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 embryos, confirming the results of previous reverse transcriptase polymerase chain reaction (RT-PCR) studies. Specificity for the IGF-I receptor was demonstrated by displacement with unlabeled IGF-I and dual-labeling experiments with colloidal gold-labeled or unlabeled insulin. Labeled IGF-I ligand is internalized by means of receptor-mediated endocytosis following its concentration in coated pits, and it can be visualized within cytoplasmic organelles. Immunocytochemical analyses at the blastocyst stage, using gold-labeled antibodies to the receptor, confirmed the expression of IGF-I receptors on all cells of the embryo. Similar studies with antibodies directed against the ligand demonstrated that IGF-I internalized by the embryo in vivo is maternally derived. Approximately 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 embryo expressed receptors on basolateral membranes of trophectoderm cells and on the surface of inner masses. Functional studies show that incubating 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.
Smith
R M
RM
Department of Pathology and Laboratory Medicine, University of Pennsylvania Medical Center, Philadelphia 19104.
Garside
W T
WT
Aghayan
M
M
Shi
C Z
CZ
Shah
N
N
Jarett
L
L
Heyner
S
S
D000818:Animals; D001755:Blastocyst / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002452:Cell Count; D004705:Endocytosis; D005260:Female; D066298:In Vitro Techniques; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000494:pharmacology; D007335:Insulin-Like Growth Factor II / Q000494:pharmacology; D051379:Mice; D016253:Microscopy, Immunoelectron; D011247:Pregnancy; D017526:Receptor, IGF Type 1 / Q000378:metabolism*
D007328:Insulin; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D017526:Receptor, IGF Type 1