Gap junctions in ciliary epithelium under conditions of oxygenation (upper) and anoxia (lower), in tissue prepared by ultrarapid freezing using liquid helium. Under oxygenated conditions, the connexons are spaced apart, and show no ordered arrangement. Under anoxia, the gap junction has hexagonally packed connexons. Images by Elio Raviola appear as Figures 100 (upper) and 101 (lower) from Chapter 3 (Junctional Specializations) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.