Fig. 8. Model of RA signaling during pectoral fin development. Expression of aldh1a2 during the formation of somites 2-7 is required to induce tbx5 in the lateral plate mesoderm (lpm) between 12-13 hpf, after which RA signaling is maintained from ventral somites flanking the pectoral fin field to establish a prepattern of posterior fates in the developing fin buds. Arrows indicate genetic interactions between somite-derived RA and fin mesenchyme, and are based on this and previous studies (Neumann et al., 1999; Yelon et al., 2000).