Scientists in Scotland say they have grown a whole, fully functional organ inside a living animal for the first time.
The team from the University of Edinburgh's Centre for Regenerative Medicine produced a working thymus - an organ found in front of the heart that produces T cells, crucial to the immune system.
Professor Clare Blackburn who led the research said her team used a technique called 'reprogramming' to turn fibroblast cells taken from a mouse embryo into a type of cell found in the thymus.
These 'reprogrammed' cells were mixed with other key thymus cell types.
When transplanted into a mouse, the cells - shown here on the right - formed a replacement thymus with the same structure and function as a healthy thymus.
Professor Blackburn hopes the breakthrough will pave the way for similar techniques to eventually be used in humans.
It's the shrinking of the thymus that contributes to the immune system becoming less effective as we age.
And people born with thymus disorders are often limited by a lack of donors for transplants and problems matching tissue to the recipient.
Blackburn says that in the future -- her lab grown cells could form the basis of new thymus treatments for people with a weakened immune system.