Pattern of the Insulin-like Growth Factor II Gene Expression During Rat Embryogenesis

The rat insulin-like growth factor II (IGF-II) gene, encoding a fetal somatomedin, expresses a family of transcripts in embryonic/fetal tissues, and also in the adult choroid plexus and the leptomeninges. We have localized IGF-II gene transcripts in sections of rat embryos of embryonic days 10-16 by performing in situ hybridization. These transcripts are present in the head mesenchyme, formed from both the mesoderm and the cephalic portion of the neural crest, and also in the majority of other tissues of mesodermal origin, predominantly those derived from the somites and the lateral mesoderm. Intense labelling was detected in muscle cells, and their precursors, throughout the examined stages, whereas in chondrocytes the initial high level of hybridization declined substantially prior to ossification. IGF-II gene transcripts are also present in derivatives of other germ layers, but in restricted sites. Thus, from the derivatives of the endoderm, only the liver and the bronchial epithelium yielded hybridization signals. Ectoderm-derived tissues, including the central and peripheral nervous system, were negative for hybridization, with the exception of the choroid plexus, the newly forming pituitary rudiment and, to a lesser extent, the auditory placode. The pattern of IGF-II gene expression during embryogenesis overlaps significantly with the reported distribution of immunohistochemically detected TGF-beta 1. A paracrine/autocrine role for IGF-II in the developmental process is discussed.