Quantitative Mechanical/Chemical Imaging of Bone from Dmp1 Null Mice
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Quantitative Mechanical/Chemical Imaging of Bone from Dmp1 Null Mice Xiaomei Yao, Lynda F. Bonewald, J. David Eick, and Yong Wang Department of Oral Biology, School of Dentistry, University of Missouri -Kansas City, Kansas City, MO 64108, USA
ABSTRACT Dentin matrix protein 1 (DMP1) is an acidic noncollagenous protein which plays an important role in mineralized tissue formation. Dmp1 null adult mice are ricketic and osteomalacic and are a model for hypophosphatemic rickets [1]. Mutation in humans results is Autosomal Recessive Hypophosphatic Rickets [1]. The degree of bone mineralization significantly contributes to bone tissue mechanical properties, but precise relationships and interactions between chemical and mechanical variables are unknown. The objective of this study was to relate the differences in chemical properties in the Dmp1 wildtype (WT) and null (KO) mouse femoral cortical bone to their mechanical properties by using FTIR imaging and Scanning Acoustic Microscopy (SAM). Interactive mechanical (elastic modulus) and chemical images (i.e., mineral/matrix ratios) were generated from the same region of bone at a lateral resolution of ~10 um. Mechanical analysis showed that elastic modulus, 75 percentile around 7.1 GPa, was ~60% less in Dmp1 KO than that in WT, in which elastic modulus was 75 percentile around 15.2 GPa. The mineral-to-matrix ratios in Dmp1KO (4.96±1.63) were ~2 times lower than that in Dmp1 WT (8.65±1.14). The mineral crystallinity and collagen crosslink ratios were not significantly different between KO and WT. Conclusions: The results relate the bone elastic modulus changes in Dmp1WT and KO mice with chemical changes within a specific bone site. These measurements provide a new tool for describing the variability of bone chemical and mechanical properties. INTRODUCTION Dentin matrix protein 1 (DMP1) is a noncollagenous matrix protein which is considered to play an important role in mineralization of bone and dentin[2,3]. In in vivo studies, Dmp1 gene knockout (KO) mice postnatally displayed reduced body weight, shorter and wider long bones, and malformed secondary ossification centers as compared to wild type (WT) controls [4] (Fig. 1). Bone tissue quality has been judged based on different aspects, such as morphological, cellular, chemical, and mechanical properties. With advances in experimental methods, measurement of chemical and mechanical properties at the microscale, even nanoscale level is being applied to a greater extent in bone research [5]. The chemical properties in the Dmp1 KO bone were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) imaging similar to previous studies [6]. The degree of bone mineralization significantly impacts bone tissue mechanical properties, but the effect of Dmp1 on the relationship between chemical and mechanical properties is unknown. FTIR imaging and Scanning Acoustic Microscopy (SAM) are useful techniques to study bone chemical and mechanical changes. Furthermore, quantitative and multivariate analysis of images will dis
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