Herring-bone print

Scientists at the RIKEN SPring-8 Center in Harima, Osaka University, Japan, and the University of California, US, are taking a close look at the structure of bone. Using an x-ray technique that provides nanometer-scale resolution, the group studied intramuscular bone in the alewife—a small herring found mostly along the North American coast (Fig. 1). Reporting their work in Physical Review Letters, the group proposes a dynamic model to explain the bone mineralization process¹.

All bones are made up of three main parts: soft collagen fibrils, proteins and a hard mineral phase. The strength and functioning of bone depend on how these three components pack together during growth, particularly at the molecular level.

Herring bone tissue contains mostly collagen and other proteins that form a rod-like matrix. Calcium phosphate mineralizes around the collagen fibrils to form a tough composite material.

Scientists would like to understand better how the mineralization nucleates and evolves with time; however conventional visible light microscopes cannot image this process with sufficient structural detail. The research group therefore used an x-ray technique called ‘lensless imaging’. A bone sample is illuminated with a beam of coherent x-rays. (Coherent x-rays have the same wavelength and travel in phase, similar to the light from a laser, but with a much shorter wavelength). When the x-rays scatter from the sample, they form an interference pattern that can, in principle, be ‘inverted’ to reveal the real image.

The inversion is a complex procedure, but the final images reveal a surprising amount of structural detail. By studying the herring bone at various stages in the mineralization process, Ishikawa and co-workers have shown that mineralization first occurs in the spaces between the collagen fibrils, ultimately expanding along the length of the fibrils. As more minerals form, the collagen expands and distorts.

The group suggests that bone strength comes mainly from those minerals that form first, since they can mix well with body fluids and have a relatively large volume in which to grow.

“These findings will enable us to obtain a better understanding of the complex structure of bone at the nanometer scale, and provide important design principles for hard tissue engineering and the development of biocompatible materials”, says team-member Tetsuya Ishikawa from RIKEN.

Beyond their specific findings, Ishikawa and colleagues expect that x-ray imaging will be an attractive alternative to electron microscopy for studying biological materials.