Estimating the cell membrane thickness.

<p><b>[A]</b> The membrane of cells freeze-substituted in acetone/uranyl acetate (UA, red) is not contrasted, whereas the cytoskeleton is highly contrasted, so that its interface (red line) with the cytoplasm (gray) is sharply resolved. In cells freeze-substituted in acetone/osmium tetroxide (OS, blue) or acetone/glutaraldehyde (GA, green), the membranes themselves (light blue ring) are highly contrasted. The difference (<i>Δd</i>/2) in projected diameters of circular cross-sections (arrows) of differentially stained cells is a good estimate of membrane thickness, <i>t_memb</i>. <b>[B]</b> Histograms of diameters of circular cross-sections of cells freeze-substituted in uranyl acetate (UA, red), osmium tetroxide (OS, blue) and glutaraldehyde (GA, green) are shown. The average diameters of the GA and OS specimens are very similar – 504±53 and 511±63 pixels (SD), respectively – whereas that of UA – 403±40 – is smaller. The difference (<i>Δd</i>) indicated by the double-headed arrow implies that the membrane thickness, <i>t_memb</i>, is ∼ 0.2<i>d</i>/2. The pixel size is 0.39 nm. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0087921#pone-0087921-g001" target="_blank"><b>Fig. 1B.</b></a>