Kinetics of the ultrastructural changes in apoptotic chondrocytes from an osteoarthrosis rat model: a window of comparison to the cellular mechanism of apoptosis in human chondrocytes.

It is conceivable that apoptosis plays a relevant role in the pathogenesis of osteoarthrosis. We have dissected certain ultrastructural changes in apoptosis when comparing chondrocytes from the rat osteoarthrosis-induced model to those of human osteoarthrosis. Chondrocytes displayed a typical ultrastructural pattern prevailing according to the days following osteoarthrosis induction. These notable modifications included cell shape changes, a distinctive nuclear chromatin condensation, prominent Golgi and rough endoplasmic reticulum (RER), an increased cytosol osmiophilic reaction, matrix vesicles blebbing from the plasma membrane, and cell fragmentation, with the consequent formation of apoptotic bodies. In addition, small particles and fibers were seen engulfed inside chondrocytes in vacuoles, which may well be related to phagocytosis. Similar modifications were observed in human osteoarthrosic cartilage, predominantly those linked to late stages in the rat model. Interestingly, we observed a peculiarly arranged structure inside a chondrocyte's mitochondrion. It consisted of aggregated particles (5-8 nm diameter) forming rounded granules (38-50 nm diameter) lined up between double membranes, probably cristae, running parallel to the long axis of the mitochondrion. All these changes could be linked to different stages in chondrocytes' apoptotic cell death in osteoarthrosic rat cartilage, similar to what happen in humans. The study of the cellular mechanism of apoptosis facilitates the understanding of the process and might shed additional light on the potential role of apoptotic chondrocyte in osteoarthrosis pathogenesis.