Electron-microscopic studies on Kupffer's stellate cells and sinusoidal endothelial cells in the liver of normal and experimental rabbits

Summary

The fine structure and functional properties of Kupffer's stellate cells and endothelial cells of the hepatic sinusoid of normal and experimental rabbits were studied using light as well as electron microscopy.

• (1) By light microscopy, it is clear that only the Kupffer cell ingests erythrocytes injected, while the endothelial cell is almost flattened, even after short-term (3 days) or long-term (10, 22 days) injection of a large dose of heterogenous erythrocytes. Both types of cell are easily distinguishable from each other in the liver of the saline solution-perfused animal. The numerical ratio of Kupffer cells to endothelial cells in normal and experimental animals is always constant (about 4:6).

• (2) Electron microscopically, the Kupffer cell is characterized by welldeveloped cytoplasmic protrusions such as numerous microvilli and pseudopods, while the endothelial cell is almost flat and smooth on its surface. All the Kupffer cells take up the heterogeneous erythrocytes injected, by phagocytosis. There is a distance of 20–30 nm between the plasma membrane of the Kupffer cell and that of the phagocytized erythrocyte, when the erythrocyte is caught by the Kupffer cell. Filamentous dense materials suggesting fuzzy coats of both plasma membranes are seen in the space. The Kupffer cell cytoplasm in contact with the erythrocyte sometimes shows microtubules running parallel or obliquely to the plasma membrane. No infiltrations of lymphocytes and plasma cells are observed in the liver of the experimental animals. No phagocytotic features are seen in any of the endothelial cells, though coated pits, small vesicles, and small lysosomelike dense bodies are found to be increased in number in the long-term erythrocyte-injected animal.

• (3) Staphylococci aureus, when injected, are also phagocytized by the Kupffer cell in the same way as erythrocytes and fuse with lysosomes in the cytoplasm.

• (4) Injected India ink particles are ingested into the coated pits of both types of cell by micropinocytosis. These coated pits become smooth vesicles and fuse with one another to form large vacuoles containing the India ink particles in the cytoplasm.

• (5) Both Kupffer cell and endothelial cell are stained with vital dyes, which are also considered to be ingested by micropinocytosis.

• (6) From the above facts it is concluded that the Kupffer cell is not derived from the endothelial cell. Both types of cell have a micropinocytotic activity, and only the Kupffer cell has a phagocytotic activity; neither India ink injection nor vital staining of the acid dye are suitable methods for detecting this phagocytotic activity. The hepatic sinusoidal region might not be a site where immunological reactions take place, though the blood stream is here cleansed of foreign materials and bacteria.