Ontogenic changes in the circulating concentrations of insulin-like growth factor (IGF)-I, IGF-II, and IGF-binding...

The ontogeny of circulating concentrations of insulin-like growth factor (IGF-)-I, IGF-II, and IGF-binding proteins (IGF-BPs) was examined in the chick embryo. Distinct ontogenic changes in the circulating concentrations of IGF-I and IGF-II were observed. The present study confirms the overall profile for circulating concentrations of IGF-I. During middevelopment, plasma concentrations of IGF-I increased to a maximum which was attained on Day 14.5 of incubation. Thereafter, plasma concentrations of IGF-I declined with decreases (P < 0.05) between Days 14.5 and 15.5 and between Days 16.5 and 17.5 of incubation. In contrast to the monophasic profile for IGF-I, plasma concentrations of IGF-II were maximal on Day 10.5 of incubation and declined to a nadir on Day 17.5 of incubation. In late developmental stages (17.5 or 18.5 days of incubation), three IGF-BPs, having molecular weights of 22, 28, and 36 kDa, were detected in the plasma of chick embryos. No significant ontogenic changes in the circulating levels of the 28- and 36-kDa IGF-BPs were observed. However, it should be noted that prior to Day 17.5 of incubation, the 22-kDa IGF-BP was nondetectable in the circulation. The role of these changes in the functioning of IGF in embryonic development is discussed.