The embryonic pattern of rat insulin-like growth factor-i gene expression suggests a role in induction and early growth of the liver

The classic experiments of LeDouarin and her colleagues demonstrated that the mesenchymal cells of the septum transversum induce both the rapid proliferation of the adjacent endodermally-derived foregut cells and their differentiation into the hepatocyte component of the liver. We have used in situ hybridization to test whether this inductive activity correlates with the developmental expression of genes encoding insulin-like growth factors-I and -II (IGF-I and -II) and/or proteins that may mediate IGF action (IGF binding protein-2 and the IGF-I receptor). We found that the onset of expression of the transcript for IGF-I in the septum transversum correlates precisely with the time that the inductive activity appears. In fact, the septum transversum is the earliest site of IGF-I mRNA expression detectable in the rat embryo by in situ hybridization.