Novel experimental effects on bone material properties and the pre- and postyield behavior of bones may be independent of bone mineralization

Biomechanical properties of anuran long bones: correlations with locomotor modes and habitat use

Long bones are subjected to mechanical loads during locomotion that will influence their biomechanical properties through a feedback mechanism (the bone mechanostat). This mechanism adapts the spatial distribution of the mineralized tissue to resist compression, bending and torsion. Among vertebrates, anurans represent an excellent group to study long bone properties because they vary widely in locomotor modes and habitat use, which enforce different skeletal loadings. In this study, we hypothesized that (a) the cortical bone mass, density and design of anuran femur and tibiofibula would reflect the mechanical influences of the different locomotor modes and habitat use, and (b) the relationships between the architectural efficiency of cortical design (cross-sectional moments of inertia) and the intrinsic stiffness of cortical tissue [cortical mineral density; the 'distribution/quality' (d/q) relationship] would describe some inter-specific differences in the efficiency of the bone mechanostat to improve bone design under different mechanical loads. To test this hypothesis, we determined tomographic (peripheral quantitative computed tomography) indicators of bone mass, mineralization, and design along the femur and tibiofibula of four anuran species with different modes of locomotion and use of habitat. We found inter-specific differences in all measures between the distal and proximal ends and mid-diaphysis of the bones. In general, terrestrial-hopper species had the highest values. Arboreal-walker species had the lowest values for all variables except for cortical bone mineral density, which was lowest in aquatic-swimmer species. The d/q relationships showed similar responses of bone modeling as a function of cortical stiffness for aquatic and arboreal species, whereas terrestrial-hoppers had higher values for moments of inertia regardless of the tissue compliance to be deformed. These results provide new evidence regarding the significant role of movement and habitat use in addition to the biomechanical properties of long bones within a morpho-functional and comparative context in anuran species.

Surface Chemistry of Nanoscale Mineralized Collagen Regulates Periodontal Ligament Stem Cell Fate

The interplay between stem cells and their extracellular microenvironment is of critical importance to the stem cell-based therapeutics in regenerative medicine. Mineralized collagen is the main component of bone extracellular matrix, but the effect of interfacial properties of mineralized collagen on subsequent cellular behaviors is unclear. This study examined the role of surface chemistry of nanoscale mineralized collagen on human periodontal ligament stem cell (hPDLSC) fate decisions. The intrafibrillarly mineralized collagen (IMC), fabricated by a biomimetic bottom-up approach, showed a bonelike hierarchy with nanohydroxyapatites (HAs) periodically embedded within fibrils. The infrared spectrum of the IMC showed the presence of phosphate, carbonate, amide I and II bands; and infrared mapping displayed uniform and higher spatial distribution of mineralization in the IMC. However, the distribution of the phosphate group differed far from that of the amide I group in the extrafibrillarly mineralized collagen (EMC), in which flowerlike HA clusters randomly depositing around the surface of the fibrils. Moreover, a large quantity of extrafibrillar HAs covered up the C═O stretch and N-H in-plane bend, resulting in substantial reduction of amide I and II bands. Cell experiments demonstrated that the hPDLSCs seeded on the IMC exhibited a highly branched, osteoblast-like polygonal shape with extended pseudopodia and thick stress fiber formation; while cells on the EMC displayed a spindle shape with less branch points and thin actin fibril formation. Furthermore, the biocompatibility of EMC was much lower than that of IMC. Interestingly, even without osteogenic induction, mRNA levels of major osteogenic differentiation genes were highly expressed in the IMC during cultivation time. These data suggest that the IMC with a similar nanotopography and surface chemistry to natural mineralized collagen directs hPDLSCs toward osteoblast differentiation, providing a promising scaffold in bone tissue regeneration.

Mechanistic modeling of a nanoscratch test for determination of in situ toughness of bone

The objective of this study was to develop a nanoscratch technique that can be used to measure the in situ toughness of bone at micro/nanostructural levels. Among the currently possible techniques, the surface scratch test may be conducted on very small regions, thus exhibiting a potential in determining the in situ failure behavior of materials. To adapt such a technique for assessing bone toughness at the micro/nanostructural levels and for limited stocks in small animal bone models (e.g. zebra finish and mice), a simple but reasonably accurate mechanistic model for the nanoscratch test was developed in this study. This model was based on the assumption that the removal energy of the tissue required during the nanoscratch test is the manifestation of the in situ toughness and the shear flow stress during the removal process is a measure of the in situ strength of bone. In addition, the experimental methodologies were developed to determine the elastic recovery force and frictional coefficients between the scratch tip and bone specimens that are required by the model. Finally, the efficacy of the nanoscratch technique was verified by testing bone samples from control (wild type), mild, and severe osteogenesis imperfecta (OI) mice, which have a distinct degree of brittleness. The experimental results indicated that the nanoscratch test could sensitively detect the in situ brittleness and strength of bone from the animal models.

Cooperative calcium phosphate nucleation within collagen fibrils

Although "chaperone molecules" rich in negatively charged residues (i.e., glutamic and aspartic acid) are known to play important roles in the biomineralization process, the precise mechanism by which type I collagen acquires intrafibrillar mineral via these chaperone molecules remains unknown. This study demonstrates a mechanism of cooperative nucleation in which three key components (collagen, chaperone molecules, and Ca(2+) and PO(4)(3-)) interact simultaneously. The mineralization of collagen under conditions in which collagen was exposed to pAsp, Ca(2+), and PO(4)(3-) simultaneously or pretreated with the chaperone molecule (in this case, poly(aspartic acid)) before any exposure to the mineralizing solution was compared to deduce the mineralization mechanism. Depending on the exact conditions, intrafibrillar mineral formation could be reduced or even eliminated through pretreatment with the chaperone molecule. Through the use of a fluorescently tagged polymer, it was determined that the adsorption of the chaperone molecule to the collagen surface retarded further adsorption of subsequent molecules, explaining the reduced mineralization rate in pretreated samples. This finding is significant because it indicates that chaperone molecules must interact simultaneously with the ions in solution and collagen for biomimetic mineralization to occur and that the rate of mineralization is highly dependent upon the interaction of collagen with its environment.