Insulin and Insulin-Like Growth Factor I in Early Development: Peptides, Receptors and Biological Events*

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Abstract
THE goal of this paper is to sketch the present understanding of the role that insulin and insulin receptors, as well as insulin-like growth factor I (IGF-I) and its receptors, seem to play in early embryogenesis. This approach overturns ideas of previous decades which have suggested that hormones function relatively late in embryogenesis, i.e. only after the individual glands envelop (1, 2); that insulin acts largely as a regulator of metabolic events in a narrow range of metabolically important target tissues (e.g. muscle, fat, and liver); and that IGF-I is mainly a postnatal growth stimulator (3). In contrast, it is now broadly accepted that 1) insulin is expressed before there is an embryonic pancreas, 2) IGFI is expressed before liver develops, and 3) both insulin and IGF-I influence metabolic, growth, and differentiation processes in a very wide range of cell types (4). We shall begin with an overview of the receptors, essential components of the hormone action cascade. Over the last decade it has been recognized that the receptors for insulin and IGF-I have similar biochemical activities, (5, 6) both in the basal and stimulated states, and both are very widely distributed throughout tissues at all ages and stages (7).

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