Fig. 4. Altered primary myogenesis in Six1^–/– embryos. X-gal staining of Six1^–/– embryos (A,C,E,G,I,J) and Six1^+/– littermates (B,D,F,H). (A,B) At E12.5, the staining observed in Six1^–/– embryos is different to that in Six1^+/– littermates in few restricted areas: at the limb level the blue staining is reduced in the distal anterior part (*), and at the interlimb level the ventral extension of epithelial somitic buds of the dermomyotome is reduced (arrows). (C,D) At E13.5 in Six1^–/– embryos, all body muscles are either absent or severely disorganized, except some deep back and head muscles. (E,F) Transverse sections of E13.5 embryos at the trunk level (dashed line in C and D) show that the external myogenic layer, the cutaneus maximus (arrow), is absent and that the internal myogenic layer is reduced and disorganized. Also specific muscle areas such as the spinotrapezius (double arrow) are missing. (G,H) Detail of E13.5 Six1^–/– embryos. Note the absence of most muscles at the shoulder level (single arrowhead), the disorganization of the latissimus dorsi (double arrowhead), and the strong reduction of abdominal and thoracic muscles (double arrow), whereas the deep back muscle, longissimus dorsi, appears correctly developed (arrow). (I,J) Detail of E14.5 Six1^–/– embryos at deep back muscle (I) and head (J) levels showing blue myotubes correctly shaped at head level, but reduced in number at the body level.