A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells.

Adult stem cells

Adult stem cells are tissue-specific; hence, they divide to develop the tissue from which they originate. One type of adult stem cell is the epithelial stem cell, which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of the skin. Adult bone marrow has three distinct types of stem cells: hematopoietic stem cells, which give rise to red blood cells, white blood cells, and platelets; endothelial stem cells, which give rise to the endothelial cell types that line blood and lymph vessels; and mesenchymal stem cells, which give rise to the different types of muscle cells.

Cancer Stem cells

Cancer stem cells are a minor subpopulation of cells present in tumors and have the potential to renew and differentiate. For this reason, they are known to share similar characteristics with normal stem cells. In 1997, Bonnet and Dick were the first to find evidence of cancer stem cells by isolating a subpopulation of cells that expressed the surface marker CD34. It was found that the CD34 subpopulation had the potential to develop tumors in severe combined immunodeficiency (SCID) mice that were histologically similar to the donor.

Whileadult stem cells are generally tissue-specific, cancer stem cells multiply to produce cancer cells with the potential to terminally differentiate and a few cancer stem cells. Adult stem cells have limited self-renewal capacity, while cancer stem cells have indefinite self-renewal capability. Adult stem cells are quiescent most of the time and have normal karyotypes. But the cancer stem cells are mitotically less active than cancer cells and often contain abnormal karyotypes.