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To clarify the histopathological progression of invasive tumors in the pituitary pars distalis due to estrogen, female Fischer 344 (F344) rats were treated subcutaneously with 5 mg/animal of estradiol dipropionate (ED) once every 2 wk for 13 wk. The animals were killed serially at 2-wk intervals during the investigation. The pituitaries with surrounding tissues were examined light microscopically. At week 7, pituitary cells showed proliferation and atypia with formation of blood-filled spaces. Lesions with these characteristics were diagnosed as adenomas. At week 9 or later, neoplastic cells exhibited extensive proliferation and infiltration into the surrounding tissues, suggesting development of carcinoma. Both proliferating cell nuclear antigen (PCNA) and 5-bromo-2'-deoxyuridine (BrdU) labeling index, markers of cell proliferation, were significantly increased in animals with adenoma or carcinoma. To detect sequential changes in pituitary weight, its signal intensity was periodically monitored in identical rats by using magnetic resonance (MR) imaging. The estimated pituitary weights revealed by MR imaging were comparable to the tumor weights obtained from rats at scheduled sacrifices. These results indicate that ED possesses the potential to cause carcinoma in rat pituitary and MR imaging is an effective tool for estimating the pituitary weight.

Invasive Pituitary Tumors in Female F344 Rats Induced by Estradiol Dipropionate