Fig. 1. AER-derived FGF signaling is required for limb hairy2 expression.

(A) Schematic representation of hairy2 expression phases in HH24 chick forelimbs, cyclically recapitulated every 6 hours (Pascoal et al., 2007). hairy2 is permanently present in the central positive domain (CPD) and in the posterior positive domain (PPD). It is not expressed in neither anterior negative (AND) nor posterior negative (PND) domains. hairy2 is cyclically expressed in the distal cyclic domain (DCD), evidenced by different intensities in each expression phase (Ai–iii). Open, partially filled and filled arrows indicate absence, intermediate pattern and full hairy2 expression in the DCD, respectively. (Bi,ii) In situ hybridization for fgf8, evidencing complete AER ablation. (Biii–viii) hairy2 down-regulation upon AER ablation or SU5402 treatment (DCD, arrow; PPD, arrowhead), and hairy2 induction by FGF8-bead in the absence of AER. (C) hairy2 is up-regulated by implantation of FGF8-beads in all limb domains (Ci–vi), except the AND (Cvii,viii). Dorsal view; anterior to the top. *Beads, delimited by dashed lines.