Adult (Somatic) stem cells are unspecialized cells that are found in different parts of the body and, depending on the source tissue, have different properties. Adult stem cells are capable of self-renewal and give rise to daughter cells that are specialized to form the cell types found in the original body part. An adult stem cell is thought to be an undifferentiated cell, found among differentiated cells in a tissue or organ that can renew itself and can differentiate to yield some or all of the major specialized cell types of the tissue or organ. In principle, such unspecialized cells are considered to be quiescent, but capable of self-renewing; these cells remain in their ‘undifferentiated’ state from suppression by some intrinsic or extrinsic factor, until stimulated. Their asymmetric division produces one identical daughter stem cell and a second progenitor cell that becomes committed to a lineage-specific differentiation program.
The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. Scientists also use the term somatic stem cell instead of adult stem cell, where somatic refers to cells of the body (not the germ cells, sperm or eggs). Unlike embryonic stem cells, which are defined by their origin (the inner cell mass of the blastocyst), the origin of adult stem cells in some mature tissues is still under investigation.
Adult stem cells are multipotent, meaning that they appear to be limited in the cell types that they can produce based on current evidence. However, recent scientific studies suggest that adult stem cells may have more plasticity than originally thought. Stem cell plasticity is the ability of a stem cell from one tissue to generate the specialized cell type(s) of another tissue. For example, bone marrow stromal cells are known to give rise to bone cells, cartilage cells, fat cells and other types of connective tissue (which is expected), but they may also differentiate into cardiac muscle cells and skeletal muscle cells (this was not initially thought possible).
While there is increased understanding of the regenerative potential of stem cells derived from adipose tissue and of resident stem cells linked to the innate repair potential of mature tissues, current experience primarily relies on bone marrow-derived stem cells. Sufficient to recapitulate the entire hematopoietic system and provide mesenchymal stem cells with nonhematopoietic differentiation potential, bone marrow-derived stem cells are a cornerstone of contemporary regenerative medicine applications.
Hematopoietic stem cells that give rise to all blood and immune cells are today the most understood of the adult stem cells. Hematopoietic stem cells from bone marrow have been providing lifesaving cures for leukemia and other blood disorders for over 40 years. Hematopoietic stem cells are primarily found in the bone marrow but have also been found in the peripheral blood in very low numbers. Compared to adult stem cells from other tissues, hematopoietic stem cells are relatively easy to obtain.
Mesenchymal stem cells are also found in the bone marrow. Mesenchymal stem cells are a mixed population of cells that can form fat cells, bone, cartilage and ligaments, muscle cells, skin cells and nerve cells.
- history of research on adult stem cells
- Where are adult stem cells found, and what do they normally do
- What tests are used for identifying adult stem cells
- What is known about adult stem cell differentiation
