Fourier Transform Infrared Imaging Analysis of Hyp Mouse Bone

Objective: To define mineral and preliminarily matrix properties alterations in separate bone compartments of the Hyp, a murine homologue of human X-linked hypophosphatemic rickets, mice bones. Materials and Methods: The proximal third of the tibias was analyzed by Fourier Transform Infrared Imaging (FTIRI) on three 2 um undecalcified semi-thin sections from four wild type (wt) and four knockout (ko) male, 2month-old mice. Background-corrected spectral images for the relative mineral (v1v3 PO43-) and matrix (Amide I) densities of the examined were produced. Growth plate, cancellous and cortical areas were spatially masked based on the images and spectra from a specific area were co-added to produce a single spectrum for analysis. Semi-quantitative calculation for mineral crystallinity, relative acidic phosphate content, carbonate substitution type and relative amount were performed. The relative presence of matrix beta sheets and alpha-helices vs triple helical collagen structure was also semi-quantitatively assessed. Results for each parameter were averaged between the two groups of animals and differences determined based on t-tests. Results: The carbonate substitution in the Hyp mouse bone mineral was higher, both in cortical (29%, p<0.01) and in trabecular (18%, p<0.05) bone, with a trend for slightly higher A-type and labile carbonate in wt mice. The acidic phosphate substitution was essentially not different between wt and Hyp mice. Mineral crystallinity was slightly higher in both cortical and trabecular bone of wt mice (p<0.05). Results for the matrix showed an altogether reduction in the noncollagenous protein content (p<0.05). Conclusions: In agreement with earlier data, the Hyp mouse bones mineral shows a slower growth, but carbonate substitution features in it are those of mature mineral. Some SIBLING and/or proteoglycan members of the matrix seem to be underexpressed in Hyp bones. These alterations happen to a different extent in cortical vs trabecular bone. Supported by NIH DE 04141.