Brainy bones: Our skeletons have a similar number of cells as the brain…

Posted: 10th December 2015

Bones are savvy. They are light, strong, and they can repair themselves when damaged. What’s more, they are continually renewing themselves, replacing old bone for new. This is not unique - other tissues (most noticeably skin) are continually replaced. But bones adjust to the body’s mechanical and physiological needs as they go. How do bones manage such a
remarkable feat? Recent advances in imaging technology are starting to reveal the brains buried deep within bone: a living cellular network composed of bone’s most abundant cell, the osteocyte.

Like the neurons in the brain, osteocytes have long finger-like projections that burrow through the bone to interconnect with each another. Inspired by the complexity of this cellular network, SVI’s Associate Professor Natalie Sims and her colleague, Monash mathematician Dr Pascal Buenzli, set out to quantify the osteocyte network in the human skeleton. What they found exceeded all expectations, paralleling even the neural network of the brain. The team estimated that the human skeleton contains 42 billion osteocytes. In comparison, the brain contains 86 billion neurons, packed in a volume (1.2 L) comparable with the volume of bone in the skeleton (1.75 L), although of course the skeleton is more spread out.

Adding together the length of the osteocytes’ projections, the team estimated that the entire network is about 175,000km long: more than four times the earth’s circumference. This is very similar to the total length of the pathways between brain cells. They then used a mathematical model to calculate the total number of connections between the cells. How many? Twenty three trillion connections exist in the human osteocyte network. So, in a way, our skeletons have a similar number of cells as the brain, interconnected in a similar sized space.

Why do our skeletons need such a complex network? The experts don’t know exactly, but they do know that the cells exchange information, just like neurons do. We know that osteocytes communicate with each other about where the skeleton is weak and needs to be strengthened, or where there is damage that needs to be fixed. These messages are transmitted to cells on the bone surface that are able to remove damaged bone (osteoclasts) and form new bone (osteoblasts).

The team at SVI will continue their research into these brainy cells, in order to find better treatments for skeletal disorders like osteoporosis or osteogenesis imperfecta, and explore ways to get people back in action more quickly after a fracture. In the meantime, the osteocytes will continue to keep our skeletons strong (and smart) enough to support us.

Image courtesy of Lynda Bonewald, University of Missouri-Kansas City