title: Association of magnesium whitlockite crystals with lipid components of the extracellular matrix in human articular cartilage.
abstract: Several basic calcium phosphate mineral phases have been reported to be 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 prevailing conditions within cartilage would not support de novo whitlockite formation. 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 microscopy (TEM), with standard processing protocols plus a malachite green-glutaraldehyde-osmium tetroxide (MGO) method, used to retain lipids normally 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 surface of cartilage specimens using light microscopy. The extent and distribution of staining correlated well with the distribution of crystals, observed by TEM, in sections of tissue from adjacent sites of the same specimens. Using standard TEM, crystals were frequently observed scattered amongst intramatrical lipidic debris, particularly pericellularly, in areas of cell necrosis and amongst close packed tangential fibers between the articular surface and initial superficial zone chondrocytes. Cartilage specimens processed using the MGO method demonstrated electron dense features, not apparent using standard techniques, identified as lipid. Such extracellular lipid deposition varied with depth, with 100 nm globular bodies present in the superficial region, where colocalization of crystals and lipid were observed in about 10% of crystal observations. The association of lipid and crystal deposition is discussed in the context of phospholipid associated mineral formation and the potential role of such magnesium whitlockite deposition assessed.
authors.affiliation: Institute of Orthopaedics, University College London Medical School, Royal National Orthopaedic Hospital, Stanmore, Middlesex, U.K.
authors.forename: C A
authors.initials: CA
authors.lastname: Scotchford
authors.forename: S Y
authors.initials: SY
authors.lastname: Ali
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001391:Azo Compounds; D002130:Calcium Phosphates / Q000032:analysis*; D002358:Cartilage, Articular / Q000737:chemistry* / Q000648:ultrastructure; D004396:Coloring Agents; D003460:Crystallization; D004577:Electron Probe Microanalysis; D005109:Extracellular Matrix / Q000737:chemistry*; D005270:Femur Head; D006801:Humans; D008055:Lipids / Q000032:analysis*; D008854:Microscopy, Electron; D008875:Middle Aged; D016707:Tissue Fixation / Q000379:methods
chemical_list: D001391:Azo Compounds; D002130:Calcium Phosphates; D004396:Coloring Agents; D008055:Lipids; C021767:whitlockite; C011049:oil red O