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.
For example, an adult stem cell in the bone marrow makes up all of our blood cells. It's important because it replaces blood cells in our body as they're lost. It cannot, however, make muscle or nerve or other kinds of specialized cells. So these cells are useful.