Fig. 4. Distinct programs underlie the development of CPM- and SpM-derived jaw muscles in the chick embryo. (A-A''') Scheme (A) of the dye labeling experiment of proximal and distal BA1 myoblasts (A'). The mandibular adductor complex is illustrated in A'' and the intermandibular muscle is shown in A'''. Co-staining of MyoD and DiO is shown in these muscles (A'',A'''). (B-B'') Transverse section of BA1 in St. 20 embryos stained for Myf5 and Isl1 (B') and Myf5 with Pax7 (B''). (C) Schematic depicting the distinct cellular origins and molecular identities of the branchiomeric muscles. (D-E') A transverse section through the adductor (add) muscle complex at St. 26 stained for the markers indicated. Note that Isl1 shows no apparent expression in this region (D''). (F-F''') A transverse section through the intermandibular (im) muscle (F) stained for Isl1 and Myf5 (F'). Note the co-localization of Isl1 and Myf5 in the enlargement (F''). Staining for MHC and Isl1 shows the lack of MHC in the Isl1⁺ cells in the intermandibular anlagen (F'''). (G-G''') Similar sections stained for Isl1 and MyoD (G'). Note the mutually exclusive expression of Isl1 and MyoD, as shown in the enlargement (G''). Isl1 was restricted to the intermandibular muscle, and was not expressed in the genioglossal (g) muscle (arrow). A sequential section (G''') was stained for Pax7 (red) and MHC (blue). Pax7 and Isl1 were not expressed in the same muscles (arrow). (H-H''') Staining for Isl1, MyoD, MHC and Pax7 at the intermandibular region of St. 31, indicating the loss of Isl1 along with the upregulation of MHC and Pax7. Scale bars: 50 µm.