text: Paracrine signaling through fibroblast growth factors and its respective receptors utilizes the receptor tyrosine pathway.  This signaling pathway has been highly studied, using Drosophila eyes and human cancers.
  Binding of FGF to FGFR phosphorylates the idle kinase and activates the RTK pathway.  This pathway begins at the cell membrane surface, where a ligand binds to its specific receptor.  Ligands that bind to RTKs include fibroblast growth factors, epidermal growth factors, platelet-derived growth factors, and stem cell factor.  This dimerizes the transmembrane receptor to another RTK receptor, which causes the autophosphorylation and subsequent conformational change of the homodimerized receptor.  This conformational change activates the dormant kinase of each RTK on the tyrosine residue.  Due to the fact that the receptor spans across the membrane from the extracellular environment, through the lipid bilayer, and into the cytoplasm, the binding of the receptor to the ligand also causes the trans phosphorylation of the cytoplasmic domain of the receptor.
  An adaptor protein (such as SOS) recognizes the phosphorylated tyrosine on the receptor.  This protein functions as a bridge which connects the RTK to an intermediate protein (such as GNRP), starting the intracellular signaling cascade.  In turn, the intermediate protein stimulates GDP-bound Ras to the activated GTP-bound Ras.  GAP eventually returns Ras to its inactive state.  Activation of Ras has the potential to initiate three signaling pathways downstream of Ras: Ras→Raf→MAP kinase pathway, PI3 kinase pathway, and Ral pathway.  Each pathway leads to the activation of transcription factors which enter the nucleus to alter gene expression.