Madin-Darby canine kidney (MDCK) cells form a polarized epithelium on the inner surface of polyethersulfone/polyvinylpyrrolidone (PES/PVP) hollow fiber membranes. (a) Confluent monolayer of MDCK cells on the inner surface of a PES/PVP hollow fiber membrane. Cracks in the monolayer (indicated by arrowheads) are artifacts resulting from sample preparation (scale bar = 100 μm). (b) Luminal surface of the MDCK cell layer on the inner surface of a PES/PVP hollow fiber membrane (scale bar = 1 μm). The cell surface is densely covered with microvilli. The inset in the lower left corner shows an enlargement of the microvilli-covered surface (scale bar = 1 μm). All images were obtained by scanning electron microscopy.