Fourier transform Infrared (FT-IR) imaging and microspectroscopy have been extensively applied to the analyses of tissues in health and disease. Spatially resolved mid-infrared data has provided insights into molecular changes that occur in diseases of connective or collagen-based tissues, including osteoarthritis, osteoporosis, osteogenesis imperfecta, osteopetrosis and pathologic calcifications. These techniques have also been used to probe chemical changes associated with load, disuse, and micro-damage in bone, and with degradation and repair in cartilage. This review summarizes the applications of FT-IR microscopy and imaging for analyses of bone and cartilage in healthy and diseased tissues, and illustrates the application of these techniques for the characterization of tissue engineered bone and cartilage.

Articular cartilage, a connective tissue that provides resistance to compressive forces during joint movements, has not been examined in detail by conventional Fourier transform infrared (FTIR) spectroscopy, microspectroscopy (FTIRM), or imaging (FTIRI). The current study reports FTIRM and FTIRI analyses of normal bovine cartilage and identifies the specific molecular components of cartilage that contribute to its IR spectrum. FTIRM data acquired through the superficial, middle, and deep zones of thin sections of bovine articular cartilage showed a variation in intensities of the absorbance bands that arise from the primary nonaqueous components of cartilage, collagen, and proteoglycan (primarily aggrecan) and thus reflected the differences in quantity of these specific components. The spectra of mixtures of model compounds, which had varying proportions of type II collagen and aggrecan, were analyzed to identify spectral markers that could be used to quantitatively analyze these components in cartilage. Collagen and aggrecan were then imaged by FTIRI based on markers found in the model compounds. Polarization experiments were also performed to determine the spatial distribution of the collagen orientation in the different zones of cartilage. This study provides a framework in which complex pathological changes in this heterogeneous tissue can be assessed by IR microscopic imaging. © 2000 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 1–8, 2001

Introduction: Hypophosphatasia (HPP) is the inborn error of metabolism that features rickets or osteomalacia caused by loss‐of‐function mutation(s) within the gene that encodes the tissue‐nonspecific isozyme of alkaline phosphatase (TNALP). Consequently, natural substrates for this ectoenzyme accumulate extracellulary including inorganic pyrophosphate (PPi), an inhibitor of mineralization, and pyridoxal 5′‐phosphate (PLP), a co‐factor form of vitamin B6. Babies with the infantile form of HPP often die with severe rickets and sometimes hypercalcemia and vitamin B6‐dependent seizures. There is no established medical treatment. Materials and Methods: Human TNALP was bioengineered with the C terminus extended by the Fc region of human IgG for one‐step purification and a deca‐aspartate sequence (D10) for targeting to mineralizing tissue (sALP‐FcD10). TNALP‐null mice (Akp2−/−), an excellent model for infantile HPP, were treated from birth using sALP‐FcD10. Short‐term and long‐term efficacy studies consisted of once daily subcutaneous injections of 1, 2, or 8.2 mg/kg sALP‐FcD10 for 15, 19, and 15 or 52 days, respectively. We assessed survival and growth rates, circulating levels of sALP‐FcD10 activity, calcium, PPi, and pyridoxal, as well as skeletal and dental manifestations using radiography, μCT, and histomorphometry. Results: Akp2−/− mice receiving high‐dose sALP‐FcD10 grew normally and appeared well without skeletal or dental disease or epilepsy. Plasma calcium, PPi, and pyridoxal concentrations remained in their normal ranges. We found no evidence of significant skeletal or dental disease. Conclusions: Enzyme replacement using a bone‐targeted, recombinant form of human TNALP prevents infantile HPP in Akp2−/− mice.

Numerous techniques are currently used to characterize biological mineralization in intact tissues and cell cultures; the von Kossa staining method, electron microscopic analysis (EM), X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR) are among the most common. In this study, we utilized three of these methods to compare the mineralization of cultured fetal rat calvarial cells (FRC) and the osteoblast cell lines 2T3 and MC3T3-E1 with the in vivo mineral of rat calvarial bone. The cells were cultured with or without ascorbic acid (100 microg/ml) and beta-glycerophosphate (2.5, 5, or 10 mM betaGP), and harvested between 16 and 21 days (FRC cells and 2T3 cells) or at 30 days of culture (MC3T3-E1 cells). In the FRC cultures, maximal von Kossa staining was observed with 2.5 and 5 mM betaGP in the presence of 100 microg/ml ascorbate. FRC cells also showed some von Kossa staining when cultured with bGP alone. In contrast, maximal von Kossa staining for MC3T3-E1 cells was observed with 10 mM betaGP. Only the cultures of MC3T3-E1 cells that received both ascorbate and betaGP produced von Kossa positive structures. The 2T3 cultures produced von Kossa positive staining only upon treatment with ascorbic acid and betaGP, which was greatly accelerated by bone morphogenic protein-2 (BMP-2). FTIR was performed on the mineral and matrix generated in FRC, MC3T3, and 2T3 cultures, and the results were compared with spectra derived from 16-day-old rat calvaria. The mineral-to-matrix ratios calculated from FTIR spectra for rat calvaria ranged from 2.97 to 7.44. FRC cells made a bonelike, poorly crystalline apatite, and, with increasing betaGP, there was a statistically significant (P

The presence of skeletal hypomineralization was confirmed in mice lacking the gene for bone alkaline phosphatase, ie, the tissue-non-specific isozyme of alkaline phosphatase (TNAP). In this study, a detailed characterization of the ultrastructural localization, the relative amount and ultrastructural morphology of bone mineral was carried out in tibial growth plates and in subjacent metaphyseal bone of 10-day-old TNAP knockout mice. Alizarin red staining, microcomputerized tomography (micro CT), and FTIR imaging spectroscopy (FT-IRIS) confirmed a significant overall decrease of mineral density in the cartilage and bone matrix of TNAP-deficient mice. Transmission electron microscopy (TEM) showed diminished mineral in growth plate cartilage and in newly formed bone matrix. High resolution TEM indicated that mineral crystals were initiated, as is normal, within matrix vesicles (MVs) of the growth plate and bone of TNAP-deficient mice. However, mineral crystal proliferation and growth was inhibited in the matrix surrounding MVs, as is the case in the hereditary human disease hypophosphatasia. These data suggest that hypomineralization in TNAP-deficient mice results primarily from an inability of initial mineral crystals within MVs to self-nucleate and to proliferate beyond the protective confines of the MV membrane. This failure of the second stage of mineral formation may be caused by an excess of the mineral inhibitor pyrophosphate (PPi) in the extracellular fluid around MVs. In normal circumstances, PPi is hydrolyzed by the TNAP of MVs' outer membrane yielding monophosphate ions (Pi) for incorporation into bone mineral. Thus, with TNAP deficiency a buildup of mineral-inhibiting PPi would be expected at the perimeter of MVs.

IL-6 is a pleiotropic cytokine involved in cell signaling in the musculoskeletal system, but its role in bone healing remains uncertain. The purpose of this study was to examine the role of IL-6 in fracture healing. Eight-week-old male C57BL/6 and IL-6 -/- mice were subjected to transverse, mid-diaphyseal osteotomies on the right femora. Sacrifice time points were 1, 2, 4, or 6 weeks post-fracture (N=14 per group). Callus tissue properties was analyzed by microcomputed tomography (micro-CT) and Fourier transform infrared imaging spectroscopy (FT-IRIS). Cartilage and collagen content, and osteoclast density were measured histologically. In intact unfractured bone, IL-6 -/- mice had reduced crystallinity, mineral/matrix ratio, tissue mineral density (TMD), and bone volume fraction (BVF) compared to wildtype mice. This suggests that there was an underlying deficit in baseline bone quality in IL-6 -/- mice. At 2 weeks post-fracture, the callus of IL-6 -/- mice had reduced crystallinity and mineral/matrix ratio. These changes were less evident at 4 weeks. At 2 weeks, the callus of the IL-6 -/- mice had an increased tissue mineral density (TMD), an increased cartilage and collagen content, and reduced osteoclast density compared to these parameters in wildtype mice. By 4 and 6 weeks, these parameters were no longer different between the two strains of mice. In conclusion, IL-6 -/- mice had delayed callus maturity, mineralization, and remodeling compared with the callus of the wildtype mice. These effects were transient indicating that the role of IL-6 appears to be most important in the early stages of fracture healing.

Thicknesses of each zone of normal equine cartilage (calculated based on differences in collagen orientation) were equivalent as determined by PLM and FT-IRIS. Comparable outcomes were obtained from the PLM and FT-IRIS analyses of repair and osteoarthritis tissues, whereby similar zonal variations in collagen orientation were apparent for the two methods. However, the PLM images of human osteoarthritic cartilage showed less obvious zonal discrimination and orientation compared to the FT-IRIS images, possibly attributable to the FT-IRIS method detecting molecular orientation changes prior to their manifestation at the microscopic level.

Increased levels of ePP i in mice deficient in TNALP (i.e., Akp2 −/− ) lead to elevated OPN concentrations. We examined the skeletal phenotype of mice lacking both OPN and TNALP and concluded that the increased OPN levels contribute to the hypophosphatasia phenotype characteristic of Akp2 −/− mice. We also found that extracellular OPN regulates the PP i output by osteoblasts. Introduction: Akp2−/− display mineralization deficiencies characterized by rickets/osteomalacia. This defect has been attributed to the increased levels of extracellular inorganic pyrophosphate (ePP i ), a substrate of tissue-nonspecific alkaline phosphatase (TNALP) and a potent inhibitor of mineral deposition. Because elevated levels of ePP i induce Opn gene expression, the Akp2 −/− mice also display increased levels of osteopontin (OPN), another inhibitor of mineralization. Materials and Methods: Akp2−/− mice were bred into the Opn −/− line. The resulting double knockout mice were analyzed for skeletal abnormalities by histology and CT. Calvarial osteoblasts were assayed for their ability to mineralize in vitro and were probed for changes in gene expression. Results: Mice lacking both Akp2 and Opn showed partial normalization at the histological level with regard to mineral deposition and BMD. However, high ePP i levels remained in Akp2 −/− mice. We found that Opn