Regression of redundant lactotrophs in rat pituitary gland after cessation of lactation.

Regressive changes occurring in the pituitary gland of the rat after removal of litters were studied. Pituitary glands of lactating rats were characterized by the presence of numerous hypertrophied lactotrophs. Interruption of lactation caused a blockade of prolactin synthesis and secretion, followed by degeneration of lactotrophs. Morphometric analysis of pituitary glands revealed that lactotrophs accounted for about 50% of the total hypophysial cell count in lactating rats. This percentage decreased progressively and reached pre-pregnant levels 7 days after removal of litters; the decrease was inversely correlated with an increase in the number of degenerating lactotrophs which comprised 30% of all lactotrophs 72 h after removal of litters. The morphological changes found in lactotrophs were closely related to changes in the prolactin content of serum and the pituitary gland. Regression of lactotrophs appeared to be the most important cause inducing the reversal of hypophysial lactotrophic activity to pre-pregnant conditions.