Novel assessment of bone using time-resolved transcutaneous Raman spectroscopy

With fragility fractures increasing as the population ages, there is a need for improved means to estimate risk of fracture. We recorded Raman spectra of both the mineral and organic phases of bone transcutaneously, a technology with potential to enhance bone quality and fracture risk assessment.

INTRODUCTION

The current "gold standard" assessment of bone quality is BMD determined by DXA. However, this accounts for only 60-70% of bone strength. X-rays are absorbed by the mineral phase of bone, whereas the organic phase remains essentially invisible; however, bone strength is critically dependent on both phases. We report, for the first time, a Raman spectroscopic technique that analyses both phases of bone beneath unbroken skin by eliminating spectral components of overlying tissues.

MATERIALS AND METHODS

We used an 800-nm laser (1-kHz, 1-ps pulses) with a synchronized 4-ps Kerr gate with variable picosecond delay that effectively shuttered out photons from overlying tissues. We measured bone Raman spectra at a point 2 mm above the carpus from two mouse genotypes with extreme differences in bone matrix quality: wildtype and oim/oim (matched for age, sex, and weight). Typical depth was 1.1 mm. We repeated the measurements with overlying tissues removed down to bone. Oim/oim mice produce only homotrimeric collagen, which results in poorly mineralized bone tissue.

RESULTS

The main spectral features were present from both bone phases. The spectral bands were in similar ratios when measured through the skin or directly from bone (in both genotypes). The band of the mineral phase (phosphate nu1) was smaller in oim/oim mice when measured directly from bone and through skin. The band associated with a particular vibrational mode of organic phase collagen (CH2 wag) showed a frequency shift between the genotypes.

CONCLUSIONS

This novel technique allowed us, for the first time, to make objective transcutaneous spectral measurements of both the mineral and the organic phases of bones and distinguish between normal and unhealthy bone tissue. After further optimization, this technology may help improve fracture risk assessments and open opportunities for screening in anticipation of the predicted increase in fragility fractures.

Anabolic actions of PTH (1-34): use of a novel tissue engineering model to investigate temporal effects on bone

PTH is in clinical use for the treatment of osteoporosis and is under intensive investigation for its potential in applications of tissue engineering, fracture healing, and implant integration. However, the mechanisms of its action to stimulate bone formation are still unclear. A novel bone tissue engineering model was used to elucidate basic mechanisms of PTH anabolic actions. Ectopic ossicles containing cortical bone, trabecular bone, and a hematopoietic marrow were generated from implanted bone marrow stromal cells (BMSC). One week after implantation, nude mice were administered PTH or vehicle for 1 week (group 1), 3 weeks (group 2), or 7 weeks (group 3). Another group was also treated for 3 weeks, initiated 12 weeks after implantation (group 4). Micro-radiography and histomorphometry revealed increased marrow cellularity in group 1 PTH-treated ossicles, increased bone in group 2 PTH-treated ossicles, and similar amounts of bone in both group 3 and 4 ossicles regardless of treatment. Incidence of phosphate mineral and phosphate mineral to hydroxyproline ratio via Raman spectroscopy were significantly higher after 3 weeks versus 1 week of PTH treatment, but there was no difference between PTH- and vehicle-treated ossicles. Early events of PTH action in group 1 ossicles and the effects of a single injection of PTH on 1- and 2-week-old ossicles were evaluated by Northern blot analysis. Osteocalcin (OC) mRNA was increased after 1 week of intermittent PTH treatment in ossicles and calvaria but an acute injection did not alter OC mRNA. In contrast, a single injection of PTH increased matrix gamma-carboxyglutamic acid protein (MGP) mRNA in 2-week-old ossicles. Differential and temporal-dependent effects of PTH on OC and MGP suggest at the molecular level, that PTH acts to inhibit osteoblast mineralization. However, this does not translate into tissue level alterations. These data indicate that anabolic actions of PTH in ectopic ossicles are temporally dependent on the BMSC implanted and suggest that cell implantation strategies are particularly responsive to PTH.

Raman imaging demonstrates FGF2-induced craniosynostosis in mouse calvaria

Craniosynostosis is a severe craniofacial disease where one or more sutures, the fibrous tissue that lies between the cranial bones, fuses prematurely. Some craniosynostosis syndromes are known to be caused by mutations in fibroblast growth factor (FGF) receptors. Mutated FGF receptors are thought to cause constitutive signaling. In this study, heparin acrylic beads released fibroblast growth factor 2 (FGF2) to mimic constitutive signaling by mutated receptors, delivering FGF2 in addition to already existing normal tissue amounts. Fetal day 18.5 mouse sutures were treated with FGF2-soaked beads and cultured in serum free media for 48 h. We have shown previously that this treatment leads to fusion and increased Msx2 expression, but here we use near-infrared Raman imaging to simultaneously examine the mineral components and matrix components of cranial tissue while providing light microscopic spatial information. FGF2-treated mouse sutures show increased v1 phosphate and v1 carbonate bandwidths, indicating a slightly chemically modified mineral being rapidly deposited. In addition, FGF2-treated mouse sutures show a marked increase in mineral-to-matrix ratios compared to control mouse sutures, typical of increased mineralization.

Subsurface probing in diffusely scattering media using spatially offset Raman spectroscopy

We describe a simple methodology for the effective retrieval of Raman spectra of subsurface layers in diffusely scattering media. The technique is based on the collection of Raman scattered light from surface regions that are laterally offset away from the excitation laser spot on the sample. The Raman spectra obtained in this way exhibit a variation in relative spectral intensities of the surface and subsurface layers of the sample being investigated. The data set is processed using a multivariate data analysis to yield pure Raman spectra of the individual sample layers, providing a method for the effective elimination of surface Raman scatter. The methodology is applicable to the retrieval of pure Raman spectra from depths well in excess of those accessible with conventional confocal microscopy. In this first feasibility study we have differentiated between surface and subsurface Raman signals within a diffusely scattering sample composed of two layers: trans-stilbene powder beneath a 1 mm thick over-layer of PMMA (poly(methyl methacrylate)) powder. The improvement in contrast of the subsurface trans-stilbene layer without numerical processing was 19 times. The potential applications include biomedical subsurface probing of specific tissues through different overlying tissues such as assessment of bone quality through skin, providing an effective noninvasive means of screening for bone degeneration, other skeletal disease diagnosis, and dermatology studies, as well as materials and catalyst research.

Highly ordered interstitial water observed in bone by nuclear magnetic resonance

NMR was used to study the nanostructure of bone tissue. Distance measurements show that the first water layer at the surface of the mineral in cortical bone is structured. This water may serve to couple the mineral to the organic matrix and may play a role in deformation.

INTRODUCTION

The unique mechanical characteristics of bone tissue have not yet been satisfactorily connected to the exact molecular architecture of this complex composite material. Recently developed solid-state nuclear magnetic resonance (NMR) techniques are applied here to the mineral component to provide new structural distance constraints at the subnanometer scale.

MATERIALS AND METHODS

NMR dipolar couplings between structural protons (OH(-) and H(2)O) and phosphorus (PO(4)) or carbon (CO(3)) were measured using the 2D Lee-Goldburg Cross-Polarization under Magic-Angle Spinning (2D LG-CPMAS) pulse sequence, which simultaneously suppresses the much stronger proton-proton dipolar interactions. The NMR dipolar couplings measured provide accurate distances between atoms, e.g., OH and PO(4) in apatites. Excised and powdered femoral cortical bone was used for these experiments. Synthetic carbonate ( approximately 2-4 wt%)-substituted hydroxyapatite was also studied for structural comparison.

RESULTS

In synthetic apatite, the hydroxide ions are strongly hydrogen bonded to adjacent carbonate or phosphate ions, with hydrogen bond (O-H) distances of approximately 1.96 A observed. The bone tissue sample, in contrast, shows little evidence of ordered hydroxide. Instead, a very ordered (structural) layer of water molecules is identified, which hydrates the small bioapatite crystallites through very close arrangements. Water protons are approximately 2.3-2.55 A from surface phosphorus atoms.

CONCLUSIONS

In synthetic carbonated apatite, strong hydrogen bonds were observed between the hydroxide ions and structural phosphate and carbonate units in the apatite crystal lattice. These hydrogen bonding interactions may contribute to the long-range stability of this mineral structure. The biological apatite in cortical bone tissue shows evidence of hydrogen bonding with an ordered surface water layer at the faces of the mineral particles. This structural water layer has been inferred, but direct spectroscopic evidence of this interstitial water is given here. An ordered structural water layer sandwiched between the mineral and the organic collagen fibers may affect the biomechanical properties of this complex composite material.

Bone chemical structure response to mechanical stress studied by high pressure Raman spectroscopy

While the biomechanical properties of bone are reasonably well understood at many levels of structural hierarchy, surprisingly little is known about the response of bone to loading at the ultrastructural and crystal lattice levels. In this study, our aim was to examine the response (i.e., rate of change of the vibrational frequency of mineral and matrix bands as a function of applied pressure) of murine cortical bone subjected to hydrostatic compression. We determined the relative response during loading and unloading of mineral vs. matrix, and within the mineral, phosphate vs. carbonate, as well as proteinated vs. deproteinated bone. For all mineral species, shifts to higher wave numbers were observed as pressure increased. However, the change in vibrational frequency with pressure for the more rigid carbonate was less than for phosphate, and caused primarily by movement of ions within the unit cell. Deformation of phosphate on the other hand, results from both ionic movement as well as distortion. Changes in vibrational frequencies of organic species with pressure are greater than for mineral species, and are consistent with changes in protein secondary structures such as alterations in interfibril cross-links and helix pitch. Changes in vibrational frequency with pressure are similar between loading and unloading, implying reversibility, as a result of the inability to permanently move water out of the lattice. The use of high pressure Raman microspectroscopy enables a deeper understanding of the response of tissue to mechanical stress and demonstrates that individual mineral and matrix constituents respond differently to pressure.

Kerr-gated time-resolved Raman spectroscopy of equine cortical bone tissue

Picosecond time-resolved Raman spectroscopy in equine cortical bone tissue is demonstrated. Using 400-nm pulsed laser excitation (1 ps at 1 kHz) it is shown that Kerr cell gating with a 4-ps window provides simultaneously time-resolved rejection of fluorescence and time-resolved Raman scatter enabling depth profiling through tissue. The Raman shifts are the same as those observed by conventional cw Raman spectroscopy using deep-red or near-infrared lasers. The time decay of Raman photons is shown to fit an inverse square root of time function, suggesting propagation by a diffusive mechanism. Using polystyrene behind a bone specimen, it is shown that the 400-nm laser light penetrates at least 0.31 mm below the surface of a fully mineralized bone tissue specimen and generates observable bone Raman scatter (approximately 415 to 430 nm) through most of this depth. These novel results demonstrate great promise for in vivo applications for studying diseased bone tissue, and ways to optimize the setup are discussed.

Bone tissue ultrastructural response to elastic deformation probed by Raman spectroscopy

Raman spectroscopy is used as a probe of ultrastructural (molecular) changes in both the mineral and matrix (protein and glycoprotein, predominantly type I collagen) components in real time of murine cortical bone as it responds to elastic deformation. Because bone is ia composite material, its mechanical properties are dependent on the structure and composition at a variety of dimensional scales. At the ultrastructural level, crystal structure and protein secondary structure distort as the tissue is loaded. These structural changes are followed as perturbations to tissue spectra. We load murine femora in a custom-made mechanical tester that fits on the stage of a Raman microprobe and can accept hydrated tissue specimens. As the specimen is loaded in tension, the shifts in mineral P-O4 v1 are followed with the microprobe. Average load and strain are measured using a load cell. These devices ensure that specimens are not loaded to or beyond the yield point. Changes occur in the mineral component of bone as a response to loading in the elastic regime. We propose that the mineral apatitic crystal lattice is deformed by movement of calcium and other ions. Raman microspectroscopy shows that bone mineral is not a passive contributor to tissue strength. The mineral active response to loading may function as a local energy storage and dissipation mechanism, thus helping to protect tissue from catastrophic damage.

Brittle IV mouse model for osteogenesis imperfecta IV demonstrates postpubertal adaptations to improve whole bone strength

The Brtl mouse model for type IV osteogenesis imperfecta improves its whole bone strength and stiffness between 2 and 6 months of age. This adaptation is accomplished without a corresponding improvement in geometric resistance to bending, suggesting an improvement in matrix material properties.

INTRODUCTION

The Brittle IV (Brtl) mouse was developed as a knock-in model for osteogenesis imperfecta (OI) type IV. A Gly349Cys substitution was introduced into one col1a1 allele, resulting in a phenotype representative of the disease. In this study, we investigate the effect of the Brtl mutation on whole bone architecture, strength, and composition across a range of age groups.

MATERIALS AND METHODS

One-, 2-, 6-, and 12-month-old Brtl and wildtype (WT) mice were analyzed. Femurs were assessed at the central diaphysis for cortical geometric parameters using microCT and were subsequently mechanically tested to failure by four-point bending. Matrix material properties were predicted using microCT data to normalize data from mechanical tests. Raman spectroscopy and DXA were used to assess matrix composition.

RESULTS

Our findings show a postpubertal adaptation in which Brtl femoral strength and stiffness increase through a mechanism independent of changes in whole bone geometry. These findings suggest an improvement in the material properties of the bone matrix itself, rather than improvements in whole bone geometry, as seen in previous mouse models of OI. Raman spectroscopic results suggest these findings may be caused by changes in mineral/matrix balance rather than improvements in mineral crystallinity.

CONCLUSIONS

Our findings parallel the currently unexplained clinical observation of decreased fractures in human OI patients after puberty. The Brtl mouse remains an important tool for investigating therapeutic interventions for OI.

Distribution of single DNA molecule electrophoretic mobilities in semidilute and dilute hydroxyethylcellulose solutions

The distribution of center of mass electrophoretic mobility mobilities and normalized migration time of up to 1080 lambda DNA molecules per experiment were measured in both semidilute hydroxyethylcellulose HEC/0.5 x Tris-borate-EDTA (TBE) solutions and dilute HEC/0.5 x TBE solution by high-speed video microscopy. Measurements were made microscopically over a short migration distance in homogeneous DNA HEC/0.5 x TBE solution and after electrophoretic migration of a plug of DNA through 7 cm. Video at 120 frames/s (semidilute HEC solution) and 236 frames/s (dilute HEC solution) allowed visualization with adequate resolution for single molecule mobility measurements. The electrophoretic migration times and band shapes predicted from the measurements corresponded well with those measured by conventional capillary electrophoresis (CE) in both semidilute and dilute HEC. In semidilute solution, the band width predicted by a square root of time scaling is in good agreement with the results of conventional CE. However, in dilute solution the precision of the measurements was not good enough to allow scaled estimates of band widths.

Compatibility of staining protocols for bone tissue with Raman imaging

We report the use of Raman microscopy to image mouse calvaria stained with hematoxylin, eosin and toluidine blue. Raman imaging of stained specimens allows for direct correlation of histological and spectral information. A line-focus 785 nm laser imaging system with specialized near-infrared (NIR) microscope objectives and CCD detector were used to collect approximately 100 x 450 micro m Raman images. Principal components analysis, a multivariate analysis technique, was used to determine whether the histological stains cause spectral interference (band shifts or intensity changes) or result in thermal damage to the examined tissue. Image analysis revealed factors for tissue components and the embedding medium, glycol methacrylate, only. Thus, Raman imaging proved to be compatible with histological stains such as hematoxylin, eosin and toluidine blue.

Earliest mineral and matrix changes in force-induced musculoskeletal disease as revealed by Raman microspectroscopic imaging

Craniosynostosis, premature fusion of the skull bones at the sutures, is the second most common human birth defect in the skull. Raman microspectroscopy was used to examine the composition, relative amounts, and locations of the mineral and matrix produced in mouse skulls undergoing force-induced craniosynostosis. Raman imaging revealed decreased relative mineral content in skulls undergoing craniosynostosis compared with unloaded specimens.

INTRODUCTION

Raman microspectroscopy, a nondestructive vibrational spectroscopic technique, was used to examine the composition, relative amounts, and locations of the mineral and matrix produced in mouse skulls undergoing force-induced craniosynostosis. Craniosynostosis, premature fusion of the skull bones at the sutures, is the second most common birth defect in the face and skull. The calvaria, or flat bones that comprise the top of the skull, are most often affected, and craniosynostosis is a feature of over 100 human syndromes and conditions.

MATERIALS AND METHODS

Raman images of the suture, the tips immediately adjacent to the suture (osteogenic fronts), and mature parietal bones of loaded and unloaded calvaria were acquired. Images were acquired at 2.6 x 2.6 microm spatial resolution and ranged in a field of view from 180 x 210 microm to 180 x 325 microm.

RESULTS AND CONCLUSIONS

This study found that osteogenic fronts subjected to uniaxial compression had decreased relative mineral content compared with unloaded osteogenic fronts, presumably because of new and incomplete mineral deposition. Increased matrix production in osteogenic fronts undergoing craniosynostosis was observed. Understanding how force affects the composition, relative amounts, and location of the mineral and matrix provides insight into musculoskeletal disease in general and craniosynostosis in particular. This is the first report in which Raman microspectroscopy was used to study musculoskeletal disease. These data show how Raman microspectroscopy can be used to study subtle changes that occur in disease.

Band-target entropy minimization (BTEM) applied to hyperspectral Raman image data

Band-target entropy minimization (BTEM) has been applied to extraction of component spectra from hyperspectral Raman images. In this method singular value decomposition is used to calculate the eigenvectors of the spectroscopic image data set. Bands in non-noise eigenvectors that would normally be used for recovery of spectra are examined for localized spectral features. For a targeted (identified) band, information entropy minimization or a closely related algorithm is used to recover the spectrum containing this feature from the non-noise eigenvectors, plus the next 5-30 eigenvectors, in which noise predominates. Tests for which eigenvectors to include are described. The method is demonstrated on one synthesized Raman image data set and two bone tissue specimens. By inclusion of small amounts of signal that would be unused in other methods, BTEM enables the extraction of a larger number of component spectra than are otherwise obtainable. An improvement in signal/noise ratio of the recovered spectra is also obtained.

Effect of urea on the polymer buffer solutions used for the electrophoretic separations of nucleic acids

The present study describes the effect of urea on the properties of hydroxyethyl cellulose (HEC) polymer solutions used for the separation of nucleic acids. Solution properties were investigated by viscosity measurements and supplemented by Raman spectroscopy of the solution components. By using viscosimetry, it was possible to identify that borate, urea, and HEC participate in an interaction causing an increase in the viscosity of dilute solutions. In addition, short polymer chains exhibited a 4-fold decrease in the entanglement threshold in the presence of 4 M urea and 0.356 M total borate concentration. These interactions were found to be specific to HEC. Raman spectroscopy monitored serial addition of 8 M urea to HEC sieving solutions combined with factor analysis indicated formation of minor urea species. Lack of change in the Raman spectrum and relative amount of borate suggested that there is no direct interaction between borate and urea. These effects on HEC sieving solution properties lead to the use of low HEC concentrations that are beneficial for the separation of nucleic acids under denaturing conditions.

Ultrastructural changes accompanying the mechanical deformation of bone tissue: a Raman imaging study

Raman spectroscopy and imaging are known to be valuable tools for the analysis of bone, the determination of protein secondary structure, and the study of the composition of crystalline materials. We have utilized all of these attributes to examine how mechanical loading and the resulting deformation affects bone ultrastructure, addressing the hypothesis that bone spectra are altered, in both the organic and inorganic regions, in response to mechanical loading/deformation. Using a cylindrical indenter, we have permanently deformed bovine cortical bone specimens and investigated the ultrastructure in and around the deformed areas using hyperspectral Raman imaging coupled with multivariate analysis techniques. Indent morphology was further examined using scanning electron microscopy. Raman images taken at the edge of the indents show increases in the low-frequency component of the amide III band and high-frequency component of the amide I band. These changes are indicative of the rupture of collagen crosslinks due to shear forces exerted by the indenter passing through the bone. However, within the indent itself no evidence was seen of crosslink rupture, indicating that only compression of the organic matrix takes place in this region. We also present evidence of what is possibly a pressure-induced structural transformation occurring in the bone mineral within the indents, as indicated by the appearance of additional mineral factors in Raman image data from indented areas. These results give new insight into the mechanisms and causes of bone failure at the ultrastructural level.

Mineralization of developing mouse calvaria as revealed by Raman microspectroscopy

Raman microspectroscopy is a nondestructive vibrational spectroscopic technique that permits the study of organic and mineral species at micron resolution, offers the ability to work with hydrated and dehydrated specimens in vivo or in vitro, and requires minimal specimen preparation. We used Raman microspectroscopy to determine the composition of the mineral environments present in mouse calvaria, the flat bones that comprise the top of the skull. We have acquired Raman transects (lines of point spectra) from mouse calvaria during a developmental time course ranging from embryonic day 13.5 (E13.5; 6 days before birth) to 6 months of age. Exploratory factor analysis (FA) reveals the presence of a variety of apatitic mineral environments throughout the tissue series. The earliest mineral is observed in the fetal day 15.5 (F15.5) mice and is identified as a carbonated apatite. The presence of a heterogeneous mineralized tissue in the postnatal specimens suggests that ionic incorporation and crystal perfection in the lattice yary as the mouse develops. This variation is indicative of the presence of both recently deposited mineral and more matured remodeled mineral. Band area ratios reveal that the mineral/matrix ratio initially increases, reaches a plateau, and then increases again. The carbonate/phosphate band area ratio remains constant from F18.5 to postnatal day 3 (PN3) and then increases with age. Insights into the chemical species, the degree of mineralization, and the multiple mineral environments that are present in normal calvarial tissue will enable us to better understand both normal and abnormal mineralization processes.