Growth hormone and prolactin are pituitary hormones that are important in normal growth and development and in lactation post-partum. These hormones have protean effects on many cells and organ systems. We showed that development of the mammary gland is controlled initially by growth hormone and that prolactin has an important effect on the mammary gland at later developmental stages. In the absence of growth hormone, mammary development in rats and monkeys does not occur. Normal mammary development occurs at puberty under the influence of growth hormone and estrogen. Our research demonstrates that growth hormone works via the growth hormone receptor and stimulates local production of insulin-like growth factor I (IGF-I) within the mammary gland. That IGF-I then stimulates development of mammary gland terminal end buds and alveolar structures is supported by the fact that IGF-I can substitute for growth hormone in this process. Additionally, animals that are deficient in IGF-I do not have normal mammary development.

Our current work is to further uncover the mechanisms by which growth hormone stimulates mammary development and determine the interaction of hormones in various tissue types within the mammary gland. We hypothesize that a full under-standing of mammary development will help develop treatment for breast cancer.