Each type of human tissue has its own stem cells, which are capable of forming all other types of cells in that particular tissue. Thus, bone marrow stem cells can only form the various types of blood and lymph cells, skin stem cells can only form skin cells and neural stem cells can only form brain and nerve cells. The cell specialization makes use of one s own stem cells impractical for most repair or replacement operations.
Stem cells are rare.
The average bone marrow transplant may contain only five or six, which is why it takes so long to restore the blood cells that provide immunity. In order to obtain a sufficient number of stem cells from the brain, so much tissue would have to be withdrawn that the individual could suffer neural damage. If the repair were treatment for cancer, there is always the possibility that the cancer has already spread to the tissue containing the stem cells, as sometimes happens with bone marrow transplants. Dr. Vescovi s Italian and Canadian team found that adult stem cells from one type of tissue can be transplanted into other tissues and can create the kinds of cells needed by the second tissue. The researchers took neural cells from mice and transplanted them into other mice whose bone marrow had been destroyed by radiation.