The relationship between calcium accumulation in osteoclast mitochondrial granules and bone resorption

In the process of bone resorption, calcium is considered to be transported within vesicles in osteoclasts and eventually released. We studied the ultramicromorphology of calcium (Ca) transport in osteoclasts by preparing samples of osteoclasts collected from rat femurs in which calcium was maximally preserved and subjected them to high-pressure quick-freezing and freeze-substitution. We then examined the localization of calcium by Electron Energy Loss Spectroscopy (EELS). The structures of cell membranes were preserved, suggesting the suitability of this high-pressure quick-freezing and freeze-substitution technique. Osteoclast mitochondria adjacent to the ruffled border were rich in mitochondrial granules and contained a large amount of Ca. In contrast, mitochondria in the basolateral region contained few granules. Moreover, by an osteoclast-culturing experiment, differences in the morphology of mitochondrial granules were noted between culturing on a dentin slice and that on a gold plate, i.e., few mitochondrial granules were noted in osteoclasts cultured on a non-dentin plate. These findings suggest an association between the morphology of mitochondrial granules in osteoclasts and bone resorption as well as a new transport route for Ca resorbed by osteoclasts. We propose that Ca accumulates in mitochondria granules to prevent increased Ca concentration in cytoplasm of osteoclasts during bone resorption.