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.
A stem cell is a type of cell that is nonspecific in its function; in contrast, for instance, to a heart or brain cell, which is functionally specific. There are two major sources of stem cells: embryonic stem cells and non-embryonic stem cells.Embryonic stem cells are obtained from 5- to 12-day old embryos.Although removal of a stem cell from an embryo kills the embryo, the stem cells are valued for their potential to produce any type of cell.hus, the heart of the stem-cell controversy centers on the aforementioned fact that the extraction of stem cells from 5- to 12-day embryos kills the embryo. But thatâs not the only issue: In addition, stem cells derived from an embryonic human may, in turn, reject the person who receives them.