Placental Lactogen and Growth Hormone

The importance of the anterior pituitary-derived hormones, growth hormone (GH), and prolactin (PRL) in regulating postnatal growth and lactogenesis is well documented in a variety of species. However, the necessity of GH and PRL in stimulating fetal growth and mammogenesis during pregnancy is more controversial. For example, the lack of effect of anencephaly or congenital absence of the pituitary gland on human fetal growth rate led to the conclusion (1) that fetal development occurs independent of fetal pituitary-derived GH. Yet infants with idiopathic GH defeciency (2) or GH receptor dysfunction (3) exhibit clinical in utero growth retardation, suggesting that in fact GH may have actions within fetal tissues. Furthermore, the importance of these hormones during pregnancy may vary between species. In cattle and sheep, the inhibition of PRL secretion during pregnancy does not impede mammary development (4), whereas in hypophysectomized rats lobulo-alveolar growth is stimulated with exogenous PRL (5). The difficulty in defining the requisite roles for GH and PRL during pregnancy at least in part results from the fact that during pregnancy placental-derived members of the GH/PRL gene family are produced, which have common functions providing sufficient redundancy to enhance the likelihood of an optimal pregnancy outcome.