Changes in the patterns of collagens and fibronectin during limb-bud chondrogenesis

The distribution and sequence of appearance of fibronectin and of type-I and type-II collagen in the developing cartilage models of embryonic chick hind-limb buds was studied by immunofluorescence, using specific antibodies directed against these proteins. Fibronectin and type-I collagen are evenly distributed throughout the intercellular space of the mesenchyme prior to condensation of core mesenchyme of the limb anlage and formation of the cartilage blastema. With the onset of the condensation process fibronectin and type-I collagen appear to increase in the cartilage blastema compared to the surrounding loose mesenchyme, reaching a maximal density at the time of cartilage differentiation. The latter process is marked by the appearance of type-II collagen in the cartilage blastema. As cartilage differentiation progresses, type-I collagen is gradually replaced by type-II collagen; fibronectin disappears and is completely absent from mature cartilage. The transient appearance of type-I collagen and fibronectin suggests a temporal role in cell-matrix or cell-cell interactions in chondrogenesis, since it had been shown that (a) type-I collagen substrates stimulate cell proliferation and cartilage differentiation in limb-bud mesenchyme cell cultures; (b) fibronectin mediates attachment of cells to collagen substrates; and (c) fibronectin is directly involved in cellular interactions in chondrocyte cultures.