Fig. 8. (Part I) Myf5 expression in E9.25 (20 somites stage) in heterozygous (A) and dKO (B) embryos. Myf5 is expressed in the epaxial somites of heterozygous embryos, and more faintly in epaxialrostral somites in dKO embryos. (Part II) Myogenin expression in E9.5 embryos in heterozygous (A) and dKO (B) embryos. A few positive myogenin-expressing cells are detected in the most rostral somites of dKO embryos (arrows in B). (Part III) Myogenin expression in E10.5 heterozygous (A,C,E) and dKO (B,D,F) embryos. Vibratome sections at the interlimb level (C,D) and forelimb level (E,F) show a strong decrease of myogenin expression that is faintly detected in the epaxial region of dKO embryos, hypaxial extension expression domain being lost. (Part IV) Myod1 expression in E10.5 heterozygous (A,C,E,G) and dKO (B,D,F,H) embryos. Most Myod1 expression is lost in Six1^–/–Six4^–/– embryos, and remaining expression can be visualized at the interlimb or forelimb levels in the central and hypaxial somite (F,H). (Part V) Myf5 expression in heterozygous (A,C,E,G) and in dKO (B,D,F,H) E10.5 embryos. Myf5 is mainly detected in caudal lips of homozygous dKO animals at interlimb levels (B, upper right), ventral and dorsal lip expression being lost (compare A with B). This expression loss is well detected on vibratome sections at hindlimb (C,D), interlimb (E,F) and forelimb (G,H) levels. In the dKO, at the forelimb level the myotome is formed only of a central region expressing Myf5 (H). In the top right-hand corners of A and B, magnification of interlimb Myf5 expression can be seen that is restricted to the caudal region of the somites in the dKO embryo (B). (Part VI) At E10.5, MRF4 expression is lost in Six1^–/–Six4^–/– embryos (B), when compared with control hetererozygous expression (A). (Part VII) Myod1 expression in heterozygous (A,C,E,G) and in Six1^–/–Six4^–/– (B,D,F,H) E11.5 embryos. A low diffuse Myod1 expression in interlimb somites of dKO embryos is detected (B) with restricted ventral extension (B, and enlargement in the top right-hand corner) when compared with Myod1 expression in heterozygous embryos (A, enlargement in the top right-hand corner). Vibratome sections (C-H) showing a faint Myod1 expression at hindlimb level, with most dorsal and ventral extension being lost (C,D). At the interlimb level, Myod1 expression is restricted to the central region of the myotome, dorsal and ventral expression is lost (E,F). A low myotomal expression is detected at the forelimb level (G,H).