Spontaneous Pituitary Carcinoma of the Pars Intermedia in a B6C3F1 Mouse

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We encountered one pituitary carcinoma derived from the pars intermedia in an untreated female B6C3F1 mouse in a carcinogenicity study. A white nodule (6 × 5 × 3 mm) was observed in the base of the brain at necropsy. Tumor cells were histologically arranged in a solid sheet or nest-like structures, and had round or oval nuclei and pale eosinophilic granular cytoplasm. Sporadic mitotic figures were seen in the tumor while necrosis was a prominent feature. In addition, distinct infiltration into the brain parenchyma was observed. Immunohistochemically, tumor cells were positive for α-melanocyte stimulating hormone (α-MSH), γ-melanocyte stimulating hormone (γ-MSH), β-endorphin and adenocorticotropic hormone (ACTH), but were negative for prolactin. However, there were no histological abnormalities such as adrenocortical hyperplasia related to the pars intermedia tumor. These results suggest that the tumor cells might not have actively secreted proopiomelanocortin (POMC)-derived peptides. The present report provides additional histopathological evidence of pituitary carcinoma of the pars intermedia in aging mice.