Signalling via type IA and type IB bone morphogenetic protein receptors (BMPR) regulates intramembranous bone formation, chondrogenesis and feather formation in the chicken embryo

Bone morphogenetic proteins (BMPs) signal via complexes of type I and type II receptors. In this study, we mapped the expression of type IA, type IB and type II receptors during craniofacial chondrogenesis and then perturbed receptor function in vivo with retroviruses expressing dominant-negative or constitutively active type I receptors. BmprIB was the only receptor expressed within all cartilages. BmprIA was initially expressed in cartilage condensations, but later decreased within cartilage elements. BmprII was expressed at low levels in the nasal septum and prenasal cartilage and at higher levels in other craniofacial cartilages. The maxillary prominence, which gives rise to several intramembranous bones, expressed both type I receptors. Misexpression of dnBMPRIB decreased the size of cartilages and bones on the treated side. In contrast, dnBMPRIA had no effect on the skeletal phenotype. The phenotypes of caBMPRIA and caBMPRIB were similar; both led to overgrowth of cartilage elements, thinner bones with fewer trabeculae and inhibition of feather development. Infection with constitutively active viruses resulted in ectopic expression of Msx1, Msx2 and Fgfr2 throughout the maxillary mesenchyme. These data suggest that the pattern of trabeculation in membranous bones derived from the maxillary prominence was related to the change in expression pattern and that Msx and Fgfr2 genes were downstream of both type I BMP receptors. We conclude that the requirement for the type IB is greater than for the type IA receptor but, when active, both receptors play similar roles in regulating bone, cartilage and feather formation in the skull.