Insulin-like growth factors and their receptors in growth

IGF-1 and IGF-II stimulate proliferation and survival of many cell types and in some cells promote differentiation and specialized function. The IGFs are synthesized throughout the body in most, if not all, tissues and at defined sites with a characteristic ontogeny of expression. They are essential stimulators of fetal and postnatal growth, with their signals being mediated predominantly by the type I IGF receptor. During organogenesis, the actions of IGF-I seem to be local (autocrine/paracrine), because it, as well as IGFBPs and the type I IGF receptor, are expressed coordinately in a highly tissue and developmentally specific fashion. IGF-I, however, likely assumes endocrine functions during postnatal life and provides signals for growth hormone and nutritional status. IGF-II on the other hand, may have only local growth-promoting functions, and its most essential actions may be restricted to in utero life. Both IGFs are involved in regenerative organ growth, and their uncontrolled expression, especially that of IGF-II, is likely involved in the formation of some tumors.