Indian hedgehog (Ihh), one of the three mammalian hedgehog (Hh) proteins, 
coordinates proliferation and differentiation of chondrocytes during 
endochondral bone development. Smoothened (Smo) is a transmembrane protein that 
transduces all Hh signals. In order to discern the direct versus indirect roles 
of Ihh in cartilage development, we have used the Cre-loxP approach to remove 
Smo activity specifically in chondrocytes. Animals generated by this means 
develop shorter long bones when compared to wild-type littermates. In contrast 
to Ihh mutants (Ihh(n)/Ihh(n)), chondrocyte differentiation proceeds normally. 
However, like Ihh(n)/Ihh(n) mice, proliferation of chondrocytes is reduced by 
about 50%, supporting a direct role for Ihh in the regulation of chondrocyte 
proliferation. Moreover, by overexpressing either Ihh or a constitutively active 
Smo allele (Smo*) specifically in the cartilage using the bigenic UAS-Gal4 
system, we demonstrate that activation of the Ihh signaling pathway is 
sufficient to promote chondrocyte proliferation. Finally, expression of cyclin 
D1 is markedly downregulated when either Ihh or Smo activity is removed from 
chondrocytes, indicating that Ihh regulates chondrocyte proliferation at least 
in part by modulating the transcription of cyclin D1. Taken together, the 
present study establishes Ihh as a key mitogen in the endochondral skeleton.