Fgfr1 and Fgfr2 have distinct differentiation- and proliferation-related roles in the developing mouse skull vault

Fibroblast growth factor receptors (FGFRs) play major roles in skeletogenesis, and activating mutations of the human FGFR1, FGFR2 and FGFR3 genes cause premature fusion of the skull bones (craniosynostosis). We have investigated the patterns of expression of Fgfr1, Fgfr2 and Fgfr3 in the fetal mouse head, with specific reference to their relationship to cell proliferation and differentiation in the frontal and parietal bones and in the coronal suture. Fgfr2 is expressed only in proliferating osteoprogenitor cells; the onset of differentiation is preceded by down-regulation of Fgfr2 and up-regulation of Fgfr1. Following up-regulation of the differentiation marker osteopontin, Fgfr1, osteonectin and alkaline phosphatase are down-regulated, suggesting that they are involved in the osteogenic differentiation process but not in maintaining the differentiated state. Fgfr3 is expressed in the cranial cartilage, including a plate of cartilage underlying the coronal suture, as well as in osteogenic cells, suggesting a dual role in skull development. Subcutaneous insertion of FGF2-soaked beads onto the coronal suture on E15 resulted in up-regulation of osteopontin and Fgfr1 in the sutural mesenchyme, down-regulation of Fgfr2, and inhibition of cell proliferation. This pattern was observed at 6 and 24 hours after bead insertion, corresponding to the timing and duration of FGF2 diffusion from the beads. We suggest (a) that a gradient of FGF ligand, from high levels in the differentiated region to low levels in the environment of the osteogenic stem cells, modulates differential expression of Fgfr1 and Fgfr2, and (b) that signalling through FGFR2 regulates stem cell proliferation whereas signalling through FGFR1 regulates osteogenic differentiation.