Naturally Occurring Polypoid Hyperplasia and Papilloma in Gallbladders of the Fat Sand Rat (Psammomys obesus terraesanctae Thomas)

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Abstract
The gallbladders of 256 fat sand rats (131 males and 125 females) seven to 41 months old were studied histologically. None of the sand rats had been used in experimental procedures prior to death. Diffuse polypoid hyperplasia or papilloma of the gallbladder mucosa was found in 49 sand rats. Of these lesions, 45 were present in 126 sand rats in the second half of their lifespan. Neither concretions nor signs of chronic inflammation were seen. Signs of focal epithelial growth activity were seen in either form of lesion. Epithelial hyperplasia was excessive in two gallbladders, and one was diagnosed as carcinoma-in-situ. The papilloma in one sand rat revealed transition to adenocarcinoma, with penetration into the deep layers of the gallbladder. Preneoplastic nodules were found in the liver as frequently as reported previously, but they were unrelated to gallbladder lesions. The unusual high incidence of spontaneous epithelial proliferation in the gallbladder mucosa in sand rats of both sexes through several generations suggests this strain as a model in the experimental study of tumorigenesis in the gallbladder.

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