Introduction: Growth is regulated by the interaction of environmental signals with endogenous neuroendocrine responses to the genetic programs that determine the body plan. The insulin-like growth factors (IGFs) are integral components of multiple systems controlling both growth and metabolism.
The igf system: The IGF system is thouht to be more complex than other endocrine systems, as genes for six IGF-binding proteins (IGFBPs) have been identified so far. The IGFs play a critical role in both cell cycle control and apoptosis, two functions involved in regulation of tumorigenesis. Insulin-like growth factor-I (IGF-I) is essential for normal growth. Confirmation of the significance of IGF-I in human physiology was obtained by the discovery of a patient with intrauterine growth retardation and postnatal growth failure associated with a mutation in the IGF-I gene.
Stages of evolution of the somatomedin hypothesis: The original somatomedin hypothesis postulated that somatic growth was regulated by growth hormone's (GH's) stimulation of hepatic IGF-I production, with IGF-I acting in an endocrine fashion to promote growth. The dual effector theory proposed an alternative view, involving direct effects by GH on peripheral tissues not mediated by IGF-I and GH-stimulated local IGF-I production for autocrine/paracrine action. It is now clear that GH stimulates the formation of ternary IGF binding complex, which stabilizes IGF-I in the serum.