR</i> (B,D,F,H) or <i>MyoD</i> (C,E,G,I). <i>MyoR</i> delineates the myogenic core of the first branchial arch (B,D,F) and is subsequently expressed in all branchiomeric muscles (H). <i>MyoD</i> transcripts are first detected in a lateral sub-region of the <i>MyoR</i> domain at HH20 (B,C, arrows) and HH22 (D,E, arrows), then spread progressively from lateral to medial regions to overlap with the <i>MyoR</i> domain in branchiomeric muscles (E,G,I). (D–G) Arrows point to the lateral domains of the core, while arrowheads show the medial domain of the core. (H,I) Arrows indicate the branchiomeric muscles expressing both <i>MyoR</i> (H) and <i>MyoD</i> (I). (A) Arrowheads indicate the hypaxial lips at the interlimb level, expressing <i>MyoR</i>. BA1, first branchial arch, di, diencephalon; e, eye; fl, forelimb; hl, hindlimb; mb, mandibular arch; mes, mesenchephalon; MR, medial rectus; mx, maxillary arch; ph, pharynx; tel, telencephalon; VO, ventral oblique; VR, ventral rectus.