text: Although the FGF family of paracrine factors has a broad range of functions, major findings support the idea that they primarily stimulate proliferation and differentiation. To fulfill many diverse functions, FGFs can be alternatively spliced or even have different initiation codons to create hundreds of different FGF isoforms.
  One of the most important functions of the FGF receptors (FGFR) is in limb development. This signaling involves nine different alternatively spliced isoforms of the receptor. Fgf8 and Fgf10 are two of the critical players in limb development. In the forelimb initiation and limb growth in mice, axial (lengthwise) cues from the intermediate mesoderm produces Tbx5, which subsequently signals to the same mesoderm to produce Fgf10. Fgf10 then signals to the ectoderm to begin production of Fgf8, which also stimulates the production of Fgf10.  Deletion of Fgf10 results in limbless mice.
  Additionally, paracrine signaling of Fgf is essential in the developing eye of chicks. The fgf8 mRNA becomes localized in what differentiates into the neural retina of the optic cup. These cells are in contact with the outer ectoderm cells, which will eventually become the lens.
  Phenotype and survival of mice after knockout of some FGFR genes: