Elevated levels of inorganic phosphate are required for formation of a localized calcified layer by deep zone articular chondrocytes in vitro

Abstract

Purpose: Articular cartilage (AC) has a depth-dependent zonal organization with a deep zone (DZ) that merges into a zone of calcified cartilage (ZCC). The ZCC is a highly mineralized region which anchors hyaline cartilage to the subchondral bone. It maintains structural integrity of the osteochondral unit and enables force transmission in the joint. The tidemark, a metabolically active structure separating the DZ from the ZCC, may function as a calcification front and represent a barrier to cartilage mineralization.Mineralization involves accumulation of phosphate and calcium in matrix vesicles, production of hydroxyapatite crystals, and propagation of these crystals into the extracellular matrix.