Making bone transparent

Scientists have developed a biochemical process that renders bones transparent and then used it to visualize the bone-cell-proliferating action of a new osteoporosis drug (Sci. Transl. Med. 2017, DOI: 10.1126/scitranslmed.aah6518). In addition to enabling the study of drug effects on bone cells, the feat could provide an unprecedented view of the processes in bone cell growth and death.

The team, led by Caltech biology and biological engineering professor Viviana Gradinaru, based its bone-clearing strategy on a soft-tissue-clarifying method, called Clarity, that Gradinaru helped develop as a postdoc in Karl Deisseroth’s lab at Stanford University.

In recent years, numerous scientists have made significant progress in being able to remove light-scattering lipids from brains and other soft-tissue constructs. The ability to visualize clear, intact soft tissue has great advantages over examining tissue slices because the structures and cells within remain connected and undisturbed.

Bones, however, have been tougher to render transparent than soft tissues because they are impregnated with minerals, which are difficult to wash away while maintaining bone structure.

To clear the calcium from bones, Gradinaru’s lab used ethylenediaminetetraacetic acid (EDTA). Then, as in the original version of Clarity, the researchers infused the bone with acrylamide monomers to form a stabilizing hydrogel mesh and finally washed out lipids with a detergent.

To see cells inside the transparent bone, the group genetically engineered mice so that their bone stem cells, known as osteoprogenitors, glowed red. Drug company Amgen provided the team with a new osteoporosis drug to investigate. Gradinaru’s team compared the bones of mice who had received the drug with those who hadn’t. The researchers saw a clear proliferation of osteoprogenitors in the vertebrae of mice who had been given the drug.

Sean Morrison, director of the Children’s Medical Center Research Institute at the University of Texas Southwestern says “improving our understanding of the localization of osteoprogenitors in the bone marrow is an important goal that will enhance our understanding of the mechanisms that regulate the maintenance of the adult skeleton.”