Electron Microscopy of Fixed Lipids
The electron microscope picture of OsO4-fixed and cross-sectioned bimolecular lipid layers in water shows two parallel dense bands separated by a light interspace. Evidence for the preservation of the lamellar structure during the fixation and embedding process has given earlier . Apparently the dense bands are caused by osmium concentrated at the hydrophilic ends of the lipid molecules. This conclusions has been supported by several independent lines of evidence [3, 4]. Quantitative studies show that enough osmium is bound by isolated membrane lipids to account for the observed contrast. A phospholipid extract from brain shows a 74% increase in mass upon reaction with OsO4 . The individual lipids after extensive purification give lower values: phosphatidylcholine, 34.8%; phosphatidylethanolamine, 49.2%, phosphatidylserine, 46.5%; phosphoinositol, 23.9%; and cerebrosides, 26.0%. This may be due to preferential loss of the more highly unsaturated lipid molecules during purification, but this point merits further study.
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