Stem cells are unique in their ability to self-renew: to divide and create two cells, each identical to the original. Understanding stem cell self-renewal is central to understanding how organisms are made and maintained, and may lead to insights that permit physicians to modulate tissue regeneration and repair in their patients with chronic diseases.
Stem cells can also produce offspring that are more specialized (differentiated) than the parental cell. Directing the differentiation of stem cells into specialized cell types will enable cell replacement therapies. Replacing pancreatic beta cells in type I diabetics, or dopaminergic neurons in Parkinson's patients, are two possible examples. It is no accident that the spectacular advances in hematology depended in large part on the discovery and manipulation of blood stem cells, including the discovery of drugs that stimulate stem cell mobilization into the blood or enhance the differentiation of cells into mature blood cells. Moreover, intensive chemotherapy or irradiation for cancer is made possible by ‘rescuing' the patient with an injection of blood stem cells. Because stem cell research is fundamentally about the ability to form, maintain and repair tissues, the insights gained from this research directly informs an understanding of abnormal processes such as cancer and degenerative disease.
There are two types of stem cells, adult stem cells and embryonic stem cells. Adult stem cells are found in mature tissues (bone marrow, skin, brain etc) that can self-renew and give rise to other cell types from their tissue of origin, thereby producing a steady supply of new cells to maintain that tissue throughout life. In general, adult stem cells from one organ do not give rise to cell types from other organs.
However, the embryonic stem (ES) cell warrants special attention as it is uniquely malleable and can make any part of the body. In some sense, ES cells represent the parent of all stem cells and provide a window into the first stages of the life. It is also important to note that mouse ES cells have precipitated a virtual revolution in our understanding of the relationship between genes and their function in intact animals. A close association of researchers working on ES cells and adult stem cells is critical to accelerate the understanding that will lead to stem cell therapies.