Electron microscopic study on the sinusoidal wall of the liver of the crucian, Carassius carassius, with special remarks on the fat-storing cell (FSC).

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Abstract
The liver of the crucian, C. carassius (teleost) was studied by EM with special reference to the sinusoidal wall. The sinusoidal endothelium partly comprises double layered endothelial sheets. The fat-storing cell (FSC) or Ito cell is the only resident in the Disse''s space. Containing several lipid droplets, it shows the same morphological characteristics as mammalian FSC. Perisinusoidal space containing abundant hepatocytic microvilli lacks collagen fibrils and fibers (type III collagen). Instead, sparse fine fuzzy filaments (possibly type IV collagen) occur among microvilli. They increased in number and formed bundles or felt-like accumulations in random places in the ample vitamin A-given crucian. After the same vitamin A administration, FSC showed dilated cisternae of its RER [rough endoplasmic reticulum] filled with a flocculent material. FSC may be responsible for production of the fuzzy filaments in the Disse''s space. The microfilaments and microtubules occur in the cytoplasm of the crucian FSC. They are increased after administration of large doses of vitamin A. The most striking feature of the crucian FSC is the existence of characteristic, membrane-bound, electron-dense bodies mingling with lipid droplets. They consist of densely packed, parallel microtubules and, in untreated crucian, may show intimate topographical relationships with the lipid droplets. After large doses of vitamin A, the crucian FSC contains, in conformity with an increase of small lipid droplets, fewer opaque dense bodies composed of loosely packed parallel microtubules and small vacuoles containing a bundle of short microtubules or microfilaments. These membrane-bound microtubular structures may be precursors of the dense bodies. Though the nature of the dense bodies is obscure, cytochemically they are distinguished from lysosomes. Desmosomal junctions frequently occur between an FSC, cell body or cytoplasmic processes and several hepatocytes. They unite several hepatocytes into 1 set and may serve to maintain the hepatic parenchymal organization. Such desmosomal junctions between 2 cell types of different developmental origin were recognized for the 1st time in the crucian. In the normal untreated and vitamin A-stimulated crucian liver, Kupffer cells are completely absent.