The fine structure of liver 3½ to 72 hours after partial hepatectomy has been compared with that of liver from sham-operated animals; all animals were 60- to 90-day old male mice of the C3H strain. Numerous small bodies with diameters ranging from 300 to 1,000 A have been observed distributed randomly throughout the cytoplasm of the hepatic parenchymal cells at early intervals after partial hepatectomy. In material fixed in osmium tetroxide and embedded in methacrylate, they appear as uniformly electron-opaque bodies, but in permanganate-fixed liver, they display only a peripheral rim of electron-opaque material surrounding a clear core. Each of these cytoplasmic bodies appears to be located within a vesicle. A few of the opaque bodies are also present in sinusoids and in the spaces of Disse; these bodies are not located within vesicular structures. Fat droplets of various sizes are easily distinguished in regenerating liver; with the increase in number of these fat droplets at later postoperative intervals, there occurs a concomitant decrease in the number of cytoplasmic bodies. It is suggested that the cytoplasmic bodies contain some lipid component. Possible explanations of the origin, nature, and fate of the cytoplasmic bodies are discussed.

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