Liver of a DEN (Diethylnitrosamine)-treated rat. DEN is a toxic chemical which quickly induces liver cirrhosis followed by HCC (Hepatocellular carcinoma, a primary liver cancer). Cirrhosis is an end result of fibrosis, the scarring of liver tissue. Fibrosis is caused by the overproduction of collagen, a component of the connective tissue forming the liver. To grade the amount of cirrhosis present in a liver sample, collagen is made visible using the dye sirius red. Under polarized light, collagen is observed as the golden to red color as shown in this image.