Expression of collagen types IX and XI and other major cartilage matrix components by human fetal chondrocytes in vivo.

Coordinate differentiation of the chondrocytes plays a crucial role during skeletal development. In the cascade of endochondral bone formation, mature chondrocytes of the fetal growth plate represent metabolically highly active cells. They show high expression levels of the major cartilage matrix genes, collagen types II, IX, and XI, the major cartilage proteoglycan aggrecan, and proteoglycan link protein. The strongest signals are found in areas of maximal growth, the proliferative and upper hypertrophic zones. The major cartilage matrix components are co-expressed by the chondrocytes of the resting and proliferative zones. Type X collagen is restricted to lower hypertrophic chondrocytes. Interestingly, in the lower hypertrophic zone type IX collagen, but not type II and XI collagen, mRNA expression is downregulated, indicating a discoordinate expression of these collagen types in hypertrophic chondrocytes. The results of this study confirm the strict zonal differentiation pattern of chondrocytes in the developing fetal growth plate, which can be monitored by the expression patterns of its major expression products, the collagen subtypes and aggrecan and proteoglycan link protein.