Researchers 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 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.
"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," Professor Clare Blackburn from the Medical Research Council Centre for Regenerative Medicine, at the University of Edinburgh, who led the research, said.
"By directly reprogramming cells we've managed to produce an artificial cell type that, when transplanted, can form a fully organised and functional organ. This is an important first step towards the goal of generating a clinically useful artificial thymus in the lab," said Blackburn.
The researchers carried out their study using cells (fibroblasts) taken from mouse embryos.
By increasing levels of a protein called FOXN1, which guides development of the thymus during normal organ development in the embryo, they were able to directly reprogramme these cells to become a type of thymus cell called thymic epithelial cells.
These are the cells that provide the specialist functions of the thymus, enabling it to make T cells.
The induced thymic epithelial cells (or iTEC) were then combined with other thymus cells (to support their development) and grafted onto the kidneys of genetically identical mice.
After four weeks, the cells had produced well-formed organs with the same structure as a healthy thymus, with clearly defined regions (known as the cortex and medulla).
"Growing 'replacement parts' for damaged tissue could remove the need to transplant whole organs from one person to another, which has many drawbacks not least a critical lack of donors," Dr Rob Buckle, Head of Regenerative Medicine at the MRC, said.
The research was published in the journal Nature Cell Biology.