ARTICLE

Wnt7b can replace Ihh to induce hypertrophic cartilage vascularization but not osteoblast differentiation during endochondral bone development Kyu Sang Joeng1,2,3 and Fanxin Long1,2,4,5 Indian hedgehog (Ihh) is an essential signal that regulates endochondral bone development. We have previously shown that Wnt7b promotes osteoblast differentiation during mouse embryogenesis, and that its expression in the perichondrium is dependent on Ihh signaling. To test the hypothesis thatWnt7bmaymediate some aspects of Ihh function during endochondral bone development,we activatedWnt7b expression from the R26-Wnt7b allele with Col2-Cre in the Ihh2/2mouse. Artificial expression ofWnt7b rescued vascularization of the hypertrophic cartilage in the Ihh2/2mouse, but failed to restore orthotopic osteoblast differentiation in the perichondrium. Similarly, Wnt7b did not recover Ihh-dependent perichondral bone formation in the Ihh2/2; Gli32/2 embryo. Interestingly, Wnt7b induced bone formation at the diaphyseal region of long bones in the absence of Ihh, possibly due to increased vascularization in the area. Thus, Ihh-dependent expression ofWnt7b in the perichondrium may contribute to vascularization of the hypertrophic cartilage during endochondral bone development