The ultrastructural localization of secretory acid phosphatase in giant cell tumor of bone

Three cases of giant cell tumor of bone were studied with the light and electron microscopes to determine the histochemical and cytochemical distribution of acid phosphatase isoenzymes. Using β-glycerophosphate as a nonspecific substrate, acid phosphatase was found in the giant cells as well as the stromal cells. Ultrastructurally, using this substrate, the enzyme was found to be associated with lysosomes in the stromal cells and giant cells and also with the profuse system of tubules and vesicles in the cytoplasm of the giant cells. Using phosphorylcholine and D-ephedrinephosphate, which are substrates for a specific secretory isoenzyme of acid phosphatase, activity was found only in the vesicles of the giant cell cytoplasm. Lysosomes did not show activity with these substrates. Multinucleated cells of giant cell tumor contain a specific secretory type of acid phosphatase which is not present in the stromal cells. This suggests that the giant cells are attempting to form a secretory system similar to osteoclasts and that the vesicles in the cytoplasm which contain this acid phosphates are the morphologic expression of an abortive secretory system.