Microcarriers and Their Potential in Tissue Regeneration

Biodegradable Gelatin Microcarriers Facilitate Re-Epithelialization of Human Cutaneous Wounds - An In Vitro Study in Human Skin

The possibility to use a suspended tridimensional matrix as scaffolding for re-epithelialization of in vitro cutaneous wounds was investigated with the aid of a human in vitro wound healing model based on viable full thickness skin. Macroporous gelatin microcarriers, CultiSpher-S, were applied to in vitro wounds and cultured for 21 days. Tissue sections showed incorporation of wound edge keratinocytes into the microcarriers and thicker neoepidermis in wounds treated with microcarriers. Thickness of the neoepidermis was measured digitally, using immunohistochemical staining of keratins as epithelial demarcation. Air-lifting of wounds enhanced stratification in control wounds as well as wounds with CultiSpher-S. Immunohistochemical staining revealed expression of keratin 5, keratin 10, and laminin 5 in the neoepidermal component. We conclude that the CultiSpher-S microcarriers can function as tissue guiding scaffold for re-epithelialization of cutaneous wounds.

Feasibility of human hair follicle-derived mesenchymal stem cells/CultiSpher(®)-G constructs in regenerative medicine

The use of human mesenchymal stem cells (hMSCs) in cell therapies has increased the demand for strategies that allow efficient cell scale-up. Preliminary data on the three-dimensional (3D) spinner culture describing the potential use of microcarriers for hMSCs culture scale-up have been reported. We exploited a rich source of autologous stem cells (human hair follicle) and demonstrated the robust in vitro long-term expansion of human hair follicle-derived mesenchymal stem cells (hHF-MSCs) by using CultiSpher(®)-G microcarriers. We analyzed the feasibility of 3D culture by using hHF-MSCs/CultiSpher(®)-G microcarrier constructs for its potential applicability in regenerative medicine by comparatively analyzing the performance of hHF-MSCs adhered to the CultiSpher(®)-G microspheres in 3D spinner culture and those grown on the gelatin-coated plastic dishes (2D culture), using various assays. We showed that the hHF-MSCs seeded at various densities quickly adhered to and proliferated well on the microspheres, thus generating at least hundreds of millions of hHF-MSCs on 1 g of CultiSpher(®)-G within 12 days. This resulted in a cumulative cell expansion of greater than 26-fold. Notably, the maximum and average proliferation rates in 3D culture were significantly greater than that of the 2D culture. However, the hHF-MSCs from both the cultures retained surface marker and nestin expression, proliferation capacity and differentiation potentials toward adipocytes, osteoblasts and smooth muscle cells and showed no significant differences as evidenced by Edu incorporation, cell cycle, colony formation, apoptosis, biochemical quantification and qPCR assays.

PCL microspheres tailored with carboxylated poly(glycidyl methacrylate)–REDV conjugates as conducive microcarriers for endothelial cell expansion

PCL microspheres were functionalized with carboxylated PGMA-REDV conjugates by a combination of surface-initiated ATRP and click reaction.

Biofabrication of tissue constructs by 3D bioprinting of cell-laden microcarriers

Bioprinting allows the fabrication of living constructs with custom-made architectures by spatially controlled deposition of multiple bioinks. This is important for the generation of tissue, such as osteochondral tissue, which displays a zonal composition in the cartilage domain supported by the underlying subchondral bone. Challenges in fabricating functional grafts of clinically relevant size include the incorporation of cues to guide specific cell differentiation and the generation of sufficient cells, which is hard to obtain with conventional cell culture techniques. A novel strategy to address these demands is to combine bioprinting with microcarrier technology. This technology allows for the extensive expansion of cells, while they form multi-cellular aggregates, and their phenotype can be controlled. In this work, living constructs were fabricated via bioprinting of cell-laden microcarriers. Mesenchymal stromal cell (MSC)-laden polylactic acid microcarriers, obtained via static culture or spinner flask expansion, were encapsulated in gelatin methacrylamide-gellan gum bioinks, and the printability of the composite material was studied. This bioprinting approach allowed for the fabrication of constructs with high cell concentration and viability. Microcarrier encapsulation improved the compressive modulus of the hydrogel constructs, facilitated cell adhesion, and supported osteogenic differentiation and bone matrix deposition by MSCs. Bilayered osteochondral models were fabricated using microcarrier-laden bioink for the bone compartment. These findings underscore the potential of this new microcarrier-based biofabrication approach for bone and osteochondral constructs.

A scalable approach to obtain mesenchymal stem cells with osteogenic potency on apatite microcarriers

Bone tissue engineering, which relies on the interactions between stem cells and suitable scaffold materials, represents a highly desirable alternative to currently used allograft or autograft strategies for the treatment of bone defects caused by injury or disease, with one of the major challenges being to generate sufficient quantities of stem cells to bring about the intended therapeutic effect. However, conventional cell culture to achieve sufficient cell numbers faces limitations of low efficiency and diminished efficacy of stem cells due to repeated passaging. Furthermore, current microcarriers available may not be suitable for therapeutic implantation. Here, the authors featured an apatite-based microcarrier intended for bone tissue engineering applications. These apatite microcarriers have a diameter of ∼230 µm, and exhibited porous and rough surface morphology. Peaks obtained from X-ray diffractometry (XRD) corresponded to hydroxyapatite (HA) with high crystallinity. Fourier transform infrared spectrophotometry (FTIR) showed that no residues of alginate remained, and all bands observed belong to phosphate and hydroxyl groups of HA. To evaluate the cytocompatibility of these microcarriers, in vitro proliferation studies were conducted and compared with conventional monolayer as well as Cytodex 3. The authors found that human foetal mesenchymal stem cells (hfMSCs) cultured on apatite microcarriers exhibited comparable growth characteristics, achieving 1.4-fold higher live cells than Cytodex 3 over a 9-day culture period. As these microcarriers were hypothesised to offer enhanced osteogenic potency over conventional monolayer culture, alkaline phosphatase (ALP), type I collagen and osteocalcin expression of hfMSCs cultured on the apatite microcarriers were evaluated over a 12-day period. ALP expression for hfMSCs seeded on apatite microcarriers was 2.7-fold higher than that of adherent monolayer culture (p < 0.001). Additionally, type I collagen and osteocalcin expression were 1.8- and 1.5-fold higher than that of adherent monolayer culture on day 12, respectively (p < 0.001).

Engineering stem cell fate with biochemical and biomechanical properties of microcarriers

Microcarriers have been widely used for various biotechnology applications because of their high scale-up potential, high reproducibility in regulating cellular behavior, and well-documented compliance with current Good Manufacturing Practices (cGMP). Recently, microcarriers have been emerging as a novel approach for stem cell expansion and differentiation, enabling potential scale-up of stem cell-derived products in large bioreactors. This review summarizes recent advances of using microcarriers in mesenchymal stem cell (MSC) and pluripotent stem cell (PSC) cultures. From the reported data, efficient expansion and differentiation of stem cells on microcarriers rely on their ability to modulate cell shape (i.e. round or spreading) and cell organization (i.e. aggregate size). Nonetheless, current screening of microcarriers remains empirical, and accurate understanding of how stem cells interact with microcarriers still remains unknown. This review suggests that accurate characterization of biochemical and biomechanical properties of microcarriers is required to fully exploit their potential in regulating stem cell fate decision. Due to the variety of microcarriers, such detailed analyses should lead to the rational design of application-specific microcarriers, enabling the exploitation of reproducible effects for large scale biomedical applications.

Differential response of human adipose tissue-derived mesenchymal stem cells, dermal fibroblasts, and keratinocytes to burn wound exudates: potential role of skin-specific chemokine CCL27

Many cell-based regenerative medicine strategies toward tissue-engineered constructs are currently being explored. Cell-cell interactions and interactions with different biomaterials are extensively investigated, whereas very few studies address how cultured cells will interact with soluble wound-healing mediators that are present within the wound bed after transplantation. The aim of this study was to determine how adipose tissue-derived mesenchymal stem cells (ASC), dermal fibroblasts, and keratinocytes will react when they come in contact with the deep cutaneous burn wound bed. Burn wound exudates isolated from deep burn wounds were found to contain many cytokines, including chemokines and growth factors related to inflammation and wound healing. Seventeen mediators were identified by ELISA (concentration range 0.0006-9 ng/mg total protein), including the skin-specific chemokine CCL27. Burn wound exudates activated both ASC and dermal fibroblasts, but not keratinocytes, to increase secretion of CXCL1, CXCL8, CCL2, and CCL20. Notably, ASC but not fibroblasts or keratinocytes showed significant increased secretion of vascular endothelial growth factor (5-fold) and interleukin-6 (253-fold), although when the cells were incorporated in bi-layered skin substitute (SS) these differences were less pronounced. A similar discrepancy between ASC and dermal fibroblast mono-cultures was observed when recombinant human-CCL27 was used instead of burn wound exudates. Although CCL27 did not stimulate the secretion of any of the wound-healing mediators by keratinocytes, these cells, in contrast to ASC or dermal fibroblasts, showed increased proliferation and migration. Taken together, these results indicate that on transplantation, keratinocytes are primarily activated to promote wound closure. In contrast, dermal fibroblasts and, in particular, ASC respond vigorously to factors present in the wound bed, leading to increased secretion of angiogenesis/granulation tissue formation factors. Our findings have implications for the choice of cell type (ASC or dermal fibroblast) to be used in regenerative medicine strategies and indicate the importance of taking into account interactions with the wound bed when developing advanced therapies for difficult-to-close cutaneous wounds.

Cellularized microcarriers as adhesive building blocks for fabrication of tubular tissue constructs

To meet demands of vascular reconstruction, there is a need for prosthetic alternatives to natural blood vessels. Here we explored a new conduit fabrication approach. Macroporous, gelatin microcarriers laden with human umbilical vein endothelial cells and aortic smooth muscle cells were dispensed into tubular agarose molds and found to adhere to form living tubular tissues. The ability of cellularized microcarriers to adhere to one another involved cellular and extracellular matrix bridging that included the formation of epithelium-like cell layers lining the lumenal and ablumenal surfaces of the constructs and the deposition of collagen and elastin fibers. The tubular tissues behaved as elastic solids, with a uniaxial mechanical response that is qualitatively similar to that of native vascular tissues and consistent with their elastin and collagen composition. Linearized measures of the mechanical response of the fabricated tubular tissues at both low and high strains were observed to increase with duration of static culture, with no significant loss of stiffness following decellularization. The findings highlight the utility of cellularized macroporous gelatin microcarriers as self-adhering building blocks for the fabrication of living tubular structures.

Array of Biodegradable Microraftsfor Isolation and Implantation of Living, Adherent Cells

A new strategy for efficient sorting and implantation of viable adherent cells into animals is described. An array of biodegradable micro-structures (microrafts) was fabricated using a polydimethylsiloxane substrate for micromolding poly(lactic-co-glycolic acid) (PLGA). Screening various forms of PLGA determined that the suitability of PLGA for microraft manufacture, biocompatibility and in vitro degradation was dependent on molecular weight and lactic/glycolic ratio. Cells plated on the array selectively attached to the microrafts and could be identified by their fluorescence, morphology or other criteria. The cells were efficiently dislodged and collected from the array using a microneedle device. The platform was used to isolate specific cells from a mixed population establishing the ability to sort target cells for direct implantation. As a proof of concept, fluorescently conjugated microrafts carrying tumor cells stably expressing luciferase were isolated from an array and implanted subcutaneously into mice. In vivo bio-luminescence imaging confirmed the growth of a tumor in the recipient animals. Imaging of tissue sections from the tumors demonstrated in vivo degradation of the implanted microrafts. The process is a new strategy for isolating and delivering a small number of adherent cells for animal implantation with potential applications in tissue repair, tumor induction, in vivo differentiation of stem cells and other biomedical research.

Manufacturing and banking of mesenchymal stem cells

INTRODUCTION

Mesenchymal stem cells (MSC) and MSC-like cells hold great promise and offer many advantages for developing effective cellular therapeutics. Current trends indicate that the clinical application of MSC will continue to increase markedly. For clinical applications, large numbers of MSC are usually required, ideally in an off-the-shelf format, thus requiring extensive MSC expansion ex vivo and subsequent cryopreservation and banking.

AREAS COVERED

To exploit the full potential of MSC for cell-based therapies requires overcoming significant cell-manufacturing, banking and regulatory challenges. The current review will focus on the identification of optimal cell source for MSC, the techniques for production scale-up, cryopreservation and banking and the regulatory challenges involved.

EXPERT OPINION

There has been considerable success manufacturing and cryopreserving MSC at laboratory scale. Surprisingly little attention, however, has been given to translate these technologies to an industrial scale. The development of cost-effective advanced technologies for producing and cryopreserving commercial-scale MSC is important for successful clinical cell therapy.

Novel macro-microporous gelatin scaffold fabricated by particulate leaching for soft tissue reconstruction with adipose-derived stem cells

The restoration of body contours as shaped by adipose tissue remains a clinical challenge specifically in patients who have experienced loss of contour due to trauma, surgical removal of tumours or congenital abnormalities. We have developed a novel macro-microporous biomaterial for use in soft tissue re-bulking and augmentation. Alginate beads provided the pore template for the construct. Incorporation, and subsequent dissolution, of the beads within a 7 % (w/v) gelatin matrix, produced a highly porous scaffold with an average pore size of 2.01 ± 0.08 mm. The ability of this scaffold to support the in vitro growth and differentiation of human adipose-derived stem cells (ADSCs) was then investigated. Histological analysis confirmed that the scaffold itself provided a suitable environment to support the growth of ADSCs on the scaffold walls. When delivered into the macropores in a fibrin hydrogel, ADSCs proliferated and filled the pores. In addition, ADSCs could readily be differentiated along the adipogenic lineage. These results therefore describe a novel scaffold that can support the proliferation and delivery of ADSCs. The scaffold is the first stage in developing a clinical alternative to current treatment methods for soft tissue reconstruction.

Delta-sleep inducing peptide entrapment and release from polymer hydrogels based on modified polyvinyl alcohol

The aim of the study was to entrap delta-sleep inducing peptide (DSIP) in cross-linked poly(vinyl alcohol)-based hydrogels of different structures and to evaluate peptide release kinetics from these hydrogels using an in vitro model. Isotropic and macroporous hydrogels on the basis of poly(vinyl alcohol) acrylic derivative (Acr-PVA) as well as macroporous hydogels containing epoxy groups which were synthesized by copolymerization of this monomer with glycidyl methacrylate. The isotropic hydrogels were fabricated at positive temperatures while the macroporous hydrogels (cryogels) were prepared at the temperatures below zero. The peptide was entrapped into macroporous modified PVA hydrogels by addition of a peptide solution on previously fabricated matrices, while into PVA-GMA hydrogels containing epoxy groups peptide immobilization was carried out by incubation of hydrogel matrices in the peptide solution. In the case of isotropic hydrogels the peptide was added into the polymer mixture at a hydrogel formation reaction. The peptide release kinetics was studied by incubation of hydrogels in PBS (pH 7.4), in physiological solution (0.9% NaCl) and in water. DSIP concentration in supernatants was determined by phase-reverse HPLC. DSIP release from the macroporous PVA hydrogel after 30 min incubation was 74, 70 и 64% in water, PBS and 0.9% NaCl, relatively, and it was completed in 3 hs. From the isotropic hydrogel the release neither peptide nor products of its degradation was not observed even after 48 hs of incubation. For freshly prepared hydrogel the release kinetics was as follows: 27 and 78% in 30 and 33 hs, relatively. In the case of the lyophilized hydrogel samples the peptide release was 63% in 30 min incubation while drying patterns at room temperature for 3 days resulted in significant peptide loss because its structure damage.

Thermally responsive microcarriers with optimal poly(N-isopropylacrylamide) grafted density for facilitating cell adhesion/detachment in suspension culture

Large-scale cell culture of anchorage-dependent cells based on microcarriers is a crucial method for industrial-scale cell culture and large-scale expansion of therapeutic cells. Previously, the authors developed temperature-responsive microcarriers bearing poly(N-isopropylacrylamide) (PIPAAm)-grafted chains on their outer surface for the non-invasive detachment of cultured cells through temperature reduction without proteolytic enzyme treatment. In this study, to further facilitate cell adhesion and thermally induced detachment efficiency, PIPAAm-grafted beads with various grafted amounts and various grafted PIPAAm chain densities were prepared. Contact angle measurements at different temperatures revealed that the magnitude of the contact angle change from 37 to 20 °C decreased with increasing brush density. Additionally, the amount of fibronectin adsorbed on the bead surface decreased with increasing brush density. Chinese hamster ovary (CHO-K1) cells adhered to the surface of PIPAAm-grafted beads at 37 °C, and a negligible difference in the cell adhesive property was observed by varying the brush density of the PIPAAm-grafted beads. When the temperature was reduced to 20 °C, the adhering cells were found to detach themselves from the PIPAAm-grafted bead surfaces. Of particular interest, PIPAAm-grafted beads with intermediate brush density exhibited the highest efficiency of thermally induced cell detachment. Thus, the brush density of PIPAAm-grafted beads strongly affected the efficiency of thermally induced cell detachment.

Stem cell-delivery therapeutics for periodontal tissue regeneration

Periodontitis, an inflammatory disease, is the most common cause of tooth loss in adults. Attempts to regenerate the complex system of tooth-supporting apparatus (i.e., the periodontal ligament, alveolar bone and root cementum) after loss/damage due to periodontitis have made some progress recently and provide a useful experimental model for the evaluation of future regenerative therapies. Concentrated efforts have now moved from the use of guided tissue/bone regeneration technology, a variety of growth factors and various bone grafts/substitutes toward the design and practice of endogenous regenerative technology by recruitment of host cells (cell homing) or stem cell-based therapeutics by transplantation of outside cells to enhance periodontal tissue regeneration and its biomechanical integration. This shift is driven by the general inability of conventional therapies to deliver satisfactory outcomes, particularly in cases where the disease has caused large tissue defects in the periodontium. Cell homing and cell transplantation are both scientifically meritorious approaches that show promise to completely and reliably reconstitute all tissue and connections damaged through periodontal disease, and hence research into both directions should continue. In view of periodontal regeneration by paradigms that unlock the body's innate regenerative potential has been reviewed elsewhere, this paper specifically explores and analyses the stem cell types and cell delivery strategies that have been or have the potential to be used as therapeutics in periodontal regenerative medicine, with particular emphasis placed on the efficacy and safety concerns of current stem cell-based periodontal therapies that may eventually enter into the clinic.

Novel magnetic fibrin hydrogel scaffolds containing thrombin and growth factors conjugated iron oxide nanoparticles for tissue engineering

Novel tissue-engineered magnetic fibrin hydrogel scaffolds were prepared by the interaction of thrombin-conjugated iron oxide magnetic nanoparticles with fibrinogen. In addition, stabilization of basal fibroblast growth factor (bFGF) was achieved by the covalent and physical conjugation of the growth factor to the magnetic nanoparticles. Adult nasal olfactory mucosa (NOM) cells were seeded in the transparent fibrin scaffolds in the absence or presence of the free or conjugated bFGF-iron oxide nanoparticles. The conjugated bFGF enhanced significantly the growth and differentiation of the NOM cells in the fibrin scaffolds, compared to the same or even five times higher concentration of the free bFGF. In the presence of the bFGF-conjugated magnetic nanoparticles, the cultured NOM cells proliferated and formed a three-dimensional interconnected network composed mainly of tapered bipolar cells. The magnetic properties of these matrices are due to the integration of the thrombin- and bFGF-conjugated magnetic nanoparticles within the scaffolds. The magnetic properties of these scaffolds may be used in future work for various applications, such as magnetic resonance visualization of the scaffolds after implantation and reloading the scaffolds via magnetic forces with bioactive agents, eg, growth factors bound to the iron oxide magnetic nanoparticles.

Magnetic scaffolds enriched with bioactive nanoparticles for tissue engineering

Novel magnetic fibrin hydrogel scaffolds for cell implantation and tissue engineering are reported. The magnetic scaffolds are produced by the interaction between thrombin-conjugated maghemite nanoparticles of narrow size distribution and fibrinogen. These scaffolds, enriched with growth factor conjugated fluorescent maghemite nanoparticles, provide a supporting 3D environment for massive proliferation of various cell types, and can be successfully visualized by MRI.

Microcarrier-based Expansion Process for hMSCs with High Vitality and Undifferentiated Characteristics

For cell therapy, a high biomass of human mesenchymal stem cells (hMSCs) is required for clinical applications, such as in the form of encapsulated implants. An easy and reproducible microcarrier-based stirred tank reactor cultivation process for hMSCs in 1.68 L scale is described. To avoid medium changes, studies comparing high-glucose DMEM (DMEM-HG) with low-glucose EMEM were performed showing that high-glucose medium has positive effects on cell proliferation and that cell differentiability remains.

Studies on the inoculation strategy and cell density, carrier concentration, volume, and stirrer speed were performed and resulted in a set of optimized parameters, inoculation strategy was found to be 45 minutes of static state and 2 minutes of stirring repeated in 4 cycles. The inoculation density was chosen to be 7×103 cells/cm2, and the carrier concentration of glass surface carrier was 25 g/L. For the described reactor system, a stirrer speed of 120 rpm for the inoculation process and a daily increase of 10 rpm up to 160 rpm were found to be suitable. Process reproducibility was shown by 3 repeated cultivations at the determined set of parameters allowing high biomass values of up to 7×108 cells per batch. With DMEM-HG, no limitation of glucose was found, and lactate and ammonia remained lower than critical inhibitory concentrations.

Comparison of the static (T-flask) and dynamic cultures in the stirred tank reactor showed for both cases, that cells were of high vitality and both maintained differentiability. In both cases, encapsulation of the cells resulted in high bead vitality, a basic requirement for cell therapy application.

Enhancement of the growth and differentiation of nasal olfactory mucosa cells by the conjugation of growth factors to functional nanoparticles

Growth factors are critical components in the tissue engineering approach. Basic fibroblast growth factor (bFGF), a representative growth factor, stimulates the cellular functions of various cells and has been used extensively for the repair and regeneration of tissues. The in vivo half-life time of free bFGF is short, about 3-10 min, due to rapid enzymatic degradation. Stabilization of the bFGF was accomplished by the covalent or physical conjugation of this factor to fluorescent maghemite (γ-Fe(2)O(3)) nanoparticles. In the present study, nasal olfactory mucosa (NOM) cells from adult rats were cultured in suspension on chitosan microcarriers (MCs) in the presence of the nonconjugated or bFGF-conjugated nanoparticles, or the free factor. The floating cells/nonconjugated, conjugated, or free bFGF/MCs aggregates were then seeded in a viscous gel. In this manuscript, we are the first to report that the stabilization of the factor by its conjugation to these nanoparticles significantly improved NOM cell-proliferation properties (migration, growth, and differentiation), compared to the same concentration, or even five times higher, of the free factor. This novel approach may significantly contribute to the advancement of the tissue engineering field.

Wound contraction is significantly reduced by the use of microcarriers to deliver keratinocytes and fibroblasts in an in vivo pig model of wound repair and regeneration

In full-thickness injuries caused by extensive burns or penetrating traumatic injuries, the natural epidermal stem cell niche is destroyed, and wound healing occurs through migration of cells from the wound edges and wound contraction. This can lead to significant contracture formation, especially in large full-thickness injuries, causing lack of mobility and pain. Contraction is reduced when wounds are treated using split-thickness skin grafts (STSG) or dermal substitutes, particularly in combination with cultured autologous keratinocytes, delivered as confluent sheets or sprayed as a single cell suspension (SAK). Here, we show that the application of keratinocytes alone or keratinocytes with fibroblasts, delivered on microcarriers, in combination with STSG or a dermal substitute, significantly reduces contraction of wounds in vivo in a porcine model of wound repair and regeneration. A decrease in alpha-smooth muscle actin-positive myofibroblasts, the cell type responsible for wound contraction, accompanies the reduction in contraction. These findings demonstrate the potential for a significant clinical advantage in the treatment of full-thickness injuries.

A fully autologous co-culture system utilising non-irradiated autologous fibroblasts to support the expansion of human keratinocytes for clinical use

Autologous keratinocytes can be used to augment cutaneous repair, such as in the treatment of severe burns and recalcitrant ulcers. Such cells can be delivered to the wound bed either as a confluent sheet of cells or in single-cell suspension. The standard method for expanding primary human keratinocytes in culture uses lethally irradiated mouse 3T3 fibroblasts as feeder cells to support keratinocyte attachment and growth. In an effort to eliminate xenobiotic cells from clinical culture protocols where keratinocytes are applied to patients, we investigated whether human autologous primary fibroblasts could be used to expand keratinocytes in culture. At a defined ratio of a 6:1 excess of keratinocytes to fibroblasts, this co-culture method displayed a population doubling rate comparable to culture with lethally irradiated 3T3 cells. Furthermore, morphological and molecular analysis showed that human keratinocytes expanded in co-culture with autologous human fibroblasts were positive for proliferation markers and negative for differentiation markers. Keratinocytes expanded by this method thus retain their proliferative phenotype, an important feature in enhancing rapid wound closure. We suggest that this novel co-culture method is therefore suitable for clinical use as it dispenses with the need for lethally irradiated 3T3 cells in the rapid expansion of autologous human keratinocytes.

A modular approach to the engineering of a centimeter-sized bone tissue construct with human amniotic mesenchymal stem cells-laden microcarriers

Tissue engineering of clinical-relevant large tissue constructs remains a big challenge due to the mass transfer limit. A modular approach via the assembling of modular tissues thus eliminating the mass transfer limit holds great promise for fabricating centimeter-sized constructs. In the present study, we investigated the feasibility of using microcarriers seeded with adult mesenchymal stem cells (MSCs) to fabricate a large bone tissue. It was demonstrated that human amniotic MSCs (hAMSCs) were efficiently seeded onto CultiSpher S microcarriers (made of porcine gelatin) in a spinner flask and quickly proliferated while retaining a great viability. Within a total culture period of 28 days, using a two-stage culture strategy, hAMSCs-laden microcarriers with a high cell density were prepared at the first stage and the cells were then directly induced to undergo osteogenic differentiation in the same culture flask. During this cultivation process, the aggregation of cell-laden microcarriers was apparent, which resulted in aggregates of 700-800 μm, a size permissive for maintaining high cell viability. The osteogenic differentiation of hAMSCs on microcarriers was confirmed with increased mineral deposition (Alizarin red S staining and quantification of calcium content), ALP activity as well as gene expression of osteogenic markers (collagen type I and osteocalcin). These modular bone-like tissues were used as building blocks to fabricate a macroscopic bone construct in a cylindrical perfusion culture chamber (2 cm in diameter). After a 7-day perfusion culture, these modular tissues readily assembled into a centimeter-sized construct (diameter × height: 2 cm × 1 cm). Both good cell viability and fairly homogenous distribution of cellular content and bone-characteristic ECM within the macrotissue were elaborated. This paper provided a proof-of-concept study for modularly engineering clinical-relevant large tissue replacements with cell-laden microcarriers.