DON W. FAWCETT / SCIENCE PHOTO LIBRARY DON W. FAWCETT / SCIENCE PHOTO LIBRARY
Transmission electron micrograph of the intestinal microvilli of a cat. The cross-section of each microvillus (in blue) is bounded by two dense lines of similar thickness (in black) separated by a lighter intermediate zone (in pink). Not all cell membranes exhibit his degree of symmetry. In some, the outer dense line is thinner than the inner.
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