The expression of IGF-II in the early post-implantation rat conceptus

Insulin-like growth factor II (IGF-II) gene expression was investigated in the early rat conceptus using in situ hybridisation. IGF-II mRNA has been detected as early as 7 days post coitum in the extraembryonic ectoderm of the egg cylinder (including the ectoplacental cone but excluding the trophoblast bordering the implantation chamber). There was no expression either in the early embryo at this stage or in the implanting blastocyst (5.5 days). At 9.5 days transcription was also detected in extra-embryonic mesodermal derivatives and the visceral yolk sac endoderm, but not in the embryonic region. At 10.5 days transcription was detected in embryonic mesodermal derivatives. High levels were found in the primitive heart, hepatic diverticulum and septum transversum; lower levels were found in the early somites and other primitive mesenchymal derivatives and midgut endoderm. No expression was detected at any stage in derivatives of the embryonic ectoderm, including the neurectoderm. Recent studies of IGF-II, the IGF-II/M-6-P receptor and enzymic extracellular matrix remodelling suggest that the expression of IGF-II in the early rat conceptus may be related to hyaluronate breakdown (with, or without, sulphated glycosaminoglycan synthesis), linked with the aggregation and differentiation of previously migrating mesenchymal cells.