| dc.description | Membrane bones arise directly from mesenchyme through intramembranous ossification without intermediate cartilage development. However, under certain circumstances, cells of membrane bones express a chondrogenic phenotype, although regulation of such chondrogenesis is poorly understood. In this study, the quadratojugal ( QJ ), a membrane bone of the chick upper jaw, was used as model system. In response to embryonic movement, the periosteum of the embryonic QJ at the posterior hook switches from osteogenesis to chondrogenesis to form secondary cartilage. To look for the regulatory mechanism modulating movement to chondrogenesis, expression of five growth factors and one cell adhesion molecule (N-CAM) was studied in vivo. N-CAM exhibits a temporal and spatial expression pattern consistent with a role during secondary chondrogenesis. N-CAM expression persists in osteogenesis but is down-regulated when periosteal cells commit to chondrogenesis. Immobilization of embryos prevents down-regulation of N-CAM and secondary cartilage formation. To test cell differentiation potential, several cell culture approaches were developed. Differentiation pathways of QJ periosteal cells are greatly influenced by culture condition. High cell density favors osteogenesis. Low density monolayer, clonal, and agarose suspension cultures elicit chondrogenesis. Fully differentiated chondrocytes were obtained in monolayer culture of low density, which is the first demonstration that chondrogenesis can be achieved from membrane bone cells in primary monolayer culture. Neither osteogenesis nor chondrogenesis occurs in moderate cell density. Furthermore, the in vitro studies suggest that secondary chondrocytes differentiate from certain cell types of the osteogenic lineage. Intramembranous differentiation may include a transient stage in which cells are able to undergo both osteo- and chondrogenesis. Osteogenesis would be the normal pathway, but chondrogenesis can be evoked under certain circumstances. Unlike primary cartilage, in which mesenchymal condensation is a prerequisite and N-CAM is up-regulated, secondary chondrogenesis occurs without a condensation process or N-CAM expression. To assess the effect of N-CAM on periosteal cell differentiation, N-CAM was overexpressed by transfection of an N-CAM-encoding plasmid into cultured periosteal cells. Enhancement of N-CAM expression inhibited chondrogenesis in low density monolayer culture. N-CAM seems to play different roles in early and later stages of chondrogenesis: it enhances cartilage formation at the mesenchymal condensation stage by increasing numbers of potential cartilage cells, but inhibits chondrocyte phenotypic expression when cell differentiation starts. In membrane bones, N-CAM may inhibit the chondrogenic pathway of bipotential periosteal cells. Down-regulation of N-CAM is a prerequisite when periosteum transforms to perichondrium. | en_US |
