Fine Structure of Hepatic Reticuloendothelial System and Lipid Uptake

Abstract

Electron microscopic studies were made in rats to elucidate fine structure of the fat-storing cell and the role of the R.E.S, cells on hepatic uptake of lipid.
1) Fat-storing cell. The fat-storing cell of normal rats is clearly diffrenciated from the R.E.S. cells lining the sinusoid in its fine structure. It is about 10μ diameter, is located in the intercellular space between two hepatocytes or in the Disse space and contains several fat droplets of fairly uniform size in the cytoplasm. However, about 30% of the fat-storing cells have no fat droplets. In rats, in which a fat emulsion was injected intravenously for two weeks, the fat-storing cell with no fat droplet disappeared and the fat droplets in the cytoplasm were significantly increased in number. The reticulum fibers were frequently, observed in close proximity to the fat-storing cell. The lack of microvilli in its cell membrane and very few pinocytic invaginations suggest that this cell has no vigorous phagocytic activity as does the littoral cell.
2) Role of the R.E.S. on hepatic uptake of lipid. Thirty minutes to three hours after forced of cream, numerous electron dense particles of lipid, measuring about 0.1μ, in diameter, -probably chylomicra-were seen in the Disse space and they appeared to be taken up into the hepatocyte by pinocytosis. However, very few or practically no lipid particles, were seen coming into the R.E.S. cell. On the other hand, when a fat emulsion was administered intravenously, lipid droplets about 0.5μ to 1.0μ in diameter, showed to be phagocytized into the Kupffer cells by the process of pinocytotic invagination of the cell membrane or engulfed by the cytoplasmic projections soon after the infusion. However, no lipid drops were seen to be taken up into the hepatocyte at this time. Thirty minutes to two hours after the infusion, numerous small dense particles of lipids, about 0.1μ in diameter, were seen in the Disse space, being pinocytosed into the hepatocytes. They seemed to be liberated from the Kupffer cells in which fat drops have been picked up. When a fat emulsion was administered intravenously under the R.E.S. blockade, uptake of lipid into the Kupffer cell and the subsequent accumulation of lipid in the hepatocyte were delayed. In the uptake of lipid by the hepatocyte, certain physicochemical propeties of the fat droplets may be the determining factors, and the R.E.S. seems to play a role in the disposal of intravenously introduced fat, probably conditioning it in such a way as to enable the hepatocyte to phagocytiz it in its altered form.