An element is a pure substance that is made from a single type of atom. Elements are the building blocks for all the rest of the matter in the world. Examples of elements include iron, oxygen, hydrogen, gold, and helium.
Since the initial discovery that the adult brain contains stem cells that generate new neurons, an important question in neurobiology has been whether or not this adult neurogenesis is functionally important.
Stem Cell Therapy. Much of the attention and all of the controversy surrounding stem cell therapy has focused on embryonic stem cells -- cells harvested four to five days after an embryo is fertilized. Adult stem cells exist to replace damaged or aging cells, and they are found in tissue throughout the body of adults and in the blood and bone marrow, where cells are much easier to harvest. Stem cell therapy has been used for many decades to treat leukemia and other cancers, but the treatment-related death rate is high due to the aggressive chemotherapy and/or radiation used to dramatically suppress the immune system and kill cancer cells.