Bone morphogenetic proteins (BMPs) are multi-functional growth factors that belong to the transforming growth factor beta (TGF-β) superfamily. BMPs play a key role in embryonic development, especially during heart, neural, and cartilage development. Around 20 BMP family members have been identified and characterized. BMPs signal through serine/threonine kinase receptors, composed of type I and II subtypes. Four type I receptors have been identified: type IA and IB BMP receptors, type IA activin receptor, and activin receptor-like kinase I. Three type II receptors have also been recognized: type II BMP receptor and type II and IIB activin receptors. Nervous system development involves several BMPs including BMP-2, 4, 5, and 7. BMP-5 is widely expressed in the nervous system throughout development and into adulthood, and BMP-5 induces multiple dendritic processes to sympathetic neurons in culture. BMP-5 plays an important role in fracture healing during the stage of cartilage formation and periosteal response. In addition, BMP-4 and BMP-5 are expressed in normal synovial tissue and have been found to be decreased in osteoarthritis and rheumatoid arthritis; therefore, it has been speculated that these BMPs are important in joint homeostasis.