Histochemical and biochemical studies on the localization and activity of lysosomal enzymes in fracture callus

Summary Quantitative biochemical studies on the activities of four lysosomal hydrolases during different stages of fracture healing in the rat were performed, and the results obtained were integrated with those of histochemical observations relating to changes in the localization of acid phosphatase in the same tissue.The findings showed presence of all the four lysosomal enzymes assayed in the callus; during early callus formation the enzyme activities calculated on a DNA basis increased up to about 12 days after the fracture. The enzyme activities appeared to be roughly reflected histochemically by the acid phosphatase staining. The increasing activity during early callus formation seemed to depend on the presence of numerous macrophage-like cells in the tissue containing many large lysosomes. A decrease in enzyme activity was found after day 12. Comparison with the histochemical and ultrastructural findings suggested that this decrease was due to a reduction in the number of macrophage-like cells and a concomitant increase in osteogenic cells with a lower enzyme content.