Fig. 7. Model of the PTHrP-dependent and PTHrP-independent function of Ihh signaling in chondrocyte differentiation. The morphology of a developing long bone is schematically shown. Chondrocytes proliferate and differentiate sequentially along the longitudinal axis. The length of the columnar chondrocyte region is determined by the rate of periarticular chondrocyte differentiation, columnar chondrocyte proliferation and hypertrophy. Ihh expression is shown in red. PTHrP expression is shown in yellow. Ihh acts through PTHrP to inhibit chondrocyte hypertrophy. Ihh acts independently of PTHrP to control chondrocyte proliferation, periarticular chondrocyte differentiation and osteoblast differentiation. Here, we show that Ihh also promotes chondrocyte hypertrophy independently of PTHrP and that this function is likely to be mediated by Bmp and Wnt/β-catenin signaling.