To investigate the gene expression and potential function of insulinlike-Iin osteophyte development.
Specimens were obtained from 25 individuals undergoing total knee arthroplasty due to severe primary osteoarthritis and from 3 healthy adults. The tissue samples were embedded in paraffin wax and made into sections to undergo HE and toluidine blue staining. Immunohistochemistry was used to detect the expression of collagen I, IIa, IIb, and X. The osteophytic tissues were grouped into different types based on the results by histomorphology and expression of different collagens and glycosaminoglycans. Immunohistochemistry and in situ hybridization were used to detect the expression of insulinlike growth factor-I in the osteophyte. The images obtained by imunohistochemistry and in situ hybridization were put into the image analysis system to be analyzed semi-quantitatively.
Based on the characteristics of histomorphology and expression of different collagens and glycosaminoglycans, the osteophytic tissues were grouped into five different types. The type I tissue mainly consists of mesenchymal fibroblast-like cells producing collagen I. The type II tissue was characterized by the existence of collagen IIA expressed in prechondrocyte and glycosaminoglycans in the extracellular mesenchyma. he type III tissue was marked by the co-expression of collagen IIB, glycosaminoglycans, collagen IIA, and collagen I. The type IV tissue showed a clear zonal organization of chondrocytes, abundant expression of collagen IIB and glycosaminoglycans, and hypertrophic deep chondrocytes expressing Collagen X. The type V tissue had a composition similar to that of the normal articular hyaline cartilage with a predominance of collagen IIB and abundant glycosaminoglycans. These five types seemed to indicate the different stages of osteophyte development: mesenchymal cell, prechondrocyte, chondrocyte, early osteophyte and mature osteophyte. Immunohistochemistry showed that the expression level of insulinlike growth factor-I in the type I tissue was 14.6 +/- 3.8, significantly higher than those in the types IV and V tissues (7.7 +/- 3.5 and 6.0 +/- 1.9 respectively, both P = 0.000); and the expression level of insulinlike growth factor-I in the type II tissue was 13.6 +/- 4.7, significantly higher than those in the types IV and V tissues (both P = 0.000). The results of in situ hybridization were similar to those by imunohistochemistry.
Insulinlike growth factor-I mRNA is mainly expressed in the early stage osteophyte, and it may play an important role in initiation of the chondrogenic differentiation of mesenchymal progenitor cells in the early stage of osteophyte development.