Hepatic sinusoidal endothelial cells (HSECs) are a morphologically distinct population of cells that form the lining of liver sinusoids. Features that distinguish HSEC from endothelial cells present in other organs and in larger liver vessels are the presence of multiple fenestrae throughout the cells and the lack of an underlying basement membrane [1-4]. The sinusoids are positioned between hepatocyte plates, and they initiate at the portal tract and terminate at the central vein. Sinusoids carry blood that converges in the liver from the portal venous supply as well as from the hepatic artery  (Fig. 5.1). Sinusoids are separated from adjacent hepatocytes by the perisinusoidal space of Disse. Due to their position, HSECs are the first cells that are in contact with blood flow into the sinusoids and serve to compartmentalize the vascular sinusoidal channels from the hepatic parenchyma [3, 4]. The hepatic sinusoids range in diameter from 4∈μm near the portal triad to 5.5∈μm near the central vein . As this is smaller than the size of both red and white blood cells, there is distortion of both cells and the sinusoid during the passage of blood cells [6, 7]. This process has been referred to as an endothelial massage that allows efficient exchange of compounds from the blood through sinusoidal fenestrae into the space of Disse [5, 6]. Also residing in the sinusoidal space are hepatic macrophages (Kupffer cells), hepatic natural killer cells (pit cells), and liver-specific pericytes (hepatic stellate cells), each of which are covered in other chapters.
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