Apoptosis is not observed following tumor necrosis factor alpha (TNF-α) and/or epidermal growth factor (EGF) treatment. Confluent monolayers of chondrocytes were treated with vehicle, TNF-α (30 ng/ml), EGF (10 ng/ml) or TNF-α + EGF for 24 hours. (a) Early stages of apoptosis were assayed by immunoblot with an antibody specific for intact and cleaved forms of poly(ADP ribose) polymerase (PARP). No cleavage of PARP (i.e. appearance of a band at 89 kDa) was detected following any of the treatments. Blot shown is representative of three independent experiments. (b) Apoptosis-induced DNA strand breaks were examined by in situ labeling (terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling [TUNEL]) and imaged using confocal microscopy. No TUNEL labeling was detected with any of the treatments. Cells treated with DNAse I to induce DNA breaks served as a positive control. Bar = 50 μm. Images are representative of three independent experiments.