Enlarged proliferative and hypertrophic zone of chondrocytes and inhibited differentiation to osteoblasts in Fgf18−/− embryos. (A–L) Longitudinal sections of the proximal tibia of wild-type (A,C,E,G,I,K) and Fgf18−/− (B,D,F,H,J,L) embryos at 18.5 dpc stained with hematoxilin-eosin (A,B), and hybridized with PTHrP-R (C,D), Ihh (E,F), ColX (G,H), Opn (I,J), and Oc (K,L) probes. (RC) Resting chondrocytes; (PC) proliferating chondrocytes; (HC) hypertrophic chondrocytes.