BMPs are required at two steps of limb chondrogenesis: Formation of prechondrogenic condensations and their differentiation into chondrocytes

Formation of the long bones requires a cartilage template. Cartilage format ion (chondrogenesis) proceeds through determination of cells and their aggr egation into prechondrogenic condensations, differentiation into chondrocyt es, and later maturation. Several studies indicate that members of the bone morphogenetic protein (BMP) family promote cartilage formation, but the ex act step(s) in which BMPs are involved during this process remains undefine d. To resolve this issue, we have used a retroviral vector to misexpress th e BMP antagonist Noggin in the embryonic chick limb. Unlike previous report s, we have characterized the resulting phenotype in depth, analyzing histol ogical and early chondrogenic markers, as well as the patterns of cell deat h and proliferation. Misexpression of Noggin prior to the onset of chondrog enesis leads to the total absence of skeletal elements, as previously repor ted (J. Capdevila and R. L. Johnson, 1998, Dev. Biol. 197, 205-217). Noggin inhibits cartilage formation at two distinct steps. First, we demonstrate that mesenchymal cells do not aggregate into prechondrogenic condensations, and additional results suggest that these cells persist in an undifferenti ated state. Second, we show that differentiation of chondroprogenitors into chondrocytes can also be blocked, concurrent with expanded expression of a presumptive joint region marker. In addition, we observed alterations in m uscle and tendon morphogenesis, and the potential role of BMPs in these pro cesses will be discussed. Our studies therefore provide in vivo evidence th at BMPs are necessary for different steps of chondrogenesis: chondroprogeni tor determination and/or condensation and subsequent differentiation into c hondrocytes. (C) 2000 Academic Press.