Fig. 7. Disruption of ventral and dorsal dermomyotomal lips in Six1^–/–Six4^–/– embryos at caudal (A-D), hindlimb (E-N), thoracic (O-R) and forelimb (S-X) levels. Section (10 µm) of heterozygous (A,B,E,F,I,J,O,P,S,T,W) and dKO (C,D,G,H,K,L,M,N,Q,R,U,V,X) E10.5 embryos hybridized with antibodies revealing Pax3 protein in red (A,C,E,G,M,O,Q,S,U), ß-gal protein in green (B,D,F,H,J,L,N,P,R,T,V), desmin (J,L) and laminin (I,K,W,X). Pax3 expression is similarly expressed in rostral somites of both heterozygous and dKO embryos, and is co-expressed with Six1 (compare C,D with A,B). At the hindlimb level in the dKO embryos, we checked desmin, laminin, Six1 and Pax3 expression in sacral somites facing the limb bud (G,H,K,L) and in more rostral somites facing the limb bud (M,N). Laminin and desmin expression is found in the myotomes in Six1-expressing cells on both wild-type and dKO sections (white arrows in I-L). At this hindlimb level, the myotome of dKO embryos is not disorganized. More rostrally, Pax3 expression is progressively lost ventrally in lumbar somites at the hindlimb level, while most cells are Six1 positive (compare G,H with M,N). At the thoracic level, Pax3 expression is restricted in the medial aspect of the dermomyotome of dKO embryos (Q), while Six1 gene expression is found in the myotomes (R). More rostrally, at the forelimb bud level, Six1-positive/Pax3-negative cells are disorganized and have lost their identity; they also fail to enter the limb bud in the double KO (compare S,T with U,V). At this level, the myotome of dKO embryos is disorganized, as revealed by the low laminin expression (white arrow in X), when compared with laminin expression in control embryos (white arrow in W).