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Oh, a flow-induced system of mechanotransduction! Of course!

PainSci » bibliography » Fritton et al 2009
Tags: biology, neat

Two articles on PainSci cite Fritton 2009: 1. Deep Friction Massage Therapy for Tendinitis2. Tissue Provocation Therapies in Musculoskeletal Medicine

PainSci notes on Fritton 2009:

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; he was interviewed by Robyn Williams on Australia’s excellent The Science Show in 2009:

“Bone cells live in caves. 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.” Under stress, fluid in the teensy tubes triggers a reaction in the bone cells. This process was identified only in the 90s, and Sheldon Weinbaum has been trying to figure out exactly how it works ever since. 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 in response to gravity.

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.

original abstract Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

Much recent evidence suggests that bone cells sense their mechanical environment via interstitial fluid flow. In this review, we summarize theoretical and experimental approaches to quantify fluid and solute transport in bone, starting with the early investigations of fluid shear stress applied to bone cells. The pathways of bone interstitial fluid and solute movement are high-lighted based on recent theoretical models, as well as a new generation of tracer experiments that have clarified and refined the structure and function of the osteocyte pericellular matrix. Then we trace how the fluid-flow models for mechanotransduction have evolved as new ultrastructural features of the osteocyte lacunar-canalicular porosity have been identified and how more recent in vitro fluid-flow and cell-stretch experiments have helped elucidate at the molecular level the possible pathways for cellular excitation in bone.

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