Skeletal Stem Cells in “Resting Zone” Actually Are Hard at Work

Dentistry Today
Image credit: Noriaki Ono


Image credit: Noriaki Ono

Skeletal stem cells are valuable because researchers believe they can heal many types of bone injury, but they’re difficult to find because no one knows exactly what they look like or where they live. 

Researchers at the University of Michigan have identified a type of skeletal stem cell in the “resting zone” of the epiphyseal growth plate, which is a special cartilaginous tissue and an important driver for bone growth.

Noriaki Ono, DDS, PhD, an assistant professor at the University of Michigan School of Dentistry, said that locating stem cells in the resting zone makes sense because it’s widely believed that stem cells stay quiet until they’re needed.

To find the cells, Ono and his colleagues used fluorescent proteins to mark specific groups of cells in mice and monitored them, revealing how the cells behaved in native conditions throughout their lifecycle rather than on a petri dish, Ono said.

The cells they found met the criteria for skeletal stem cells because they become cells that make cartilage and bone while supporting blood cell production. This also may be only one type of skeletal stem cells, Ono said, but it’s an important start. 

“Understanding these special stem cells in the growth plate will help understand why some types of bone deformities and fragile bone diseases can happen in some patients,” Ono said. 

The growth plate comprises different layers, with the resting zone in the top layer. It’s long been thought that cells in the resting zone don’t divide, but Ono’s group found that some cells there wake up and start to make rapidly dividing chondrocytes—cells that produce “beautiful” columns resembling a stack of pancakes and maintain bone growth. 

Some of the cells in the resting zone go from the top to the bottom layer of the growth plate. Some go through the growth plate and into the bone marrow cavity, creating osteoblasts (cells that make bone) and bone marrow stromal cells, which support blood cells. 

Ono said he was surprised that the cells in the resting zone “weren’t just lazy and doing nothing. They’re very hardworking cells. They can occasionally wake up and keep making chondrocytes.”

It has been long hypothesized that chondrocytes at the bottom of the growth plate die, but these findings show that they survive and continue to make bone, Ono said.

The study, “Resting Zone of the Growth Plate Harbors a Unique Class of Skeletal Stem Cells,” was published by Nature.

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