Electron microscopic studies on the sinusoidal cells in the monkey liver.
The sinusoidal wall was observed by transmission electron microscopy in crab-eating monkey livers. The perikarya of sinusoidal endothelial cells were characterized by numerous macropinocytotic vacuoles and curved smooth-surfaced tubules of high electron density. Size and spacing of fenestrae in endothelial sieve plates corresponded to essentially those in other mammalian species including the human. It was verified that the high concentration of microfilaments was responsible for the electron dense appearance of the sieve plate in tangential sections. Besides occasional overlapping of endothelial sheets, complicated interdigitations of several short lamellae originating from endothelial processes occasionally caused a layered structure of the endothelial lining. Kupffer cells were strikingly rich in lysosomes and contained large mitochondria and phagosomes. They were fixed to the endothelial lining by patches of junctional complexes identical with those between endothelial cells. Ito cells of the monkey liver demonstrated, like those of the human liver, many smooth-surfaced caveolae and vesicles along their perisinusoidal surface, suggesting their micropinocytotic activity. They also contained glycogen beta-particles which were partly gathered around lipid vacuoles. The electron dense droplets enclosed by glycogen particles as revealed in human Ito cells and regarded as immature lipid droplets retaining the chemical properties of glycogen, could rarely be confirmed in the present study. Between the Ito cell and hepatocyte there occurred many junctional complexes. The space of Disse contained, besides abundant collagen fibrils, numerous fine filaments forming irregular meshworks or bundles which resembled fibrillar material (precursor of collagen) in appearance in the dilated cisternae of the rough endoplasmic reticulum (RER) of the Ito cell. These filaments often entwined the collagen fibrils in the Disse's space as if to participate by apposition in their development. The question, whether pinocytosis-like structures of the Ito cell might be involved in the precursor transport from the cisternae of the RER to the Disse's space, remained unanswered.