Growth hormone as an early embryonic growth and differentiation factor.

In this review we consider the evidence that growth hormone (GH) acts in the embryo as a local growth, differentiation, and cell survival factor. Because both GH and its receptors are present in the early embryo before the functional differentiation of pituitary somatotrophs and before the establishment of a functioning circulatory system, the conditions are such that GH may be a member of the large battery of autocrine/paracrine growth factors that control embryonic development. It has been clearly established that GH is able to exert direct effects, independent of insulin-like growth factor-I (IGF-I), on the differentiation, proliferation, and survival of cells in a wide variety of tissues in the embryo, fetus, and adult. The signaling pathways behind these effects of GH are now beginning to be determined, establishing early extrapituitary GH as a bona fide developmental growth factor.