Labeling of articular cartilage surface with cationized ferritin: Aged human normal and osteoarthritic cartilage

The labeling of the articular surface with cationized ferritin (CF), an electron–dense marker, visualizes the anionic sites and may disclose abnormal penetration of the large CF molecule into the subsurface layers. Various areas of cartilage selected by unaided eye examination were taken from femoral heads excised in three cases of osteoarthritis and two cases of hip fracture. The fragments were examined by optical microscopy and by electron microscopy after labeling with CF. The labeling with and the penetration of CF were correlated with the morphological features of the surface. The surfaces belonging to the erosion border were disrupted and the CF penetrated approximately 2 μm into the matrix along the collagen fibers and in areas containing a patchy dense material. Prefixation with Karnovsky's fixative prevents CF penetration. The fragments taken at a distance from the erosion border showed at electron microscopical examination either an intact appearance of the surface that was labeled without penetration or a disrupted surface with penetration of the label. The osteophytes and the regeneration buds surface were labeled showing little or no penetration. The fragments from cartilage of hip fractures had either an intact surface regularly labeled or a slightly or moderately disrupted surface with moderate penetration of CF. The penetration of large molecules of CF in damaged cartilage demonstrates important permeability changes that may be significant for the pathogenetic mechanism of osteoarthritis. Similar permeability changes were previously shown in mice femoral heads treated in vitro with collagenase or trypsin and labeled with CF.