Scientists target individual tendon cells to reduce scarring after surgery

Long overlooked in the field of musculoskeletal study, the tendon is a highly complex anatomical structure that triggers a dynamic but mysterious cellular response when it's injured. In a new study in Nature Communications, University of Rochester researchers for the first time trace and manipulate a key player in the healing process—the epitenon cells that form a thin outer layer surrounding the tendon—and show the remarkable similarity between these cells' activity in mice and humans. Their discovery may pave a path to new translational treatments for a common, highly consequential but poorly understood health issue.

Injuries to the flexor tendon in the hand require approximately 300,000 surgeries a year and represent $400 million annually in health care costs.

Severed tendons are generally repaired surgically but often heal poorly due to the weak cell regeneration capacity of tendon tissue and the likelihood of excessive scar formation. As many as 24% of patients with injured flexor tendons require a second surgery to remove excess scar tissue. Currently there is no post-operative intervention to improve tendon healing beyond physical therapy, and until a decade ago, few researchers were exploring tendon cell biology as a viable therapeutic approach.

UR's Center for Musculoskeletal Research (CMSR) is one of a handful of centers around the world focusing on tendon healing at the cellular level, and their work is expanding appreciation for the tendon's complex, intricate healing processes.

Tendon: Not a 'rope' but a complex structure

Tendon has long been seen as a simplistic body part, a "rope" of stringy, tough tissue. Recent research in the CMSR has focused on uncovering the great diversity and complexity of tendon cells, and on dissecting their interplay at the injury site. CMSR researchers are revealing how these cells induce healing or interfere with it at different points in the process.

"One of the challenges in tendon is being able to target all these different cell populations," said lead author Anne Nichols, Ph.D., assistant professor of Orthopaedics at the Center for Musculoskeletal Research at the University of Rochester. "Tendons in general heal poorly, but not uniformly. In different regions of the healing tendon, you'll see different types of fibrosis develop. An effective therapy requires targeting these specific areas in different ways to modulate the healing process."

In tendon surgery, the two severed ends are sewn back together. The scar that forms between the tendon ends is crucial for function, but excessive scar tissue that builds up outside the repair site can restrict finger range of motion. Nichols and her team aimed to show that shutting down certain cellular processes in specific cells during a precise healing timeframe can reduce excess scar formation without compromising tendon stability.

A new player: The epitenon

Most previous studies examined tenocytes, specialized cells found in tendons that produce and maintain the tendon's structure. The study focused on another source of cells, the epitenon. Previous research identified it as a primary orchestrator of early tendon healing, possibly contributing to collagen formation in and around the injury.

"The epitenon contains a cell population that we knew was important, but could not target," Nichols said. Her team focused on identifying a genetic driver that could be used to target, track, and manipulate these cells.

Researchers used genetic lineage tracing and single cell RNA sequencing to target epitenon cells and for the first time, were able to trace the behavior and outcome of this population following tendon injury in mice.

When it comes to tendon healing, Nichols said, timing isn't everything. Location matters, too.

The team looked at the location of epitenon-derived cells and found that a portion of them followed tenocytes into the crucial bridging tissue that helps tendons repair, but a larger percentage migrated to form a capsule of scar tissue around the injury site. This suggests that epitenon cells are involved in both productive healing and undesired scar tissue depending on where they end up during healing.

"We know there is a fibrotic burst of activity in an injured tendon between days 7 and 10 post-injury; we wanted to target pro-fibrotic epitenon cells in that time window to see how removing them from the injury site impacts healing."

When researchers killed epitenon-derived cells using a genetically encoded toxin, they confirmed that suppressing epitenon-derived cells at the right time and place in the healing trajectory significantly improved tendon range of motion in mice.

Identifying the similarity of mouse, human cells

To demonstrate the translational value of their work, researchers looked at data on epitenon cells in mice and compared that to donated tissue from healing tendons in humans.

"The cells we identified as problematic for forming scar tissue in a mouse also exist in the human scar, which suggest they play a similar scar-forming role in humans following tendon injury," Nichols said. "In theory, if we were able to target these cells in humans, we could get the same outcome in them as we could from a beneficial treatment in mice."