Rat pituitary changes observed with magnetic resonance imaging following removal of estrogen stimulus: correlation with histopathology and immunohistology.

The effect of estrogen withdrawal on pituitary glands of rats treated with estradiol-17 beta for various lengths of time was monitored by magnetic resonance imaging (MRI) and histological examination. Estrogen pellets were removed at seven different time points ranging from 4 to 206 days after pellet implantation. High-resolution mid-sagittal MR images of the rat head were made 1 day before pellet implantation, immediately following pellet withdrawal, and 14 and 28 days after pellet withdrawal. Twenty-eight days after pellet withdrawal pituitary glands were fixed and processed for histological examination. Enlarged pituitaries were detected by MRI from 16 days after implantation and onwards. Twenty-eight days after estrogen withdrawal the typical triangular shape of the normal pituitary had returned and pituitary morphology was indistinguishable from that of normal pituitaries in all rats that had been treated with estrogen for up to 114 days. Pituitaries of rats that had received estrogen for 186 days had a normal MR image 28 days after estrogen withdrawal, but microscopic examination revealed multifocal hyperplasias of prolactin-positive cells throughout the pars distalis. MRI of rats treated for 206 days showed tumorously enlarged pituitaries. There was no evidence of tumor regression in these rats 28 days after pellet removal. It was concluded that hypertrophic pituitaries regained a normal size, shape and morphology after estrogen withdrawal, except for a remarkable type of hyperplasia following estrogen treatment for 186 days and a recovery period of 28 days. In tumorous pituitaries no regression of lesions was noticed.