Molecular biological techniques are being used to discern gene expression of insulin, insulin-like growth factors (IGFs) and their receptors and actions in several vertebrate model systems. Insulin is apparently a requirement for normal development, and the chicken embryo is one of the suitable models for studying the role of insulin in development. The ontogeny of expression of the insulin gene has been studied and the gene for IGF-I and IGF-I receptor are cloned. The role of insulin in cell differentiation and gene expression in the eye lens of chick embryo is also being used as a cell model to understand the action of insulin in development. The amphibian, Xenopus laevis, is also a model system used to study development. The amphibian insulin and insulin-like growth factor I genes have been isolated and their sequences determined and confirmed by molecular cloning. Studies are in progress to define the expression of these peptides during Xenopus development using the polymerase chain reaction. A transgenic mouse line with multiple copies of the human insulin gene integrated into its genome has been established. The degree of hyperinsulinemia correlates with human gene copy numbers. The transgenic mice provide a model system for studies in regulation of insulin gene expression and the effects of chronic hyperinsulinemia on glucose homeostasis.
