Scientists can already grow organs in a dish (termed in vitro), but new research shows that a thymus can be grown in a mouse (termed in vivo) after injection of genetically modified cells. The thymus is particularly important for our health since it's the sight of T cell development (one of the main fighters of the immune system), and it tends to shrink with age (see image, showing the loss of the main parts of the thymus). As a result, the number of new T cells being produced decreases, which increases the chance of succumbing to infections and cancers.
To grow the thymus in vivo, researchers forced the expression of a transcription factor, FOXN1, in a specific type of cell in mice. This transcription factor is important for the development of cells in the thymus, and the researchers hypothesized that turning it on would create thymic-like cells. They found that this was the case, and were able to support the development of T cells in vitro.
Next, they took these modified cells, and injected them into the kidney capsule of adult mice (a common method of growing new cells in a mouse). They found that these cells were able to create a proper thymus, complete with the correct tissue architecture and the ability to help make mature T cells.
Of course, there are the usual limitations to mouse studies. Genetic engineering was needed to turn on FOXN1, and things that work in mice don't necessarily work the same way in humans.
Still, recent advancements have shown that we can create malleable stem cells from adult humans. Studies like these show how we could potentially manipulate those cells to create organs, in a lab or in the patients themselves. Fingers crossed that with the steady accumulation of knowledge, we'll be seeing patient-derived organs and treatments in the next several decades.