Quantitative studies on the preservation of choline and ethanolamine phosphatides during tissue preparation for electron microscopy

The loss of C-14 ethanolamine-and H-3 choline-labelled phospholipids from rat liver during tissue preparation for electron microscopy has been examined. Column and thin-layer chromatography combined with double-label scintillation spectrometry were used to analyse the radioactive phospholipid content of the livers of rats injected simultaneously with C-14 aminoethanol and H-3 choline chloride. After 4 h (in vivo) the C-14 and H-3 labels were mainly incorporated into phosphatidyl ethanolamine and phosphatidyl choline respectively but some C-14 label had been incorporated into phosphatidyl choline. Chopped rat liver was fixed in glutaraldehyde or osmium tetroxide or both sequentially and tissues were dehydrated in ethanol and embedded in Araldite. In each procedure examined the choline label proved more labile than the ethanolamine. After glutaraldehyde fixation alone complete loss of phosphatidyl choline occurred and half of the phosphatidyl ethanolamine was also lost. Following osmium tetroxide fixation negligible loss of either phosphatide occurred. In terms of phospholipid retention, no advantage was gained by glutaraldehyde fixation prior to osmium tetroxide fixation. The results show that both ethanols and embedding monomers are potent phospholipid solvents. The data also suggests that EM autoradiography of these two phosphatides may be carried out with reasonable confidence although it must be pointed out that a high degree of retention does not necessarily imply retention in situ.