Fine structural localization of alkaline phosphatase in the fracture callus of the rat

Summary The fine structural localization of nonspecific alkaline phosphomonoesterase in the different cells constituting the fracture callus in the rat was studied by incubating sections of glutaraldehyde-fixed callus tissue of variable age in media containing -glycerophosphate and either lead or calcium ions. The specificity of the reactions were tested by exposing the tissues to inhibitors of alkaline phosphatase.The results showed presence of final product on the plasma membranes and associated structures (subplasmalemmal endocytotic vesicles) of fibroblasts, pre-osteoblasts, osteoblasts, and cartilaginous cells in the callus. With the calcium method, reaction product was demonstrated in vesicular elements of the Golgi apparatus in osteoblasts and chondrocytes. Precipitates indicating presence of alkaline phosphatase activity were also observed on the membranes bordering cytoplasmic projections and fragments of cytoplasm located adjacent to enzyme-containing cells. Furthermore, the globule-shaped bodies in the matrix (Bonucci-bodies) showed evidence of alkaline phosphatase activity.The evidence obtained supported the view that alkaline phosphatase plays a role in calcification. It is suggested that transfer of cellular alkaline phosphatase to the sites of initial calcification in the extracellular matrix occurs by way of pinched off vesicular fragments of the cytoplasm and plasma membrane of osteogenic enzyme-producing cells; these structures appear to move awy from their cells of origin to form the Bonucci bodies in the matrix.