During the development of long bones in the limbs, chondrocytes lay down a cartilage template. Subsequently, osteoblasts form a bone collar in the perichondrium - the connective tissue that covers the cartilage - and deposit bone matrix throughout the cartilage matrix to form the primary spongiosa, the central part of the bone. On p. 1309, Long et al. propose that Indian hedgehog (Ihh) signalling directly promotes osteogenesis in long bones, probably in conjunction with endogenous factors such as bone morphogenetic proteins. In support of their hypothesis, the researchers show that the removal from perichondrial cells of smoothened (Smo), which encodes a transmembrane protein essential for transducing Ihh signals, prevents the formation of both the bone collar and primary spongiosa. Based on this and other results, the researchers suggest that Ihh acts in vivo on an `osteochondroprogenitor' cell to promote osteoblast differentiation and prevent chondroctye differentiation.

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