Two closely related fibroblast growth factor receptors, FGFR1 and FGFR2, have been cloned from a newt (Notophthalmus viridescens) limb blastema cDNA library. Sequence analysis revealed that we have isolated both the bek and KGFR variants of FGFR2. These two variants differ only in the second half of the last of their three Ig-like domains. The expression patterns of FGFR1 and FGFR2 during limb regeneration have been determined by in situ hybridization. During the preblastema stages of regeneration, FGFR2 expression is observed in the basal layer of the wound epithelium and in the cells of the periosteum. As regeneration progresses to the blastema stages, FGFR2 expression continues to be observed in the basal layer of the wound epithelium with additional hybridization seen in the blastema mesenchyme closely associated with the bisected bones. From the early bud to the mid-bud blastema stage, FGFR1 expression is observed throughout the blastema mesenchyme but, unlike FGFR2, is distinctly absent from the wound epithelium. In the differentiation stages of regeneration, the mesenchymal expression of FGFR2 becomes restricted to the cells of the condensing cartilage and later to the perichondrium. During these later stages of regeneration, the wound epithelium hybridization to the FGFR2 probe is no longer observed. The expression patterns of these receptors suggest that FGFR1 and FGFR2 have distinct roles in limb regeneration, despite their sharing a number of the FGF ligands. Further investigation regarding the potential sources of the FGF ligands will help establish the role that FGFs and FGFRs play in limb regeneration.

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