Any of certain polypeptides (see growth factor) that are structurally similar to the hormone insulin and promote division and growth of cells. There are two principal types: IGF-I (somatomedin C) and IGF-II (somatomedin A). Growth hormone stimulates the liver to release IGF-I, which circulates in the bloodstream bound to several specific IGF-binding proteins. It exerts its effect by binding to IGF-I receptors on target cells. IGF-I is also produced locally in various tissues, including kidney, muscle, bone, and the gastrointestinal tract, where it can mediate effects by autocrine or paracrine signalling.