The morphology and composition of the superficial zone of mammalian articular cartilage

Literature available on the morphology and composition of the articular surface appears to be inconsistent and contradictory. The characteristics of the superficial zone of articular cartilage are critical to understanding the wear and lubrication processes occurring in synovial joints and the anomalies associated with arthritic disorders. The superficial zone was therefore re-examined to assess the properties of the region which relate to the wear and lubrication processes. The presence of a transitory osmiophilic layer, 0.1-0.4 mum in thickness, was established. Combined observations from optical microscopy, transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) indicated that the layer contained phospholipid. Phospholipid is an effective lubricant and is hydrophobic. It is likely that the phospholipid acts as a lubricant in synovial joints, and its presence on the articular surface may be important to the flow of solutes in articular cartilage. The presence of the transitory stratum may account for the variety of surface morphologies reported when different methods of examination were used. Observations were also made on the collagenous matrix of the superficial zone of articular cartilage.