Preimplantation mouse embryos internalize maternal insulin via receptor-mediated endocytosis: pattern of uptake and functional correlations.

High resolution microscopy in conjunction with colloidal gold-labeled insulin has been used to provide evidence that insulin is internalized by preimplantation mouse embryos by means of receptor-mediated endocytosis and concentration in coated pits. In addition, immunocytochemical analyses at the blastocyst stage, using gold-labeled anti-insulin receptor immunoglobulin (IgG) have confirmed the expression of insulin receptors on all cells of the embryo, including the inner cell mass. Immunocytochemical studies using gold-labeled anti-insulin IgG have provided evidence that the insulin internalized by the embryo is maternally derived. Functional studies show that incubating embryos in physiological levels of insulin results in increased synthesis of RNA and DNA. We conclude that insulin may play a role in early mammalian development, although the precise function of this hormone remains to be defined.