The functional role of insulin in fertility and embryonic development-What can we learn from the bovine model?

Insulin is a key metabolic hormone that plays a crucial role in regulating energy homeostasis in the body. In addition, insulin-dependent signaling has important functions in reproduction and early embryo development. As metabolism and reproduction are closely linked, metabolic challenges may be the source of reproductive disorders and decreased fertility. This is known for the dairy cow and for other species including the human. Although metabolic disorders in the dairy cow often derive from a failure to adapt to a high milk production, the situation in the human is often linked to emerging conditions and associated diseases in our modern society such as obesity and diabetes, where an excess energy intake causes decreased fertility in women. Both energy excess and energy deficit are associated with a deviation of insulin concentrations in serum and follicular fluid from normal levels. Although many studies have shown that extreme variation in energy supply can negatively influence early embryo development by inducing changes in circulating concentrations of several metabolites or hormones like insulin, several in vitro culture media are still supplemented with insulin in high concentrations. In this review, direct and indirect effects of insulin on fertility will be described. Differences between the in vivo and in vitro situations will also be discussed.