Ultrastructural findings in a case of CEOT are reported. It was found that the tumour cells consisted of two populations. One population was composed of polyhedral epithelial cells of the type described previously by other authors. These cells showed variation in size and abundant cytoplasm. They contained large numbers of electron-dense bundles of tonofilaments and numerous mitochondria. The other population was a cell having the ultrastructural characteristics of myoepithelial cells. These cells were arranged juxtaposed to the tumour epithelial cells and had very elongated profiles. They showed a lamina densa which was continuous with that in relation to the basal plasma membrane of the tumour epithelial cells and also a large number of hemidesmosomes. These cells also contained large numbers of fine filaments with electron-dense bodies similar to those described in smooth muscle cells. The amyloid-like material was found to consist of two types of structures which are probably related. One type showed sheets of low electron-dense filaments, the other aggregates of lamina densa fragments.

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