Scanning electron micrograph of the lumenal surface of the toad bladder shows the boundaries of four polygonal squamous epithelial cells surrounding a mitochondrial-rich cell bearing short globular microvilli. Boundaries between cells are demarcated by what appears to be a continuous ridge. Image from Davis, Goodman, & Schuster (1974) J. Cell Biol. 61:544-547 and reprinted with permission as Figure 39 from Chapter 2 (Specializations of the Free Surface) 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.