Problem: Investigations to date on localization of placental insulin receptors (IR) at the ultrastructural level focused on the maternal facing side of the placenta but did not allow localization of IRs to intracellular compartments.
Method: The ultrastructural localization of IR in placentae from early and term gestation was studied using the immunogold technique and a monoclonal antibody against the external domain of the IR (clone MA20).
Results: At 10 wk gestation, there were high levels of IR in both cytotrophoblast and syncytiotrophoblast, with sparse microvillous labeling. Fetal vessels contained various amounts of IR while stromal cells were also labeled. By 13 wk, this pattern was unchanged; however, at term, fewer IR were present in the syncytiotrophoblast, with more label on microvilli than before. Fetal vessels were well labeled, while stromal cells appeared unchanged. Intense labeling of fetal erythrocytes provided an internal positive control, while omission of the primary antiserum produced loss of binding.
Conclusion: The data suggest a spatiotemporal alteration in placental IR distribution during pregnancy, possibly reflecting a shift in control of placental growth and metabolism from mother to fetus.