Normally, new cells are divide at a controlled rate, keeping the overall number of cells nearly constant. Feedback mechanisms that stimulate or inhibit cell division regulate the growth of normal cells. In contrast, the body's normal regulatory mechanisms are usually unable to control the proliferation of cancer cells once malignant transformation has taken place. Cancer cells may also lose the ability to differentiate. Differentiation is the process by which normal cells undergo physical and structural changes as they develop to form different tissues of the body.
Differentiated cells specialize in different physiological functions.
However, microscopically and functionally they are unique from other cell types in the body. The degree of differentiation of malignant cells is variable. Cancer cells that closely resemble the tissue of origin are called well-differentiated, while bizarre tumor cells bearing little similarity to the tissue of origin are termed undifferentiated or anaplastic. Undifferentiated or anaplastic malignancies are usually more aggressive in their growth and behavior than well-differentiated types. There are also intermediate degrees of differentiation.