Ultrastructural localization of transferrin, transferrin receptor, and iron-binding sites on human placental and duodenal microvilli.

Ultrastructural methods were used to determine the subcellular location of the transferrin receptor, transferrin and iron-binding sites on human term placenta and human duodenum microvillus surfaces. The transferrin receptor and transferrin were localized by immunocytochemical methods employing either OKT9, a human transferrin receptor monoclonal antibody, or mouse anti-human transferrin (ATfn), both followed by a horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG (GAM-HRP) and diaminobenzidine (DAB) sequence. Iron-binding sites were localized by acid ferrocyanide (AF) staining after saturation of tissue specimens with iron, accomplished with iron nitrilotriacetate (FeNTA), a known transferrin iron donor. Placental microvillus surfaces demonstrated staining for the OKT9-GAM-HRP-DAB-reactive transferrin receptor, ATfn-GAM-HRP-DAB-reactive transferrin, and FeNTA-AF-reactive iron acceptor, whereas enterocyte microvillus surfaces lacked significant staining with each of these methods. FeNTA-AF stained iron-binding substance in placental and enterocyte microvilli and cytoplasmic matrix. Thus using the same ultrastructural immunostaining and cytochemical methods transferrin receptor, transferrin, and nitrilotriacetate iron acceptor sites can be demonstrated on the microvillus surface of human placenta but not on the microvillus surface of human duodena.