Effect of IGF‐I on pig oocyte maturation, fertilization, and early embryonic development in vitro, and on granulosa and cumulus cell biosynthetic activity

Porcine granulosa cells have been shown previously to both secrete and respond to insulin‐like growth factor‐I (IGF‐I), suggesting an autocrine function of this peptide in the follicle. The present work was undertaken to determine possible effects of IGF‐I on in vitro maturation, in vitro fertilization, and early embryonic development in culture. Granulosa and cumulus cell proliferation and differentiation based on 3 H‐thymidine uptake and progesterone production, respectively, were also assessed. The results showed that the cleavage rate of oocytes was markedly stimulated in a dose‐dependent manner by the addition of IGF‐I to the oocyte maturation medium ( P < 0.05). Embryo development beyond the 8‐cell stage was improved by IGF‐I, reaching a maximum of 22% at 200 ng/ml IGF‐I. Treatment with IGF‐I after fertilization increased the percentage of total oocyte cleavage ( P < 0.05) to approximately 52%, 43%, and 57% at, respectively, 25, 50, and 100 ng/ml IGF‐I. 3 H‐thymidine incorporation by granulosa cells was significantly increased in cultures treated with FSH (3‐fold) or IGF‐I (6‐fold) compared to the control. For the cumulus cells, FSH caused a similar increase (3‐fold) in 3 H‐thymidine incorporation while IGF‐I stimulated a 15‐fold increase. Progesterone production by the granulosa cells was increased to the same extent by treatment with FSH or IGF‐I (4.7 and 5.1‐fold, respectively). However, for the cumulus cells, while FSH caused a marked 16‐fold increase in progesterone production, IGF‐I caused only a marginal increase of 2.5‐fold. These results indicate a beneficial effect of IGF‐I on in vitro porcine oocyte maturation and pre‐implantation embryo development, suggesting a physiological role for IGF‐I in vivo. The in vivo effect of IGF‐I may be indirect via autocrine stimulation of cumulus and/or granulosa cells resulting in enhanced oocyte maturation and fertilization.