Transmission electron microscopy of the stem/progenitor cell niche in neonatal rabbit kidney after fixation of specimens by (a,c) conventional glutaraldehyde and (b,d) glutaraldehyde including tannic acid. a) Neither in a collecting duct ampulla (A) nor in surrounding mesenchymal stem/progenitor cells or organ capsule (Cap) a dark cell type is detected. The epithelial-mesenchymal interface is bright (asterisk with bar). The basal aspect of a CD ampulla is marked by a cross. c) At the ampulla neck maturation of light ‘Principal Cells’ and grey ‘Intercalated Cells’ is seen. b) Fixation of specimens in glutaraldehyde including tannic acid shows intense label at the epithelial-mesenchymal interface (asterisk with bar) as it was earlier described . d) Label of tannic acid is lost at the lateral side of the ampulla (arrow head). Neither in embryonic, maturing or matured renal parenchyma dark cells within a tubule epithelium are present.