Morphofunctional investigations on spontaneous pituitary tumors in Wistar rats

Adenoma of the pituitary gland represents one of the commonest spontaneous tumors in strains of laboratory rats. In a retrospective survey of pituitary glands from 2165 albino Wistar rats, the total number of pituitary adenomas was 501, representing an overall incidence of 23% with females showing a higher incidence (32%) than males (13%). Pituitary adenoma was rare from 6-52 weeks of age and accounted for only 0.2% of the total incidence. The first pituitary tumors in this series were observed at 32 weeks in 2 female rats and at 40 weeks in a male rat. From 52-85 weeks of age, incidence remained low, and then increased progressively in animals that died from 85-110 weeks of age. In a series of 200 Wistar rats, detailed evaluation was completed for neoplastic and hyperplastic lesions of the pituitary gland. The immunocytochemical characteristics of the pituitary adenoma were investigated using markers for prolactin, growth hormone, and thyrotropic hormone. Positive immunoperoxidase staining revealed an incidence of 59% prolactinomas in both male and female rats. Forty-one percent of pituitary adenomas did not stain for prolactin, thyrotropic, or growth hormone and showed no specific morphologic differences from prolactinomas. The application of immunoperoxidase-staining techniques offers a useful tool for characterizing secretory activity of pituitary adenomas and evaluating histopathologic changes of the pituitary gland.