Abstract
The presence and distribution of alkaline phosphatase, adenosine triphosphatase (ATPase) and acid phosphatase activities in the epiphyseal plates of young mice were studied by electron microscopic cytochemical methods. Alkaline phosphatase and ATPase activities were associated with the plasma membranes of chondrocytes and with the investing membranes of matrix vesicles. These vesicles contain the earliest recognized deposits of hydroxyapatite and may promote calcification through an active process. Alkaline phosphatase and ATPase activities were greatest in the hypertrophic zone of the epiphysis, which is an area of beginning calcification. Acid phosphatase activity was demonstrable in the cytoplasm of chondrocytes in association with dense bodies which were larger than matrix vesicles and were devoid of alkaline phosphatase and ATPase activities. These cytoplasmic lysosome-like bodies were slightly more numerous in the hypertrophic zone but disappeared in the underlying zone of chondrocyte degeneration and matrix calcification. Our observations do not support a previous suggestion by others that matrix vesicles represent lysosomes. The presence of ATPase and alkaline phosphatase is compatible with an enzymatic calcium- and/or phosphate-concentrating mechanism in the matrix vesicles.
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