Adult stem cells
Adult stem cells are undifferentiated cells found in tissues and organs. They are capable of self-renewal and can differentiate to form the major specialised cell types of that tissue or organ. Their main role 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 probably all tissues contain adult stem cells but only in very small numbers. Scientists think they remain dormant until activated by disease or injury to that tissue. Because of their small numbers, adult stem cells have proven difficult to isolate but to date adult stem cells have been derived from various tissues, such as the brain, bone marrow, blood, muscle, skin, pancreas and liver. Most research has been done on haematopoietic (blood forming) stem cells isolated from bone marrow and blood.
National Institute of Health, US
Adult stem cells appear to 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 this type of stem cell has been known for a long time and bone marrow transplants have been used for over 30 years to treat people with a variety of life-threatening blood disorders such as leukaemia and thalassaemia.
Adult stem cells are attractive as research tools and for treating disease as they do not involve the destruction of embryos. They are also attractive as it may be possible to use a patient’s own stem cells to generate tissue for transplant, thus avoiding problems with immune rejection common to other types of transplantation.
However, 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 certain types of adult stem cells from one tissue may be able to generate cell types of a completely different tissue if exposed to the right conditions. This phenomenon is called plasticity and some researchers believe that adult stem cells may be as potentially useful as ES cells in generating tissue for transplants. Research into the factors and conditions that control their differentiation in the laboratory is proceeding.
Researchers in the UK have reported success in using adult bone marrow stem cells to reverse the effects of cirrhosis of the liver, negating the need for a transplant of rarely available organs. The experiment involved separating blood from the patient into its components, isolating stem cells from white blood cells and injecting them into the liver’s hepatic artery, returning red blood cells to the body.