Model of developmental regulation of FGFRs in MSCs and skeletal tissues. Simplified working model based on findings presented here and in other studies.^4,5,33  Undifferentiated mesenchymal cells expressed FGFR1/2 in the perichondrium and in vitro and appeared to self-renew in vitro upon FGF-2 signaling (proliferation with maintenance of MSC properties). Upon chondrogenic differentiation in the growth plate or in vitro, mesenchymal cells lost FGFR1/2 expression but up-regulated FGFR3. However, FGFR1 was expressed in hypertrophic chondrocytes. More committed mesenchymal progenitors in cortical and trabecular perivascular space and in vitro expressed FGFR1 to various degrees, but rarely expressed FGFR2. MSC-derived differentiated osteoblasts and stromal elements (eg, adipocytes) did not express FGFRs.