Expression and roles of connective tissue growth factor in Meckel's cartilage development

Meckel's cartilage is a prominent feature of the developing mandible, but its formation and roles remain unclear. Because connective tissue growth factor (CTGF, CCN2) regulates formation of other cartilages, we asked whether it is expressed and what roles it may have in developing mouse Meckel's cartilage. Indeed, CTGF was strongly expressed in anterior, central, and posterior regions of embryonic day (E) 12 condensing Meckel's mesenchyme. Expression decreased in E15 newly differentiated chondrocytes but surged again in E18 hypertrophic chondrocytes located in anterior region and most-rostral half of central region. These cells were part of growth plate-like structures with zones of maturation resembling those in a developing long bone and expressed such characteristic genes as Indian hedgehog (Ihh), collagen X, MMP-9, and vascular endothelial growth factor. At each stage examined perichondrial tissues also expressed CTGF. To analyze CTGF roles, mesenchymal cells isolated from E10 first branchial arches were tested for interaction and responses to recombinant CTGF (rCTGF). The cells readily formed aggregates in suspension culture and interacted with substrate-bound rCTGF, but neither event occurred in the presence of CTGF neutralizing antibodies. In good agreement, rCTGF treatment of micromass cultures stimulated both expression of condensation-associated macromolecules (fibronectin and tenascin-C) and chondrocyte differentiation. Expression of these molecules and CTGF itself was markedly up-regulated by treatment with transforming growth factor-beta1, a chondrogenic factor. In conclusion, CTGF is expressed in highly dynamic manners in developing Meckel's cartilage where it may influence multiple events, including chondrogenic cell differentiation and chondrocyte maturation. CTGF may aid chondrogenesis by acting down-stream of transforming growth factor-beta and stimulating cell-cell interactions and expression of condensation-associated genes.