A total of 12 undecalcified and decalcified oto-sclerotic stapes containing active spongiotic lesions were examined using an electron microscope. Evidence of osteolytic osteolysis was seen in all specimens, but osteoclasts were observed in only four. The advancing front in the spongiotic lesion was 'moth-eaten' due to the demineralization of canaliculi and lacunae. The de-mineralizing process appeared to be initiated by degranula-tion of lysosomes by the degenerating osteocytes. As bone resorption continued, poorly mineralized new bone was laid down by osteoblasts which showed mitochondrial swelling. The possible implication of this mitochondrial swelling is discussed in relation to the pathology of the otoscierosis.
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