Changes in alkaline phosphatase activity in rabbit articular cartilage associated with ageing and joint contracture

Changes in alkaline phosphatase (ALPase) activity in rabbit articular cartilage induced by joint contracture were studied by light and electron microscopy. The knee joint was plaster-immobilized for 3--9 weeks in the flexion position. Three different age groups of rabbits were examined, at the age of 1 month (young), 1 year (adult), and over 2.5 years (old). ALPase activity was detected in the cartilage taken from the distal end of the femur, employing the lead citrate method. Positive activity was primarily localized in the plasma membrane of the chondrocytes and also in the pericellular matrix vesicles. Before immobilization the positive cells were distributed exclusively in the deep zone of the cartilage, whereas after immobilization activity was present in cells both in the middle and deep zones. This phenomenon was most evident in the old group.