Supply of nutrients to cells in engineered tissues

Migrating Myofibroblastic Iliotibial Band-Derived Fibroblasts Represent a Promising Cell Source for Ligament Reconstruction

The iliotibial band (ITB) is a suitable scaffold for anterior cruciate ligament (ACL) reconstruction, providing a sufficient mechanical resistance to loading. Hence, ITB-derived fibroblasts attract interest for ligament tissue engineering but have so far not been characterized. This present study aimed at characterizing ITB fibroblasts before, during, and after emigration from cadaveric ITB explants to decipher the emigration behavior and to utilize their migratory capacity for seeding biomaterials. ITB and, for comparison, ACL tissues were assessed for the content of alpha smooth muscle actin (αSMA) expressing fibroblasts and degeneration. The cell survival and αSMA expression were monitored in explants used for cell isolation, monolayer, self-assembled ITB spheroids, and spheroids seeded in polyglycolic acid (PGA) scaffolds. The protein expression profile of targets typically expressed by ligamentocytes (collagen types I-III, elastin, lubricin, decorin, aggrecan, fibronectin, tenascin C, CD44, β1-integrins, vimentin, F-actin, αSMA, and vascular endothelial growth factor A [VEGFA]) was compared between ITB and ACL fibroblasts. A donor- and age-dependent differing percentage of αSMA positive cells could be detected, which was similar in ITB and ACL tissues despite the grade of degeneration being significantly higher in the ACL due to harvesting them from OA knees. ITB fibroblasts survived for several months in an explant culture, continuously forming monolayers with VEGFA and an increased αSMA expression. They shared their expression profile with ACL fibroblasts. αSMA decreased during the monolayer to spheroid/scaffold transition. Using self-assembled spheroids, the migratory capacity of reversible myofibroblastic ITB cells can be utilized for colonizing biomaterials for ACL tissue engineering and to support ligament healing.

Mechanically robust cationic cellulose nanofibril 3D scaffolds with tuneable biomimetic porosity for cell culture

Robust 3D modified cellulose scaffolds, with exquisite tuneable structure, in the form of foams, with meso and macro scale pores were prepared by a “bottom-up” approach.

A mosaic structure multi-level vascular network design for skull tissue engineering

In human skull tissue engineering scaffolds, cell growth and osteogenesis are limited due to the lack of vascular structure. Therefore, a mosaic structure vascular parameterized design method is proposed according to the scanning characteristics of the diploic vein. Using micro-CT scans of skull samples, the features of the diploic vein were extracted, and a multi-level vascular network model was established based on a power diagram. Considering the characteristics of blood flow in the veins, finite element analysis (FEA) of the fluid-solid coupling was established to analyze the effect of blood on vessels with four-level mosaic structures. The results showed that the deformation and stress distribution of vessels were reasonable, and the blood pressure, velocity and shear stress in the designed vascular structure could meet the cell growth requirements. The mosaic structure was prepared by PDMS and cultured in vitro using HUVECs. It was found that most of the cells survived after 48 h, and some cells were attached to the surface mosaic structure. In this method, different levels of vessels nest together, with a curvature that matches the shape of the skull, forming a similar morphology to the native diploic vein, and the local structures can be adjusted flexibly. This mosaic structure vascular design method can be used for network vascular design and experimental studies in hard tissues.

Angiogenic Self-Assembling Peptide Scaffolds for Functional Tissue Regeneration

Implantation of acellular biomimetic scaffolds with proangiogenic motifs may have exciting clinical utility for the treatment of ischemic pathologies such as myocardial infarction. Although direct delivery of angiogenic proteins is a possible treatment option, smaller synthetic peptide-based nanostructured alternatives are being investigated due to favorable factors, such as sustained efficacy and high-density epitope presentation of functional moieties. These peptides may be implanted in vivo at the site of ischemia, bypassing the first-pass metabolism and enabling long-term retention and sustained efficacy. Mimics of angiogenic proteins show tremendous potential for clinical use. We discuss possible approaches to integrate the functionality of such angiogenic peptide mimics into self-assembled peptide scaffolds for application in functional tissue regeneration.

Bone regeneration strategy by different sized multichanneled biphasic calcium phosphate granules: In vivo evaluation in rabbit model

A variety of synthetic materials are currently in use as bone substitutes, among them a new calcium phosphate-based multichannel, cylindrical, granular bone substitute that is showing satisfactory biocompatibility and osteoconductivity in clinical applications. These cylindrical granules differ in their mechanical and morphological characteristics such as size, diameter, surface area, pore size, and porosity. The aim of this study is to investigate whether the sizes of these synthetic granules and the resultant inter-granular spaces formed by their filling critical-sized bone defects affect new bone formation characteristics and to determine the best formulations from these individual types by combining the granules in different proportions to optimize the bone tissue regeneration. We evaluated two types of multichanneled cylindrical granules, 1 mm and 3 mm in diameter, combined the granules in two different proportions (wt%), and compared their different mechanical, morphological, and in vitro and in vivo biocompatibility characteristics. We assessed in vitro biocompatibility and cytotoxicity using MC3T3-E1 osteoblast-like cells using MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and confocal imaging. In vivo investigation in a rabbit model indicated that all four samples formed significantly better bone than the control after four weeks and eight weeks of implantation. Micro-computed tomography analysis showed more bone formation by the 1 mm cylindrical granules with 160 ± 10 µm channeled pore and 50% porosity than the other three samples ( p<.05), which we confirmed by histological analysis.

Oxygen-distribution within 3-D collagen I hydrogels for bone tissue engineering

Tissue engineering (TE) approaches typically envisage the structural and functional reconstitution of previously damaged tissue in situ. An adequate three-dimensional environment is therefore of fundamental importance for the designated cells associated to the scaffold material. The sufficient supply with nutrients and oxygen in vitro and in vivo mark thereby critical challenges of TE. In this study, we intended to analyse the level of locally dissolved oxygen within 3-D cell-loaded collagen I gels in vitro. For the analysis of the oxygen levels in situ, we employed an optical fibre-based micro sensor setup, as well as a camera supported non-invasive optical sensor foil based technique. These complementary analytical tools enable the identification, localization, and temporal follow-up investigation of specified regions of interest within TE constructs. Human adipose-derived mesenchymal stem cells (hAdMSCs) cultured in collagen I gels under normoxic conditions were analysed periodically and kinetically up to 70 days - thereby revealing dynamic changes of the level of dissolved oxygen inside the gel constructs. Dependent on the applied cell concentration, the in vitro oxygen concentration (cO₂) within the gels reached physiological ranges (7-9%) after 21 days, or 35 days of culture. The minimal cO₂ was measured after 35 days in vitro, featuring an oxygen level of 4.8 ± 1.3%. Upon prolonged culture, a plateau-like status of the cO₂ around 8-9% established, indicating a change in the physiological activity of the cells under investigation. The expression patterns of BCL2, CASP3 and MCM5 revealed significant differences among the proliferative and apoptotic stages of the cell-loaded samples at the investigated time points of 7 and 70 days in culture. In summary, these data show the temporary dynamic nature of the oxygen distribution in cell-loaded gel constructs. The applied technique is an ideal tool for the evaluation of multiple parameters affecting the oxygen distribution in vitro. We conclude that it takes 5 weeks for establishing an equilibrium of cO₂. Levels reached in a 3-D gel construct are comparable with physiological oxygenation ranges in bone-associated tissues.

Accelerated and Improved Differentiation of Retinal Organoids from Pluripotent Stem Cells in Rotating-Wall Vessel Bioreactors

Pluripotent stem cells can be differentiated into 3D retinal organoids, with major cell types self-patterning into a polarized, laminated architecture. In static cultures, organoid development may be hindered by limitations in diffusion of oxygen and nutrients. Herein, we report a bioprocess using rotating-wall vessel (RWV) bioreactors to culture retinal organoids derived from mouse pluripotent stem cells. Organoids in RWV demonstrate enhanced proliferation, with well-defined morphology and improved differentiation of neurons including ganglion cells and S-cone photoreceptors. Furthermore, RWV organoids at day 25 (D25) reveal similar maturation and transcriptome profile as those at D32 in static culture, closely recapitulating spatiotemporal development of postnatal day 6 mouse retina in vivo. Interestingly, however, retinal organoids do not differentiate further under any in vitro condition tested here, suggesting additional requirements for functional maturation. Our studies demonstrate that bioreactors can accelerate and improve organoid growth and differentiation for modeling retinal disease and evaluation of therapies.

Challenges and Strategies for Improving the Regenerative Effects of Mesenchymal Stromal Cell-Based Therapies

Cell-based therapies have the potential to revolutionize current treatments for diseases with high prevalence and related economic and social burden. Unfortunately, clinical trials have made only modest improvements in restoring normal function to degenerating tissues. This limitation is due, at least in part, to the death of transplanted cells within a few hours after transplant due to a combination of mechanical, cellular, and host factors. In particular, mechanical stress during implantation, extracellular matrix loss upon delivery, nutrient and oxygen deprivation at the recipient site, and host inflammatory response are detrimental factors limiting long-term transplanted cell survival. The beneficial effect of cell therapy for regenerative medicine ultimately depends on the number of administered cells reaching the target tissue, their viability, and their promotion of tissue regeneration. Therefore, strategies aiming at improving viable cell engraftment are crucial for regenerative medicine. Here we review the major factors that hamper successful cell engraftment and the strategies that have been studied to enhance the beneficial effects of cell therapy. Moreover, we provide a perspective on whether mesenchymal stromal cell-derived extracellular vesicle delivery, as a cell-free regenerative approach, may circumvent current cell therapy limitations.

Biological Interaction Between Human Gingival Fibroblasts and Vascular Endothelial Cells for Angiogenesis: A Co-culture Perspective

Advancement in cell culture protocols, multidisciplinary research approach, and the need of clinical implication to reconstruct damaged or diseased tissues has led to the establishment of three-dimensional (3D) test systems for regeneration and repair. Regenerative therapies, including dental tissue engineering, have been pursued as a new prospect to repair and rebuild the diseased/lost oral tissues. Interactions between the different cell types, growth factors, and extracellular matrix components involved in angiogenesis are vital in the mechanisms of new vessel formation for tissue regeneration. In vitro pre-vascularization is one of the leading scopes in the tissue-engineering field. Vascularization strategies that are associated with co-culture systems have proved that there is communication between different cell types with mutual beneficial effects in vascularization and tissue regeneration in two-dimensional or 3D cultures. Endothelial cells with different cell populations, including osteoblasts, smooth muscle cells, and fibroblasts in a co-culture have shown their ability to advocate pre-vascularization. In this review, a co-culture perspective of human gingival fibroblasts and vascular endothelial cells is discussed with the main focus on vascularization and future perspective of this model in regeneration and repair.

Inspiration from heart development: Biomimetic development of functional human cardiac organoids

Recent progress in human organoids has provided 3D tissue systems to model human development, diseases, as well as develop cell delivery systems for regenerative therapies. While direct differentiation of human embryoid bodies holds great promise for cardiac organoid production, intramyocardial cell organization during heart development provides biological foundation to fabricate human cardiac organoids with defined cell types. Inspired by the intramyocardial organization events in coronary vasculogenesis, where a diverse, yet defined, mixture of cardiac cell types self-organizes into functional myocardium in the absence of blood flow, we have developed a defined method to produce scaffold-free human cardiac organoids that structurally and functionally resembled the lumenized vascular network in the developing myocardium, supported hiPSC-CM development and possessed fundamental cardiac tissue-level functions. In particular, this development-driven strategy offers a robust, tunable system to examine the contributions of individual cell types, matrix materials and additional factors for developmental insight, biomimetic matrix composition to advance biomaterial design, tissue/organ-level drug screening, and cell therapy for heart repair.

Stem Cell Spheroids and Ex Vivo Niche Modeling: Rationalization and Scaling-Up

Improved protocols/devices for in vitro culture of 3D cell spheroids may provide essential cues for proper growth and differentiation of stem/progenitor cells (S/PCs) in their niche, allowing preservation of specific features, such as multi-lineage potential and paracrine activity. Several platforms have been employed to replicate these conditions and to generate S/PC spheroids for therapeutic applications. However, they incompletely reproduce the niche environment, with partial loss of its highly regulated network, with additional hurdles in the field of cardiac biology, due to debated resident S/PCs therapeutic potential and clinical translation. In this contribution, the essential niche conditions (metabolic, geometric, mechanical) that allow S/PCs maintenance/commitment will be discussed. In particular, we will focus on both existing bioreactor-based platforms for the culture of S/PC as spheroids, and on possible criteria for the scaling-up of niche-like spheroids, which could be envisaged as promising tools for personalized cardiac regenerative medicine, as well as for high-throughput drug screening.

3D Bioprinting for Vascularized Tissue Fabrication

3D bioprinting holds remarkable promise for rapid fabrication of 3D tissue engineering constructs. Given its scalability, reproducibility, and precise multi-dimensional control that traditional fabrication methods do not provide, 3D bioprinting provides a powerful means to address one of the major challenges in tissue engineering: vascularization. Moderate success of current tissue engineering strategies have been attributed to the current inability to fabricate thick tissue engineering constructs that contain endogenous, engineered vasculature or nutrient channels that can integrate with the host tissue. Successful fabrication of a vascularized tissue construct requires synergy between high throughput, high-resolution bioprinting of larger perfusable channels and instructive bioink that promotes angiogenic sprouting and neovascularization. This review aims to cover the recent progress in the field of 3D bioprinting of vascularized tissues. It will cover the methods of bioprinting vascularized constructs, bioink for vascularization, and perspectives on recent innovations in 3D printing and biomaterials for the next generation of 3D bioprinting for vascularized tissue fabrication.

Diffusion chamber system for testing of collagen-based cell migration barriers for separation of ligament enthesis zones in tissue-engineered ACL constructs

A temporary barrier separating scaffold zones seeded with different cell types prevents faster growing cells from overgrowing co-cultured cells within the same construct. This barrier should allow sufficient nutrient diffusion through the scaffold. The aim of this study was to test the effect of two variants of collagen-based barriers on macromolecule diffusion, viability, and the spreading efficiency of primary ligament cells on embroidered scaffolds. Two collagen barriers, a thread consisting of a twisted film tape and a sponge, were integrated into embroidered poly(lactic-co-caprolactone) and polypropylene scaffolds, which had the dimension of lapine anterior cruciate ligaments (ACL). A diffusion chamber system was designed and established to monitor nutrient diffusion using fluorescein isothiocyanate-labeled dextran of different molecular weights (20, 40, 150, 500 kDa). Vitality of primary lapine ACL cells was tested at days 7 and 14 after seeding using fluorescein diacetate and ethidium bromide staining. Cell spreading on the scaffold surface was measured using histomorphometry. Nuclei staining of the cross-sectioned scaffolds revealed the penetration of ligament cells through both barrier types. The diffusion chamber was suitable to characterize the diffusivity of dextran molecules through embroidered scaffolds with or without integrated collagen barriers. The diffusion coefficients were generally significantly lower in scaffolds with barriers compared to those without barriers. No significant differences between diffusion coefficients of both barrier types were detected. Both barriers were cyto-compatible and prevented most of the ACL cells from crossing the barrier, whereby the collagen thread was easier to handle and allowed a higher rate of cell spreading.

Simultaneous regeneration of full-thickness cartilage and subchondral bone defects in vivo using a three-dimensional scaffold-free autologous construct derived from high-density bone marrow-derived mesenchymal stem cells

BACKGROUND

In recent years, several methods have been developed for repairing full-thickness cartilage defects by tissue engineering using mesenchymal stem cells. Most of these use scaffolds to achieve sufficient thickness. However, considering the potential influence of scaffolds on the surrounding microenvironment, as well as immunological issues, it is desirable to develop a scaffold-free technique. In this study, we developed a novel technique, a scaffold-free autologous construct derived from bone marrow-derived mesenchymal stem cells (BM-MSCs), and successfully use this technique to regenerate cartilage and subchondral bone to repair an osteochondral defect in rabbit knees.

METHODS

BM-MSCs were isolated from bone marrow liquid aspirated from the iliac crest of rabbits. After expansion in culture dishes and re-suspension in 96-well plates, the cells spontaneously aggregated into a spheroid-like structure. The spheroids were loaded into a tube-shaped Teflon mold with a 5-mm height and maintained under air-liquid interface conditions. These loaded spheroids fused with each other, resulting in a cylinder-shaped construct made of fused cells that conformed to the inner shape of the mold. The construct was implanted into an osteochondral defect in rabbit knees and histologically analyzed 24 and 52 weeks after implantation using Wakitani's scoring system.

RESULTS

Both bone and cartilage were regenerated, maintaining a constant thickness of cartilage. The mean histological score was 10 ± 1.7 in the 24-week group and 9.7 ± 0.6 in the 52-week group. There was no significant difference between the 24- and 52-week groups in either parameter of the score, indicating that no deterioration of the repaired tissue occurred during the intervening period.

CONCLUSIONS

Using our novel technique, which employs a three-dimensional scaffold-free autologous construct derived from BM-MSCs, we successfully achieved simultaneous regeneration of bone and cartilage for up to 1 year in vivo. This method has potential for clinical use as a safe and effective method for repairing bone and cartilage defects.

Effects of pulsatile bioreactor culture on vascular smooth muscle cells seeded on electrospun poly (lactide-co-ε-caprolactone) scaffold

Electrospun nanofibrous scaffolds have several advantages, such as an extremely high surface-to-volume ratio, tunable porosity, and malleability to conform over a wide variety of sizes and shapes. However, there are limitations to culturing the cells on the scaffold, including the inability of the cells to infiltrate because of the scaffold's nano-sized pores. To overcome the limitations, we developed a controlled pulsatile bioreactor that produces static and dynamic flow, which improves transfer of such nutrients and oxygen, and a tubular-shaped vascular graft using cell matrix engineering. Electrospun scaffolds were seeded with smooth muscle cells (SMCs), cultured under dynamic or static conditions for 14 days, and analyzed. Mechanical examination revealed higher burst strength in the vascular grafts cultured under dynamic conditions than under static conditions. Also, immunohistology stain for alpa smooth muscle actin showed the difference of SMC distribution and existence on the scaffold between the static and dynamic culture conditions. The higher proliferation rate of SMCs in dynamic culture rather than static culture could be explained by the design of the bioreactor which mimics the physical environment such as media flow and pressure through the lumen of the construct. This supports regulation of collagen and leads to a significant increase in tensile strength of the engineered tissues. These results showed that the SMCs/electrospinning poly (lactide-co-ε-caprolactone) scaffold constructs formed tubular-shaped vascular grafts and could be useful in vascular tissue engineering.

Improved recellularization of ex vivo vascular scaffolds using directed transport gradients to modulate ECM remodeling

The regeneration of functional, clinically viable, tissues from acellular ex vivo tissues has been problematic largely due to poor nutrient transport conditions that limit cell migration and integration. Compounding these issues are subcellular pore sizes that necessarily requires extracellular matrix (ECM) remodeling in order for cells to migrate and regenerate the tissue. The aim of the present work was to create a directed growth environment that allows cells to fully populate an ex vivo-derived vascular scaffold and maintain viability over extended periods. Three different culture conditions using single (one nutrient source) or dual perfusion bioreactor systems (two nutrients sources) were designed to assess the effect of pressure and nutrient gradients under either low (50/30 mmHg) or high (120/80) relative pressure conditions. Human myofibroblasts were seeded to the ablumenal periphery of an ex vivo-derived vascular scaffold using a collagen/hydrogel cell delivery system. After 30 days culture, total cell density was consistent between groups; however, significant variation was noted in cell distribution and construct mechanics as a result of differing perfusion conditions. The most aggressive transport gradient was developed by the single perfusion low-pressure circuits and resulted in a higher proportion of cells migrating across the scaffold toward the vessel lumen (nutrient source). These investigations illustrate the influence of directed nutrient gradients where precisely controlled perfusion conditions significantly affects cell migration, distribution and function, resulting in pronounced effects on construct mechanics during early remodeling events.

In vitro platforms for tissue engineering: implications for basic research and clinical translation

Clinical translation of engineered tissues into regenerative medicine applications, and the effort to reduce the use of animals for the screening of drugs and other compounds, result in an increasing demand for human tissues engineered in vitro for implantation, in vitro screening systems and basic research. Further development and optimization of in vitro models for quantitative studies of biophysical stimulation and mass transport in engineered tissues is seen as one of the high research priorities. A recent international symposium held in The Netherlands discussed the state of the art and key challenges in the development of bioreactor platforms and microscale technologies. Here we summarize the current status and future development of in vitro tissue models, based on the findings presented at this symposium.