Stem cells are primitive and unspecialized cells that have the ability to regenerate and differentiate, which could proliferate into the same original cells, and could also differentiate into other functional cells. Because of these properties, stem cells could be used to repair damaged tissue or organs, or even construct a complete organ.

The zygote is the first human stem cell, and then proliferated and differentiated into embryo and adult. However, the proportion of stem cells in human cells decreased gradually from 100% to 0.01% of newborns, 0.001% of young people, one millionth in middle age people and half millionth in elderly. Aging occurs when the body's self-renewal capacity is decreased, and consequently leading to disease by aging one after another.