This electron micrograph shows the plasma membrane of epithelial cells of the brush border of the cat intestine, obtained from a thin section parallel to the free surface. The ability to visualize cell membranes, and other ultrastructural elements of the cell with the high resolution of the transmitting electron microscope provided critical views of cell ultrastructure, and fueled debate about composition and organization of the plasma membrane. The transverse plane of section reveals membranes composed of 2 symmetrical thick lines separated by an intermediate zone of lighter density. Not all membranes show this parallel symmetry. This image by Susumu Ito is Figure 2 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.