Lipid solvent extraction of aldehyde-fixed hepatic tissue of rats caused disappearance of all intravascular and hepatocellular osmiophilic droplets normally present, thus indicating their lipid content. Intramitochondrial dense granules and osmiophilic droplets in lysosomes also disappeared after this treatment. Lipid solvents extracted 43.8 to 92.6% of the radioactivity from aldehyde-fixed rat liver with C14-labeled lipids. Only 0.7 to 5.8% of the radioactivity was extracted when the hepatic proteins were labeled. When tissue was fixed with OsO4, the lipid solvents extracted only 0.7 to 7.2% of the radioactivity from lipid-labeled liver and only 0 to 0.7% when proteins were labeled. Thin layer chromatography of the lipid solvents used in extraction of formaldehyde-fixed tissue revealed that triglyceride, phospholipid, and cholesterol and other lipid classes had been removed. However, acetone extracted less phospholipids than did ethanol or methanol-chloroform. During fat absorption the number and size of osmiophilic droplets increased in the nongranular endoplasmic reticulum. In animals fasted up to 5 days, 250-A osmiophilic particles were still present in the Golgi vesicles, other cytoplasmic vesicles, and in the space of Disse. These were considered possibly to represent lipoprotein being synthesized in the liver cell and secreted into the blood.

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