Skeletons reveal that chondrocyte differentiation needs BMPs

Bone morphogenetic protein (BMP) signalling is an essential regulator of chondrogenesis. But although in vitro studies have shown that BMPs promote proliferation in cartilage growth plates, whether they inhibit or stimulate chondrocyte differentiation has remained unclear. Now, Karen Lyons and colleagues report that antagonistic BMP and FGF signalling controls the progression of chondrocytes through the growth plate in vivo (see p. 4667). The researchers characterized the skeletal phenotypes of Bmpr1a^CKO mice (which lack BMP receptor type IA in chondrocytes) and double Bmpr1a^CKO; Bmpr1b^+/- mice (which also lack one BMP receptor type IB gene in their chondrocytes). From these studies, the authors discovered that BMP signalling is essential for multiple aspects of chondrogenesis, including proliferation and the completion of differentiation, and that it both promotes the expression of Indian hedgehog (which co-ordinates chondrocyte proliferation and differentiation) and inhibits FGF signalling (which negatively regulates these processes). Together, these results greatly clarify the complex role that BMP signalling plays in chondrogenesis in vivo.