Renal tubules that have formed spontaneously on 2-D surfaces in vitro. Renal cells were cultivated on the bottom of the wells of multiwell plates for several days until spontaneous tubule formation occurred. (a) Part of a renal tubule formed by human primary renal proximal tubule cells (HPTCs). The left end of the tubule is attached to the edge of the well (dark rim in the lower left corner). The epithelium on the bottom of the well was partially disrupted during the process of tubule formation (area devoid of cells in the upper left corner). The image was obtained by bright field microscopy, and the appearance of areas with bright and dark illumination is due to optical effects close to the edge of the well and around the tubule. (b) Part of a renal tubule formed by LLC-PK1 cells. The cell nuclei were stained with 4', 6'-diamidino-2'-phenylindole (DAPI, blue). The image was obtained by epifluorescence microscopy. Scale bars = (a) 400 μm and (b) 100 μm.