Expression of chondrocyte markers by cells of normal and degenerate intervertebral discs

Aims: To investigate the phenotype of cells in normal and degenerate intervertebral discs by studying the expression of molecules characteristic of chondrocytes in situ. Methods: Human intervertebral discs taken at surgery were graded histologically, and classified on this basis as normal or degenerate. Eighteen of each type were selected, and in situ hybridisation was performed for the chondrocytic markers Sox9 and collagen 11 using S-35 labelled cDNA probes. Aggrecan was located by immunohistochemistry, using the monoclonal antibody HAG7E1, and visualised with an avidin-biotin peroxidase system. Results: In the normal discs, strong signals for Sox9 and collagen 11 mRNA, and strong staining for the aggrecan protein were seen for the cells of the nucleus pulposus (NP), but reactions were weak or absent over the cells of the annulus fibrosus (AF). In degenerate discs, the Sox9 and collagen 11 mRNA, signals remained visible over the cells of the NP and were again absent in the AF. Aggrecan staining was not visible in the NP cells, and was again absent in the AF. Conclusions: Cells of the normal NP showed expression of all three markers, clearly indicating a chondrocytic phenotype. In degeneration, there was evidence of a loss of aggrecan synthesis, which may contribute to the pathogenesis of disc degeneration. AF cells showed no evidence of a chondrocytic phenotype in either normal or degenerate discs.