Fig. 9. Model showing FGF regulation of joint development. The formation of the joint interzone is marked by the appearance of Wnt14 and Gdf5 expression, and decreased type II collagen expression. Joint interzone cells also have an elongated cell morphology. Adjacent cartilage elements express higher levels of type II collagen and express the mutant FGFR. At later stages in ColII-FGFR31^ach and ColII-FGFR3^ach mice, chondrification expands into the joint interzone owing to over activation of the FGFR signaling pathway in adjacent chondrocytes, resulting in a failure of joint formation.