Characterization of IGF-I-receptors in human placenta: identification with autoradiography and in-situ hybridization. • 230

The importance of insulin-like growth factor I and II in the regulation of human fetal growth is well established, but their exact impact on prenatal development is not. Both peptides are produced by many tissues and have not only endocrine but also paracrine and autocrine actions and act mainly by means of the IGF-I-receptor. Much less than their role in fetal tissues is known about their expression in the human placenta. The aim of the study was firstly to characterize IGF-I binding in human placenta and secondly to localize IGF-I binding at the cellular level by both receptor autoradiography and in-situ hybridization (ISH). In order to elucidate IGF-binding in human placenta we incubated cryosections of placenta of the 18th to 26th week of gestation with [¹²⁵I]IGF-I in the presence and absence of unlabeled IGF-I. Binding of [¹²⁵I]IGF-I to cryosections of human term placenta at 15°C was specific and halfmaximally inhibited by unlabeled IGF-I at 2.1± 0.3 × 10⁹ M. Unlabeled IGF-II and insulin were 18-fold and 900-fold respectively, less potent in displacing [¹²⁵I]IGF-I from placental sections. Autoradiography showed particularly dense binding of[¹²⁵I]IGF-I in the decidua, which was 4 × greater than in the chorion. At the microscopic level, binding was predominantly localized to the cytotrophoblast ISH was performed with a digoxigenin-labeled cRNA probe for IGF-I receptor: findings were in accordance with binding studies for IGF-I-receptor. We conclude that the methods employed for the first time provide an accurate tool in the identification of IGF-I receptors in the placenta. Specific binding of IGF-I to growth sensitive areas of the placenta indicate its role in placental and fetal growth, its localization predominantly to the decidua supports the idea that IGF-I regulates fetal growth from the maternal side of the placenta.