Abstract
A case of spontaneous trichoepithelioma having a partial granular cell appearance, which was found in a 109-week-old male F344Du/Crj rat, is described. Grossly, a white soft subcutaneous nodule approximately 16×13mm in diameter was found at the right axilla of the animal. Histologically, the tumor showed two structural patterns, one composed of eosinophilic granular cell nests and the other resembling trichoepithelioma composed of basal cells in ribbon or solid pattern nests occasionally with horny cysts or whorls. The cytoplasm of the granular cells was positively stained with PAS and was diastase-resistant. Trichoepithelioma components were strongly immunostained for keratin, but granular cells were weakly stained for keratin. None of the tumor cells were stained for S-100 protein, desmin, and vimentin. Ultrastructurally, the granular cell cytoplasm was filled with lysosome-like structures containing numerous electron-dense bodies. Many of the tumor cells had a prominant desmosome with attached tonofilament. These findings appeared to be essentially similar to those of the granular cell basal cell tumor in dogs and humans. However, the most appropriate diagnosis of this case may be “trichoepithelioma with granular cell appearance or trichoepithelioma, granular cell type”, based on its characteristic his-tological features showing a differentiation to hair follicles.
