Hormonal Development of the Mammary Gland of Dairy Heifers

The important activities of the secretions from the endocrine glands in the development of the mammary glands of several species have been indicated by a large body of experimental data.Recent reviews by Petcrsen (17), Turner (21) and Folley (2, 3) have summarized and integrated most of these studies.The predominant role of the ovarian hormones--the ~strogens and progesterone--and of certain pituitary hormones has been emphasized.In most species both estrogen and progesterone are necessary for duct and lobule-alveolar growth in intact animals.The guinea pig and possibly the monkey are exceptions in that complete mammary development can be induced by estrogen alone.In hypophysectomized animals, pituitary hormones (notably prolactin) are necessary, in addition to estrogen and progesterone, for mammary growth comparable to that developed during pregnancy.Very few detailed studies have been conducted with the larger domestic animals.In 1941, Walker and Stanley (22) induced udder development and copious lactation in a castrate heifer by injecting steroid hormones.Subsequently, Folley and Malpress (5), Hammond, and Day (9) and Spriggs (19) on an extensive scale showed that marked udder development and copious lactation could be induced in unbred heifers by subcutaneous implantation of synthetic estrogen tablets.The copious lactation induced in many of these animals led to the obvious inference that complete mammary development had been induced by estrogen and that progesterone was not necessary for udder development in the bovine.No detailed histological examination of the udder tissues of these animals was made.Milk yield was the main criterion upon which estimates of the normality of mammary development were based.The above studies were characterized by great variability in the gross appearance of the udder and in the lactation responses obtained, indicating that estrogen alone will not invariably induce udder development and lactation in the bovine.The lactation responses also usually were quantitatively smaller than might normally be expected.Therefore, on this basis also, it might be questioned whether completely normal udder development had taken place.The generally poor responses of freemartins (5, 9) to estrogen implants likewise would cast some doubt on the assumption that progesterone is unnecessary for complete mammary development in the bovine.Results similar to the above have been obtained with the goat (6, 10, 14, 16).More detailed histological examination of the udder tissue was made in these Received for publication Dec.