Scaffolds are an essential component of bone tissue engineering to provide support and create a physiological environment for cells. Biomimetic scaffolds are a promising approach to fulfill the ...requirements. Bone allografts are widely used scaffolds due to their mechanical and structural characteristics. The scaffold geometry is well known to be an important determinant of induced mechanical stimulation felt by the cells. However, the impact of allograft geometry on permeability and wall shear stress distribution is not well understood. This information is essential for designing biomimetic scaffolds that provide a suitable environment for cells to proliferate and differentiate. The present study investigates the effect of geometry on the permeability and wall shear stress of bone allografts at both macroscopic and microscopic scales. Our results concluded that the wall shear stress was strongly correlated with the porosity of the allograft. The level of wall shear stress at a local scale was also determined by the surface curvature characteristics. The results of this study can serve as a guideline for future biomimetic scaffold designs that provide a mechanical environment favorable for osteogenesis and bone repair.
•The mean wall shear stress felt by the cells is linearly predicted by the porosity, in contrast to permeability.•The local curvature of the scaffold surface plays an important role in the wall shear stress felt by the cells.•Bone remodeling limits the appearance of extreme wall shear stress by creating an optimized trabecular microarchitecture.•This study can serve as a guideline for scaffold designs that provide a mechanical environment favorable for osteogenesis.
Mesenchymal stem cells (MSCs) hold considerable promise in tissue engineering (TE). However, their poor survival when exogenously administered limits their therapeutic potential. Previous studies ...from our group demonstrated that lack of glucose (glc) (but not of oxygen) is fatal to human MSCs because it serves as a pro‐survival and pro‐angiogenic molecule for human MSCs (hMSCs) upon transplantation. However, which energy‐providing pathways MSCs use to metabolize glc upon transplantation? Are there alternative energetic nutrients to replace glc? And most importantly, do hMSCs possess significant intracellular glc reserves for ensuring their survival upon transplantation? These remain open questions at the forefront of TE based‐therapies. In this study, we established for the first time that the in vivo environment experienced by hMSCs is best reflected by near‐anoxia (0.1% O2) rather than hypoxia (1%–5% O2) in vitro. Under these near‐anoxia conditions, hMSCs rely almost exclusively on glc through anerobic glycolysis for ATP production and are unable to use either exogenous glutamine, serine, or pyruvate as energy substrates. Most importantly, hMSCs are unable to adapt their metabolism to the lack of exogenous glc, possess a very limited internal stock of glc and virtually no ATP reserves. This lack of downregulation of energy turnover as a function of exogenous glc level results in a rapid depletion of hMSC energy reserves that explains their poor survival rate. These new insights prompt for the development of glc‐releasing scaffolds to overcome this roadblock plaguing the field of TE based‐therapies. Stem Cells 2018;36:363–376
This study demonstrated that human mesenchymal stem cells (hMSCs) located at the core of hydrogels construct implanted in vivo face a near‐anoxic microenvironment which quickly becomes ischemic when extracellular nutrients (especially exogenous glucose glc) are exhausted. In this ischemic environment, failure of hMSC to adapt their glc consumption to glc shortage as well as their inability to use alternative energy substrates result in a scenario where implanted hMSCs consume their glycolytic reserves through glycolysis in less than 24 hours. Once these glycolytic reserves are exhausted, hMSCs cannot maintain their ATP content and are unable to meet their bioenergetic requirements ultimately leading to an early and massive cell‐death within 3 to 7 days post‐implantation. Red arrows indicate functions that are downregulated, Green arrows indicate functions that are upregulated. Abbreviations: TCA, tricarboxylic acid cycle; PPP, pentose phosphate pathway; HIF, hypoxia‐inducible factor; LDH, lactate dehydrogenase.
In tissue engineering and regenerative medicine, stem cell—specifically, mesenchymal stromal/stem cells (MSCs)—therapies have fallen short of their initial promise and hype. The observed marginal, to ...no benefit, success in several applications has been attributed primarily to poor cell survival and engraftment at transplantation sites. MSCs have a metabolism that is flexible enough to enable them to fulfill their various cellular functions and remarkably sensitive to different cellular and environmental cues. At the transplantation sites, MSCs experience hostile environments devoid or, at the very least, severely depleted of oxygen and nutrients. The impact of this particular setting on MSC metabolism ultimately affects their survival and function. In order to develop the next generation of cell‐delivery materials and methods, scientists must have a better understanding of the metabolic switches MSCs experience upon transplantation. By designing treatment strategies with cell metabolism in mind, scientists may improve survival and the overall therapeutic potential of MSCs. Here, we provide a comprehensive review of plausible metabolic switches in response to implantation and of the various strategies currently used to leverage MSC metabolism to improve stem cell‐based therapeutics.
A major impediment to the development of therapies with mesenchymal stem cells/multipotent stromal cells (MSC) is the poor survival and engraftment of MSCs at the site of injury. We hypothesized that ...lowering the energetic demand of MSCs by driving them into a quiescent state would enhance their survival under ischemic conditions. Human MSCs (hMSCs) were induced into quiescence by serum deprivation (SD) for 48 hours. Such preconditioned cells (SD‐hMSCs) exhibited reduced nucleotide and protein syntheses compared to unpreconditioned hMSCs. SD‐hMSCs sustained their viability and their ATP levels upon exposure to severe, continuous, near‐anoxia (0.1% O2) and total glucose depletion for up to 14 consecutive days in vitro, as they maintained their hMSC multipotential capabilities upon reperfusion. Most importantly, SD‐hMSCs showed enhanced viability in vivo for the first week postimplantation in mice. Quiescence preconditioning modified the energy‐metabolic profile of hMSCs: it suppressed energy‐sensing mTOR signaling, stimulated autophagy, promoted a shift in bioenergetic metabolism from oxidative phosphorylation to glycolysis and upregulated the expression of gluconeogenic enzymes, such as PEPCK. Since the presence of pyruvate in cell culture media was critical for SD‐hMSC survival under ischemic conditions, we speculate that these cells may utilize some steps of gluconeogenesis to overcome metabolic stress. These findings support that SD preconditioning causes a protective metabolic adaptation that might be taken advantage of to improve hMSC survival in ischemic environments. Stem Cells 2017;35:181–196
A 48‐hours serum deprivation preconditioning was used to drive human mesenchymal stem cells/multipotent stromal cells (hMSCs) into a quiescent state. Such preconditioning modified the energy‐metabolic profile of hMSCs: it suppressed energy‐sensing mTOR signaling, stimulated autophagy, promoted a shift in their bioenergetic metabolism from oxidative phosphorylation to glycolysis and upregulated the expression of gluconeogenesis enzymes. As a result, quiescent hMSCs adopt a metabolic profile favorable for sustaining ischemia and were able to survive for up to two weeks in vitro and one week in vivo when implanted in mice. Red arrows indicate functions that are downregulated, Green arrows indicate functions that are upregulated.
Over the past 25 years, we have demonstrated the feasibility of airway bioengineering using stented aortic matrices experimentally then in a first‐in‐human trial (n = 13). The present TRITON‐01 study ...analyzed all the patients who had airway replacement at our center to confirm that this innovative approach can be now used as usual care. For each patient, the following data were prospectively collected: postoperative mortality and morbidity, late airway complications, stent removal and status at last follow‐up on November 2, 2021. From October 2009 to October 2021, 35 patients had airway replacement for malignant (n = 29) or benign (n = 6) lesions. The 30‐day postoperative mortality and morbidity rates were 2.9% (n = 1/35) and 22.9% (n = 8/35) respectively. At a median follow‐up of 29.5 months (range 1–133 months), 27 patients were alive. There have been no deaths directly related to the implanted bioprosthesis. Eighteen patients (52.9%) had stent‐related granulomas requiring a bronchoscopic treatment. Ten among 35 patients (28.6%) achieved a stent free survival. The actuarial 2‐ and 5‐year survival rates (Kaplan–Meier estimates) were respectively 88% and 75%. The TRITON‐01 study confirmed that airway replacement using stented aortic matrices can be proposed as usual care at our center.
Clinicaltrials.gov Identifier: NCT04263129.
The TRITON‐01 study confirms that airway replacement using stented aortic matrices can be proposed as usual care. Patterson and Rendina comments page 2721.
Scaffolds are an essential component of bone tissue engineering. They provide support and create a physiological environment for cells to proliferate and differentiate. Bone allografts extracted from ...human donors are promising scaffolds due to their mechanical and structural characteristics. Bone microarchitecture is well known to be an important determinant of macroscopic mechanical properties, but its role at the microscopic, i.e., the trabeculae level is still poorly understood. The present study investigated linear correlations between microarchitectural parameters obtained from X-ray computed tomography (micro-CT) images of bone allografts, such as bone volume fraction (BV/TV), degree of anisotropy (DA), or ellipsoid factor (EF), and micromechanical parameters derived from micro-finite element calculations, such as mean axial strain (ε
) and strain energy density (W
). DAEF, a new parameter based on a linear combination of the two microarchitectural parameters DA and EF, showed a strong linear correlation with the bone mechanical characteristics at the microscopic scale. Our results concluded that the spatial distribution and the plate-and-rod structure of trabecular bone are the main determinants of the mechanical properties of bone at the microscopic level. The DAEF parameter could, therefore, be used as a tool to predict the level of mechanical stimulation at the local scale, a key parameter to better understand and optimize the mechanism of osteogenesis in bone tissue engineering.
Use of human induced pluripotent stem cells (h-iPSCs) for bone tissue engineering is most appealing, because h-iPSCs are an inexhaustible source of osteocompetent cells. The present study ...investigated the contribution of undifferentiated h-iPSCs and elucidated aspects of the underlying mechanism(s) of the involvement of these cells to new bone formation. Implantation of undifferentiated h-iPSCs seeded on coral particles in ectopic sites of mice resulted in expression of osteocalcin and DMP-1, and in mineral content similar to that of the murine bone. The number of the implanted h-iPSCs decreased with time and disappeared by 30 days post-implantation. In contrast, expression of the murine osteogenic genes at day 15 and 30 post-implantation provided, for the first time, evidence that the implanted h-iPSCs affected the observed outcomes via paracrine mechanisms. Supporting evidence was provided because supernatant conditioned media from h-iPSCs (h-iPSC CM), promoted the osteogenic differentiation of human mesenchymal stem cells (h-MSCs) in vitro. Specifically, h-iPSC CM induced upregulation of the BMP-2, BMP-4 and BMP-6 genes, and promoted mineralization of the extracellular matrix. Given the current interest in the use of h-iPSCs for regenerative medicine applications, our study contributes new insights into aspects of the mechanism underlying the bone promoting capability of h-iPSCs.
The purpose of this study was to evaluate potential osteoarthritic alterations within the ankle using a surgically-induced chronic lateral ankle instability (CLAI) model. Twelve rats were assigned ...randomly to either the control (n = 4) or CLAI group (n = 8). Surgery was performed on the right ankle. Osteoarthritis was assessed through in-vivo micro-CT at 8 weeks and a clinical analysis. Macroscopic analysis, high-resolution ex-vivo micro-CT and histological examination were conducted after euthanasia at 12 weeks. Three subgroups (SG) were analyzed. SG1 comprised the operated ankles of the CLAI group (n = 8). SG2 consisted of the non-operated ankles of the CLAI group (n = 8). SG3 included both sides of the control group (n = 8). In-vivo micro-CT revealed no significant differences among the three subgroups when analyzed together (p = 0.42), and when comparing SG1 with SG2 (p = 0.23) and SG3 (p = 0.43) individually. No noticeable clinical differences were observed. After euthanasia, macroscopic analysis employing OARSI score, did not demonstrate significant differences, except between the medial tibia of SG1 and SG3 (p = 0.03), and in the total score comparison between these two subgroups (p = 0.015). Ex-vivo micro-CT did not reveal any differences between the three subgroups regarding bony irregularities and BV/TV measurements (SG1 vs. SG2 vs. SG3: p = 0.72; SG1 vs. SG2: p = 0.80; SG1 vs. SG3: p = 0.72). Finally, there was no difference between the three subgroups regarding OARSI histologic score (p = 0.27). These findings indicate that the current model failed to induce significant osteoarthritis. However, they lay the groundwork for improving the model’s effectiveness and expanding its use in CLAI research, aiming to enhance understanding of this pathology and reduce unnecessary animal sacrifice.
Local tissue ischemia is a prime cause responsible for the massive cell death in tissue-engineered (TE) constructs observed postimplantation. To assess the impact of ischemia on the death of ...implanted human multipotent stromal cells (hMSCs), which have great potential for repairing damaged tissues, we hereby investigated the in vivo temporal and spatial fate of human Luc-GFP-labeled MSCs within fibrin gel/coral scaffolds subcutaneously implanted in nude mice. In vivo bioluminescence imaging monitoring and histological analyses of the constructs tested confirmed the irremediable death of hMSCs over 30 days postimplantation. The kinetics of expression of three hypoxic/ischemic markers (HIF-1α, LDH-A, and BNIP3) was also monitored. Our results provided evidence that hMSCs located within the core of implanted constructs died faster and predominantly and strongly expressed the aforementioned ischemic markers. In contrast, cells located in the outer regions of TE constructs were reperfused by neovascularization and were still viable (as evidenced by their ex-vivo proliferative potential) at day 15 postimplantation. These results support the explanation that in the central part of the constructs tested, death of hMSCs was due to ischemia, whereas in the periphery of these constructs, cell death was due to another mechanism that needs to be elucidated.
Although physical exercise has unquestionable benefits on bone health, its effects on bone healing have been poorly investigated. This study evaluated the effects of preemptive moderate continuous ...running on the healing of non-critical sized bone defects in rats by µCT. We hypothesized that a preemptive running exercise would quicken bone healing. Twenty 5-week-old, male, Wistar rats were randomly allocated to one of the following groups (
= 10): sedentary control (SED) or continuous running (EX, 45 min/d, 5 d/week at moderate speed, for 8 consecutive weeks). A 2 mm diameter bone defect was then performed in the right tibia and femur. No exercise was performed during a 4 week-convalescence. Healing-tissue trabecular microarchitectural parameters were assessed once a week for 4 weeks using µCT and plasma bone turnover markers measured at the end of the study protocol (time point T12). At T12, bone volume fraction (BV/TV; BV: bone volume, TV: tissue volume) of the healing tissue in tibiae and femurs from EX rats was higher compared to that in SED rats (
= 0.001). BV/TV in EX rats was also higher in tibiae than in femurs (
< 0.01). The bone mineral density of the healing tissue in femurs from EX rats was higher compared to that in femurs from SED rats (
< 0.03). N-terminal telopeptide of collagen type I in EX rats was decreased compared to SED rats (
< 0.05), while no differences were observed for alkaline phosphatase and parathyroid hormone. The study provides evidence that preemptive moderate continuous running improves the healing of non-critical sized bone defects in male Wistar rats.
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