Diffractive small angle X-ray scattering imaging for anisotropic structures.

Insights into the micro- and nano-architecture of materials is crucial for understanding and predicting their macroscopic behaviour. In particular, for emerging applications such as meta-materials, the micrometer scale becomes highly relevant. The micro-architecture of such materials can be tailored to exhibit specific mechanical, optical or electromagnetic behaviours. Consequently, quality control at micrometer scale must be guaranteed over extended areas. Mesoscale investigations over millimetre sized areas can be performed by scanning small angle X-ray scattering methods (SAXS). However, due to their long measurement times, real time or operando investigations are hindered. Here we present a method based on X-ray diffractive optics that enables the acquisition of SAXS signals in a single shot (few milliseconds) over extended areas. This method is applicable to a wide range of X-ray sources with varying levels of spatial coherence and monochromaticity, as demonstrated from the experimental results. This enables a scalable solution of spatially resolved SAXS.

No Signature of Osteocytic Osteolysis in Cortical Bone from Lactating NMRI Mice.

The roles of osteocytes in bone homeostasis have garnered increasing attention since it has been realized that osteocytes communicate with other organs. It has long been debated whether and/or to which degree osteocytes can break down the bone matrix surrounding them in a process called osteocytic osteolysis. Osteocytic osteolysis has been indicated to be induced by a number of skeletal challenges including lactation in CD1 and C57BL/6 mice, whereas immobilization-induced osteocytic osteolysis is still a matter of controversy. Motivated by the wish to understand this process better, we studied osteocyte lacunae in lactating NMRI mice, which is a widely used outbred mouse strain. Surprisingly, no trace of osteocytic osteolysis could be detected in tibial or femoral cortical bone either by 3D investigation by synchrotron nanotomography, by studies of lacunar cross-sectional areas using scanning electron microscopy, or by light microscopy. These results lead us to conclude that osteocytic osteolysis does not occur in NMRI mice as a response to lactation, in turn suggesting that osteocytic osteolysis may not play a generic role in mobilizing calcium during lactation.

Canalicular Junctions in the Osteocyte Lacuno-Canalicular Network of Cortical Bone.

The osteocyte lacuno-canalicular network (LCN) is essential for bone remodeling because osteocytes regulate cell recruitment. This has been proposed to occur through liquid-flow-induced shear forces in the canaliculi. Models of the LCN have thus far assumed that it contains canaliculi connecting the osteocyte lacunae. However, here, we reveal that enlarged spaces occur at places where several canaliculi cross; we name these spaces canalicular junctions. We characterize them in detail within mice cortical bone using synchrotron nanotomography at two length scales, with 50 and 130 nm voxel size, and show that canalicular junctions occur at a density similar to that of osteocyte lacunae and that canalicular junctions tend to cluster. Through confocal laser scanning microscopy, we show that canalicular junctions are widespread as we have observed them in cortical bone from several species, even though the number density of the canalicular junctions was not universal. Fluid flow simulations of a simple model system with and without a canalicular junction clearly show that liquid mass transport and flow velocities are altered by the presence of canalicular junctions. We suggest that these canalicular junctions may play an important role in osteocyte communication and possibly also in canalicular fluid flow. Therefore, we believe that they constitute an important component in the bone osteocyte network.

Organ and tissue level properties are more sensitive to age than osteocyte lacunar characteristics in rat cortical bone.

Modeling and remodeling induce significant changes of bone structure and mechanical properties with age. Therefore, it is important to gain knowledge of the processes taking place in bone over time. The rat is a widely used animal model, where much data has been accumulated on age-related changes of bone on the organ and tissue level, whereas features on the nano- and micrometer scale are much less explored. We investigated the age-related development of organ and tissue level bone properties such as bone volume, bone mineral density, and load to fracture and correlated these with osteocyte lacunar properties in rat cortical bone. Femora of 14 to 42-week-old female Wistar rats were investigated using multiple complementary techniques including X-ray micro-computed tomography and biomechanical testing. The body weight, femoral length, aBMD, load to fracture, tissue volume, bone volume, and tissue density were found to increase rapidly with age at 14-30 weeks. At the age of 30-42 weeks, the growth rate appeared to decrease. However, no accompanying changes were found in osteocyte lacunar properties such as lacunar volume, ellipsoidal radii, lacunar stretch, lacunar oblateness, or lacunar orientation with animal age. Hence, the evolution of organ and tissue level properties with age in rat cortical bone is not accompanied by related changes in osteocyte lacunar properties. This suggests that bone microstructure and bone matrix material properties and not the geometric properties of the osteocyte lacunar network are main determinants of the properties of the bone on larger length scales.