Fig. 6. (A-D) In Ihh^–/– limbs distal chondrocytes differentiate directly into prehypertrophic chondrocytes. Sections of E14.5 wild-type (A,C) and Ihh^–/– (B,D) limbs were hybridized with Fgfr1 (A,B) and Pthr1 (C,D) riboprobes. In Ihh^–/– mutants Fgfr1 is expressed in a stripe of distal chondrocytes, which is flanked by Pthr1 expressing hypertrophic chondrocytes. Black dashed lines indicate the border between Fgfr1 and Pthr1 expression domains; red dashed lines indicate the end of the cartilage elements. (E-H) Loss of Gli3 restores columnar chondrocyte differentiation in Ihh^–/– mice. Sections of E16.5 wild-type (E,G) and Gli3^–/–;Ihh^–/– (F,H) limbs were hybridized with riboprobes for Fgfr1 (E,F) and Pthr1 (G,H). In Gli3^–/–;Ihh^–/– mutants a reduced zone of Fgfr1 expressing distal chondrocytes is separated from the Pthr1 expression domain by a population of columnar chondrocytes. B,D and F,H display serial sections.