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Mixed Muscle Messages

Posted on March 1, 2019

A new study from the University of Michigan investigates why fat accumulates at the site of rotator cuff injuries instead of new muscle.

Rotator cuff tears are common injuries, often caused by overuse. These injuries can take a long time to heal – if they heal at all.

“Rotator cuff injuries as very small tears in the muscle. Over time, as a result of overuse – or simply normal use – these tears get bigger, making the healing process long and difficult,” said Dr. Joel Singer, a New York personal cell physician.

In many cases, surgery is required to repair damaged muscle tissue, a situation that comes with its own set of complications.

“Surgery for an injured rotator cuff means a long time for complete healing, and even then many patients experience weakening or atrophy of the muscle,” Singer said.

This situation makes rotator cuff injuries among the most difficult injuries to rebound from for most people.

One explanation could be that fat accumulation develops after a tear more readily in the rotator cuff muscles than in other injured muscles.

But, why does that happen?

The Michigan researchers set out to answer this question.

Their investigation took them into the cellular, molecular and genetic reasons for why the rotator cuff muscles develop fat accumulation.

Using a mouse model, the researchers isolated specific stem cells known as satellite cells in the rotator cuff muscles tissue. They also extracted these special stem cells in calf muscles.

Once the cells were collected, the researchers performed genetic studies to understand how the pathways of the two cells in the two types of muscle were different.

While stem cells taken from the rotator cuff muscles and the cells taken from the calf muscle tissue were generally thought to be based on the same type of muscle stem cells, the researchers believed there was a difference in their cell behavior or development.

They were right.

They found that the stem cells in the rotator cuff muscle group behaved differently and had different genetic markers.

The rotator cuff stem cells differentiated into 23 percent fewer muscle cells than the calf muscle stem cells. These cells also showed an 87 percent decrease in a marker for muscle formation compared to their calf muscle counterparts.

The stem cells taken from the rotator cuff also had more genetic markers that lent to fat cell generation – an up to 65-fold increase compared to calf muscle cells.

The researchers also examined the cells to determine what caused fat cell production to become activated.

They found that there were 355 different regions of DNA between the stem cells taken from the rotator cuff and those collected from the calf muscle.

The researchers were able to use a pathway enrichment analysis to identify what genes triggered adipogenesis, or the formation of fatty tissue.

Their findings suggest that rotator cuff muscle stem cells simply have DNA that makes them designed to become fat cells.

The U of M study is the first of its kind to study DNA and how it relates to the development of rotator cuff issues. The study authors hope that their research inspires future studies and new ways to treat musculoskeletal conditions.

“Many orthopedic and muscle injuries can take a long time to heal for reasons such as the development of fatty tissue at the injury site, as wells as lack of adequate blood flow and oxygen to the injured area, continued use and chronic inflammation,” said Singer.

Source: Michigan Medicine – University of Michigan. “Stem cells provide greater insight into rotator cuff disease.” ScienceDaily. ScienceDaily, 26 February 2019. 

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