In recent research published in the Journal of Nature Cell Biology, scientists discovered stem cells that could help regenerate tendon cells following an injury.
Tendons are connective tissue in the body that join muscles to the bones. They facilitate movement and help to promote stability. However, tendons are prone to injuries.
Knee Injury
Tendon injuries, including rotator cuff tears and jumper’s knee, are usually painful. They take time to heal and could even lead to secondary ruptures.
In fact, people rarely fully recover from these injuries. This means that patients may have to endure long-term pain and reduced mobility.
Scientists have battled to understand why tendon injuries are difficult to recover from compared to other types of injury. They found out that the buildup of fibrous scar tissue is what stands in the way. Some also thought that the connective tissue probably lacks stem cells, making it hard for injury to fully heal.
Now, researchers at the Carnegie Institution for Science have discovered that tendon stem cells do exist. This finding could help boost healing and possibly remove the need for surgery in case of these injuries.
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“Because tendon injuries rarely heal completely, it was thought that tendon stem cells might not exist,” said lead author Tyler Harvey, a developmental cell biologist. “Many searched for them to no avail, but our work defined them for the first time.”
Potentially game-changing stem cells
The research team was able to identify all cell types in the Patellar tendon, located below the kneecap. It identified tendon stem cells that were not known to exist previously among these cell types.
This discovery was made in mice.
Stem cells are cells that have yet to differentiate fully to serve a specific purpose or function. They are capable of regenerating to form new cells that support the function of the tissue they are associated with.
Almost all tissue types were known to have these cells, but none were linked to the tendon.
The Carnegie team found that the Patellar tendon harbors a group of cells called Tppp3+. That name is the short form for tubulin polymerization-promoting protein family member 3-expressing cells.
These cells help to generate new tendon cells known as tenocytes, the researchers said. Also, they are capable of self-renewing when there is an injury.
This discovery is an interesting one. It means tendon stem cells could potentially find use in promoting natural regeneration.
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However, the scientists found that a receptor may stand in the way of tendon healing, though. Recovery may slow to a halt when platelet-derived growth factor receptor alpha becomes inactive.
Certain Tppp3+ cells express the PDFgra receptor.
A protein known as platelet-derived growth factor-A activates the receptor. When this happens, it stimulates both the tendon’s stem cells and fibrous scar tissue cells. These effects make it somewhat clearer to see why tendon injury does not heal easily.
When the receptor on tendon stem cells gets turned off, the formation of new tendon cells following an injury stops. This gives room for only scar tissue to form.
“Tendon stem cells exist, but they must outcompete the scar tissue precursors to prevent the formation of difficult, fibrous scars,” lead researcher Chen-Ming Fan said.
The team leader said that being able to find a way of blocking scar-forming cells while boosting Tppp3+ cells may help immensely. He said such a therapy could turn out to be a “game-changer” in the treatment of tendon injuries.
Before such therapy could emerge, it is needed to replicate these results in humans.
References
Harvey, T., Flamenco, S. & Fan, CM. A Tppp3+Pdgfra+ tendon stem cell population contributes to regeneration and reveals a shared role for PDGF signalling in regeneration and fibrosis. Nat Cell Biol 21, 1490–1503 (2019). https://doi.org/10.1038/s41556-019-0417-z
