Tumors that look very like normal tissue are well differentiated.
Normal tissues are "differentiated". The liver consists largely of liver cells - and they look the same in everyone's liver. A hepatocellular carcinoma arising in the liver may resemble very closely normal or benign liver cells. Such a tumor is described as well differentiated. Some hepatocellular carcinomas consist of masses of epithelial cells that bear no resemblance to liver cells. These tumors are undifferentiated. If they bear some (little) resemblance they are poorly differentiated. Often there is a spectrum of differentiation and the tumor may appear better differentiated in some areas than others. This variation is often called "heterogeneity" . Tumors with variable differentiation are usually evaluated on the "worst" areas - the least differentiated parts.
Why is differentiation important? In general, poorly differentiated and undifferentiated cancers are more aggressive than well differentiated cancers.
Concepts relating to differentiation are discussed in the tutorial.
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