FIGURE 1.

Liver anatomy and the hepatic lobule. The liver is composed of hexagonal units called lobules. Oxygen and nutrient rich blood flows directionally from the hepatic vessels (red) in the corners of the lobule toward a central vein in the middle (blue). Periportal hepatocytes (zone 1) are at the periphery of the lobule, followed by mid-lobular cells, and finally pericentral hepatocytes surround the central vein (zone 3). The variable microenvironment along the periportal-pericentral axis results in graded gene expression and the spatial separation of certain metabolic processes to periportal (red) and pericentral (blue) regions. Certain liver injuries are also zone-dependent, with some originating or confined to periportal (yellow) regions and some to pericentral regions of the lobule (brown). NAFLD, non-alcoholic fatty liver disease.