Ultrastructural, cytochemical, and radioautographic localization of placental iron.

Maternal-fetal iron transfer across the guinea pig hemomonochorial placenta during the first, second, and third trimesters was examined using ultrastructural, cytochemical, radioautographic, and ferrokinetic methods. Acid ferrocyanide stained inorganic ferric iron on and in sinusoidal microvilli, cytoplasmic matrix or ground substance, and the outer basal plasmalemma of epithelial cells. Some stain deposits were observed within and on either side of the basement membrane. The extraluminal outer plasmalemma, intercellular junctions, and cytoplasm of endothelial cells frequently contained numerous stain deposits. Staining of trophoblast sinusoidal microvilli was similar during early and late gestation, whereas the staining of the basement membrane and endothelial cells was most prominent during the second and third trimesters. Staining of ferric iron was encountered in rare cytoplasmic granules of epithelial cells during late gestation, but not during early gestation. Placental macrophages contained acid-ferrocyanide-reactive ferric iron in large heterophagosomes and hyaloplasm. Acid ferricyanide failed to localize ferrous iron in either epithelial cells or macrophages. Light-microscopic radioautographic studies localized radioiron in placental epithelial cells and Prussian-blue-positive macrophages in specimens obtained 30 minutes after injection of radioiron ((55)Fe, (59)Fe) into the maternal saphenous vein. At the ultrastructural level labeling was observed (in order of decreasing grain density) in or on the epithelial basal plasmalemma and basement membrane, endothelial cytoplasm, epithelial sinusoidal microvilli, and epithelial cytoplasm. Significant staining or radiolabeling was not observed in mitochondria, trophoblast granules, or nuclei. These results indicate that placental non-heme iron is trivalent and moves from the maternal to the fetal circulation by passing through trophoblast microvilli, cytoplasmic matrix, basal plasmalemma, basement membrane, endothelial cell junctions, and cytoplasm.