Tartrate-resistant acid phosphatase is not an exclusive marker for mouse osteoclasts in cell culture.

The method of Barka and Anderson was used for the demonstration of tartrate-resistant acid phosphatase (TRAcP) in cultures of bone marrow, spleen, lung, and peritoneal cells of the mouse. The staining was performed either in the usual way by adding both substrate (naphthol-AS-BI-phosphate) and coupler (hexazonium pararosanilin) together (the simultaneous-coupling technique) or by adding first the substrate and then the coupler (the post-coupling technique). We measured TRAcP-activity fluorometrically after extraction of the product naphthol-AS-BI, using the same staining solution as in cytochemical method, but without the coupler. In bone marrow, spleen, lung, and peritoneal cell cultures a biochemically measurable TRAcP-activity was detected. Post-coupling generally gave a higher level of staining and larger numbers of TRAcP-positive cells than simultaneous-coupling. In bone marrow cultures macrophages, identifiable by their ability to phagocytose microspheres, became TRAcP-positive during culture. In lung cell cultures cells capable of phagocytosis of bacteria were shown to be TRAcP-positive. Peritoneal macrophages remained TRAcP-negative in the simultaneous-coupling technique. Using the post-coupling technique a small number stained TRAcP-positive. In spleen cell cultures TRAcP-positive cells containing hemosiderin were visible. In cultures of all four cell types, F4/80 positive cells staining also for TRAcP were present. F4/80 is a well known marker for macrophages, whereas osteoclasts are negative. In conclusion, mouse macrophages originating from various tissues can become TRAcP-positive in vitro. TRAcP activity alone is not a reliable marker for osteoclasts in bone marrow cultures.