LOCALIZATION AND QUANTIFICATION OF INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) AND IGF-II MANNOSE-6-PHOSPHATE (IGF-II/M6P) RECEPTORS IN PIG EMBRYOS DURING EARLY-PREGNANCY

To assess a potential role of insulin-like growth factor-I (IGF-I) and II (IGF-II) in early embryonic development, the presence of their receptors was investigated by both immunohistochemistry and autoradiography experiments on whole embryos at Days 4 and 6 of pregnancy, on embryo sections at Days 8 and 10, and on placenta at Day 20 of pregnancy. Immunohistochemistry experiments were performed by using specific polyclonal antibodies raised against human IGF I and IGF-II/mannose-6-phosphate (IGF-II/M6P) receptors. By autoradiography, specificity of [I-125]-IGF-I and [I-125]-IGF-II binding on embryonic cells was assessed by competition with unlabeled IGF-I and IGF-II, and quantification of the autoradiographic signal was performed by image analysis. The presence of IGF-I receptors on porcine trophectoderm cells was detected neither by immunohistochemistry nor by autoradioradiography on whole embryos or embryo sections. IGF-I receptors were present in the placenta at Day 20 of pregnancy, but only on endometrial cells. In contrast, IGF-II/MGP receptors were detected on porcine trophectoderm cells by both immunohistochemistry and autoradiography on whole embryos, on embryo sections at Day 8 and Day 10 of pregnancy, and on fetal and maternal compartments of the placenta at Day 20. The number of IGF-II/MGP receptors on trophectoderm cells was greatly heterogeneous between embryos within the same litters. There was no relationship between the number of IGF-II/MGP receptors on trophectoderm cells and the age or size of embryos between Day 8 and Day 10 of pregnancy. The involvement of the IGF-II/M6P receptor in early embryonic development in the pig remains to be determined.