Growth hormone and mammary development

Classic studies in rodents conducted in the 1950s showed that growth hormon e (GH) is essential for mammary development both in the pubertal phase and during pregnancy. Since then, a considerable number of experiments have bee n carried out in ruminants to investigate the role of GH for regulation of normal mammary development and to examine the possibility of enhancing mamm ary growth by administration of GH. The available evidence demonstrates tha t GH treatment stimulates mammary growth before puberty, but the data do no t convincingly support the idea that the effect is translated into increase d milk yield. GH treatment during late pregnancy seems to stimulate both ma mmary growth and milk yield during lactation. The limited data concerning t he effect of GH on mammary growth during lactation indicate that mammary gr owth is unaffected by GH treatment in early lactation, whereas GH seems to increase the amount of mammary parenchyma in mid-lactation. The mechanism o f action of GH remains a puzzle, but the effect of exogenous GH most likely involves insulin-like growth factor-I (IGF-I). Full understanding of the r ole of endogenous GH for regulation of normal mammary development requires more knowledge about the interaction between GH and IGF-I and the interplay between the GH-IGF-I axis and locally produced factors, including receptor s, binding proteins, and growth factors. (C) 1999 Elsevier Science Inc. All rights reserved.