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%; phosphatidylethanol-amine, 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.
KeywordsHydrophilic Group Lipid Molecule Unsaturated Lipid Individual Lipid Preferential Loss
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