The history of research on adult stem cells began about 40 years ago. In the 1960 s, researchers discovered that the bone marrow contains at least two kinds of stem cells. One population, called stem cells, forms all the types of blood cells in the body. A second population, called bone marrow stroma l cells was discovered a few years later. Stromal cells are a mixed cell population that generates bone, cartilage, fat, and fibrous connective tissue. Also in the 1960 s, scientists who were studying rats discovered two regions of the brain that contained dividing cells, which become nerve cells.

Despite these reports, most scientists believed that new nerve cells could not be generated in the adult brain. It was not until the 1990 s that scientists agreed that the adult brain does contain stem cells that are able to generate the brain's three major cell types - (star shaped neurological cell) and, which are non-neuronal cells, and neurons or nerve cells. ABOUT ADULT/EMBRYONIC STEM CELLS AND DIFFERENCES Adult Stem cells are thought to reside in a specific area of each tissue where they may remain (non-dividing) for many years until they are activated by disease or tissue injury. The adult tissues reported to contain stem cells include brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.

Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for cell-based regenerative therapies. Of course, adult and embryonic stem cells differ in the number and type of differentiated cells types they can become. Embryonic stem cells can become all cell types of the body because they are (single cell becoming many different cell types). Adult stem cells are generally limited to differentiating into different cell types of their tissue of origin Large numbers of embryonic stem cells can be relatively easily grown in culture, while adult stem cells are rare in mature tissues and methods for expanding their numbers in cell culture have not yet been worked out. This is an important distinction, as large numbers of cells are needed for stem cell replacement therapies. A potential advantage of using stem cells from an adult is that the patient's own cells could be expanded in culture and then reintroduced into the patient.

The use of the patient's own adult stem cells would mean that the cells would not be rejected by the immune system and would continue to grow.