Growth factors in preimplantation development: role of insulin and insulin-like growth factors

In vitro studies of preimplantation embryos from a number of mammalian species have shown that the oviduct and uterus contain growth factors that stimulate cellular proliferation and differentiation of preimplantation embryos. The mammalian preimplantation embryo was first viewed as an autonomous entity, due to the ease with which mouse embryos could be cultured from the two-cell stage on to the blastocyst. This view changed following studies in other species, notably domestic animals, which revealed the presence of 'blocks' to development, when embryos were cultured in vitro. Another line of evidence leading to the view that the maternal environment is crucial for optimal development, was the finding that embryos cultured in vitro lag developmentally behind their in vivo counterparts. This developmental retardation could be ameliorated when washings from the reproductive tract or specific growth factors were added to the media. The expression of genes for growth factors and their receptors is regulated in a tissue-specific manner as well as temporally and spatially during mammalian development. In this review, information regarding the expression and role of polypeptide growth factors of the insulin family during preimplantation mammalian development is summarized.