The expression of senescence biomarkers in vivo. (a) Mean telomere length (MTL) in cells directly extracted from non-degenerate and degenerate human intervertebral discs (IVDs): correlation with age. Samples are from 20 non-degenerate discs (6 aged 37 years, 7 aged 47 years, 2 aged 59 years, 4 aged 62 years, and 1 aged 74 years), 10 intermediate degenerate discs (4 aged 37 years, 1 aged 44 years, 1 aged 49 years, 2 aged 62 years, and 2 aged 74 years), and 1 severely degenerate disc (aged 49 years). Spearman rank correlation P < 0.05. (b) MTL in cells directly extracted from non-degenerate and degenerate human IVDs: effect of degree of degeneration. *Intermediate degenerate samples are significantly different from non-degenerate samples (P < 0.05). Disc samples are as described in (a). Data are shown as average MTL ± standard error of the mean (SEM) for each disease state. (c) Quantification and localisation of p16INK4a immunopositivity in human IVDs correlated with degree of degeneration. *Samples are significantly different from non-degenerate samples (P < 0.05). Samples are from 11 non-degenerate discs, 6 intermediate degenerate discs, and 5 severely degenerate discs. Averages ± SEM are presented. (d) p16INK4a immunopositive cells in human IVDs correlated with age. Samples are as detailed in (c). Intermediate degenerate (grades 4 to 7) and severely degenerate (grades 8 to 12) samples are grouped for correlation analysis. Spearman rank correlation for non-degenerate samples P < 0.05 and for degenerate samples P = 0.26. IAF, inner annulus fibrosus; kbp, kilobase pairs; NP, nucleus pulposus; OAF, outer annulus fibrosus.