Normal growth, differentiation, and function of the mammary gland are under the influence of hormones-insulin, estrogen, progesterone, thyroid hormone as well as growth hormone, prolactin (PRL), and placental lacto· gens. While some of these hormones may affect primarily growth and development of the mammary gland, others, such as PRL, are of major importance in stimulating mammary gland function. A comprehensive treatment of endocrine influences on the mammary gland may be found in (12). Here we highlight progress in understanding the mechanism of action of PRL on mammary gland function. Since large gaps in this understanding remain, we indicate areas where research may be fruitful. The role played by PRL on mammary gland function varies from one species to another. For example, lactational changes in the mammary gland can be induced in rabbits by administering PRL alone, while in rats other hormones are required. In the cow the peripartum increase in serum PRL is crucial for successful lactation; but if PRL secretion is inhibited at later periods postpartum, milk secretion is hardly affected. In women, however, inhibition of PRL secretion by similar pharmacological means at any time postpartum causes prompt cessation of lactation. Generalizations and ex· trapolations from one species to another must thus be made with consider· able caution. The possible role of PRL in the etiology of breast cancer, which is not covered here, has been reviewed extensively (29, 33, 7 1).
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Shiu et al. (1980) studied this question.