Scientists reverse stroke damage using stem cells

Scientists at the University of Zurich and the University of Southern California say stem cell-derived brain cells may help repair damage caused by stroke and improve recovery after severe brain injury.

This study offers new hope for treating stroke damage that is currently considered permanent.

Stroke is one of the leading causes of long-term disability worldwide. When blood flow to the brain is blocked, brain cells die quickly from lack of oxygen. Unlike other parts of the body, the brain cannot easily replace damaged tissue, leaving many survivors with lasting problems such as paralysis, speech difficulties and memory loss.

In the study in Nature Communication, researchers used neural progenitor cells, early-stage cells that can develop into different types of brain tissue. The cells were transplanted into mice one week after a stroke, a timing the researchers found to be important.

Over five weeks, the transplanted cells survived, spread into nearby brain tissue, and developed mainly into working brain cells. Many became inhibitory brain cells, which help control brain activity and are often lost after stroke. These cells are important for coordinating movement and balancing brain signals.

The treated mice also developed more blood vessels around the damaged area, improving blood flow.

Researchers found reduced inflammation and a stronger blood-brain barrier, which protects the brain from harmful substances in the bloodstream.

The transplanted cells also appeared to support nerve repair. Some extended connections into areas linked to movement and sensation, suggesting they may have begun integrating into existing brain circuits.

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To test recovery, scientists used AI-assisted motion tracking to study how the mice moved. Mice treated with stem cells showed better coordination, balance and movement compared to untreated mice. Improvement became more noticeable over time, suggesting the treatment may support long-term healing.

Researchers also focused on safety. The stem cells were produced using animal-free methods suitable for future clinical use, and scientists are developing built-in safety measures to stop abnormal cell growth if needed.

For now, the cells must be implanted directly into the brain, but researchers are exploring less invasive delivery methods through blood vessels.

However, important challenges remain. The study was carried out in genetically modified mice that would not reject human cells, and scientists still need to prove the transplanted cells can fully integrate into the human brain network over the long term.