Functional recovery coexists with progressive chondrogenic-like matrix remodeling during murine Achilles tendon healing: a longitudinal histological, functional, and exploratory single-cell study

The Achilles tendon is the body's strongest cable, a thick band of connective tissue that links the powerful muscles of the calf to the heel bone. It acts as a spring, storing energy when we walk or run and releasing it to propel us forward. When this cable tears, the body attempts to repair it, but the process is rarely a perfect restoration. Instead of growing back exactly as it was, the injured tissue often heals with a scar that is weaker and less organized than the original. For decades, doctors and scientists have judged the success of a tendon repair by how well the limb functions again. If a person can walk without pain and regain their strength, the treatment is considered a success. However, this focus on movement has left a gap in our understanding: does a limb that works well actually contain healthy tendon tissue, or is it simply a functional patchwork of different materials?

A team of researchers at the Chinese PLA General Hospital set out to investigate this question by watching the healing process unfold over time in mice. They created a controlled injury in the Achilles tendon of young adult mice, cutting about half of the tendon's width to simulate a partial tear. Then, they tracked the recovery over four weeks, checking the tissue at regular intervals. They looked at the tissue under microscopes to see how the fibers were arranged and measured how well the mice could walk and grip with their injured legs. To get a deeper look at the cells involved, they also analyzed the genetic activity of the cells within the healing tissue at two specific points in time. Their goal was to see if the return of function matched the return of healthy tendon structure, or if something else was happening beneath the surface.

What the researchers found was a surprising disconnect between how the leg felt and what the tissue actually looked like. As the weeks passed, the mice's limbs clearly improved. By the end of the study, the animals were walking with a more normal gait, and their hind legs were significantly stronger. The tissue itself showed signs of healing too; the collagen fibers, which are the main structural threads of the tendon, began to line up more neatly, and the chaotic inflammation seen in the first week largely disappeared. On the surface, this looked like a successful recovery. Yet, when the scientists examined the chemical makeup of the tissue, they discovered a different story. While the fibers were organizing, the tissue was also accumulating a growing amount of a slippery, jelly-like substance called proteoglycan. This substance is not typical for a healthy tendon; it is usually found in cartilage, the smooth material that cushions joints.

The amount of this cartilage-like material grew steadily throughout the healing period. At the start, it made up a tiny fraction of the tissue, but by the fourth week, it had expanded to cover more than twenty percent of the repair area. At the same time, the researchers tracked two specific types of cells. One type, marked by a protein called scleraxis, is responsible for making tendon tissue. The other type, marked by a protein called SOX9, is responsible for making cartilage. Both types of cells increased in number as the tendon healed, but the cartilage-making cells grew much faster, increasing more than five times their original number, while the tendon-making cells grew by less than two times. The cartilage-making cells were found exactly where the jelly-like substance was accumulating, suggesting that the tissue was actively building a cartilage-like structure alongside the healing tendon fibers.

To understand how these different cell types interacted, the team used a technique that reads the genetic instructions of thousands of individual cells at once. This analysis revealed that the healing tissue was not just a single mass of scar tissue, but a complex mix of different cell states. Some cells were acting like tendon builders, others were acting like stem cells ready to help, and a significant group was acting like cartilage builders. The researchers used computer models to trace how these cells might be related, finding that the stem-like cells could potentially develop into either tendon or cartilage cells. They also identified chemical signals, specifically pathways involving proteins like WNT and TGF-beta, that might be telling these cells to switch roles. However, because the researchers studied a single pool of cells from each time point rather than multiple separate groups, these genetic interactions remain a strong hypothesis rather than a confirmed fact.

The study concludes that a tendon can heal enough to restore function while simultaneously undergoing a transformation into a tissue that is chemically different from a healthy tendon. The return of strength and movement does not guarantee that the tissue has returned to its original, pure state. Instead, the repair process involves a progressive shift where cartilage-like material builds up alongside the recovering fibers. This finding challenges the common assumption that a functional limb means a fully restored tendon. It suggests that when doctors evaluate a healed tendon, looking only at how well a patient moves might miss the underlying changes in the tissue's composition. The study implies that future treatments might need to focus not just on getting the limb to work, but on ensuring the tissue itself returns to its proper form, preventing the long-term risks that might come from a tendon that is functional but chemically altered.