Hormonal regulation of breast development and activity

The prepubertal mammary gland consists of simple ducts or tubes whose walls are composed of epithelial cells. The process of differentiation seems to include cellular proliferation at irregular sites along the wall of these ducts. The proliferating regions ultimately give rise to the alveoli, the site of milk secretion. Which hormone or hormones serve as the primary mitogenic agent or agents is not certain. However, under in vivo conditions, prolactin seems to be an especially potent mitogenic substance. Under in vitro conditions, insulin stimulates cell division for a time. During the proliferative phase, an increase in DNA indicates increased activity of the replicative process. Increased RNA synthesis also indicates an enhancement of the transcriptional process. Cortisol stimulates synthesis of the rough endoplasmic reticulum and ribosomes and stabilizes the rough endoplasmic reticulum ribosome complex. There is evidence that prolactin also stimulates secretory protein formation. Several hormones are involved in the complete differentiation process of the secretory epithelial cell. It is not clear whether a given hormone acts at a single focus or at several foci. There is evidence that several hormones do affect equivalent events in the differentiation process.