Scientists turn bones transparent to let them see into marrow

Now you see it, now you don’t: Scientists have used a chemical technique to make mouse bones turn transparent. The technique has been used in the past to make brains and kidneys see-through, but this marks the first time it’s been used in hard tissues.

The ability to see within a bone could have implications for research into bone diseases, by letting researchers get a more accurate picture of bone’s internal structure.

The technique is called CLARITY, and since 2013, when it was first described, it has been deployed on a wide variety of mammalian tissues and in plants. Caltech neuroscientist Viviana Gradinaru, an original developer of the technique, even cleared an entire mouse’s body in 2014 (except for its bones, which were unaffected, she said).

The approach works by chemically locking proteins and DNA in place with a hydrogel, after which researchers wash away fats within the tissue. Lipids refract light, so this washing step makes CLARITY-treated tissues transparent.

In this case, Gradinaru wanted to look at bone marrow and count the number of stem cells that could ultimately produce new bone cells.

“Bone is not a static organ. It is continuously changed. The bones we have in our body, we didn’t have them 10 years ago,” she explained. A continuous process of bone cell death and bone cell growth is happening, spurred by progenitor cells in a bone’s soft, spongy marrow.

But looking for these cells can be challenging. There aren’t that many progenitor cells, so extrapolating the number and distribution based on a small sample isn’t ideal. Researchers can slice the bone, but cutting can damage the edges. Putting images of the sliced bones back together into a coherent, 3-D picture is very difficult, too. A clear bone avoids slicing altogether.

Doug Richardson, director of imaging at the Harvard Center for Biological Imaging, said the paper represented “a step forward in bone clearing.” (Richardson was not involved in this research.)

“This technique has the potential to monitor bone health or disease progression over larger volumes with greater accuracy,” he said.

A clear path forward

Gradinaru’s team has already demonstrated one possible application. They found that a drug for osteoporosis, currently being developed by Amgen, triggered an increase in the number of stem cells in CLARITY-treated bone. Some Amgen scientists were coauthors of the paper.

Using CLARITY let the team more effectively measure the rate of this increase. “This is very important, because you want a controlled increase — too much of an increase can lead to tumors,” Gradinaru said. 

Other uses could be on the horizon. Being able to make a mouse or rat skull see-through could be useful for Gradinaru’s fellow neuroscientists who use implants in their research and want to establish the exact position of the impact after experiments are done.

There’s still more work to be done. For instance, finding a way to tag the samples with antibodies — without having to cut a bone in half, as researchers did in this paper — would be ideal. Gradinaru also wouldn’t mind some speed improvements: In this case, the CLARITY process took nearly a month.

“It’s not a fast method, by any means,” Gradinaru said. “However, the result — there’s no substitute for getting 3-D access to the intact bone marrow.”