Pioneering treatments to repair damaged tissue and organs have been boosted by a major advance in stem-cell research.
The cells, which can grow into a range of different tissue types, either reproduce to increase supply or transform into specific types for which there is a need.
Scientists at the University of Edinburgh have found that the "decision" is taken when stem cells reach a crucial stage of development.
Dr Ian Chambers, from the University's Medical Research Council Centre for Regenerative Medicine, said: "If there is too much self-renewal, the capacity to generate specialist cells to repair damaged tissues and organs is diminished.
"Yet, if you don't have enough self-renewal going on you will run out of stem cells, which could otherwise have been converted into specialist cells to help the body heal.
"This is a fine balance, but gaining greater understanding of how stem cells work will help us develop therapies for a range of diseases."
The research uses embryonic stem cells taken from early-stage human embryos, but the researchers believe the findings could also apply to adult cells.