Cell death in human articular chondrocytes: an ultrastructural study in micromass model

Chondrocyte apoptosis is known to contribute to articular cartilage damage in osteoarthritis (OA) and is correlated to a number of cartilage disorders. Micromass cultures represent a convenient means for studying chondrocyte biology, and, in particular, their death. In this study, we present the first ultrastructural analysis on chondrocyte death experimentally induced by different agents, all known to be powerful apoptotic triggers. Chondrocytes were obtained from subjects undergoing joint arthroplasty. At the end of the maturation, they were treated as follows: 10 or 30 μM etoposide for 24h, UV-B for 30 min followed by 4h recovery, 200 or 500 nM staurosporine for 24h, hyperthermia for 1h at 43°C followed by 4h recovery. They were processed for TEM and immunofluorescence. Control chondrocyte morphology appears similar to that of human articular cartilage. Proteoglycans and collagen fibers are present in the intercellular space, indicating a good extracellular matrix (ECM) production. Etoposide, when effective, induces necrosis. UV-B treated cells show chromatin condensation and pore clustering, typical of apoptotic nuclei. Hyperthermia seems to induce apoptosis in the presence of abundant ECM, giving, differently, a general necrosis when ECM is scarse. Staurosporine has no effect at 200 nM concentration, but frequently, at 500 nM, chromatin condensation appears. Chromatin clumping appears different from that of classical apoptotic models: Roach et al. proposed the term “chondroptosis” to indicate this type of cell death. Cells appear shrunk and the nucleus contains condensed chromatin, not marginated into large solid masses, but in small patches, mainly at nuclear periphery. Chondrocytes seem so to undergo apoptosis in a peculiar typical manner.