Chondrocyte necrosis and apoptosis in impact damaged articular cartilage

A decrease in chondrocyte numbers is one characteristic of osteoarthritic c artilage. This decrease may be the result of apoptosis or other forms of ce ll death induced by mechanical damage. Furthermore, cell death may contribu te to the structural and metabolic changes found in osteoarthritic cartilag e. Therefore, we investigated cell viability and the mode of cell death in cartilage subjected to an increasing severity of impact loads expected to c ause compositional damage and osteoarthritic-like metabolic alterations. Ca nine cartilage explants were subjected to cyclic indentation impacts of 5 m egapascals at 0.3 Hz for 0, 2, 20, and 120 min and then kept in culture for 2, 4, 48, and 144 h. Cell death was assessed by the TUNEL assay and by upt ake of propidium iodide. Viable cells were detected by the ability to metab olize fluorescein diacetate. Nuclear morphology and ultrastructure of the c ell were examined using Hoechst 33342 fluorescent staining and transmission electron microscopy (TEM). As controls for necrosis and apoptosis, cartila ge was, respectively, frozen and thawed or incubated with mitomycin-C, an a poptosis inducer. In cartilage that had been loaded for 2 h, 32% of the cho ndrocytes in the loaded core took up propidium iodide within 2 h after load ing. Most of these were in the middle to superficial zones and reflected le aky cell membranes usually characteristic of necrosis. Less than 1% of thes e chondrocytes were positive in the TUNEL assay after 4 h. After additional culture for 2 days, however, the proportion of chondrocytes which were pos itive in the TUNEL assay reached 73%. A dose dependent response to duration of loading was detected with the TUNEL assay at this time. The TUNEL assay was not specific for apoptosis since 92% of chondrocytes in freeze/thawed cartilage were TUNEL positive. However, some cells with apoptotic bodies an d chromatin condensation characteristic of apoptosis were found in the tran sition zone between necrotic and normal chondrocytes, but not in the superf icial and upper zones, in impact damaged cartilage. We concluded that in th is study, necrosis occurred first, followed by apoptosis. (C) 2001 Orthopae dic Research Society. Published by Elsevier Science Ltd. All rights reserve d.