Developmental expression of two closely related fibroblast growth factor receptors, bek and flg, is described from early postimplantation until advanced organogenesis. Transcripts of bek and flg were first seen in the primitive ectoderm of egg-cylinder-stage embryos. Later, starting with somitogenesis, and then throughout embryogenesis, they were actively transcribed both in the mesoderm and neuroectoderm. Bek was expressed also in the surface ectoderm and in various epithelia, whereas flg expression was restricted mainly to the mesenchyme. In the limb bud bek transcripts displayed a gradient-like distribution and appeared earlier than flg. The two receptors, in contrast to their almost identical ligand binding specificity, displayed distinct spatial specificities throughout development, suggesting that developmental localization may contribute to functional specificity. The role of bek and flg in gastrulation and in epithelial-mesenchymal interactions of organogenesis will be discussed.

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