Enzyme histochemical studies of induced heterotopic cartilage and bone formation in guinea pigs with special reference to acid phosphatase.

Heterotopic bone and cartilage formation was studied in a bone induction model in abdominal muscles of guinea pigs using the demineralized allogenic dentin as inductive material. The light microscopic study showed high activity of acid phosphatase (AcP) and leucine aminopeptidase (LAP) in young matrix-producing cells. The highest activity was seen in cells surrounded by unmineralized matrix. In the surrounding matrix there was strong staining of AcP, especially in matrix with high glycosaminoglycan content. Electron microscopic studies of AcP in sections from unfixed frozen tissue showed intracellular activity mainly located in lysosomes. Extracellular AcP activity was found in matrix vesicles in the newly formed hard tissue. It was most pronounced in areas of cartilaginous differentiation. By means of energy dispersive X-ray microanalysis in a scanning transmission electron microscope, the deposits were confirmed to contain led precipitates. With the exception of some lysosomal staining, prefixation with formaldehyde prevented demonstration of the enzyme activity. Both AcP and LAP may be involved in degradation of organic components of the matrix which enable the matrix to mineralize.