Immunoelectron microscopic localization of gp-Fy in principal cells of collecting duct and peritubular capillaries in kidney medulla. Antibody 6615 was used for labeling. (a) Strong labeling of the apical plasma membrane and small vesicles of the principal cells of the collecting duct is seen. (Original magnification × 39,000.) (b) The basolateral membrane domain, including membrane infolding, of the principal cells of the collecting duct was also heavily labeled. (Original magnification × 60,000.) (c) Peritubular fenestrated endothelial cells of capillaries of the inner medulla exhibited strong labeling. (Original magnification × 48,000.) (d and e). When preimmune serum was used, no immunolabeling of the principal cells of the collecting duct (d) and capillary endothelial cells (e) was observed. (Original magnification × 39,000.) BM, basement membrane.