Cell clusters in intervertebral disc degeneration:An attempted repair mechanism aborted via apoptosis

Abstract

Abstract Cell clusters are a histological hallmark feature of intervertebral disc degeneration. Clusters arise from cell proliferation, are associated with replicative senescence, and remain metabolically active. Their precise role in stages of disc degeneration remain obscure. The aim of this study was therefore to investigate small, medium, and large size cell-clusters during degeneration. For this purpose, human disc samples were collected from 55 subjects, aged 37-72 years, 21 patients had disc herniation, 10 had degenerated non-herniated discs, and 9 had degenerative scoliosis of spinal curvature < 45°. 15 non-degenerated control discs were from cadavers. To corelate, abnormal loading on clustering process, 18-month-old, 10 male and 10 female sprague dawley rat spines were compressed with a custom-built loading device to induce tissue damage and stimulate attempted repair response. Clusters and matrix changes were investigated with histology, immunohistochemistry, and SDS-PAGE. Data obtained were analyzed with spearman rank correlation and ANOVA. Results revealed, small and medium-sized clusters were positive for cell proliferation markers Ki-67 and proliferating cell nuclear antigen (PCNA) in control, slightly degenerated human, and rat discs loaded for <= 7 days. Large cell clusters were typically more abundant in severely degenerated and herniated human discs, and in rat discs loaded for <=30 days. Large clusters associated with matrix fissures, proteoglycan loss, MMP-1, and Caspase-3. Spatial association findings were reconfirmed with SDS-PAGE that showed the presence of target markers within the dense matrix by its molecular weight. Controls, slightly degenerated discs showed smaller clusters, less proteoglycan loss, MMP-1, and caspase 3. In conclusion, this study shows cell clusters in the early stages of degeneration could be indicative of repair, however sustained loading and inflammatory catabolism alters cellular metabolism facilitating formation of large clusters, thus attempted repair process fails and is aborted at least in part via apoptosis.