Ultrastructural localization of TfRs in SCF progenitors versus transformed HD3E22 erythroblasts.

Subcellular localization of TfRs in primary SCF progenitors (A) and AEV-transformed HD3E22 erythroblasts (B) was determined by immunoelectron microscopy of ultrathin cryosections labeled with a monoclonal murine antibody to chicken TfR and detected with Protein A/gold (10 nm) complex (see “Materials and methods”. The pictures show a section of the plasma membrane and intracellular compartments. Cisternal structures (double membranes) that enclose electron-lucent areas and electron-dense structures resembling endosome carrier vesicles are part of the early endosomal compartments. PM indicates plasma membrane; E, endosome; N, nucleus. TfR-specific labeling in coated pits (large arrowheads), coated vesicles (medium arrowheads), and small vesicular structures (small arrowheads). Note the differential amounts and distribution of the 10 nm Protein A/gold particles in the SCF progenitors versus the erythroleukemic HD3E22 cells. Magnifications: (A) × 75 000, (B) × 50 000; bars represent a length of 200 nm.