This electron micrograph highlights a darkly-stained glycocalyx rim of the brush border of the intestinal epithelium of the cat, stained en bloc with colloidal thorium. The glycocalyx is composed of negatively charged acidic polysaccharides, which bind the thorium. The enhanced staining of the polysaccharides shows that the glycocalyx is not just superficial to the microvilli, but rather, extends into the clefts between them. Figure 19, courtesy of Susumu Ito, from Chapter 1 (The Cell 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.