MOUSE PREIMPLANTATION EMBRYOS EXHIBIT RECEPTOR-MEDIATED BINDING AND TRANSCYTOSIS OF MATERNAL INSULIN-LIKE GROWTH FACTOR-I

High-resolution microscopy in conjunction with colloidal gold-labeled insulin-like growth factor I (IGF-I) has been used to provide evidence that the IGF-I receptor is first detected in 8-cell-stage mouse embry os, confirming the results of previous reverse transcriptase polymeras e chain reaction (RT-PCR) studies. Specificity for the IGF-I receptor was demonstrated by displacement with unlabeled IGF-I and dual-labelin g experiments with colloidal gold-labeled or unlabeled insulin. Labele d IGF-I ligand is internalized by means of receptor-mediated endocytos is following its concentration in coated pits, and it can be visualize d within cytoplasmic organelles. Immunocytochemical analyses at the bl astocyst stage, using gold-labeled antibodies to the receptor, confirm ed the expression of IGF-I receptors on all cells of the embryo. Simil ar studies with antibodies directed against the ligand demonstrated th at IGF-I internalized by the embryo in vivo is maternally derived. App roximately 40% of blastocysts showed apical plasma membrane binding of gold-labeled ligand (''responders''), while approximately 60% did not demonstrate binding (''nonresponders''); however, both classes of emb ryo expressed receptors on basolateral membranes of trophectoderm cell s and on the surface of inner masses. Functional studies show that inc ubating embryos in physiological levels of IGF-I (40 ng/ml) results in increased numbers of cells in the inner cell mass (p < 0.05), but not the trophectoderm, as compared to controls.