Noninvasive in situ evaluation of osteogenic differentiation by time-resolved laser-induced fluorescence spectroscopy

Collagen: quantification, biomechanics, and role of minor subtypes in cartilage

Collagen is a ubiquitous biomaterial in vertebrate animals. Although each of its 28 subtypes contributes to the functions of many different tissues in the body, most studies on collagen or collagenous tissues have focussed on only one or two subtypes. With recent developments in analytical chemistry, especially mass spectrometry, significant advances have been made toward quantifying the different collagen subtypes in various tissues; however, high-throughput and low-cost methods for collagen subtype quantification do not yet exist. In this Review, we introduce the roles of collagen subtypes and crosslinks, and describe modern assays that enable a deep understanding of tissue physiology and disease states. Using cartilage as a model tissue, we describe the roles of major and minor collagen subtypes in detail; discuss known and unknown structure-function relationships; and show how tissue engineers may harness the functional characteristics of collagen to engineer robust neotissues.

Real-time multispectral fluorescence lifetime imaging using Single Photon Avalanche Diode arrays

Autofluorescence spectroscopy has emerged in recent years as a powerful tool to report label-free contrast between normal and diseased tissues, both in vivo and ex vivo. We report the development of an instrument employing Single Photon Avalanche Diode (SPAD) arrays to realize real-time multispectral autofluorescence lifetime imaging at a macroscopic scale using handheld single-point fibre optic probes, under bright background conditions. At the detection end, the fluorescence signal is passed through a transmission grating and both spectral and temporal information are encoded in the SPAD array. This configuration allows interrogation in the spectral range of interest in real time. Spatial information is provided by an external camera together with a guiding beam that provides a visual reference that is tracked in real-time. Through fast image processing and data analysis, fluorescence lifetime maps are augmented on white light images to provide feedback of the measurements in real-time. We validate and demonstrate the practicality of this technique in the reference fluorophores and in articular cartilage samples mimicking the degradation that occurs in osteoarthritis. Our results demonstrate that SPADs together with fibre probes can offer means to report autofluorescence spectral and lifetime contrast in real-time and thus are suitable candidates for in situ tissue diagnostics.

Autofluorescence Lifetime Reports Cartilage Damage in Osteoarthritis

Osteoarthritis (OA) is the most common arthritis and its hallmark is degradation of articular cartilage by proteolytic enzymes leading to loss of joint function. It is challenging to monitor the status of cartilage in vivo and this study explores the use of autofluorescence lifetime (AFL) measurements to provide a label-free optical readout of cartilage degradation that could enable earlier detection and evaluation of potential therapies. We previously reported that treatment of ex vivo porcine cartilage with proteolytic enzymes resulted in decreased AFL. Here we report changes in AFL of ex vivo mouse knee joints, porcine metacarpophalangeal joints, normal human metatarsophalangeal articular tissue and human OA tibial plateau tissues measured with or without treatment using a compact single-point time resolved spectrofluorometer. Our data show that proteolytically damaged areas in porcine metacarpophalangeal joints present a reduced AFL and that inducing aggrecanases in mouse and human joints also significantly reduces AFL. Further, human cartilage from OA patients presents a significantly lower AFL compared to normal human cartilage. Our data suggest that AFL can detect areas of cartilage erosion and may potentially be utilised as a minimally-invasive diagnostic readout for early stage OA in combination with arthroscopy devices.

Multimodal Label-Free Imaging for Detecting Maturation of Engineered Osteogenic Grafts

There is a critical need to develop noninvasive, nondestructive methods for assessing the quality of engineered constructs prior to implantation. Currently, the composition and maturity of engineered tissues are assessed using destructive, costly, and time-consuming biochemical and mechanical analyses. The goal of this study was to use noninvasive, multimodal imaging to monitor osteogenic differentiation and matrix deposition by human mesenchymal stem/stromal cells (MSCs) during in vitro culture. MSCs were encapsulated in alginate hydrogels and cultured in osteogenic conditions for 4 weeks. Samples were evaluated using fluorescence lifetime imaging (FLIm) and ultrasound backscatter microscopy (UBM) prior to traditional biochemical and mechanical testing. Using linear regression analysis, we identified strong correlations between imaging parameters (e.g., fluorescence lifetime and acoustic attenuation coefficient) and destructive mechanical and biochemical tests to assess the maturation of osteogenically induced constructs. These data demonstrate the promise of nondestructive label-free imaging techniques to noninvasively ascertain the progression and maturity of tissue engineered bone grafts.

Noninvasive Optical Assessment of Implanted Engineered Tissues Correlates with Cytokine Secretion

Fluorescence lifetime sensing has been shown to noninvasively characterize the preimplantation health and viability of engineered tissue constructs. However, current practices to monitor postimplantation construct integration are either qualitative (visual assessment) or destructive (tissue histology). We employed label-free fluorescence lifetime spectroscopy for quantitative, noninvasive optical assessment of engineered tissue constructs that were implanted into a murine model. The portable system was designed to be suitable for intravital measurements and included a handheld probe to precisely and rapidly acquire data at multiple sites per construct. Our model tissue constructs were manufactured from primary human cells to simulate patient variability based on a standard protocol, and half of the manufactured constructs were stressed to create a range of health states. Secreted amounts of three cytokines that relate to cellular viability were measured in vitro to assess preimplantation construct health: interleukin-8 (IL-8), human β-defensin 1 (hBD-1), and vascular endothelial growth factor (VEGF). Preimplantation cytokine secretion ranged from 1.5 to 33.5 pg/mL for IL-8, from 3.4 to 195.0 pg/mL for hBD-1, and from 0.1 to 154.3 pg/mL for VEGF. In vivo optical sensing assessed constructs at 1 and 3 weeks postimplantation. We found that at 1 week postimplantation, in vivo optical parameters correlated with in vitro preimplantation secretion levels of all three cytokines (p < 0.05). This correlation was not observed in optical measurements at 3 weeks postimplantation when histology showed that the constructs had re-epithelialized, independent of preimplantation health state, supporting the lack of a correlation. These results suggest that clinical optical diagnostic tools based on label-free fluorescence lifetime sensing of endogenous tissue fluorophores could noninvasively monitor postimplantation integration of engineered tissues.

Fiber-based fluorescence lifetime imaging of recellularization processes on vascular tissue constructs

New techniques able to monitor the maturation of tissue engineered constructs over time are needed for a more efficient control of developmental parameters. Here, a label-free fluorescence lifetime imaging (FLIm) approach implemented through a single fiber-optic interface is reported for nondestructive in situ assessment of vascular biomaterials. Recellularization processes of antigen removed bovine pericardium scaffolds with endothelial cells and mesenchymal stem cells were evaluated on the serous and the fibrous sides of the scaffolds, 2 distinct extracellular matrix niches, over the course of a 7 day culture period. Results indicated that fluorescence lifetime successfully report cell presence resolved from extracellular matrix fluorescence. The recellularization process was more rapid on the serous side than on the fibrous side for both cell types, and endothelial cells expanded faster than mesenchymal stem cells on antigen-removed bovine pericardium. Fiber-based FLIm has the potential to become a nondestructive tool for the assessment of tissue maturation by allowing in situ imaging of intraluminal vascular biomaterials.

Detection of Pentosidine Cross-Links in Cell-Secreted Decellularized Matrices Using Time Resolved Fluorescence Spectroscopy

Hyperglycemia-mediated, nonenzymatic collagen cross-links such as pentosidine (PENT) can have deleterious effects on cellular interactions with the extracellular matrix (ECM). Present techniques to quantify PENT are limited, motivating the need for improved methods to study the accumulation and contribution of PENT toward diabetic clinical challenges such as impaired bone healing. Current methods for studying PENT are destructive, laborious, and frequently employ oversimplified collagen films that lack the complexity of the native ECM. The primary goal of this study was to evaluate the capacity of time-resolved fluorescence spectroscopy (TRFS) to detect PENT in cell-secreted ECMs possessing enhanced compositional complexity. To demonstrate an application of this method, we assessed the response of human mesenchymal stem cells (MSCs) to cross-linked substrates to explore the role of detected PENT on osteogenic differentiation. We exposed MSC-secreted decellularized matrices (DMs) to 0.66 M ribose for 2 weeks and used TRFS to detect the accumulation of PENT. Ribose treatment resulted in a 30 nm blue shift in peak fluorescence emission and a significant decrease in average lifetime compared to that of control DMs (4.4 ± 0.3 ns vs 3.5 ± 0.09 ns). Evaluation of samples with high performance liquid chromatography (HPLC) confirmed that changes in observed fluorescence were due to PENT accumulation. A strong correlation was found between TRFS parameters and the HPLC measurement of PENT, validating the use of TRFS as an alternative method of PENT detection. Osteoblastic gene expression was significantly reduced in MSCs seeded on ribose DMs at days 7 and 14. However, no significant differences in calcium deposition were detected between control and ribose DMs. These data demonstrate the efficacy of nondestructive fluorescence spectroscopy to examine the formation of nonenzymatic collagen cross-links within biomimetic culture platforms and showcase one example where an improved biomimetic substrate can be used to probe cell-ECM interactions in the presence of collagen cross-links.

Two-photon autofluorescence lifetime imaging of human skin papillary dermis in vivo: assessment of blood capillaries and structural proteins localization

The papillary dermis of human skin is responsible for its biomechanical properties and for supply of epidermis with chemicals. Dermis is mainly composed of structural protein molecules, including collagen and elastin, and contains blood capillaries. Connective tissue diseases, as well as cardiovascular complications have manifestations on the molecular level in the papillary dermis (e.g. alteration of collagen I and III content) and in the capillary structure. In this paper we assessed the molecular structure of internal and external regions of skin capillaries using two-photon fluorescence lifetime imaging (FLIM) of endogenous compounds. It was shown that the capillaries are characterized by a fast fluorescence decay, which is originated from red blood cells and blood plasma. Using the second harmonic generation signal, FLIM segmentation was performed, which provided for spatial localization and fluorescence decay parameters distribution of collagen I and elastin in the dermal papillae. It was demonstrated that the lifetime distribution was different for the inner area of dermal papillae around the capillary loop that was suggested to be due to collagen III. Hence, we propose a generalized approach to two-photon imaging of the papillary dermis components, which extends the capabilities of this technique in skin diagnosis.

Fluorescent lifetime imaging microscopy using Europium complexes improves atherosclerotic plaques discrimination

The objective of this study is to characterize arterial tissue with and without atherosclerosis by fluorescence lifetime imaging microscopy (FLIM) using Europium Chlortetracycline complex (EuCTc) as fluorescent marker. For this study, twelve rabbits were randomly divided into a control group (CG) and an experimental group (EG), where they were fed a normal and hypercholesterolemic diet, respectively, and were treated for 60 days. Cryosections of the aortic arch specimens were cut in a vertical plane, mounted on glass slides, and stained with Europium (Eu), Chlortetracycline (CTc), Europium Chlortetracycline (EuCTc), and Europium Chlortetracycline Magnesium (EuCTcMg) solutions. FLIM images were obtained with excitation at 405 nm. The average autofluorescence lifetime within plaque depositions was ~1.36 ns. Reduced plaque autofluorescence lifetimes of 0.23 and 0.31 ns were observed on incubation with EuCTc and EuCTcMg respectively. It was observed a quenching of collagen, cholesterol and TG emission spectra increasing EuCTc concentration. The drastic reduction in fluorescence lifetimes is due to a resonant energy transfer between collagen, triglycerides, cholesterol and europium complexes, quenching fluorescence.

Three-dimensional printed miniaturized spectral system for collagen fluorescence lifetime measurements

Various types of collagens, e.g., type I and III, represent the main load-bearing components in biological tissues. Their composition changes during processes such as wound healing and fibrosis. When excited by ultraviolet light, collagens exhibit autofluorescence distinguishable by their unique fluorescent lifetimes across a range of emission wavelengths. Here, we designed a miniaturized spectral-lifetime detection system as a noninvasive probe for monitoring tissue collagen compositions. A sine-modulated LED illumination was applied to enable frequency domain fluorescence lifetime measurements under three wavelength bands, separated via a series of longpass dichroics at 387, 409, and 435 nm. We employed a lithography-based three-dimensional (3-D) printer with <50  μm resolution to create a custom designed optomechanics in a handheld form factor. We examined the characteristics of the optomechanics with finite element modeling to simulate the effect of thermal (from LED) and mechanical (from handling) strain on the optical system. The geometry was further optimized with ray tracing to form the final 3-D printed structure. Using this device, the phase shift and demodulation of collagen types were measured, where the separate spectral bands enhanced the differentiation of their lifetimes. This system represents a low cost, handheld probe for clinical tissue monitoring applications.

Current concepts and future of noninvasive procedures for diagnosing oral squamous cell carcinoma--a systematic review

Background

Oral squamous cell carcinoma (OSCC) has a remarkably high incidence worldwide, and a fairly serious prognosis, encouraging further research into advanced technologies for noninvasive methods of making early diagnoses, ideally in primary care settings.

Objectives

Our purpose was to examine the validity of using advanced noninvasive technologies in diagnosis of OSCC by identifying and evaluating relevant published reports.

Data source

MEDLINE, EMBASE, and CINAHL were searched to identify clinical trials and other information published between 1990 and 10 June 2014; the searches of MEDLINE and EMBASE were updated to November 2014. Study selection: Studies of noninvasive methods of diagnosing OSCC, including oral brush biopsy, optical biopsy, saliva-based oral cancer diagnosis, and others were included.

Data extraction

Data were abstracted and evaluated in duplicate for possible relevance on two occasions at an interval of 2 months before being included or excluded.

Data synthesis

This study identified 163 studies of noninvasive methods for diagnosing OSCC that met the inclusion criteria. These included six studies of oral brush biopsy, 42 of saliva-based oral diagnosis, and 115 of optical biopsy. Sixty nine of these studies were assessed by the modified version of the QUADAS instrument. Saliva-based oral cancer diagnosis and optical biopsy were found to be promising noninvasive methods for diagnosing OSCC.

Limitation

The strength of evidence was rated low for accuracy outcomes because the studies did not report important details required to assess the risk for bias.

Conclusions

It is clear that screening for and early detection of cancer and pre-cancerous lesions have the potential to reduce the morbidity and mortality of this disease. Advances in technologies for saliva-based oral diagnosis and optical biopsy are promising pathways for the future development of more effective noninvasive methods for diagnosing OSCC that are easy to perform clinically in primary care settings.

Application of time-resolved autofluorescence to label-free in vivo optical mapping of changes in tissue matrix and metabolism associated with myocardial infarction and heart failure

We investigate the potential of an instrument combining time-resolved spectrofluorometry and diffuse reflectance spectroscopy to measure structural and metabolic changes in cardiac tissue in vivo in a 16 week post-myocardial infarction heart failure model in rats. In the scar region, we observed changes in the fluorescence signal that can be explained by increased collagen content, which is in good agreement with histology. In areas remote from the scar tissue, we measured changes in the fluorescence signal (p < 0.001) that cannot be explained by differences in collagen content and we attribute this to altered metabolism within the myocardium. A linear discriminant analysis algorithm was applied to the measurements to predict the tissue disease state. When we combine all measurements, our results reveal high diagnostic accuracy in the infarcted area (100%) and border zone (94.44%) as well as in remote regions from the scar (> 77%). Overall, our results demonstrate the potential of our instrument to characterize structural and metabolic changes in a failing heart in vivo without using exogenous labels.

Fluorescence lifetime techniques in medical applications

This article presents an overview of time-resolved (lifetime) fluorescence techniques used in biomedical diagnostics. In particular, we review the development of time-resolved fluorescence spectroscopy (TRFS) and fluorescence lifetime imaging (FLIM) instrumentation and associated methodologies which allow in vivo characterization and diagnosis of biological tissues. Emphasis is placed on the translational research potential of these techniques and on evaluating whether intrinsic fluorescence signals provide useful contrast for the diagnosis of human diseases including cancer (gastrointestinal tract, lung, head and neck, and brain), skin and eye diseases, and atherosclerotic cardiovascular disease.

Nondestructive evaluation of tissue engineered articular cartilage using time-resolved fluorescence spectroscopy and ultrasound backscatter microscopy

The goal of this study is to evaluate the ability of a bimodal technique integrating time-resolved fluorescence spectroscopy (TRFS) and ultrasound backscatter microscopy (UBM) for nondestructive detection of changes in the biochemical, structural, and mechanical properties of self-assembled engineered articular cartilage constructs. The cartilage constructs were treated with three chemical agents (collagenase, chondroitinase-ABC, and ribose) to induce changes in biochemical content (collagen and glycosaminoglycan [GAG]) of matured constructs (4 weeks); and to subsequently alter the mechanical properties of the construct. The biochemical changes were evaluated using TRFS. The microstructure and the thickness of the engineered cartilage samples were characterized by UBM. The optical and ultrasound results were validated against those acquired via conventional techniques including collagen and GAG quantification and measurement of construct stiffness. Current results demonstrated that a set of optical parameters (e.g., average fluorescence lifetime and decay constants) showed significant correlation (p<0.05) with biochemical and mechanical data. The high-resolution ultrasound images provided complementary cross-section information of the cartilage samples morphology. Therefore, the technique was capable of nondestructively evaluating the composition of extracellular matrix and the microstructure of engineered tissue, demonstrating great potential as an alternative to traditional destructive assays.

Noninvasive multimodal evaluation of bioengineered cartilage constructs combining time-resolved fluorescence and ultrasound imaging

A multimodal diagnostic system that integrates time-resolved fluorescence spectroscopy, fluorescence lifetime imaging microscopy, and ultrasound backscatter microscopy is evaluated here as a potential tool for assessing changes in engineered tissue composition and microstructure nondestructively and noninvasively. The development of techniques capable of monitoring the quality of engineered tissue, determined by extracellular matrix (ECM) content, before implantation would alleviate the need for destructive assays over multiple time points and advance the widespread development and clinical application of engineered tissues. Using a prototype system combining time-resolved fluorescence spectroscopy, FLIM, and UBM, we measured changes in ECM content occurring during chondrogenic differentiation of equine adipose stem cells on 3D biodegradable matrices. The optical and ultrasound results were validated against those acquired via conventional techniques, including collagen II immunohistochemistry, picrosirius red staining, and measurement of construct stiffness. Current results confirm the ability of this multimodal approach to follow the progression of tissue maturation along the chondrogenic lineage by monitoring ECM production (namely, collagen type II) and by detecting resulting changes in mechanical properties of tissue constructs. Although this study was directed toward monitoring chondrogenic tissue maturation, these data demonstrate the feasibility of this approach for multiple applications toward engineering other tissues, including bone and vascular grafts.

Noninvasive evaluation of tissue-engineered cartilage with time-resolved laser-induced fluorescence spectroscopy

Regenerative medicine requires noninvasive evaluation. Our objective is to investigate the application of time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) using a nano-second-pulsed laser for evaluation of tissue-engineered cartilage (TEC). To prepare scaffold-free TEC, articular chondrocytes from 4-week-old Japanese white rabbits were harvested, and were inoculated at a high density in a mold. Cells were cultured for 5 weeks by rotating culture (RC) or static culture (SC). The RC group and SC group at each week (n = 5), as well as normal articular cartilage and purified collagen type II (as controls), were analyzed by TR-LIFS. The peak wavelength was compared with those of type II collagen immunostaining and type II collagen quantification by enzyme-linked immunosorbent assay and tensile testing. The fluorescence peak wavelength of the TEC analyzed by this method shifted significantly in the RC group at 3 weeks, and in the SC group at 5 weeks (p < 0.01). These results correlated with changes in type II collagen (enzyme-linked immunosorbent assay) and changes in Young's modulus on tensile testing. The results were also supported by immunohistologic findings (type II collagen staining). Our findings show that TR-LIFS is useful for evaluating TEC.

Fluorescence lifetime in cardiovascular diagnostics

We review fluorescence lifetime techniques including time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) and fluorescence lifetime imaging microscopy (FLIM) instrumentation and associated methodologies that allow for characterization and diagnosis of atherosclerotic plaques. Emphasis is placed on the translational research potential of TR-LIFS and FLIM and on determining whether intrinsic fluorescence signals can be used to provide useful contrast for the diagnosis of high-risk atherosclerotic plaque. Our results demonstrate that these techniques allow for the discrimination of important biochemical features involved in atherosclerotic plaque instability and rupture and show their potential for future intravascular applications.

Optical spectroscopy and imaging for the noninvasive evaluation of engineered tissues

Optical spectroscopy and imaging approaches offer the potential to noninvasively assess different aspects of the cellular, extracellular matrix, and scaffold components of engineered tissues. In addition, the combination of multiple imaging modalities within a single instrument is highly feasible, allowing acquisition of complementary information related to the structure, organization, biochemistry, and physiology of the sample. The ability to characterize and monitor the dynamic interactions that take place as engineered tissues develop promises to enhance our understanding of the interdependence of processes that ultimately leads to functional tissue outcomes. It is expected that this information will impact significantly upon our abilities to optimize the design of biomaterial scaffolds, bioreactors, and cell systems. Here, we review the principles and performance characteristics of the main methodologies that have been exploited thus far, and we present examples of corresponding tissue engineering studies.

Application of laser induced auto-fluorescence spectrum in the diagnosis of cancer

Along with the development of cross-discipline of the fluorescence (physics) and medicine, laser induced auto-fluorescence spectrum integrated with endoscopic technique became a new way to diagnose cancer in recent years. It is sensitive and non-invasive. In this review, we discuss the mechanism, clinical application and development tendency of laser induced auto-fluorescence spectrum.

Line-scanning microscopy for time-gated and spectrally resolved fluorescence imaging

Laser-scanning fluorescence microscopy for efficient acquisition of time-gated and spectrally resolved fluorescence images was developed based on line illumination of the laser beam and detection of the fluorescence image through a slit. In this optical arrangement, the fluorescence image was obtained by scanning only one axis perpendicular to the excitation line, and the acquisition time was significantly reduced compared with conventional laser-scanning confocal microscopy. A multidimensional fluorescence dataset consisting of fluorescence intensities as a function of x-position, y-position, fluorescence wavelength, and delay time after photoexcitation was analyzed and decomposed based on the parallel factor analysis model. The performance of the line-scanning microscopy was examined by applying it to the analysis of one of the plant defense responses, accumulation of antimicrobial compounds of phytoalexin in oat (Avena sativa), induced by the elicitor treatment.

Laser induced autofluorescence studies of animal skin used in modeling of human cutaneous tissue spectroscopic measurements

BACKGROUND/PURPOSE

Laser-induced autofluorescence spectroscopy provides excellent possibilities for medical diagnostics of different tissue pathologies including cancer. However, to create the whole picture of pathological changes, investigators collect spectral information from patients in vivo or they study different tumor models to obtain objective information for fluorescent properties of every kind of healthy and diseased tissue. Therefore, it is very important to find the most appropriate, and close to the human skin, animal samples from the fluorescence point of view, which will allow the extrapolation of the animal data to human spectroscopic diagnostics.

METHODS

In the present work, we examined the autofluorescence properties of different animal skin tissues, which are considered as the most common skin models. A nitrogen laser was used as an excitation source. Samples of healthy mouse, chicken and pig skin in vivo and/or ex vivo were studied and were compared with results obtained from investigations of healthy human skin in vivo.

RESULTS AND CONCLUSION

Specific features of the recorded spectra are discussed and the possible origin of the obtained fluorescence signals is proposed. Quantitative evaluation of data extrapolation for each skin type is also depicted.

Regenerative Medicine Bioprocessing: Building a Conceptual Framework Based on Early Studies

This paper reviews early studies of regenerative medicine using human cells and engineered tissues progressing from a laboratory-centered manual procedure toward automated manufacture. It then examines the distinctive bioprocesses by which autologous human material must be produced, the degree of simplification allowed by use of allogeneic cell lines and engineered tissue derived from them, and issues that affect both cell types. The paper concludes by drawing upon this discussion to suggest some factors that will determine how regenerative medicine bioprocessing can progress to provide many units of material economically.

Nondestructive evaluation of osteogenic differentiation in tissue-engineered constructs

Conventional measurements of osteogenesis in tissue-engineered constructs are destructive to living cells and incapable to provide three-dimensional information. In the present study, noninvasive magnetic resonance (MR) microscopy was used to evaluate osteogenic differentiation in vitro in human mesenchymal stem cell-based tissue-engineered constructs. The constructs were prepared by seeding the cells (10(6)cells/ml) on 4 x 4 x 4 mm gelatin sponge cubes and subsequently exposing them to osteogenic differentiation or basic medium. During the 4-week experiment, alkaline phosphatase (ALP) activity and calcium content of differentiated constructs were significantly increased compared to the basic medium controls. The T1, T2, and apparent diffusion coefficient (ADC) of differentiated constructs were significantly lower than those of the control group at each time point (p < 0.05). The MR parameters of constructs were significantly correlated to their ALP activities (r to T1, T2, and ADC = -0.57, -0.78, and -0.81, respectively) and calcium content (r to T1, T2, and ADC = 0.48, 0.90, and 0.92, respectively) measured by biochemical techniques. MR microscopy can be a promising tool for noninvasive assessment of osteogenic differentiation and to provide three-dimensional information about tissue-engineered constructs.

MR Assessment of Osteogenic Differentiation in Tissue-Engineered Constructs

Bone marrow stromal cells (MSC) are a promising source of osteoprogenitor cells for bone tissue engineering. However, the population of the osteoprogenitor cells and their differentiation potentials change with the gender, age, and health of the donor. Development of a noninvasive method to assess osteogenic progression is critical for successful bone tissue regeneration. High-resolution magnetic resonance imaging (MRI) (at 11.7 T, with spatial resolution of 62.5 x 62.5 microm in 500 microm slices) is used in the present study to monitor osteogenic differentiation of tissue-engineered constructs prepared by seeding human bone MSCs on gelatin sponge scaffolds. Quantitative measurements of the MR relaxation times (T1, T2) and the apparent diffusion coefficient (ADC) were performed for four successive weeks on control tissue constructs and constructs exposed to osteogenic differentiation medium. The T1 and T2 relaxation times and ADC were found to decrease as osteogenic progression proceeded in samples exposed to osteogenic differentiation medium. At week 4, the T1, T2, and ADC of TE constructs were 1.81 +/- 0.11 s, 19.5 +/- 11.02 ms, and 1.01 +/- 0.47 x 10(3) mm(2)/s, respectively, for osteogenic differentiated constructs, significantly different from control constructs 2.22 +/- 0.08 s, 50.39 +/- 5.57 ms, and 1.86 +/- 0.18 x 107(3) mm(2)/s (p < 0.05). The MR parameters were also highly correlated with the cell seeding densities and alkaline phosphatase (ALP) activities of the osteogenic constructs. In conclusion, periodic measurements of MR parameters (T1, T2, and ADC) provide a promising method for noninvasive monitoring of the status of tissue-engineered bone growth and differentiation.