The mechanisms of mineralization

OKAYAMA, JAPAN—Researchers at Okayama University have published two studies which uncover more about the early stages of bone formation. During endochondral ossification, the chondrocytes which form cartilage secrete matrix proteins and mineralization factors that optimize the environment for mineralization. Cartilage acts as a precursor, and is progressively degraded and replaced by bone.

The mechanisms of bone formation are not completely elucidated, and manipulating mineralization is challenging. Gaining control of this process is relevant, as it would result in improved bioengineering techniques for cartilage tissue synthesis and reconstruction, and bone formation.

To gain insight into the initial steps of mineral formation, Professor Takuya Matsumoto and Assistant Professor Emilio Satoshi Hara from the Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences studied bone formation in the femur epiphysis in mice during secondary ossification in the first post-natal days.

In the first paper, researchers observed that chondrocytes burst near the mineralized area, which might be a space-making mechanism for mineral expansion. The space created after the cell burst matches the space later occupied by the minerals at the end of the process, as demonstrated by time-lapse images of the initial bone formation process.

To demonstrate the link between the burst, mineral formation and expansion, Professor Matsumoto and colleagues used external stimuli to induce the burst and manipulate bone tissue formation. In particular, two external factors were linked with triggering the burst: mechanical and osmotic pressure. Ex-vivo culture of the femur epiphysis in hypotonic condition or under mechanical pressure enhanced mineral formation; in-vivo investigations of the role of mechanical pressure showed that reduced pressure on the joints results in suppressed bone formation in the femur epiphysis.

As the second paper notes, the scientists used a variety of techniques to observe the dynamic changes in organic and inorganic material in the cartilage in a time- and stage-specific way, confirming that the early steps of mineralization are based on the activity of chondrocytes.

A careful analysis of nanofragments observed near the mineralized area revealed that they were nanofragments of chondrocyte membrane, and could be the nucleation sites for amorphous calcium phosphate, which then transforms into apatite crystals. The phospholipids in the fragments could provide the phosphate needed for this process. The researchers also synthesized artificial cell nanofragments, and showed that they promote mineral formation in vitro.

Because this research reveals two ways to induce the burst of chondrocytes, which in turn controls bone formation by making space for mineralized tissue via mechanical and osmotic pressure, it opens the way to new methods of engineering bone tissue. The authors note that “manipulation of chondrocyte burst with external mechano-chemical stimuli could be an additional approach for cartilage and bone tissue engineering.”

The unveiling of the role of membrane fragments as nucleation centers for bone formation also provides a new avenue for the development of biomaterials for bone tissue engineering and regeneration. The authors say that “in the future, cell membrane fragment-based materials can also be developed and applied in bone tissue engineering and regeneration.”