Crystallization by Amorphous Particle Attachment: On the Evolution of Texture.

Crystallization by particle attachment (CPA) is a gradual process where each step has its own thermodynamic and kinetic constrains defining a unique pathway of crystal growth. An important example is biomineralization of calcium carbonate through amorphous precursors that are morphed into shapes and textural patterns that cannot be envisioned by the classical monomer-by-monomer approach. Here, a mechanistic link between the collective kinetics of mineral deposition and the emergence of crystallographic texture is established. Using the prismatic ultrastructure in bivalve shells as a model, a fundamental leap is made in the ability to analytically describe the evolution of form and texture of biological mineralized tissues and to design the structure and crystallographic properties of synthetic materials formed by CPA.

The role of mural mechanics on cephalopod palaeoecology.

Cephalopods transformed the molluscan shell into a buoyancy device that must be strong enough to resist external water pressure. Historically, unique features of the shell have been interpreted on the basis that the strength of the shell presents a hard limit on maximum habitat depth. One such feature is the mural flap, which is a semi-prismatic layer deposited on the inner surface of some coleoid septa that has been suggested to strengthen the shell and permit colonization of deeper waters. We test this hypothesis by constructing finite-element models that show how mural modifications affect the response of the shell to hydrostatic pressure. The mural flaps are found to have no notable structural function. Another mural modification discovered here is the adapical ridge flap that initially seemed to have a potential function in shifting peak stress away from the attachment site of the septum; however, the irregular distribution of this feature casts any functional interpretation in doubt. Ecological separation of belemnites and decabrachians is likely not mediated by the presence/absence of mural flaps. This work illustrates a potential caveat that not all unique septal features formed in response to increasing hydrostatic pressure and deeper habitats.

Increased Retention of Gadolinium in the Inflamed Brain After Repeated Administration of Gadopentetate Dimeglumine: A Proof-of-Concept Study in Mice Combining ICP-MS and Micro- and Nano-SR-XRF.

OBJECTIVES: The aim of this study was to determine in vivo if brain inflammation leads to increased gadolinium (Gd) retention in brain tissue after repeated applications of Gd-based contrast agents (GBCAs). MATERIALS AND METHODS: Experimental autoimmune encephalomyelitis (EAE) was induced in female SJL/J mice (n = 6). Experimental autoimmune encephalomyelitis and healthy control mice (n = 4) received 2.5 mmol/kg Gd-DTPA over 10 days (8 injections, cumulated dose of 20 mmol/kg), starting at day 14 post immunization when EAE mice reached the maximal clinical disability. In a group of mice, T1-weighted 2-dimensional RARE images were acquired before the first GBCA injection and 1 day after the last injection. Mice were killed either 1 day or 10 days after the last Gd application. From each single animal, a brain hemisphere was used for Gd detection using inductively coupled plasma mass spectrometry, whereas the other hemisphere was processed for histology and synchrotron x-ray fluorescence spectroscopy (SR-XRF) analysis. RESULTS: Gadolinium deposition in inflamed brains was mapped by SR-XRF 1 day after the last Gd-DTPA injections, although only mild signal hyperintensity was found on unenhanced T1-weighted images. In addition, using inductively coupled plasma mass spectrometry, we detected and quantified Gd in both healthy and EAE brains up to 10 days after the last injections. However, EAE mouse brains showed higher levels of Gd (mean ± SD, 5.3 ± 1.8 µg/g; range, 4.45-8.03 µg/g) with respect to healthy controls (mean ± SD, 2.4 ± 0.6 µg/g; range, 1.8-3.2 µg/g). By means of micro-SR-XRF, we identified submicrometric Gd hotspots in all investigated samples containing up to 5893 µg Gd/g tissue. Nano-SR-XRF further indicated that Gd small hotspots had an average size of ~160 nm diameter and were located in areas of high inflammatory activity. CONCLUSIONS: After repeated administrations of Gd-DTPA, ongoing inflammation may facilitate the retention of Gd in the brain tissue. Thus, neuroinflammation should be considered as a risk factor in the recommendation on use of linear GBCA-enhanced MRI.

Polarization induced contrast X-ray fluorescence at submicrometer resolution reveals nanometer apatite crystal orientations across entire tooth sections.

For biomedical research, successful imaging of calcified microstructures often relies on absorption differences between features, or on employing dies with selective affinity to areas of interest. When texture is concerned, e.g. for crystal orientation studies, polarization induced contrast is of particular interest. This requires sufficient interaction of the incoming radiation with the volume of interest in the sample to produce orientation-based contrast. Here we demonstrate polarization induced contrast at the calcium K-edge using submicron sized monochromatic synchrotron X-ray beams. We exploit the orientation dependent subtle absorption differences of hydroxyl-apatite crystals in teeth, with respect to the polarization field of the beam. Interaction occurs with the fully mineralized samples, such that differences in density do not contribute to the contrast. Our results show how polarization induced contrast X-ray fluorescence mapping at specific energies of the calcium K-edge reveals the micrometer and submicrometer crystal arrangements in human tooth tissues. This facilitates combining both high spatial resolution and large fields of view, achieved in relatively short acquisition times in reflection geometry. In enamel we observe the varying crystal orientations of the micron sized prisms exposed on our prepared surface. We easily reproduce crystal orientation maps, typically observed in polished thin sections. We even reveal maps of submicrometer mineralization fronts in spherulites in intertubular dentine. This Ca K-edge polarization sensitive method (XRF-PIC) does not require thin samples for transmission nor extensive sample preparation. It can be used on both fresh, moist samples as well as fossilized samples where the information of interests lies in the crystal orientations and where the crystalline domains extend several micrometers beneath the exposed surface.