Adult stem cells are undifferentiated cells found in the tissues and organs of the body. They are capable of self-renewal. Their differentiation is mainly restricted to forming the cell types of that tissue or organ.
The chief role of adult stem cells is to maintain and repair the tissue in which they are found.
Skin stem cells, for example, give rise to new skin cells, ensuring that old or damaged skin cells are replenished.
It now appears that all tissues probably contain adult stem cells, but only in very small numbers. In each tissue, they are used to produce new mature cells as old ones die in the natural processes of senescence. They may also remain dormant until activated by disease or injury. Their small numbers make adult stem cells difficult to isolate but they have been successfully isolated from the brain, bone marrow, blood, muscle, skin, pancreas and liver. Most research has been carried out on haematopoietic (blood forming) stem cells isolated from bone marrow and blood.
Within the body, adult stem cells normally only generate the cell types of the tissue in which they are found. Haematopoietic stem cells, for example, are found in the bone marrow and give rise to the many types of cells found in the blood, including red and white blood cells and platelets. The existence of these types of stem cells has been known for a long time and bone marrow transplants containing such cells have been used for over 50 years to treat people with a variety of life-threatening disorders such as lymphomas, leukaemia and thalassemia.
Although adult stem cells are less versatile than embryonic stem cells, their use in research is less topical as it does not involve the destruction of embryos. Their potential use for cell-based therapies is also attractive as it may be possible to use a patient’s own stem cells to generate tissue for transplantation, thus avoiding problems with immune rejection.
One of the potential hurdles for the use of adult stem cells for transplants is their limited ability to generate different cell types. Recent experiments, however, have revealed that, under some experimental conditions, certain types of adult stem cells from one tissue may be able to colonise a completely different tissue. This phenomenon is called plasticity and some researchers believe that adult stem cells may, in the future, may be as useful as embryonic stem cells in generating tissue for transplants. A major focus of current research is the investigation of factors and conditions that control the differentiation of adult stem cells in the laboratory.
The image on this page shows adult stem cells isolated from bone marrow as seen under a microscope and magnified 400 times. The cells express green fluorescent protein used to examine cell shape and the DNA within the nucleus is stained blue. These cells are used to research stem cell treatments for heart disease.