Nondestructive evaluation of osteogenic differentiation in tissue-engineered constructs

Methods of Monitoring Cell Fate and Tissue Growth in Three-Dimensional Scaffold-Based Strategies for In Vitro Tissue Engineering

In the field of tissue engineering, there is a need for methods that allow assessing the performance of tissue-engineered constructs noninvasively in vitro and in vivo. To date, histological analysis is the golden standard to retrieve information on tissue growth, cellular distribution, and cell fate on tissue-engineered constructs after in vitro cell culture or on explanted specimens after in vivo applications. Yet, many advances have been made to optimize imaging techniques for monitoring tissue-engineered constructs with a sub-mm or μm resolution. Many imaging modalities have first been developed for clinical applications, in which a high penetration depth has been often more important than lateral resolution. In this study, we have reviewed the current state of the art in several imaging approaches that have shown to be promising in monitoring cell fate and tissue growth upon in vitro culture. Depending on the aimed tissue type and scaffold properties, some imaging methods are more applicable than others. Optical methods are mostly suited for transparent materials such as hydrogels, whereas magnetic resonance-based methods are mostly applied to obtain contrast between hard and soft tissues regardless of their transparency. Overall, this review shows that the field of imaging in scaffold-based tissue engineering is developing at a fast pace and has the potential to overcome the limitations of destructive endpoint analysis.

Characterization of time-course morphological features for efficient prediction of osteogenic potential in human mesenchymal stem cells

Human bone marrow mesenchymal stem cells (hBMSCs) represents one of the most frequently applied cell sources for clinical bone regeneration. To achieve the greatest therapeutic effect, it is crucial to evaluate the osteogenic differentiation potential of the stem cells during their culture before the implantation. However, the practical evaluation of stem cell osteogenicity has been limited to invasive biological marker analysis that only enables assaying a single end-point. To innovate around invasive quality assessments in clinical cell therapy, we previously explored and demonstrated the positive predictive value of using time-course images taken during differentiation culture for hBMSC bone differentiation potential. This initial method establishes proof of concept for a morphology-based cell evaluation approach, but reveals a practical limitation when considering the need to handle large amounts of image data. In this report, we aimed to scale-down our proposed method into a more practical, efficient modeling scheme that can be more broadly implemented by physicians on the frontiers of clinical cell therapy. We investigated which morphological features are critical during the osteogenic differentiation period to assure the performance of prediction models with reduced burden on image acquisition. To our knowledge, this is the first detailed characterization that describes both the critical observation period and the critical number of time-points needed for morphological features to adequately model osteogenic potential. Our results revealed three important observations: (i) the morphological features from the first 3 days of differentiation are sufficiently informative to predict bone differentiation potential, both activities of alkaline phosphatase and calcium deposition, after 3 weeks of continuous culture; (ii) intervals of 48 h are sufficient for measuring critical morphological features; and (iii) morphological features are most accurately predictive when early morphological features from the first 3 days of differentiation are combined with later features (after 10 days of differentiation). Biotechnol. Bioeng. 2014;111: 1430–1439.

Novel mineral contrast agent for magnetic resonance studies of bone implants grown on a chick chorioallantoic membrane

Magnetic resonance imaging (MRI) studies of tissue engineered constructs prior to implantation clearly demonstrate the utility of the MRI technique for studying the bone formation process. To test the utility of our MRI protocols for explant studies, we present a novel test platform in which osteoblast-seeded scaffolds were implanted on the chorioallantoic membrane of a chick embryo. Scaffolds from the following experimental groups were examined by high-resolution MRI: (a) cell-seeded implanted scaffolds (CIM), (b) unseeded implanted scaffolds (UCIM), (c) cell-seeded scaffolds in static culture (CIV) and (d) unseeded scaffolds in static culture (UCIV). The reduction in water proton transverse relaxation times and the concomitant increase in water proton magnetization transfer ratios for CIM and CIV scaffolds, compared to UCIV scaffolds, were consistent with the formation of a bone-like tissue within the polymer scaffold, which was confirmed by immunohistochemistry and fluorescence microscopy. However, the presence of angiogenic vessels and fibrotic adhesions around UCIM scaffolds can confound MRI findings of bone deposition. Consequently, to improve the specificity of the MRI technique for detecting mineralized deposits within explanted tissue engineered bone constructs, we introduce a novel contrast agent that uses alendronate to target a Food and Drug Administration-approved MRI contrast agent (Gd-DOTA) to bone mineral. Our contrast agent termed GdALN was used to uniquely identify mineralized deposits in representative samples from our four experimental groups. After GdALN treatment, both CIM and CIV scaffolds, containing mineralized deposits, showed marked signal enhancement on longitudinal relaxation time-weighted (T1W) images compared to UCIV scaffolds. Relative to UCIV scaffolds, some enhancement was observed in T1W images of GdALN-treated UCIM scaffolds, subjacent to the dark adhesions at the scaffold surface, possibly from dystrophic mineral formed in the fibrotic adhesions. Notably, residual dark areas on T1W images of CIM and UCIM scaffolds were attributable to blood inside infiltrating vessels. In summary, we present the efficacy of GdALN for sensitizing the MRI technique to the deposition of mineralized deposits in explanted polymeric scaffolds.

Steroid regulation of proliferation and osteogenic differentiation of bone marrow stromal cells: a gender difference

Bone marrow mesenchymal stem cells (MSCs) are considered a potential cell source for stem cell-based bone tissue engineering. However, noticeable limitations of insufficient supply and reduction of differentiation potential impact the feasibility of their clinical application. This study investigated the in vitro function of steroids and gender differences on the proliferation and differentiation of rat MSCs. Bone marrow MSCs of age-matched rats were exposed to proliferation and osteogenic differentiation media supplements with various concentrations of 17beta-estradiol (E2) and dexamethasone. Cell proliferation was measured by MTS assay; osteogenic markers and steroid-associated growth factors and receptors were evaluated by ELISA and real-time PCR. The results revealed that supplements of E2 and dexamethasone increase MSC proliferation in a biphasic manner. The optimal dose and interaction of steroids required to improve MSC proliferation effectively varied depending on the gender of donors. Supplementation of E2 effectively improves osteogenic differentiation markers including ALP, osteocalcin and calcium levels for MSCs isolated from both male and female donors. The mRNA of TGF-beta1 and BMP-7 are also up-regulated. However, effective doses to maximally improve osteogenic potentials and growth factors for MSCs are different between male and female donors. The relationship between steroid receptors, osteogenic markers and cytokines are also varied by genders. The outcomes of the present study strongly indicate that steroids potentially function as an effective modulator to improve the capacity of MSCs in bone regeneration. It provides crucial information for improving and optimizing MSCs for future clinical application of bone regeneration.

Evaluation of bioreactor-cultivated bone by magnetic resonance microscopy and FTIR microspectroscopy

We present a three-dimensional mineralizing model based on a hollow fiber bioreactor (HFBR) inoculated with primary osteoblasts isolated from embryonic chick calvaria. Using non-invasive magnetic resonance microscopy (MRM), the growth and development of the mineralized tissue around the individual fibers were monitored over a period of 9 weeks. Spatial maps of the water proton MRM properties of the intact tissue, with 78 microm resolution, were used to determine changes in tissue composition with development. Unique changes in the mineral and collagen content of the tissue were detected with high specificity by proton density (PD) and magnetization transfer ratio (MTR) maps, respectively. At the end of the growth period, the presence of a bone-like tissue was verified by histology and the formation of poorly crystalline apatite was verified by selected area electron diffraction and electron probe X-ray microanalysis. FTIR microspectroscopy confirmed the heterogeneous nature of the bone-like tissue formed. FTIR-derived phosphate maps confirmed that those locations with the lowest PD values contained the most mineral, and FTIR-derived collagen maps confirmed that bright pixels on MTR maps corresponded to regions of high collagen content. In conclusion, the spatial mapping of tissue constituents by FTIR microspectroscopy corroborated the findings of non-invasive MRM measurements and supported the role of MRM in monitoring the bone formation process in vitro.