Unique type of skeletal stem cells found in bones

Scientists have discovered in the growth plate of bone a new type of stem cells, which forming bones, cartilage and tissue of the spongy or cancellous portions of bones, containing bone marrow. Prepared reserves of these cells can one day help doctors restore or replace cartilage in the joints, heal fractures faster, build up bone tissue in osteoporosis, and even grow new bones and cartilage for plastic surgery.

Skeletal stem cells are of great value because they can heal many types of bone injury, but they’re difficult to find because researchers don’t know exactly what they look like or where they live.

Previously discovered mesenchymal stem cells (MSCs) can also be differentiated into bone and cartilage cells, but they are able to transform into fat, muscle and connective tissue cells. Those MSCs are more universal, in contrast to “highly specialized” skeletal stem cells.

American researchers from the University of Michigan have identified a type of skeletal stem cell in the “resting zone” of the epiphyseal growth plate, responsible for bone growth. The discovery results are published on October 31, 2018 in Nature.

Noriaki Ono, an associate professor of dentistry at the University of Michigan, said that the location of stem cells in the resting zone is logical, since it is widely believed that stem cells stay quiet until they’re needed.

To find these cells, Noriaki and colleagues used fluorescent proteins to mark specific groups of cells in mice and tracked the fate of these cells over time.

The cells found in this way met all the necessary criteria for skeletal stem cells, since they fulfilled three main conditions:  they become cells that forming both cartilage and bone and support blood cell generation.

“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”, – said Noriaki.

The growth plate of the bone consists of various layers. In the top layer is a zone of rest. It’s long been thought that the cells in this zone do not divide. But the Noriaki group found that some cells wake up and quickly differentiate into chondrocytes that support bone development by forming stacks along the long axis of the developing skeletal element.

Some of the stem cells of the resting zone go all the way from the upper to the lower layer of the growth plate. Others actually pass through the growth plate into the bone marrow cavity, creating osteoblasts (cells that form bone) and stromal cells of the bone marrow that support the population of blood cells.

Noriaki 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.”

For many years, it has been suggested that chondrocytes at the base of the growth plate die over time, but the data show some evidence that they survive and continue to form bone, he said.