Bones seem pretty hard for the most part, but recently scientists from the University of Cambridge have discovered a “goo” between minerals in the bone that allow the tissue to flex slightly and resist breaking. The discovery was made by analyzing bone with NMR spectroscopy, x-ray diffraction, and other computer models. 

The viscous ‘goo’ is made from water and citrate, which is generated during cellular respiration. Though this goo was previously unknown before this study, it could make up about half of bone mineral. The goo is sandwiched between layers of minerals, which gives the minerals a bit of cushion when pressure is applied and keeps everything from drying out. Citrate molecules suspended in the goo easily bond with calcium, which is abundant in bone minerals, and helps to keep them in place but doesn’t allow them to fuse together. Additionally, the citrate also helps hold the water in place so it doesn’t leak out.

This discovery has already kicked the door wide open on what is known about bone disease, such as osteoporosis. If not for the goo, the minerals sheets would clump together and become a dry, brittle crystal, easily breaking. The sheet structure is required to maintain the flexibility and resist fracturing.

When calcium for the bone is transported through the body, it comes wrapped in a citrate sheath, which ensures it doesn’t end up in the wrong spot and create brittle lumps. The rest of the process is a chemical reaction, and not a biological process. When the calcium becomes integrated with the bone, it fits into tiny holes in a protein mesh. The accompanying citrate is unable to get back out of the holes, and thus becomes integrated as a layer in the bone tissue, providing tensile strength.

With age or after injury, the holes in the protein mesh aren’t quite as small as they used to be, because the tissue does not get repaired as effectively and is lost more quickly than it is repaired. The larger holes allows citrate to drop of the necessary calcium, but doesn’t keep them trapped in order for it to bond with the bone mineral and provide that cushion. In order to properly address bone diseases like osteoporosis, the researchers believe the focus should shift toward repairing the holes in the protein mesh and return them to their smaller size, which would allow the citrate to bond and create a strong, (ever so slightly) flexible bone. 

The paper, which was published in the Proceedings of the National Academy of Sciences is the first in a series on bone chemistry that will be released by the team this year.

Diagram of how the citrate layers fit in with the mineral platelets. Credit: PNAS