Fig. 1. Expression profile of RACK1 during limb development and its induction by ectopic FGF. (A) At stage 13, RACK1 was expressed uniformly in the lateral plate. (B) By stage 17 the expression of RACK1 was augmented in the limb-forming region but was low in the interlimb flank. (C,D) By stage 17, RACK1 protein detected by immunostaining (brown deposit) was high in the wing bud (C) but was low in the interlimb region (D). (E) RACK1 was expressed in the limb buds of a stage 18 embryo. (F,G) RACK1 mRNA (F) and protein (G) in sections through a stage 22 wing bud. (H) Ectopic expression of RACK1 (white arrows) induced by FGF released from a bead (red arrow) was observed after 12 hours of treatment. (I,J) Two different embryos showing strong ectopic expression of RACK1 (black arrows) upon FGF4 treatment for 24 hours. (K) A plain heparin bead (red arrow) did not induce RACK1. AER, apical ectodermal ridge; dm, dermomyotome; ec, ectoderm; fl, forelimb; hl, hindlimb; if, interlimb flank; lp, lateral plate; m, mesenchyme.