Electron Microscope Studies on the Fine Structure of the Sinusoidal Wall and Fat-Storing Cells of Rabbit Livers

Abstract

The ultrastructure of the sinusoidal wall in the adult rabbit livers and associated structures such as the stellate cell of KUPFFER and the fat-storing cell of ITO (1951) was observed with the electron microscope, in thin sections after osmium tetroxide fixation and methacrylate embedding. The results were discussed in comparison with those observed by light microscopy.1. The wall of the hepatic sinusoid is composed of the endothelial cells, whose cytoplasm is relatively rich in the perinuclear portion but is abruptly or gradually reduced in amount at the parts distant from the nucleus. In the latter portion, the endothelial cytoplasm becomes a thin film 50-300mμ thick. Occasional discontinuities (holes) of 100-300mμ in distance are occured in the filmy endothelial lining, through which the lumen of the sinusoid is directly continuous to the perisinusoidal space.2. A perisinusoidal space of about 1μ in width is formed between the endothelial lining and the hepatic cell surfaces. The matrix of this space appears in a similar nature to the sinusoidal lumen (blood space), but none of the blood corpuscles can be observed in the perisinusoidal space. This space may not strictly correspond to the so-called DISSE's space.3. Numerous fine filiform projections or the microvilli extend from the hepatic cell surface into the perisinusoidal space. The disposition of the microvilli is rather irregular, and hence the actual length of each microvillus is difficult to be measured but the cross diameter is relatively uniform and measures about 0.2μ. In a portion of rather wide perisinusoidal space, the microvilli are abundant and crowded; while in narrow parts of the space, the microvilli are scanty or often lacking at all, making smooth the hepatic cell surfaces.None of the endothelial cell, stellate cell and fat-storing cell sends out the microvilli.4. There is no corresponding structure in the perisinusoidal space to the basement membrane of ordinary blood capillaries. Among the microvilli in this space, solid dotts of moderate density are scattered or clumped, while in part delicate fibrillar bundles are observed. These may be represented in light microscopy as reticular fibers, but such cross or longitudinally cut fibrils are far few in electron micrographs comparing with the light microscopic results. This difference may be caused by the thinness of the section for electron microscopy, because the gross meshed network of reticular fibers may not appear in electron micrographs as the complete reticulum.5. Stellate cells of KUPFFER are situated inside the lumen of the sinusoid and are usually detached from the sinusoidal wall but sometimes fixed to it by the cytoplasmic processes. It may be noted that this cell is not involved in the construction of the sinusoidal wall, unlike the endothelial cells. The cytoplasmic processes of KUPFFER cells are either like a hillock or a tongue and sometimes are much longer. These are described by light microscopy as the pseudopods.6. In the KUPFFER cell cytoplasm, round inclusions of various shapes and sizes (0.2-2μ in diameter) may be contained. Relatively large bodies of 1-2μ large are dense, and possess fine granular or filamentous texture in the interior and apparent limiting membranes on the outer surfaces. These are probably red blood cells phagocytosed by the KUPFFER cell. In some cases, granules 0.2-1μ large with extremely high density are gathered into an irregular clump looking like grapes. The center of each dense granule is often cleared up appearing as a ring.7. The fat-storing cell is always localized in the perisinusoidal space outside the sinusoid, namely between the endothelial cells and the parenchymatous hepatic cells. Therefore, the fat-storing cell is separated from the sinusoidal lumen (blood space) and is apparently different from the stellate cell, which is situated in the sinusoidal lumen.