What keeps bones able to remodel themselves and stay healthy? A team from Charité Berlin has discovered clues to the key function of non-collagen protein compounds and how they help bone cells react to external load. The scientists used fish models to examine bone samples with and without bone cells to elucidate differences in microstructures and the incorporation of water. Using 3D neutron tomography at the Berlin research reactor BER II, they succeeded for the first time in precisely measuring the water diffusion across bone material — with a surprising result.
Around 500 million years ago, early vertebrates in the seas became fish, adopting an inner skeleton and a flexible spine based on a nanocmposite of fibers and mineral, known as bone material. This “invention” of evolution was so successful that the basic structure was also adopted for later vertebrates that lived on land. However, while the bones of all terrestrial vertebrates are basically equipped with bone cells (osteocytes), certain fish species continued to evolve and finally managed to create a more energy efficient material: bone lacking bone cells, found today for example in fish such as salmon, medaka or tilapia.
Samples with and without bone cells
“We asked ourselves how bone samples with and without bone cells actually differ in their microstructures and properties,” says Prof. Paul Zaslansky, who heads a research group at Charité Berlin and specializes in mineralized biomaterials including teeth and bones. Together with PhD student Andreia Silvera and international partners, they have now compared bone samples from zebrafish and medaka. Both fish species are of similar size and live in similar conditions, so their skeletons must withstand similar stresses. However, while zebrafish have bone cells, the skeleton of medaka do not.
“The background to the question is that the function of bone cells in bone and how they change with age is of great interest to the aging population,” Silvera explains. Bone cells can respond to physical stress by sending biochemical signals that lead to the formation or resorption of bone tissue, adapting to load. But with age or in diseases such as osteoporosis, this mechanism no longer seems to work. “With our basic research, we want to find out how bones with and without bone cells differ and cope with the challenges of external stress,” Zaslansky says.
Strength and elasticity
Bones have a complex structure: they comprise nanofibers of collagen and nanoparticles of mineral but also other minor ingredients. Certain protein compounds, so called Proteoglycans (PGs), are embedded in a tissue of collagen fibers and nanocrystals and play important roles in tissue formation and maintenance. “PGs may be compared to salt in the soup. Too little or too much of it is not good,” Zaslansky says. The PGs can retain water, and there are plenty of PGs in healthy cartilage, making it as elastic as a sponge. Together, these components form an extracellular matrix (ECM), a 3D structure that provides strength and elasticity, ensuring function for many years. In bones, an open network (Lacunar Channel Network or LCN) of channels and pores with diameters ranging from a few hundred nanometers to micrometers is created in this 3D structure. This LCN hosts the bone osteocytes, cells that sense load and orchestrate bone remodeling. In the LCN and within the nanocomposite, bone contains up to 20% of its volume in water, with many functions including toughening and adaptation to mechanical stress.
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