A definition of the role of IGF-I in differentiation and development requires a detailed understanding of its expression and tissue-specific regulation in embryogenesis. Standard techniques for analysis of IGF-I gene expression are not sufficiently sensitive for studies in early embryos. We have used the highly sensitive polymerase chain reaction (PCR) to study IGF-I gene expression in whole chick embryos from the late blastula stage (E0=laying) through the end of organogenesis (day 8), and in liver, brain and pancreas during mid-late embryogenesis and perinatally (hatching = day 21). Although at low levels in the blastoderm and gastrula, IGF-I mRNA was detectable in the whole embryo in all stages studied, with a tendency of the signal to increase with age during the first week of embryogenesis. In mid- and late embryogenesis, we easily detected IGF-I mRNA transcripts in pancreas and brain while the levels in the liver were barely detectable. Liver IGF-I mRNA increased markedly at the peak of postnatal growth (day 50). These studies suggest that while the major source of postnatal IGF-I may be the liver, extrahepatic tissues may be the predominant source of IGF-I during prenatal chicken development.
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