Adult or somatic stem cells exist in the body after embryonic development. These undifferentiated cells are found among differentiated tissues, such as brain, bone marrow, blood, blood vessels, skeletal muscles, testis, ovary, skin and the liver. The primary role of adult stem cells is to maintain and repair the tissue where they are found. They remain in a dormant or non-dividing state for many years until activated by disease or tissue injury. Adult stem cells can divide or self-renew indefinitely, to allow them to generate a wide range of cell types from the organ of origin, or regenerate the entire original organ. Adult stem cells reside in a specific area of each tissue (stem cell niche) and only a small number of cells are found in each tissue. However, their capacity to divide is limited once they are removed from the body. Therefore, cell culturing of the adult stem cell is rather difficult.
These adult stem cells are not completely unspecialized. Although it is well accepted that adult stem cell differentiation is limited to the organ of origin, other evidence suggests that adult stem cells can differentiate into other cell types. An example of an adult stem cell is the blood forming (haematopoietic) stem cells found in bone marrow. The red blood cells carry oxygen throughout the body, which only survive for about 120 days. The blood-forming stem cells continue to divide and produce cells for the function of red blood cells at the same time. These stem cells divide to maintain the cell reserve, as well as replacing the old or damaged red blood cells and differentiating to other blood cells. Bone marrow transplants have been used since 1968 to treat patients with leukemia and other blood-related diseases.
Adult stem cells or somatic stem cells are two common names used in scientific field. Somatic cells refer to the cells of body apart from germ cells, sperm or eggs). However, the definition of adult stem cells requires further investigation, as there are many sources and origins of stem cells in mature tissues.