Quantitative and distributional changes in the activity of alkaline phosphatase during the maturation of cartilage.

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Abstract
The quantitative changes in the activity of alkaline phosphatase during the maturation of cartilage cells were evaluated based on morphometric measurements at the ultrastructural level. Undifferentiated chondroprogenitor cells revealed positive reaction products for alkaline phosphatase in their nuclei and along their plasma membrane. With the differentiation of the progenitor cells to chondroblasts an intensification of enzyme activity took place along the plasma membrane; the cell nuclei lost their reactivity to the enzyme. A significant increase in the cellular (plasma membrane) enzyme activity was noted in the mature hypertrophic chondrocytes. Enzyme activity was seen in fully matured chondrocytes located deep within the mineralization zone. A different pattern of enzyme activity was observed in the pericellular matrix along the various zones of the maturing cartilage. A significant decline in enzyme activity accompanied the transition of the chondroprogenitor zone to the chondroblastic zone, followed by an increase in the premineralizing hypertrophic zone. In the former zones, the enzyme was closely associated with the matrical collagen, whereby in the latter zone, the enzyme''s reaction products were consistently associated with the many matrix vesicles that characterized the premineralizing matrix. Despite the fact that the precise role of alkaline phosphatase in ossifying cartilage is not as yet fully understood, in addition to its apparent evolvement in the mineralization process, it apparently is actively involved in the metabolism of cartilage cells throughout their life cycle.