Pituicytoma: primary astrocytic tumor of the pars nervosa in aging Fischer 344 rats.

We describe 2 cases of a relatively rare tumor diagnosed as pituicytoma in the pars nervosa of rat pituitary. This tumor was spontaneously noted in one 110-week-old female and one 109-week-old male Fischer 344 (F344)/DuCrj rats during 2-year carcinogenicity studies. Although no gross abnormality of the pituitary was detected in the female rat, whitish discoloration and enlargement of the pituitary were observed in the male. Histopathologically, neoplastic cells in both animals possessed pale eosinophilic, often abundant irregular cytoplasm with nuclei of variable size. The tumor cells were arranged in the spindle or sheet cell pattern with indistinct cell boundaries and showed compression or invading proliferation of surrounding tissues. Prominent pleomorphism of the cells was noted in the tumor in the female rat, and mitotic figures were detected in several portions of the tumor in the male rat. Small-sized cells having scanty cytoplasm with deeply staining nuclei seen in the mass were suspected to be microglia. Moreover, isolated single native pars distalis cells were distributed throughout the tumor masses. Immunohistochemically, cytoplasmic foot process of the tumor cells showed a positive immunoreactivity for glial fibrillary acidic protein. On the basis of morphologic characteristics and glial fibrillary acidic protein staining, this tumor is consistent with astrocytoma.