The insulin/insulin growth factor (IGF) system plays a critical role in embryo growth and development. We have investigated the expression of insulin and IGF1 receptor (IR, IGF1R) and the activation of their downstream pathways in rabbit 6 day old blastocysts. IR was expressed in embryoblast (Em, ICM) and trophoblast (Tr) cells, whereas IGF1R was localized mainly in Em. Isoform A (IR-A) represents the main insulin isoform in blastocysts and was found in Em and Tr cells. IR-B was detectable only in Tr. IR/IGF1R signalling pathways were analysed after stimulation with insulin (17 nM) or IGF1 (1.3 nM) in cultured blastocysts. Insulin stimulated Erk1/2 in Em and Tr, and Akt in Tr but not in Em. IGF1 activated both kinases exclusively in Em. The target genes c-fos (for MKK1/Erk signalling) and phosphoenolpyruvate carboxykinase (PEPCK; for PI3K/Akt signalling) were also specifically regulated. Insulin downregulated PEPCK RNA amounts in Tr by activation of the PI3K/Akt pathway. Expression of c-fos by insulin and IGF1 was different in respect to time and fortitude of expression, mirroring again the specific IR and IGF1R expression patterns in Em and Tr.

Taken together we show that IGF1 acts primarily mitogenic, an effect which is cell lineage-specifically restricted to the Em. By contrast, insulin is the growth factor of the Tr stimulating mitogenesis and downregulating metabolic responses. As soon as blastocyst differentiation in embryoblast and trophoblast has been accomplished, insulin and IGF1 signalling is different in both cell lineages, implying a different developmental impact of both growth factors.