Journal of reproduction and fertility. Supplement
Development in vitro is influenced by embryo density, serum, somatic cell co-culture and the production of 'embryotrophic' paracrine and autocrine factors. Research in our laboratory has focussed principally on the insulin-like growth factor (IGF) family. We have demonstrated that pre-attachment bovine and ovine embryos express mRNAs encoding a number of growth factor ligand and receptor genes including all members of the IGF ligand and receptor family throughout this developmental interval. In addition, early embryos express mRNAs encoding IGF-binding proteins (IGFBPs) 2-5 from the one-cell to the blastocyst stage and IGFBP5 mRNA at the blastocyst stage. Cultured bovine blastocysts release up to 35 pg per embryo in 24 h, whereas release of IGF-I was below detectable values. Analysis extended to bovine oviductal cultures has also demonstrated that mRNAs encoding these IGF family members are present throughout an 8 day culture period. Transcripts encoding IGFBPs 2-6 were also present. Release of both IGFs was recorded over an 8 day culture period. IGF-II release was significantly greater than that observed for IGF-I. Therefore, the IGFs are present throughout the maternal environment during early embryo development. The oocyte, within the follicle, is held in an environment high in IGFs and IGFBPs. The zygote, after fertilization, is maintained in an IGF-rich environment while free-living in the oviduct and the uterus. This review is focused on the IGF family and IGFBPs and their roles in enhancing development up to the blastocyst stage.