Histopathological findings of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is based on the concept of pathological morphology as well as clinical findings, and is broadly categorized into nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH). The differential diagnosis between NAFL and NASH is important because NASH has the potential to progress to cirrhosis and hepatocellular carcinoma. NAFL is simple hepatic steatosis without hepatocellular injury, while NASH is characterized by macrovesicular steatosis, inflammation, and ballooning hepatocytes with a predominantly centrilobular (zone 3) distribution. Liver biopsy is a useful test for diagnosing NAFLD, but it is invasive. Therefore, various noninvasive methods including diagnostic imaging have been developed in recent years. To verify their usefulness, it is necessary to clarify in detail how the pathological findings are reflected in the image findings as imaging and histopathological findings are closely related. We describe the main histological features of NAFLD, i.e., steatosis, inflammation, ballooning hepatocytes, Mallory-Denk bodies, and fibrosis, as well as the evolutional process to liver cirrhosis.