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Researchers discovered the underlying mechanisms behind how the body cures injured muscles. The research team from University of California San Francisco studied how muscle repair works in mice and they discovered an immune cell called the eosinophil. These cells work in conjunction with other cells responsible for creating fat during the process of healing muscles.

The study's head researcher, Ajay Chawla, M.D., Ph.D., an associate professor of medicine at the University's Cardiovascular Research Institute, and colleagues experimented on mice. They found that during a muscle injury, the eosinophil cells travel to the injured site and work with a type of progenitor cell, which are immature cells that have similar features to stem cells. The combination of these two cells work together and help promote the growth of muscle fibers. The researchers stated that both cells could not repair muscles on their own.

"Without eosinophils you cannot regenerate muscle," Chawla stated. "They wake up the cells in muscle that divide and form muscle fibers."

The progenitors, also known as fibro/adipogenic cells (FAP), are known for producing fat during aging and for forming connective tissues. This new discovery in mice suggests that different combinations of cells can help with multiple tasks within the body. The research team found that eosinophil cells release a molecule known as IL-4, which causes FAP cells to multiply their numbers. However, in this case, the FAP cells do not manufacture fat, but rater, they start acting like muscle stem cells and trigger the healing process of muscles. The researchers also found that even before a muscle injury, the combination of cells was responsible for another task within the body.

"Eosinophils, acting via FAPs, are needed for the rapid clearance of necrotic debris, a process that is necessary for timely and complete regeneration of tissues," Chawla explained. "We want to know if eosinophils and FAPs are universally employed in these situation as a way to get rid of debris without triggering severe reactions such as anaphylactic shock."

The study was published in Cell.