Hepatic sinusoids differ from other sinusoids in that a second cell type, the stellate sinusoidal macrophage, or Kupffer cell (Fig. 18.8 and Plate 66, page 658), is a regular part of the vessel lining.
Kupffer cells belong to the mononuclear phagocytotic system.
Like other members of the mononuclear phagocytotic system, Kupffer cells are derived from monocytes. The scanning electron microscope (SEM) and transmission electron microscope (TEM) clearly show that the Kupffer cells form part of the lining of the sinusoid. Previously, they had been described as lying on the luminal surface of the endothelial cells. This older histologic description was probably based on the fact that processes of the Kupffer cells occasionally overlap endothelial processes on the luminal side. Kupffer cells do not form junctions with neighboring endothelial cells.
Processes of Kupffer cells often seem to span the sinusoidal lumen and may even partially occlude it. The presence of red cell fragments and iron in the form of ferritin in the cytoplasm of Kupffer cells suggests that they may be involved in the final breakdown of some damaged or senile red blood cells that reach the liver from the spleen. Some of the ferritin iron may be converted to hemosiderin granules and stored in the cells. This function is greatly increased after splenectomy when it is then essential for red blood cell disposal.
Perisinusoidal Space (Space of Disse)
The perisinusoidal space is the site of exchange of materials between blood and liver cells.
The perisinusoidal space (space of Disse) lies between the basal surfaces of hepatocytes and the basal surfaces of endothelial cells and Kupffer cells that line the sinusoids. Small, irregular microvilli project into this space from the basal surface of the hepatocytes (Fig. 18.9).
The microvilli increase the surface area available for exchange of materials between hepatocytes and plasma by as much as six times. Because of the large gaps in the endothelial layer and the absence of a continuous basal lamina, no
FIGURE 18.8 • Electron micrograph of two hepatic sinusoids of the liver. One hepatic sinusoid (top) displays a stellate sinusoidal macrophage (Kupffer cell). The remainder of the sinusoid as well as the other sinusoid is lined by thin endothelial cell cytoplasm. Surrounding each sinusoid is the perisinusoidal space (space of Disse), which contains numerous hepatocyte microvilli. Also present in the perisinusoidal space is a hepatic stellate cell (Ito cell) with a large lipid droplet and several smaller droplets. Its nucleus conforms to the curve of the lipid droplet. 6,600.
significant barrier exists between the blood plasma in the sinusoid and the hepatocyte plasma membrane. Proteins and lipoproteins synthesized by the hepatocyte are transferred into the blood in the perisinusoidal space; this pathway is for liver secretions other than bile.
In the fetal liver, the space between blood vessels and hepatocytes contains islands of blood-forming cells. In cases of chronic anemia in the adult, blood-forming cells may again appear in the perisinusoidal space.
FIGURE 18.9 • Electron micrograph showing the perisinusoidal space (of Disse). The perisinusoidal space (D) is located between the hepatocytes (H) and the sinusoid. A gap (large arrow) separates the endothelial cells (En) that line the sinusoid. Such gaps allow easy passage of small substances between the sinusoid and the perisinusoidal space. Numerous microvilli extend from the hepatocytes into the perisinusoidal space. These processes are long and frequently branch (small arrow). A red blood cell (RBC) is within the sinusoid. 18,000.