Fig. 1. FGFR4, but not FGFR1 is expressed in embryonic limb skeletal muscles. E5 embryos were hybridized to a FGFR4-specific RNA probe (A,C,E), or to a FGFR1-specific probe (B,D,F). Whole-mount in situs indicate that FGFR4 is specifically expressed in the developing muscles (A), whereas FGFR1 is widely expressed in the entire limb (B). Sections confirm that FGFR4 is specifically expressed in the skeletal muscles (C), which are recognized after immunohistochemistry with an embryonic myosin heavy chain-specific monoclonal antibody (E). In contrast, FGFR1 is strongly expressed in the dermis (D) and poorly, if at all, in the muscles (F). ec, ectoderm; de, dermis; mu, skeletal muscles; me, mesenchyme.