IGF-I AND THE IGF-I RECEPTOR IN DEVELOPMENT OF NONMAMMALIAN VERTEBRATES

Extracellular signals are likely to be involved in the control of growth and differentiation during embyrogenesis of vertebrates. These signals include, among others, several members of the insulin family: insulin-like growth factor (IGF)-I, IGF-II, and insulin. In the chick embryo, maternal IGF-I is stored in the yolk. In addition, the embryonic IGF-I gene is expressed very early and in late development in multiple tissues. We have used reverse-transcribed (RT) RNA and amplification by the polymerase chain reaction (PCR) to detect IGF-I gene expression. IGF-I was preferentially expressed in cephalic regions during late neurulation and early organogenesis. During late organogenesis, in some tissues, such as the eye lens, IGF-I gene expression is compartmentalized to a subset of cells, the epithelial cells. In these lens cells, IGF-I stimulates transcription of the 8-crystallin gene. Competence to respond to IGF-I exists in multiple cell types, since, based on binding studies, receptors for IGF-I are widespread in the gastrulating and neurulating embryo. Target tissues in which an autocrine/paracrine role for IGF-I appears more likely are the developing eye lens and retina, which are avascular organs rich in IGF-I receptors. In late development, IGF-I may have an additional endocrine role, with an impact on the general growth of the chick embryo. In embryos developed ex ovo, that show growth retardation after day 10 of embyrogenesis, IGF-I serum levels are very low. By day 8, expression of IGF-I mRNA in these embryos is markedly reduced in multiple tissues. Future studies in which IGF-I and its receptor are overexpressed or abolished should clarify the function of this growth factor in development. (C) 1993 Wiley-Liss, Inc.