Fig. 7. Six1 is needed for MyoD and myogenin expression in distal territories. Whole-mount in situ hybridization of Six1^–/– embryos (A,C,E,G,I) and Six1^+/– littermates (B,D,F,H,J) revealed that Six1-deficient mice fail to activate MyoD and myogenin genes in distal territories. (A-D) Hybridization with the MyoD mRNA probe shows the absence of MyoD expression in the limb buds (arrowheads) and the reduced ventrolateral extension of the dermomyotome (arrows), and the absence of MyoD expression in the epaxial most domain (double arrowheads). (E-H) Hybridization with the myogenin mRNA shows the absence of myogenin expression in the limb buds (arrowheads) and the altered organisation of the ventrolateral part of the dermomyotome (arrows). A broken line separates the epaxial and hypaxial myotome showing that myogenin expression is reduced in the epaxial most domain (double arrowheads). (I-J) At E12.5, hybridization with a MyoD mRNA probe reveals a decrease of MyoD-expressing cells at the shoulder level (arrowheads). (K-L) Detail of the Six1^–/– (left) and Six1^+/– (right) forelimbs (K) and hindlimbs (L) of E12.5 embryos showing that from this stage forelimb muscles are more affected than hindlimb muscles. (K) Dorsal view of forelimb buds shows a few MyoD-expressing cells in the Six1^–/– forelimb (arrowhead). (L) Lateral view of hindlimb buds shows a few MyoD-expressing cells restricted to the dorsal region of Six1^–/– hindlimb (arrowhead).

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