Growing Muscle’s Own Stem Cells Again with 3D Mini-Muscles

Skeletal muscles can repair themselves because they keep a small number of dedicated stem cells called satellite cells. Satellite cells are muscle stem cells that live in a special spot next to each muscle fiber. When muscle is injured they wake up, divide to make new cells, fuse with damaged fibers to rebuild them, and then return to a resting state ready for the next repair job.

The challenge is that satellite cells are very rare. If they are grown in normal flat dishes they lose key properties and become myoblasts. Myoblasts are muscle precursor cells committed to become muscle fibers once, but they cannot return to stem-cell status or regenerate muscle multiple times.

A team of researchers described a simple three-dimensional organoid method to convert abundant myoblasts back into genuine muscle stem cells, called in vitro-derived satellite cells or idSCs. Here is how it works. First, they take mouse myoblasts from muscle tissue. Second, they put those cells into a spinning bioreactor vessel that keeps them floating and encourages them to stick together into small 3D clusters called skeletal muscle organoids or SkMOs. Third, they feed the organoids for about thirty days in the right nutrient mix.

Over that time some of the cells in each organoid switch off a protein called MyoD that normally drives them to commit to becoming myoblasts. At the same time they keep the protein Pax7 that marks real satellite cells. They shrink to the right size for a stem cell, stop dividing into new cells and enter a quiescent state, which means they are resting but still alive. These idSCs match natural satellite cells in patterns of gene activity, chemical tags on their DNA called epigenetic marks and on their cell-surface proteins.

The real test is function. When researchers transplanted just ten thousand idSCs into damaged or genetically dystrophic mouse muscles they fused into new muscle fibers, repopulated the satellite cell niche and after further injuries repaired muscle as well as freshly isolated satellite cells. By contrast, the same number of myoblasts barely stayed in the muscle and did not regenerate tissue or refill the stem-cell pool.

Importantly the team did not rely on genetic labels to isolate idSCs. They found that idSCs carry two marker proteins on their surfaces called CD9 and CD104. By using sorting methods that look for CD9 positive and CD104 negative cells they could purify idSCs for transplantation.

To make this clinically relevant the researchers also tried the protocol with human muscle cells from patient biopsies and from commercial cell lines. In just fifteen days of organoid culture in a defined, serum-free medium they observed human Pax7 positive MyoD negative cells that were quiescent and resembled actual human satellite cells in key ways. While further studies in larger animals will be needed, this 3D organoid strategy offers a potentially scalable route to grow true muscle stem cells for therapies. Possible uses include treating genetic muscle diseases such as muscular dystrophy, healing serious injuries from accidents or surgery and reversing age-related muscle loss.

By rebuilding the physical environment that muscle stem cells experience in living tissue the organoid culture reawakens each cell’s own latent repair program. This advance offers not only a new way to generate cells for regenerative medicine but also a useful platform to study the molecular switches that tell a cell when to sleep and when to spring into action to fix muscle.

Works Cited

Price, Feodor D., Mark N. Matyas, Andrew R. Gehrke, William Chen, Erica A. Wolin, Kristina M. Holton, Rebecca M. Gibbs, et al. “Organoid Culture Promotes Dedifferentiation of Mouse Myoblasts into Stem Cells Capable of Complete Muscle Regeneration.” Nature Biotechnology, 2024, https://doi.org/10.1038/s41587-024-02344-7.