The effect of oxygen tension on the in vitro assay of human osteoblastic connective tissue progenitor cells

Influence of Glucose Concentration on Colony-Forming Efficiency and Biological Performance of Primary Human Tissue-Derived Progenitor Cells

OBJECTIVE

Glucose concentrations used in current cell culture methods are a significant departure from physiological glucose levels. The study focuses on comparing the effects of glucose concentrations on primary human progenitors (connective tissue progenitors [CTPs]) used for cartilage repair.

DESIGN

Cartilage- (Outerbridge grade 1, 2, 3; superficial and deep zone cartilage), infrapatellar fatpad-, synovium-, and periosteum-derived cells were obtained from 63 patients undergoing total knee arthroplasty and cultured simultaneously in fresh chondrogenic media containing 25 mM glucose (HGL) or 5 mM glucose (NGL) for pairwise comparison. Automated ASTM-based quantitative image analysis was used to determine colony-forming efficiency (CFE), effective proliferation rates (EPR), and sulfated-proteoglycan (GAG-ECM) staining of the CTPs across tissue sources.

RESULTS

HGL resulted in increased cell cultures with CFE = 0 compared with NGL in all tissue sources (P = 0.049). The CFE in NGL was higher than HGL for superficial cartilage (P < 0.001), and contrary for synovium-derived CTPs (P = 0.046) when CFE > 0. EPR of the CTPs did not differ between the media in the 6-day assay time period (P = 0.082). The GAG-ECM area of the CTPs and their progeny was increased in presence of HGL (P = 0.027).

CONCLUSION

Glucose concentration is critical to progenitor's physiology and should be taken into account in the setting of protocols for clinical or in vitro cell expansion strategies.

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

The Efficiency of Bone Marrow Aspiration for the Harvest of Connective Tissue Progenitors from the Human Iliac Crest

BACKGROUND

The rational design and optimization of tissue engineering strategies for cell-based therapy requires a baseline understanding of the concentration and prevalence of osteogenic progenitor cell populations in the source tissues. The aim of this study was to (1) define the efficiency of, and variation among individuals in, bone marrow aspiration as a means of osteogenic connective tissue progenitor (CTP-O) harvest compared with harvest from iliac cancellous bone, and (2) determine the location of CTP-Os within native cancellous bone and their distribution between the marrow-space and trabecular-surface tissue compartments.

METHODS

Eight 2-mL bone marrow aspiration (BMA) samples and one 7-mm transcortical biopsy sample were obtained from the anterior iliac crest of 33 human subjects. Two cell populations were obtained from the iliac cancellous bone (ICB) sample. The ICB sample was placed into αMEM (alpha-minimal essential medium) with antibiotic-antimycotic and minced into small pieces (1 to 2 mm in diameter) with a sharp osteotome. Cells that could be mechanically disassociated from the ICB sample were defined as marrow-space (IC-MS) cells, and cells that were disassociated only after enzymatic digestion were defined as trabecular-surface (IC-TS) cells. The 3 sources of bone and marrow-derived cells were compared on the basis of cellularity and the concentration and prevalence of CTP-Os through colony-forming unit (CFU) analysis.

RESULTS

Large variation was seen among patients with respect to cell and CTP-O yield from the IC-MS, IC-TS, and BMA samples and in the relative distribution of CTP-Os between the IC-MS and IC-TS fractions. The CTP-O prevalence was highest in the IC-TS fraction, which was 11.4-fold greater than in the IC-MS fraction (p < 0.0001) and 1.7-fold greater than in the BMA fraction. However, the median concentration of CTP-Os in the ICB (combining MS and TS fractions) was only 3.04 ± 1.1-fold greater than that in BMA (4,265 compared with 1,402 CTP/mL; p = 0.00004).

CONCLUSIONS

Bone marrow aspiration of a 2-mL volume at a given needle site is an effective means of harvesting CTP-Os, albeit diluted with peripheral blood. However, the median concentration of CTP-Os is 3-fold less than from native iliac cancellous bone. The distribution of CTP-Os between the IC-MS and IC-TS fractions varies widely among patients.

CLINICAL RELEVANCE

Bone marrow aspiration is an effective means of harvesting CTP-Os but is associated with dilution with peripheral blood. Overall, we found that 63.5% of all CTP-Os within iliac cancellous bone resided on the trabecular surface; however, 48% of the patients had more CTP-Os contributed by the IC-MS than the IC-TS fraction.

A prospective comparison of 3 approved systems for autologous bone marrow concentration demonstrated nonequivalency in progenitor cell number and concentration

OBJECTIVES

To evaluate the efficacy of 3 commercially available systems: the Harvest SmartPReP 2 BMAC, Biomet BioCUE, and Arteriocyte Magellan systems. We compared the number and concentration of progenitor cells achieved both before and after centrifugation and the percentage of progenitor cells salvaged after centrifugation.

METHODS

Forty patients, mean age 47 ± 18 years (range: 18-92 years, 19 male/21 female) were prospectively consented for bilateral iliac crest aspiration. The first 20 aspirations compared the Harvest and Biomet systems, and based on those results, the second 20 compared the Harvest and Arteriocyte systems. One system was randomly assigned to each iliac crest. Each system's unique marrow acquisition process and centrifugation mechanism was followed. Samples for analysis were taken both immediately before the marrow was put into the centrifugation system (after acquisition), and after centrifugation. The number of progenitor cells in each sample was estimated by counting the connective tissue progenitors (CTPs).

RESULTS

The Harvest system achieved a significantly greater number and concentration of CTPs both before and after centrifugation when compared to the Biomet system. There was no difference in the percent yield of CTPs after centrifugation. There was no significant difference in the number and concentration of CTPs between the Harvest and Arteriocyte systems before centrifugation, but the Harvest system had a significantly greater number and concentration of CTPs after centrifugation. The Harvest system also had a significantly higher percent yield of CTPs after centrifugation compared with the Arteriocyte system.

CONCLUSIONS

The Harvest system resulted in a greater CTP number and concentration after centrifugation when compared with the Biomet and Arteriocyte systems and may thus provide increased osteogenic and chondrogenic capacity.

Hypoxia: how does the monocyte-macrophage system respond to changes in oxygen availability?

Hypoxia is an important feature of inflamed tissue, such as the RA joint. Activated monocytes/macrophages and endothelial cells play a pivotal role in the pathogenesis of RA, implicated in the mechanism of inflammation and erosion. During development, myeloid progenitor cells sequentially give rise to monoblasts, promonocytes, and monocytes that are released from the bone marrow into the bloodstream. After extravasation, monocytes differentiate into long-lived, tissue-specific macrophages or DCs. The effect of different oxygen concentrations experienced by these cells during maturation represents a novel aspect of this developmental process. In inflamed joint tissue, the microvascular architecture is highly dysregulated; thus, efficiency of oxygen supply to the synovium is poor. Therefore, invading cells must adapt instantaneously to changes in the oxygen level of the microenvironment. Angiogenesis is an early event in the inflammatory joint, which is important in enabling activated monocytes to enter via endothelial cells by active recruitment to expand the synovium into a "pannus", resulting in cartilage degradation and bone destruction. The increased metabolic turnover of the expanding synovial pannus outpaces the dysfunctional vascular supply, resulting in hypoxia. The abnormal bioenergetics of the microenvironment further promotes synovial cell invasiveness. In RA, joint hypoxia represents a potential threat to cell function and survival. Notably, oxygen availability is a crucial parameter in the cellular energy metabolism, itself an important factor in determining the function of immune cells.

Slowing the Onset of Hypoxia Increases Colony Forming Efficiency of Connective Tissue Progenitor Cells In Vitro

BACKGROUND

Survival and colony formation by transplanted tissue derived connective tissue progenitor cells (CTPs) are thought to be important factors in the success of clinical tissue engineering strategies for bone regeneration. Transplantation of cells into defects larger than a few millimeters expose cells to a profoundly hypoxic environment. This study tested the hypothesis that delaying the onset of hypoxia will improve the survival and performance of CTPs in vitro.

METHODS

To mimic declines seen in an avascular in vivo bone defect, colony forming efficiency by marrow derived nucleated cells was assessed under osteogenic conditions. Variation in the rate of oxygen decline from an oxygen tension of 21% to 0.1% oxygen was explored using an incubator with programmable active control of gas concentrations. The effect of doping cultures with defined concentrations of RBCs was also used to evaluate the potential for RBCs to serve as a natural buffer in the setting of declining oxygen levels.

RESULTS

A delay in onset of hypoxia over 96 hours resulted in a 3-fold increase in the relative colony forming efficiency (rCFE) of CTPs as compared to an immediate onset of hypoxia. The presence of RBCs in vitro inhibited the rCFE of CTPs. Given the negative effects of RBCs, methods of RBC removal were evaluated and compared for their effectiveness of RBC removal and retention of colony forming efficiency.

CONCLUSIONS

These data suggest that conditions of hypoxia compromise colony forming efficiency in marrow derived CTPs. However, slowing the rate of decline of oxygen preserved colony forming efficiency at levels achieved in a stable normoxic (3% O₂) environment. These data also suggest that RBCs are detrimental to the rCFE of CTPs and that buffy coat is an effective and preferred method for removing RBCs from marrow aspirates while preserving CTPs. These findings may inform clinical strategies for CTP transplantation.

In vivo transplantation of autogenous marrow-derived cells following rapid intraoperative magnetic separation based on hyaluronan to augment bone regeneration

INTRODUCTION

This project was designed to test the hypothesis that rapid intraoperative processing of bone marrow based on hyaluronan (HA) could be used to improve the outcome of local bone regeneration if the concentration and prevalence of marrow-derived connective tissue progenitors (CTPs) could be increased and nonprogenitors depleted before implantation.

METHODS

HA was used as a marker for positive selection of marrow-derived CTPs using magnetic separation (MS) to obtain a population of HA-positive cells with an increased CTP prevalence. Mineralized cancellous allograft (MCA) was used as an osteoconductive carrier scaffold for loading of HA-positive cells. The canine femoral multidefect model was used and four cylindrical defects measuring 10 mm in diameter and 15 mm in length were grafted with MCA combined with unprocessed marrow or with MS processed marrow that was enriched in HA(+) CTPs and depleted in red blood cells and nonprogenitors. Outcome was assessed at 4 weeks using quantitative 3D microcomputed tomography (micro-CT) analysis of bone formation and histomorphological assessment.

RESULTS

Histomorphological assessment showed a significant increase in new bone formation and in the vascular sinus area in the MS-processed defects. Robust bone formation was found throughout the defect area in both groups (defects grafted with unprocessed marrow or with MS processed marrow.) Percent bone volume in the defects, as assessed by micro-CT, was greater in defects engrafted with MS processed cells, but the difference was not statistically significant.

CONCLUSION

Rapid intraoperative MS processing to enrich CTPs based on HA as a surface marker can be used to increase the concentration and prevalence of CTPs. MCA grafts supplemented with heparinized bone marrow or MS processed cells resulted in a robust and advanced stage of bone regeneration at 4 weeks. A greater new bone formation and vascular sinus area was found in defects grafted with MS processed cells. These data suggest that MS processing may be used to enhance the performance of marrow-derived CTPs in clinical bone regeneration procedures. Further assessment in a more stringent bone defect model is proposed.

Overcoming hypoxia to improve tissue-engineering approaches to regenerative medicine

The current clinical successes of tissue engineering are limited primarily to low-metabolism, acellular, pre-vascularized or thin tissues. Mass transport has been identified as the primary culprit, limiting the delivery of nutrients (such as oxygen and glucose) and removal of wastes, from tissues deep within a cellular scaffold. While strategies to develop sufficient vasculature to overcome hypoxia in vitro are promising, inconsistencies between the in vitro and the in vivo environments may still negate the effectiveness of large-volume tissue-engineered scaffolds. While a common theme in tissue engineering is to maximize oxygen supply, studies suggest that moderate oxygenation of cellular scaffolds during in vitro conditioning is preferable to high oxygen levels. Aiming for moderate oxygen values to prevent hypoxia while still promoting angiogenesis may be obtained by tailoring in vitro culture conditions to the oxygen environment the scaffold will experience upon implantation. This review discusses the causes and effects of tissue-engineering hypoxia and the optimization of oxygenation for the minimization of in vivo hypoxia.

Oxygen consumption, acidification and migration capacity of human primary osteoblasts within a three-dimensional tantalum scaffold

A major clinical problem within synthetic, large-scaled scaffolds is the insufficient nutrient supply resulting in inhomogeneous cell proliferation and differentiation. The aim of this study was to analyse pH value, oxygen consumption and migration of human osteoblasts within a 3D tantalum scaffold, clinically used for larger bone defects. After 24 h the oxygen concentration within the scaffold decreased significantly and remained low during incubation. Monitoring of the pH value inside the tantalum scaffold showed a slightly acidification under static culture conditions. However, cell migration within the 3D scaffold was detected. Hence, in clinical application it can be assumed that porous tantalum scaffolds can be settled by osteoblasts under critical oxygen and nutrient supply. In general, monitoring of cell migration, oxygen consumption and acidification can be a suitable instrument for creating advanced 3D bone scaffolds.

One-step recovery of marrow stromal cells on nanofibers

This study describes the one-step isolation and expansion of marrow stromal cells (MSCs) directly onto the implantable nanofibrous scaffolds. Coverslips were first coated with either aligned or random configurations of poly L,D lactic acid, poly lactic-glycolic acid, and poly-epsilon-caprolactone and then seeded with fresh bone marrow aspirate. Colony-forming units were quantified and the differentiation capacities of the recovered cells were explored. Further optimization was provided by exploring the impact of hyperoxic (21% O(2)) and physiologically approximate (2% O(2)) on cell recovery. Aligned nanofibers in 2% O(2) were identified as being superior for isolation of MSCs. Isolated cells formed colonies following the direction of nanofibers, indicating potential for guided tissue regeneration. The isolated MSCs demonstrated retention of multipotency. These findings offer a rapid, cost-effective method of producing a stem-cell-seeded scaffold for regeneration of multiple tissue types.

The design and use of animal models for translational research in bone tissue engineering and regenerative medicine

This review provides an overview of animal models for the evaluation, comparison, and systematic optimization of tissue engineering and regenerative medicine strategies related to bone tissue. This review includes an overview of major factors that influence the rational design and selection of an animal model. A comparison is provided of the 10 mammalian species that are most commonly used in bone research, and existing guidelines and standards are discussed. This review also identifies gaps in the availability of animal models: (1) the need for assessment of the predictive value of preclinical models for relative clinical efficacy, (2) the need for models that more effectively mimic the wound healing environment and mass transport conditions in the most challenging clinical settings (e.g., bone repair involving large bone and soft tissue defects and sites of prior surgery), and (3) the need for models that allow more effective measurement and detection of cell trafficking events and ultimate cell fate during the processes of bone modeling, remodeling, and regeneration. The ongoing need for both continued innovation and refinement in animal model systems, and the need and value of more effective standardization are reinforced.

Vascularization in bone tissue engineering: physiology, current strategies, major hurdles and future challenges

The lack of a functional vascular supply has, to a large extent, hampered the whole range of clinical applications of 'successful' laboratory-based bone tissue engineering strategies. To the present, grafts have been dependent on post-implant vascularization, which jeopardizes graft integration and often leads to its failure. For this reason, the development of strategies that could effectively induce the establishment of a microcirculation in the engineered constructs has become a major goal for the tissue engineering research community. This review addresses the role and importance of the development of a vascular network in bone tissue engineering and provides an overview of the most up to date research efforts to develop such a network.