Cartilage-specific matrix protein, chondromodulin-I (ChM-I), is a strong angio-inhibitor in endochondral ossification of human neonatal vertebral tissues in vivo: relationship with angiogenic factors in the cartilage

Although cartilage contains many angiogenic factors during endochondral ossification, it is an avascular tissue. The cartilage-specific non-collagenous matrix protein chondromodulin-I (ChM-I) has been shown to be a strong angio-inhibitor. To elucidate whether ChM-I plays an essential role in angio-inhibition during endochondral ossification in man, we investigated the expression and localization of ChM-I in comparison with those of angiogenic factors and the endothelial cell marker CD34 in human neonatal vertebral tissues. Although invasion of CD34-positive endothelial cells was observed in primary subchondral spongiosa, expression of the marker of endothelial cells, CD34, was not found in neonatal vertebral cartilage matrix. Type 11 collagen was deposited in all matrices during endochondral ossification, whereas aggrecan was deposited in the matrix of hypertrophic cartilage, especially around lacunae. Vascular endothelial growth factor (VEGF), which is known to be a strong angiogenic factor, was localized in chondrocytes in mature to hypertrophic cartilage and also in bone marrow. Fibroblast growth factor-2 (FGF-2; basic fibroblast growth factor), which is also known to be a strong angiogenic factor, was localized in the cytoplasm of chondrocytes of mature cartilage in human vertebral cartilage tissues. Transforming growth factor (TGF)-beta has been reported to have many functions including angiogenesis, and TGF-beta1 was also localized in mature chondrocytes in endochondral tissues undergoing ossification. On the other hand, the novel cartilage-specific matrix protein ChM-I was localized in inter-territorial regions of the matrix in mature to hypertrophic cartilage, especially around lacunae. In conclusion, these observations indicate that ChM-I may serve as a barrier against the angiogenic properties of VEGF, FGF-2 and TGF-beta1 during endochondral ossification, and this matrix molecule may play an essential role in determining the avascular nature of cartilage in vivo.