Spontaneous Tumors and Atypical Proliferation of Pancreatic Acinar Cells in Mastomys (Praomys) natalensis2

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Abstract
In 17 of 90 untreated 2-year-old Mastomys (Praomys) natalensis, an acinar cell tumor was observed, and atypical acinar proliferation was noted in 30. The lesions, either neoplastic or proliferative, consisted of large polygonal cells with prominent nucleoli in the enlarged hyperchromatic nuclei. The cytoplasms of these cells were polychromatic and were characterized by a central eosinophilic granular area and a peripheral basophilic fibrillar zone around an eccentric nucleus. The differentiated cells retained acinar organization with occasional formation of small luminal spaces, but the less-differentiated cells, particularly in the anaplastic tumor nodules, lacked polarity and lost almost completely the normal acinar architecture. Electron microscopy of 1 tumor nodule revealed numerous zymogen granules of various sizes and an abundance of often dilated, rough endoplasmic reticulum. No definite ductlike structure was found within the lesions, and metastatic spread was not evident in any of the organs examined. Instead, normal pancreatic tissues of Mastomys frequently contained small foci of degenerated acini in which thin pseudoducts proliferated.