The major function of adult stem cells is to maintain homeostasis in the body in terms of replacing dead or injured cells with new ones that function correctly. At this point in time, no one truly knows the origin of adult stem cells in mature tissue. It is said that adult stem cells are clonogenic and unilineal. Clonogenic means that the cells generate identical copies of themselves, then diffirentiate into mature cells of the tissue in which it resides. Unilineal means that a cell differentiates into a specific, more mature cell, using one or more intermediate steps. To say that adult stem cells are unilineal would be to say that there is no such thing as a pluri, or multipotent stem cell in adults. (4)

Are all adult stem cells only unilineal?

Many scientists, however, would dispute this claim. Many experiments have been performed on mice to try and prove that certain populations of adult stem cells are multipotent, that they have plasticity. For example, stem cells derived from the bone marrow are believed to have the ability to form not only erythrocytes, but also skeletal and cardiac muscle. (5) In addition, it is thought that neural derived stem cells can differentiate into not only brain tissue, but also hematopoietic cells and skeletal muscle. (Look at adult stem cell plasticity) (5)

How about progenitor cells?

Another topic of controversy concerning adult stem cells is whether certain populations of adult stem cells are truly stem cells, or whether they are actually progenitor cells of certain tissues, derived from an unknown population of stem cells. In addition, some scientists declare that progenitor cells in adult organisms are actually true stem cells. (Look at the difference between a stem cell and a progenitor cell) (4)