DEVELOPMENTAL-CHANGES IN EMBRYONIC AND EXTRAEMBRYONIC INSULIN-LIKE GROWTH-FACTOR-I TISSUE CONCENTRATIONS IN THE TURKEY EMBRYO

The ontogeny of insulin-like growth factor-I (IGF-I) embryonic and ext ra-embryonic tissue concentrations were determined in the developing t urkey embryo. At 2-d intervals, starting on Day 6 of incubation, indiv idual tissues (n = 8 for each stage of incubation) were removed, weigh ed, pulverized, and extracted in 1 M acetic acid for IGF-I determinati on. Amniotic and allantoic fluid were collected starting on Day 8, ser um on Day 12, and analyzed for IGF-I levels. Serum IGF-I levels were t he lowest on Days 12 and 28 of incubation (5.9 and 9.5 ng/mL), respect ively, and the highest on Day 20 (16.2 ng/mL). Allantoic and amniotic fluid IGF-I concentrations were essentially unchanged during incubatio n. Extra-embryonic tissue IGF-I levels increased in both the yolk sac and chorioallantoic membranes as incubation advanced with concentratio ns being 8- to 10-fold greater in the chorioallantois. Embryo tissue I GF-I concentrations varied greatly with regard to tissue and stage of development. Brain IGF-I levels were the highest on Day 8 and lowest o n Day 26 (423 vs 35 pg/mg protein, respectively). Tissue IGF-I pattern in the heart mirrored that of brain. Liver IGF-I was low (< 40 pg/mg protein) from Day 10 to 20 and undetectable on Days 22 to 28. Muscle I GF-I levels were similar in the final days of development to those obs erved in early incubation. Bone IGF-I levels were highest in mid-incub ation and lowest on Day 26. Peptide levels in the gastrointestinal tra ct (GI) tract and gizzard were dissimilar in that IGF-I was elevated i n the GI tract in early incubation and declined with advancing incubat ion, whereas gizzard IGF-I levels peaked in late incubation. It is app arent that tissue synthesis of IGF-I is differentially regulated withi n a given tissue and stage of incubation during embryogenesis in the t urkey embryo.