In a major forward leap in regenerative medicine research, a group of British scientists report that they have successfully grown a complete, functional organ from harvested mouse cells. Even better, the new organ functioned normally after it was transplanted into a recipient mouse.

Researchers at the University of Edinburgh produced the world’s first working thymus, not from stem cells but from connective tissue cells called fibroblasts. Although the work was done entirely in mice, the success is nonetheless an extraordinary achievement in regenerative medicine, which endeavors to one day be able to replace diseased or injured organs with fresh ones grown in the lab.

Up to now, only small portions of functional organs have been successfully grown in the lab. Bits of hearts, livers, and even brains have been grown from stem cells, but no complete, functional organs have been produced this way. In the new study, the investigators created the new thymus from fibroblast cells, not stem cells.

The thymus gland is a kind of command center for the body’s immune system. Two kinds of immune cells stop by the thymus to be armed and equipped to fight off foreign invaders once they move back into the circulating blood. These so –called T-cells, particularly CD4 or “helper” T-cells and CD8 or “killer” T-cells orchestrate and carry out attacks on infected or abnormal cells, respectively.

The Edinburgh scientists coaxed mouse fibroblast cells to overproduce a transcription factor, or gene “switch” called FOXN1, which guides the formation of the thymus during normal embryonic development. The fibroblasts with enhanced FOXN1 levels transformed into induced thymic epithelial cells that when combined with other supportive thymus cells and grafted onto the kidneys of mice, grew into well-formed, structurally intact organs.

“This research is an exciting early step towards that goal, and a convincing demonstration of the potential power of direct reprogramming technology, by which once cell type is converted to another,” said Dr Rob Buckle, Head of Regenerative Medicine at the Medical Research Council, the source of funding for the study. “However, much more work will be needed before this process can be reproduced in the lab environment, and in a safe and tightly controlled way suitable for use in humans.”

The breakthrough accomplishment was published earlier this week in the journal Nature Cell Biology.