Hormonal Effects on Mammary Cytology

Abstract

This reviews cytological studies of hormonal effects on the mammary gland. Normal lobuloalveolar growth in the intact animal requires the stimulation of estrogen and progesterone. Exogenous estrogen alone brings about limited growth in ovariectomized mice but a combination of exogenous estrogen and progesterone brings about growth similar to early pregnancy. Proliferation of mammary epithelium is under the direct influence of estrogen and progesterone. Alveolar and terminal duct epithelia undergo a marked morphological change during lactogenesis. Ultrastructural differentiation of the epithelium occurs immediately before and after parturition. The need for pituitary and adrenal hormones in addition to ovarian steroids for the progression of lactogenesis has been shown by endocrine ablation studies. Midpregnancy mouse mammary tissue exposed in vitro to the lactogenic hormone combination of glucocorticoid insulin and prolactin develops alveolar cells displaying all the accepted ultrastructural correlates of lactogenesis and lactation. This emphasizes the primary action of these 3 hormones on mammary epithelial structure. Human chorionic somatomammotrophin will substitute for pituitary prolactin in initiating secondary development of mouse and rabbit mammary tissue in vitro. Growth hormone is not needed in culture for lactogenesis unless insulin cortocoid and prolactin are in minimal amounts. Nonsecretory midpregnancy mouse mammary cells treated with insulin divided in vitro into cells ultrastructurally identical to parent cells. These daughter cells when treated with hydrocortisone developed a granular endoplasm reticulum but not milk protein or lactose synthesis. They divide only once and cannot be maintained long in a secretory state. Cytomorphological studies of induced lactogenesis in pseudopregnant rabbits showed that mammary cells do not develop to full synthesizing capacity until 72 hours after prolactin treatment and some hours after organelle development. Steroid hormones may inhibit lactogenesis either alone or in concert with lactogenic hormones. The observation that dairy cows yield significant milk prepartum shows that mammary growth and development are not absolutely parallel with elevated hormones at partuition. Also a gland-specific lactogenic inhibitor mechanism independent of hormones has been shown in cows.