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The avian embryo has been a useful model system for studies on the role of insulin and its close relative insulin-like growth factor-I (IGF-I) in development. The unfertilized chicken egg contains both peptides from maternal origin, and the embryo expresses insulin and IGF-I before the major organs are formed. Insulin receptors and IGF-I receptors are found in the blastoderm and in all tissues examined during organogenesis. When exogenous insulin or IGF-I are added to the embryo, growth and differentiation events are stimulated. By contrast, insulin antibodies and insulin receptor antibodies retard embryo development. In embryos cultured ex ovo, in which growth is impaired, the levels of serum IGF-I are decreased.

Insulin-like growth factor-I (IGF-I) is the primary mediator of GH action after birth, but its role as a regulator of prenatal growth is unclear. In a previous study we showed that IGF-I mRNA was expressed in chicken embryos beginning at the blastoderm stage (day 0, newly laid egg). Here we present the ontogeny of serum IGF-I in normal and growth-retarded chicken embryos. Serum samples were pooled from multiple embryos starting on day 4 of development in ovo until hatching (day 21). Extracts of day 2 and 3 whole embryos were also studied. IGF-binding proteins were removed by filtration on Sep-Pak C-18 cartridges. IGF-I was quantitated by a heterologous RIA validated for chicken species. Embryonic IGF-I showed a HPLC profile similar to that of adult chicken serum IGF-I. Serum IGF-I was measurable on day 6 of development (approximately 0.04 ng/ml), reached a peak on day 15 (18 ng/ml), and decreased to a low concentration (0.2 ng/ml) the day before hatching. Embryos cultured ex ovo showed progressive growth retardation after day 10 of development, and by day 20 their weight was 50% of normal. The serum IGF-I concentration of ex ovo cultured embryos was normal on day 10, but remained low until day 21, without the midembryogenesis rise observed in normal embryos. These results support the concept that IGF-I may have a role in general embryonic growth in addition to any paracrine/autocrine action in individual tissues.

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