Astronauts lose about 20% of their leg bone mass in a three-month stay in space, no matter how hard they work out on the Stair Master®. Why? To figure it out, scientists needed to crack a century-old mystery. We’ve known since the late 1800s that bone’s microscopic structure is perfectly adapted for the stresses it has to endure — but how does it do it? Bone cells are trapped deep in rigid bone. How do they know what’s going on?
Sheldon Weinbaum is one of the scientists who cracked the bone code. Dr. Weinbaum was interviewed recently by Robyn Williams on Australia’s excellent The Science Show.
“Bone cells live in caves,” Dr. Weinbaum explains. “The mystery has always been how a tissue that’s as stiff as bone can communicate that it’s being loaded to the cells that live within it.”
And the solution to the mystery is “tiny little tubes.” Just like your inner ear uses fluid-filled tubes to detect motion, bone uses (much, much smaller) microscopic tubes to detect forces on bone: “a flow-induced system of mechanotransduction.” Of course it does! D’oh! How could we have missed that for so long? Under stress, fluid in the teensy tubes triggers a reaction in the bone cells. Piece of cake.Just like your inner ear uses fluid-filled tubes to detect motion, bone uses (much, much smaller) microscopic tubes to detect forces on bone.
This process was identified only about fifteen years ago, and Sheldon Weinbaum has been trying to figure out exactly how it works ever since. Just five years ago they made a major breakthrough: “we showed that the bone cell processes were actually tethered along their length and attached to these rigid canalicular walls.” It is these “tethers” that are responding to the fluid motion, tugging ever-so-slightly on the cell every time your foot hits the ground.
Er, right. No wonder it took a hundred years to figure it out.
So, astronauts lose bone mass because the tube system simply doesn’t work in zero-G: the fluid floats randomly in the tubes! Precisely the same reason that astronauts get dizzy in zero-G, and have to learn how to orient themselves visually.
A few well-educated readers might ask “What about piezoelectric effect?” But that’s the answer to a slightly different puzzle: piezoelectric effect is a tiny electrical current produced by deformation of the crystalline structure of bone, another amazing system that the body also uses to control bone development in response to stresses. However, I believe the difference is that piezeoelectric effect regulates overall bone mass and shape — i.e. “put bone here” and “remove some from here,” while the tube system is providing information for the management of the microscopic structure of whatever bone is there. I think that’s the basic idea, but any experts reading this are invited to correct me!