The hepatic stellate cells (Ito cells) store vitamin A; however, in pathologic conditions, they differentiate into myofibroblasts and synthesize collagen.
The other cell type found in the perisinusoidal space is the hepatic stellate cell (commonly called an Ito cell). These cells of mesenchymal origin are the primary storage site for hepatic vitamin A in the form of retinyl esters within cytoplasmic lipid droplets. Vitamin A is released from the hepatic stellate cell as retinol (alcohol form) bound to retinol chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas LIVE R 637 binding protein (RBP). It is then transported from the liver to the retina, where its stereoisomer 11-cis retinal binds to the protein opsin to form rhodopsin, the visual pigment of rods and cones of the retina. For many years, fish liver oils (e.g., cod liver oil) were medically and economically important nutritional sources of vitamin A.
In certain pathologic conditions, such as chronic inﬂammation or liver cirrhosis, hepatic stellate cells lose their lipid and vitamin A storage capability and differentiate into cells with characteristics of myofibroblasts. These cells appear to play a significant role in hepatic fibrogenesis; they synthesize and deposit type I and type III collagen within the perisinusoidal space, resulting in liver fibrosis. This collagen is continuous with the connective tissue of the portal space and the connective tissue surrounding the central vein. An increased amount of perisinusoidal fibrous stroma is an early sign of liver response to toxic substances. The cytoplasm of hepatic stellate cells contains contractile elements, such as desmin and smooth muscle -actin filaments. During cell contraction, they increase the vascular resistance within the sinusoids by constricting the vascular channels, leading to portal hypertension. In addition, hepatic stellate cells play a role in remodeling the extracellular matrix during recovery from liver injury.
Hepatic lymph originates in the perisinusoidal space.
Plasma that remains in the perisinusoidal space drains into the periportal connective tissue, where a small space, the periportal space (space of Mall) (see Fig. 18.10b), is described between the stroma of the portal canal and the outermost hepatocytes. From this collecting site, the fluid then enters lymphatic capillaries that travel with the other components of the portal triad.
The lymph moves in progressively larger vessels, in the same direction as the bile (i.e., from the level of the hepatocytes, toward the portal canals and eventually to the
FIGURE 18.10 • Canals of Hering and the intrahepatic ductile. a. Photomicrograph showing an area near a portal canal. Arrows indicate regions where bile canaliculi are draining into canals of Hering. Note that the canal of Hering is partially lined by hepatocytes and partially by cholangiocytes. It drains into intrahepatic bile ductule surrounded by hepatocytes, in contrast to the interlobular bile duct, which is embedded in the connective tissue of the portal canal. The terminal branch of a portal vein (lower right) accompanied by a small bile ductule are evident.800. b. Electron micrograph showing an intrahepatic bile ductule. The ductule collects bile from the canals of Hering. It is close to the hepatocytes, but the actual connection between bile canaliculi and the intrahepatic ductule is not evident in this plane of section. The ductule is composed of cholangiocytes (CH) surrounded by a complete basal lamina (BL). The narrow space (asterisks) into which microvilli of hepatocytes project is the periportal space (of Mall), not the perisinusoidal space (of Disse). 6,000.