Hydroxyapatite formation in bovine serum: Implications for pathological calcification and biomaterials

The formation of small, mineralized structures in incubated serum under standard cell culture conditions was first introduced in 1997 (Kajander et al., SPIE Proc. 3111: 420–8) and was attributed to a previously unknown microorganism that was subsequently assigned to the aα-2 subgroup of Proteobacteria. Despite extensive controversy surrounding the existence of these “nanobacteria”, the possibility exists that these microorganisms might be involved in pathological calcification such as kidney stones. Using a combined morphological and biochemical approach, we have investigated further the formation of these mineralized structures in incubated serum. Transmission electron microscopy, X-ray microanalysis, high-resolution lattice imaging, and freeze-fracturing techniques were used to demonstrate at the ultrastructural level the crystalline nature and composition of these structures. Additional studies were performed to elucidate the relationship of nanoforms with mineralizing bone cell cultures. The formation of HA in incubated serum represents an otherwise unknown calcification process that offers a unique opportunity to study mineralization under laboratory conditions. In addition, information gained from understanding this process might be useful for bone tissue engineering where mineralization is desirable within synthetic bone substitutes, and within which mineral-binding bioactive factors may also be incorporated. (Abstract shortened by UMI.)...