Electron microscopic study on the hepatic sinusoidal wall and the fat-storing cells in the normal human liver.

In normal human livers obtained in surgical operations the sinusoidal wall has been observed with the electron microscope.The “endothelial cells” and the “Kupffer cells” lining the sinusoid are regarded as two functional states of the same cell lineage and show alike the ultrastructures suggesting phagocytic activity. They differ from each other especially in that the endothelial cells consist of two distinct portions, the perikaryon and the sheet-like cytoplasmic extension, the latter occupying the major part of the sinusoidal lining, while the Kupffer cells are of simple rounded-up form bulking into the sinusoidal lumen; the endothelial cell sheets extend to the Kupffer cell body to contact with the latter. On the other hand, the marginal ends on the neighboring endothelial sheets may overlap like roof tiles. In both connections there occur no functional structures like terminal bars and desmosomes.The endothelial lining of the sinusoid which is discontinuous because of the presence of intracellular gaps or pores and lacking in general in continuous basement membrane, is as a rule a simple layer. Frequent figures of stratified endothelial sheets are caused for the most part by the occurrence of subendothelial fat-storing cells which extend thin processes as if to reinforce the endothelial lining. The perisinusoidal or Disse's space is filled with blood plasm filtered through the discontinuous endothelial lining of the sinusoid. This space which represents the blood-tissue barrier of the liver contains, besides abundant microvilli of the hepatocytes and collagen or reticular fibers and fibrils, fat-storing cells, unmyelinated nerve fibers with their Schwann cells and occasional round, probably hemopoietic cells. The fat-storing cells are constant residents in the Disse's space and frequently extend into the recesses between the hepatocytes. They are lacking in their own basement membrane and contact directly with the collagen fibers, which are sometimes embraced in the invaginations of their plasma membrane. They send out a considerable number of cytoplasmic processes in the Disse's space which interlace with the microvilli of the hepatocytes. Close contacts between the even surfaces of the fat-storing cells and of the hepatocytes are occasionally encountered.The fat-storing cells possess relatively well-developed Golgi complex containing a diplosome, sparse small mitochondria, fairly well-developed elements of granular endoplasmic reticulum, small lysosomes, glycogen particles and, occasionally, a single cilium into the Disse's space which arises from one of the paired centrioles of the diplosome. Though the fat-storing cells exhibit no cytological signs indicating a phagocytic activity, many invaginations of the plasma membrane and vesicles including the bristle-coated ones are found along their free surfaces suggesting a vigorous pinocytotic activity. As constant and characteristic cytoplasmic inclusion bodies, the fat-storing cells contain small fat droplets, each of which (about 2μ in diameter) is bounded by a weak limiting membrane and does not fuse with each other into larger ones. The fat-storing cells may occasionally contain no fat droplets; the “pericytes” observed by some authors in the Disse's space, probably correspond to these empty fat-storing cells.The following possible functions of the fat-storing cells are proposed: 1) Fat synthesis and fat-droplet formation, 2) the storing of fat droplet and fat-soluble vitamin A, 3) the participation in intralobular fibrilogenesis and 4) the reinforcement of the endothelial lining of the sinusoid by sending subendothelial processes.