ASSOCIATION OF MAGNESIUM WHITLOCKITE CRYSTALS WITH LIPID COMPONENTS OF THE EXTRACELLULAR-MATRIX IN HUMAN ARTICULAR-CARTILAGE

Several basic calcium phosphate mineral phases have been reported to b e associated with osteoarthritis joint disease. Magnesium whitlockite crystal deposition has been reported in both osteoarthritic and normal human articular cartilage. Existing data suggest that likely prevaili ng conditions within cartilage would not support de novo whitlockite f ormation. It would appear, therefore, that additional factors must be extant at sites of crystal deposition. In this study normal articular cartilage specimens were examined for the presence and distribution of lipids relative to crystal deposition within the extracellular matrix . Specimens were examined using light and transmission electron micros copy (TEM), with standard processing protocols plus a malachite green- glutaraldehyde-osmium tetroxide (MGO) method, used to retain lipids no rmally removed from tissues during preparation for electron microscopy . Elemental maps of sections produced using this method were also made using X-ray microanalysis. Positive oil red O staining for lipid was clearly apparent immediately below and parallel to the articular surfa ce of cartilage specimens using light microscopy. The extent and distr ibution of staining correlated well with the distribution of crystals, observed by TEM, in sections of tissue from adjacent sites of the sam e specimens. Using standard TEM, crystals were frequently observed sca ttered amongst intramatrical lipidic debris, particularly pericellular ly, in areas of cell necrosis and amongst close packed tangential fibe rs between the articular surface and initial superficial zone chondroc ytes. Cartilage specimens processed using the MGO method demonstrated electron dense features, not apparent using standard techniques, ident ified as lipid. Such extracellular lipid deposition varied with depth, with 100nm globular bodies present in the superficial region, where c olocalization of crystals and lipid were observed in about 10% of crys tal observations. The association of lipid and crystal deposition is d iscussed in the context of phospholipid associated mineral formation a nd the potential role of such magnesium whitlockite deposition assesse d.