DISTRIBUTION OF ACID AND ALKALINE PHOSPHATASE ACTIVITY IN UNDEMINERALIZED SECTIONS OF THE RAT TIBIAL DIAPHYSIS

Abstract

The distribution of acid and alkaline phosphatase activity in the rat tibia has been studied in transverse sections sawed from fresh undemineralized diaphyses. The endosteal resorbing surfaces had the highest acid phosphatase activity as demonstrated with 50 mM α-naphthol phosphate as the substrate. Strong activity was present not only in osteoclasts and adjacent osteocytes but extracellularly lining Howship's lacunae. In areas of bone formation, moderate acid phosphatase activity was present in osteoblasts and young osteocytes. In addition, a zone of activity was found at the mineralizing front where mineralization is initiated. This active zone ( a) was separated from the active periosteal cell layer by a zone of inactive osteoid and ( b) in formol-calcium-fixed demineralized sections extended into young bone. The identity of the mineralizing front was confirmed by tetracycline labeling. The activity at the mineralizing front had the properties of an enzyme. Alkaline phosphatase activity demonstrated with naphthol ASTR phosphate was largely associated with the osteoblasts and other mesenchymal cells at forming surfaces.