Any orthopedist can tell you that a tendon injury is worse than a muscle injury. Part of the reason is that tendon tissue heals very slowly and incompletely.

Now, a group of scientists has identified unique cells within the adult tendon that can regenerate in the same way as stem cells in other kinds of tissue. They were able to isolate these cells and regenerate tendon-like tissue in animals, a discovery that holds great promise for the treatment of human tendon injuries.

The results of their research will be published in the October 2007 issue of the journal Nature Medicine.

Tendons, the specialized tissues that connect bone to muscle, are made up of strong collagen fibers that allow the body to move. Tendon injuries are a common problem in sports and in daily life, as damaged tendons heal slowly and rarely regain their original strength or flexibility.

"Clinically, tendon injury is a difficult one to treat, not only for athletes but for patients who suffer from tendon rupture or ectopic ossification [bony substances that can develop within muscle or tendon tissue]," says Songtao Shi, a researcher at the University of Southern California and co-leader of the research team, "This research demonstrates that we can use stem cells to repair tendons. We now know how to collect them from tissue and how to control their formation into tendon cells." The team also included scientists from the National Institute of Dental and Craniofacial Research at NIH, Johns Hopkins University and the University of Maryland School of Medicine.

There was little previous research on the cellular makeup of tendons and their precursors. By looking at tendons at the molecular level, the research team identified tendon stem/progenitor cells (TSPCs) in both mice and adult humans. When guided by a certain molecular environment, these TSPCs are able to form into tendon cells.