At present, there are 16 members of the FGF ligand family. The typical FGF receptor has a split intracellular tyrosine kinase domain and an extracellular domain containing up to three ligand-binding immunoglobin-like loops. Alternative splicing within the extracellular domains of FGFR genes can generate great diversity of ligand-binding domains, which can result in different ligand-binding specificities. Additionally, each FGF receptor may transduce a particular mitogenic potential. However, as individual receptors can be activated by several different ligands, specificity may partly depend upon spatial and temporal expression patterns. An exception is FGF7 (keratinocyte growth factor, KGF), which selectively activates a differentially spliced form of FGFR-2. Using RNase protection analysis, we have detected KGF receptor transcripts in urogenital ridge RNA (unpublished results). Neither germ cell nor gonadal defects have been observed in KGF-deficient mice. buy cheap antibiotics
Do FGFs contribute to the in vivo development of the germ line? The pattern of expression of several FGFs, but most notably FGF4, suggests that it may be expressed in or near PGCs. In the early postimplantation embryo, FGF4 is expressed in the embryonic ectoderm at the egg cylinder stage, then along the primitive streak and in the primitive dorsal mesoderm.