Most cells in the adult body are terminally differentiated cells, namely they are highly specialized cells that perform a specific function and do not divide and form new cells. The body relies on stem cells to proliferate and produce new cells, some of which then differentiate into specialized cells to perform specific functions. Stem cells are most abundant in tissues that regularly replace aging cells, such as the bone marrow, the dermis underlying the skin, and the lining of the intestine. However, stem cells can also be found throughout the body, such as in the blood and in fat deposits.
The first stem cell transplants used stem cells from the bone marrow to rebuild a damaged hematopoietic system, and today stem cell transplants are a routine treatment for certain cancers and other conditions. More recently, scientists have been using stem cells to orthopedic conditions, such as arthritis and damaged tendons.