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A small bone structure developed from human skeletal stem cells. The blue color indicates the cartilage, the brown represents the bone marrow and the yellow shows the bone. (Credit: Chan and Longaker et al.)
If only we could push back our broken bones like Harry Potter, the Skele-gro style. Or, at the very least, heal like a triton that regenerates the limbs. Alas, we humans do not have such abilities. Although our bodies can repair broken bones, the older we get, the more messy the patchwork is. When it comes to cartilage – the essential padding that keeps our bones from rubbing – once everything is gone, it's gone for good.
But a new discovery by researchers could change this perspective. A team from Stanford University has finally discovered skeletal stem cells – cells that give birth to bones, cartilage, and the spongy interior of a bone called stroma – in the body. 39, man for the first time. And the hope is that doctors can someday use these stem cells to help people fight back broken bones and missing cartilage.
A break in the case
First of all, a quick introduction to some types of stem cells (of which there are many).
In general, stem cells have the ability to divide and grow into specialized variations, such as blood or muscle cells. As you may have guessed, embryonic stem cells are found in embryos. They have the ability to develop primarily in any type of cell in the body. Adult stem cells, on the other hand, are not as rich in potential. They always grow in different cells, but usually these types of cells are restricted to the organ from which they originate. For example, neural stem cells reside in the brain, where they divide and generate brain cells such as neurons.
Previously, experts had only discovered so-called mesenchymal cells in their search for human skeletal stem cells. Mesenchymal cells are adult stem cells that can grow into a number of other more specialized cells, such as bones, cartilage, fats, and muscles. But researchers still could not focus specifically on skeletal stem cells.
In 2015, the Stanford team, led by Michael Longaker, professor of plastic and reconstructive surgery at the university, benefited from a break. They announced that they had found skeletal stem cells in mice. To do this, they looked at genetically modified mice so that different stem cell subtypes of the mesenchymal mixture all produce different colors. By coding these subtypes by color, the team was able to track their lineage – their division and evolution into more specialized cells.
Virtually all cells have surface proteins and sometimes cells of different species share similar surface proteins. The team therefore thought they would only look at the proteins of the mouse skeletal stem cells they had discovered and look for similar ones on human mesenchymal cells. Then, they would follow the line of these cells to see if they were turning into specialized bone cells. Easy, right? Nope. They could not find any human cells with enough surface proteins in common with what they had found in mouse cells.
Change search tactics
Instead, Longaker and his team have done reverse engineering. They examined the bones given by the fetal remains, focusing specifically on the still-growing extremities, giving the researchers a better chance of finding the stem cells they were hunting.
Since surface proteins were prohibited, they searched for cells with genetic signatures similar to those of mouse skeletal stem cells. Once they found human cells close enough, they isolated them and put them in a petri dish. And that's it: the cells just produced new pieces of bone, cartilage and spongy stroma. Even though Longaker and his company were convinced that they were finding what they were looking for, they still had to make sure.
Thus, eventually, the group turned to adult bone pieces that they had acquired from people who had undergone procedures such as hip replacements. The group spotted cells with this same genetic profile and extracted them. And they too turned into bone, cartilage and stroma in Petri dishes. The team published its findings in Cell.
Put into practice
Now that we have found these stem cells, what is the next step? Longaker hopes that approximately 10 years from now, doctors will be able to use these cells. "The United States has a population that is aging rapidly and suffering nearly 2 million joint replacements every year," he said in a news release. "If we can use this stem cell for relatively non-invasive therapies, it could be a dream come true."
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