Apoptosis of tenocytes. (a) As determined by the levels of cytoplasmic histone-associated DNA fragments, tenocytes from osteoarthritis (OA) patients (n = 5) exhibited a 1.6-fold higher rate of spontaneous apoptosis than did normal control tenocytes (n = 5). Stimulation of OA tenocytes with tumour necrosis factor (TNF)-α for 24 hours resulted in a dose dependent suppression of programmed cell death in OA tenocytes but had no effects in tenocytes from normal controls. (b) Stimulation of OA and control tenocytes with 100 ng/ml recombinant human Fas ligand (rhFasL) for 16 hours strongly induced apoptosis. OA tenocytes (n = 5) exhibited a significantly higher susceptibility to FasL-induced cell death, and TNF-α prevented these cells from undergoing FasL-induced cell death in a dose-dependent manner. No such inhibitory effects of TNF-α on FasL-induced apoptosis were seen in control tenocytes (n = 5). *P < 0.05, versus control. (c) Fluorescence activated cell sorting analysis of two representative cell cultures revealed that 36% of non-OA tenocytes stained positive with the APO-BRDU™ assay (Pharmingen; San Diego, CA, USA) following stimulation with rhFasL (black line) versus unstimulated cells (grey line). (d) In contrast, 95% of OA tenocytes showed signs of apoptosis.