tumoral differentiation

Differentiation refers to the extent to which neoplastic cells resemble comparable normal cells, both morphologically and functionally; lack of differentiation is called anaplasia.

Well-differentiated tumors are composed of cells resembling the mature normal cells of the tissue of origin of the neoplasm.

Poorly differentiated or undifferentiated tumors have primitive-appearing, unspecialized cells. In general, benign tumors are well differentiated.

The neoplastic cell in a benign smooth muscle tumor-a leiomyoma-so closely resembles the normal cell that it may be impossible to recognize it as a tumor by microscopic examination of individual cells. Only the massing of these cells into a nodule discloses the neoplastic nature of the lesion. One may get so close to the tree that one loses sight of the forest.

Malignant neoplasms, in contrast, range from well differentiated to undifferentiated. Malignant neoplasms composed of undifferentiated cells are said to be anaplastic.

Lack of differentiation, or anaplasia, is considered a hallmark of malignant transformation.

Anaplasia literally means "to form backward," implying a reversion from a high level of differentiation to a lower level.

There is substantial evidence, however, that most cancers do not represent "reverse differentiation" of mature normal cells but, in fact, arise from stem cells that are present in all specialized tissues.

The well-differentiated cancer evolves from maturation or specialization of undifferentiated cells as they proliferate, whereas the undifferentiated malignant tumor derives from proliferation without complete maturation of the transformed cells.

Lack of differentiation, or anaplasia, is marked by a number of morphologic changes.