LONDON (TIP): Scientists have for the first time grown a fully functional organ from scratch inside a living animal by transplanting cells that were originally created in a laboratory. The advance could in future aid the development of ‘lab-grown’ replacement organs. Scientists created a working thymus, a vital immune system – “nerve centre” located near the heart, with connective tissue cells from a mouse embryo which were converted into a completely different cell strain by flipping a genetic “switch” in their DNA.

The resulting cells grew spontaneously into the whole organ when injected into the mouse with other similar cells. Researchers from the MRC Centre for Regenerative Medicine at the University of Edinburgh, took cells called fibroblasts from a mouse embryo and converted them directly into a completely unrelated type of cell – specialised thymus cells – using a technique called ‘reprogramming’.

When mixed with other thymus cell types and transplanted into mice, these cells formed a replacement organ that had the same structure, complexity and function as a healthy native adult thymus. The reprogrammed cells were also capable of producing T cells – a type of white blood cell important for fighting infection – in the lab. The researchers hope that with further refinement their lab-made cells could form the basis of a readily available thymus transplant treatment for people with a weakened immune system. They may also enable the production of patient-matched T cells.

The thymus, located near the heart, is a vital organ of the immune system. It produces T cells, which guard against disease by scanning the body for malfunctioning cells and infections. When they detect a problem, they mount a coordinated immune response that tries to eliminate harmful cells, such as cancer, or pathogens like bacteria and viruses. People without a fully functioning thymus can’t make enough T cells and as a result are very vulnerable to infections. This can be a particular problem for some patients who need a bone marrow transplant (for example to treat leukaemia), as a functioning thymus is needed to rebuild the immune system once the transplant has been received.

The problem can also affect children; around one in 4,000 babies born each year in the UK have a malfunctioning or completely absent thymus. Thymus disorders can sometimes be treated with infusions of extra immune cells, or transplantation of a thymus organ soon after birth, but both are limited by a lack of donors and problems matching tissue to the recipient. Being able to create a complete transplantable thymus from cells in a lab would be a huge step forward in treating such conditions.

And while several studies have shown it is possible to produce collections of distinct cell types in a dish, such as heart or liver cells, scientists haven’t yet been able to grow a fully intact organ from cells created outside the body. Professor Clare Blackburn from the MRC Centre for Regenerative Medicine at the University of Edinburgh, who led the research, said “The ability to grow replacement organs from cells in the lab is one of the ‘holy grails’ in regenerative medicine. But the size and complexity of lab-grown organs has so far been limited. By directly reprogramming cells we’ve managed to produce an artificial cell type that, when transplanted, can form a fully organised and functional organ.