Insulin-like growth factor axis during embryonic development

The insulin-like growth factor (IGF) axis has been studied extensively in t he developing vertebrate embryo. Knockout experiments have demonstrated tha t both IGF-I and -II are required for normal development in the mouse embry o, and mRNA and protein expression patterns for both growth factors, togeth er with those for the type I ICF receptor and the six IGF-binding proteins, have been analysed in embryos from different species. Although the unique temporal and spatial expression patterns of these genes indicates important roles for the ICF axis during organ and whole animal development, the vari ation and complexity of expression makes these roles difficult to unravel. However, one possible mechanism unifying the IGF system in development is p rogrammed cell death (apoptosis), which has been shown to be important in s culpting embryonic tissues, and, in particular, the developing limb bud. In addition, the very early onset of expression of various IGF family members in chicken embryos further emphasizes the fundamental importance of this s ystem in development. This article reviews the work that has been carried o ut in this area in the context of current understanding of the IGF system.