DIFFERENTIAL EXPRESSION OF COLLAGEN TYPE-I, TYPE-II, TYPE-III, AND TYPE-X IN HUMAN OSTEOPHYTES

BACKGROUND: Osteophytes are neoplastic cartilaginous and osseous protr usions growing at the margins of osteoarthritic joints. Their formatio n involves complex patterns of cellular proliferation, differentiation , as well as matrix synthesis and turnover that are poorly understood. EXPERIMENTAL DESIGN: Here we report on an experimental approach using in situ hybridization and immunohistology to elucidate pathways of ch ondrocyte differentiation in human osteophytes. Ab and cDNA probes for collagen types were used as specific parameters for chondrocyte pheno types. RESULTS: In early precartilaginous mesenchymal tissue, cytoplas mic mRNA for alpha 1(I) and alpha 1(III) collagen genes (Col1A1 and Co l3A1) were found by in situ hybridization, correlating with the distri bution of type I and III collagen as revealed by Ab staining. Strong e xpression of type II collagen both at mRNA and protein levels was the hallmark of chondrogenic differentiation in the cartilaginous zone of osteophytes. Type II collagen expression increased in all cartilaginou s and fibrocartilaginous areas with growth and maturation of osteophyt es. The signal intensity obtained after in situ hybridization with a C OL2A1 probe was high and corresponded to that obtained in fetal cartil age, whereas normal adult articular cartilage usually did not show mea surable type II collagen expression. In fibrocartilaginous areas, the most abundant, but heterogeneous tissue type seen in osteophytes, type II and III collagen mRNA expression overlapped considerably. Type III collagen was scattered, both pericellularly and interterritorially, o ver the whole osteophyte, excluding bone and chondrocytic cells of the deep zone. The strongest type I collagen expression was seen in bone and in the superficial fibrous layer. In areas of endochondral ossific ation, large chondrocytes were found expressing type X collagen, a spe cific marker for hypertrophic chondrocytes. CONCLUSIONS: These results show that discrete stages of cartilage differentiation can be precise ly followed in osteophytes using collagen type-specific cDNA probes an d Ab as markers. In addition, a fibrocartilaginous chondrocyte phenoty pe was identified that expresses type II and III, but not type I colla gen.