Expression of insulin-like growth factor system genes in liver and brain tissue during embryonic and post-hatch development of the turkey.
The ontogeny of insulin-like growth factor-I (IGF-I) embryonic and extra-embryonic tissue concentrations were determined in the developing turkey embryo. At 2-d intervals, starting on Day 6 of incubation, individual tissues (n = 8 for each stage of incubation) were removed, weighed, pulverized, and extracted in 1 M acetic acid for IGF-I determination. Amniotic and allantoic fluid were collected starting on Day 8, serum on Day 12, and analyzed for IGF-I levels. Serum IGF-I levels were the lowest on Days 12 and 28 of incubation (5.9 and 9.5 ng/mL), respectively, and the highest on Day 20 (16.2 ng/mL). Allantoic and amniotic fluid IGF-I concentrations were essentially unchanged during incubation. Extra-embryonic tissue IGF-I levels increased in both the yolk sac and chorioallantoic membranes as incubation advanced with concentrations being 8- to 10-fold greater in the chorioallantois. Embryo tissue IGF-I concentrations varied greatly with regard to tissue and stage of development. Brain IGF-I levels were the highest on Day 8 and lowest on 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 IGF-I levels were similar in the final days of development to those observed in early incubation. Bone IGF-I levels were highest in mid-incubation and lowest on Day 26. Peptide levels in the gastrointestinal tract (GI) tract and gizzard were dissimilar in that IGF-I was elevated in the GI tract in early incubation and declined with advancing incubation, whereas gizzard IGF-I levels peaked in late incubation. It is apparent that tissue synthesis of IGF-I is differentially regulated within a given tissue and stage of incubation during embryogenesis in the turkey embryo.