Characterization of the functional stages of osteoclasts by enzyme histochemistry and electron microscopy

Abstract To investigate the functional stages of osteoclasts, the ultrastructural histochemical distribution of the lysosomal enzymes [acid phosphatase (tartrate‐sensitive) and neutral phosphatase], the plasma membrane enzymes [alkaline phosphatase, Ca ++ ‐ATPase, and alkaline ouabain‐insensitive p‐nitrophenylphosphatase (alkaline p‐NPPase)], and the mitochondrial enzyme (cytochrome C oxidase) was evaluated in the chicken tibial metaphysis. Both active‐appearing and detached (resting) osteoclasts were studied. Serial sectioning was used to identify detached osteoclasts which were present in the perivascular space. The ultrastructure of detached osteoclasts was similar to that of active osteoclasts, except for the lack of a ruffled border and clear zone, and an altered distribution pattern of small vesicles. Small vesicles were uniformly distributed in the cytoplasm of resting osteoclasts, whereas they were concentrated beneath the ruffled border of active osteoclasts. Alkaline p‐NPPase, a marker enzyme for the basal ruffled border, was also apparent on the membrane of small vesicles. However, the vesicles did not possess Ca ++ ‐ATPase, a marker enzyme for the apical plasma membrane. These findings support the concept that small vesicles serve as a membrane reservoir for the ruffled border membrane. Pre‐osteoclasts contained abundant mitochondria and lysosomes, prominent Golgi complexes, moderately developed endoplasmic reticulum, and lacked small vesicles. Pre‐osteoclasts appear to fuse with osteoclasts which are attached to the bone surface, but not with detached osteoclasts. The small vesicles, from which the ruffled border arises, are absent from pre‐osteoclasts, suggesting that they develop after fusion with pre‐existing osteoclasts or after attachment to the bone surface. Alkaline p‐NPPase appears to be a marker for differentiation of pre‐osteoclasts to mature osteoclasts.