Immunohistochemical characterization of the small proteoglycans decorin and proteoglycan-100 in heterotopic ossification.

Heterotopic ossification is a metabolically active process which shares several properties of orthotopic bone formation and, therefore, represents an excellent model for studying bone matrix components. Immunohistochemical methods were used to investigate the distribution pattern of the small proteoglycans decorin and proteoglycan-100 during different stages of heterotopic ossification of pressure sores of paraplegic patients. Decorin and proteoglycan-100 exhibited a substantially divergent distribution pattern. Decorin was detectable in the perivascular matrix of granulation tissue as well as in the stroma of heterotopic ossification. The ossification zone was stained most strongly. In contrast, proteoglycan-100 was predominantly detectable in fibroblasts and preosteoblasts in early areas of osteogenesis. In more mature forms of heterotopic ossification immunostaining was markedly reduced in osteoblasts and osteocytes and even absent in so-called bone-lining cells. However, at least some osteoclasts were strongly positive. These results suggest indicate that decorin and proteoglycan-100 are important components during the formal pathogenesis of heterotopic ossification. The expression of the small proteoglycans, especially of proteoglycan-100, correlates with different phases during heterotopic ossification, showing a maximum for proteoglycan-100 in matrix-forming cells in early phases of bone formation, but in osteoclasts in mature bone.