The hepatic sinusoidal endothelial cell (EC) was not recognized as a highly differentiated cell type until the sinusoid was examined by perfusion fixation combined with electron microscopy, as described in Wisse's seminal papers in 1970 and 1972 (1,2). Our understanding of hepatic sinusoidal EC characteristics took an additional step forward with the first description of a method to isolate a highly pure population of these cells (3,4).

MORPHOLOGY OF HEPATIC SINUSOIDAL ENDOTHELIAL CELL AND THE SINUSOID

The hepatic sinusoids (Figure 133.1) form the equivalent of a capillary system. The hepatic sinusoid lacks an organized basement membrane on the abluminal side of the hepatic sinusoidal ECs (SECs). The virtual space between the hepatic SECs and the hepatocytes is called the space of Disse. The space of Disse contains loosely organized extracellular matrix and resident pericytes that surround the hepatic SECs, the so called stellate cells. Stellate cells are vitamin A–storing cells. In addition, they are contractile and thus capable of regulating sinusoidal diameter. On the luminal side of the hepatic sinusoidal endothelium are the residentmacrophages, the Kupffer cells. When measured in vivo by light microscopy, the diameter of the hepatic sinusoid ranges from 6 to 7 μm, increasing slightly from the periportal to the centrilobular area. Sinusoids are smaller than neutrophils, which measure approximately 8.5 μm. It has been hypothesized that, because leukocytes are larger than the sinusoids, the entry of leukocytes into the sinusoid compresses the space of Disse and forces fluid out through fenestrations within hepatic SECs (discussed later), whereas with passage of the leukocyte back out of the sinusoid the space of Disse will restore the original shape and cause suction of fresh fluid back into the space (forced sieving).