An adult stem cell is an undifferentiated cell, found among differentiated cells in tissue or an organ. The adult stem cell can renew itself and can differentiate to yield some or all of the major specialized cell types of the tissue or organ. The primary role of adult stem cells in a living organism is to maintain and repair the tissue in which they are found. Scientists also use the term somatic stem cell to describe adult stem cells, 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 (cells from the preimplantation-stage embryo), the origin of adult stem cells in some mature tissues is still under investigation.
Research on adult stem cells
Research on adult stem cells has generated a great deal of excitement. Scientists have found adult stem cells in many more tissues than they once thought possible. This finding has led researchers and clinicians to consider whether adult stem cells could be used for transplants. In fact, adult hematopoietic—or blood-forming—stem cells from bone marrow have been used in transplants for more than 40 years. Scientists now have evidence that stem cells exist in the brain and the heart, two locations where adult stem cells were not at first expected to reside. If the differentiation of adult stem cells can be controlled in the laboratory, these cells may become the basis of transplantation-based therapies.
The history of research on adult stem cells began more than 60 years ago. In the 1950s, researchers discovered that the bone marrow contains at least two kinds of stem cells. Hematopoietic stem cells form all the types of blood cells in the body. Bone Marrow stromal stem cells are a multipotent subset of bone marrow stromal cells that are able to form bone, cartilage, stromal cells that support blood formation, fat, and fibrous tissue.
Types of adult stem cells
Bone marrow stem cells are also called mesenchymal stem cells, and were discovered a few years later. These non-hematopoietic stem cells make up a small proportion of the stromal cell population in the bone marrow and can generate bone, cartilage, and fat cells that support the formation of blood and fibrous connective tissue.
In the 1960s, scientists who were studying rats discovered two regions of the brain that contained dividing cells that ultimately become nerve cells, but despite these reports most scientists believed that the adult brain could not generate new nerve cells. It was not until the 1990s that scientists agreed that the adult brain does contain stem cells that are able to generate the brain’s three major cell types—astrocytes and oligodendrocytes, which are non-neuronal cells, and neurons, or nerve cells.
Where are adult stem cells found, and what do they normally do?
Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis. They are thought to reside in a specific area of each tissue (called a “stem cell niche”). In many tissues, current evidence suggests that some types of stem cells are pericytes, cells that compose the outermost layer of small blood vessels. Stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain tissues, or by disease or tissue injury.
What tests are used to identify adult stem cells?
Scientists often use one or more methods to identify adult stem cells:
• Label the cells in a living tissue with molecular markers and then determine the specialized cell types they generate;
• Remove the cells from a living animal, label them in cell culture, and transplant them back into another animal to determine whether the cells replace (or “repopulate”) their tissue of origin.
Importantly, scientists must demonstrate that a single adult stem cell can generate a line of genetically identical cells that then gives rise to all the appropriate differentiated cell types of the tissue. To confirm experimentally that a putative adult stem cell is indeed a stem cell, scientists show either that the cell can give rise to these genetically identical cells in culture, and/or that a purified population of these candidate stem cells can repopulate or reform the tissue after transplant into an animal.