Chick embryos after 48 h of development (day 2) maintained in ovo provide an adequate model to study hormonal influences in early organogenesis in vertebrates. In previous studies at this (prepancreatic) stage of chick embryogenesis we demonstrated not only the presence of an insulin-related material but also insulin receptors and insulin-like growth factor I (IGF-I) receptors. Further, when embryos developed in the presence of antiinsulin antibodies, we showed retardation in both morphological and biochemical events which strongly suggested a physiological requirement for insulin in normal embryogenesis. In the present study we have evaluated the effects of insulin, proinsulin, desoctapeptide insulin, and IGF-I when applied to day 2 chick embryos. At day 4 of development biochemical indices were compared in treated vs. control groups. Insulin (10-100 ng/embryo) increased the content of protein, total creatine kinase, creatine kinase MB isozyme, triglycerides, cholesterol, phospholipids, DNA, and RNA, in a dose-dependent fashion. IGF-I had a lower potency than insulin in stimulating both metabolic and growth indices and was nearly equipotent in stimulating the creatine kinase MB content (marker of muscle differentiation). The high relative potency of insulin together with the effects of proinsulin (less than 15%) and desoctapeptide insulin (less than 10%) compared to insulin on the chick embryo, led us to infer that at low doses (nanograms per embryo) insulin stimulates developmental processes mainly through the insulin receptor, with the possible exception of muscle differentiation. The broad range of metabolic, growth, and differentiation indices stimulated by insulin and IGF-I in chick embryos, at a stage when specific receptors for both peptides are present, suggests that insulin and IGF-I may have a regulatory, complementary, or overlapping role in normal chick embryo early development.

We previously reported specific cross-linking of '2SI-labeled insulin and 12SI-labeled insulin-like growth fac- The signal transduction capability of the receptors in early organogenesis was assessed by their ability to phosphorylate the exogenous substrate poly(Glu8OTyr2'). Ligand-dependent tyrosine phosphorylation was demonstrable with both insulin and IGF-I in glycoprotein-enriched preparations from embryos at days 2 through 6 of embryogenesis. There was a developmentally regulated change in ligand-dependent tyrosine kinase activity, with a sharp increase from day 2 to day 4, in contrast with a small increase in the ligand binding. Binding of 12SI-labeled IGF-I was, with the solubilized receptors, severalfold higher than binding of 125I-labeled insulin. However, the insulin-dependent phosphorylation was as high as the IGF-

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