Annulate lamellae in spontaneous prolactin cell adenomas of the rat pituitary.

Electron microscopy of 13 pituitary prolactin cell adenomas, removed at autopsy from untreated female aging Long-Evans rats, revealed the presence of annulate lamellae in 3 tumors. No annulate lamellae were detected in 10 non-tumorous pituitary glands of adult untreated rats. Direct continuity was demonstrated between annulate lamellae and rough-surfaced endoplasmic reticulum, consistent with the assumption that annulate lamellae may originate from and/or transform to endoplasmic reticulum membranes. The results indicate that annulate lamellae are not uncommonly formed in prolactin cell adenomas, i.e. in cells which have a faster growth rate than non-tumorous adenohypophysiocytes.