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ow you see it, now you dont: Scientists have used a chemical technique to make mouse bones turn transparent. The technique has been used in the past to make brains and kidneyssee-through, but this marks the first time its been used in hard tissues.

The ability to see within a bone couldhave implications for research into bone diseases, by letting researchers get a more accurate picture of bones 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 inplants. Caltech neuroscientistViviana Gradinaru, an original developer of the technique, even cleared an entire mouses body in 2014 (except for its bones, which were unaffected, she said).

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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.

Flexible 3-D printed scaffolds could mend broken bones

In this case, Gradinaruwanted 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 iscontinuously changed. The bones we have in our body, we didnt have them 10 years ago, she explained. Acontinuous process of bone cell death and bone cell growth ishappening, spurred by progenitor cells in a bones soft, spongy marrow.

But looking for these cells can bechallenging. There arent that many progenitor cells, soextrapolating the number and distribution based ona small sample isnt ideal. Researchers can slice the bone, but cuttingcan damage the edges. Putting images of the sliced bones back together into a coherent, 3-D picture is very difficult, too.A clear bone avoidsslicing 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.

Gradinarus 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, Gradinarusaid.

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

Theres still more work to be done. For instance, finding a way to tagthe samples with antibodies without having to cut a bone in half, as researchers did in this paper would be ideal.Gradinaru also wouldnt mind some speed improvements:In this case, the CLARITY process took nearly a month.

Its not a fast method, by any means, Gradinaru said. However, the result theres no substitute for getting 3-D access to the intact bone marrow.

Kate Sheridan can be reached at kate.sheridan@statnews.com Follow Kate on Twitter @sheridan_kate

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Scientists turn bones transparent to let them see into marrow - Stat - STAT

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