Fig. 3. Retinoic acid rescues pectoral fin field induction in the absence of paraxial mesoderm. Whole-mount in situ hybridizations. Anterior is towards the left, dorsal views. (A-D) Expression of tbx5 in pectoral fin buds is unchanged in ntl(B), but reduced in spt (C) and absent in ntl/spt double-mutant embryos (D). (E-M) Application of 10^-8 M RA, beginning at 10 hpf, rescues tbx5 expression in ntl/spt pectoral fins (arrowheads in I,K); rescue occurs in the absence of myogenic differentiation, demonstrated by the absence of myod (J,K) and -tropomyosin-1 (tpm1) expression (L,M) in RA-treated double mutants. Four rescue experiments, each with 40 embryos, were performed (total numbers of phenotypes: 32 spt, 35 ntl, 11 ntl/spt, 82 siblings without mutant phenotype).