Pro-apoptotic caspase activation occurs predominantly in non-apoptotic cells. Caspase-3 (A), caspase-6 (C), caspase-7 (E), caspase-8 (G), and caspase-9 (I) expression in the ossification zone, apoptotic cells detected by TUNEL assay on serial sections (B, D, F, H, J). Arrows point to positive cells (brown). Scale bar = 100 µm. Abbreviation: TUNEL, TdT-mediated biotin-dUTP nick end labeling.