Regulation of apoptosis in the bovine blastocyst by insulin and the insulin-like growth factor (IGF) superfamily

Insulin and the insulin-like growth factors, IGF-I and IGF-II, have been reported to exert a mitogenic effect on the preimplantation mammalian embryo. Furthermore, it has been proposed that loss of imprinting of the insulin-like growth factor II receptor gene and the consequent over-production of IGF-II may be involved in the aetiology of the Enlarged Offspring Syndrome, which occurs as an artefact of in vitro embryo production. We have previously shown that apoptosis occurs in the preimplantation bovine embryo and is influenced by in vitro culture conditions. We have therefore sought to establish the effects of insulin, IGF-I and IGF-II on apoptosis and cell proliferation in bovine blastocysts in vitro. Zygotes, obtained by in vitro maturation and fertilization of follicular oocytes, were cultured to blastocysts, with or without exogenous growth factors. Embryos were stained with propidium iodide to label all nuclei and by TUNEL to label apoptotic nuclei and analyzed by epifluorescent and confocal microscopy. IGF-I and IGF-II, but not insulin, were found to increase the proportion of embryos which formed blastocysts. Insulin decreased the incidence of apoptosis without affecting blastocyst cell number. IGF-I acted to decrease apoptosis and increase total cell number and IGF-II increased cell number alone. These data suggest roles for insulin and the IGFs as mitogens and/or apoptotic survival factors during early bovine development. Perturbation of IGF-II regulated growth may be involved in fetal oversize.