Development of a Closed Bioreactor System for Culture of Tissue-Engineered Skin at an Air–Liquid Interface

Recreating Human Skin In Vitro: Should the Microbiota Be Taken into Account?

Skin plays crucial roles in the human body: besides protecting the organism from external threats, it acts as a thermal regulator, is responsible for the sense of touch, hosts microbial communities (the skin microbiota) involved in preventing the invasion of foreign pathogens, contains immunocompetent cells that maintain a healthy immunogenic/tolerogenic balance, and is a suitable route for drug administration. In the skin, four defense levels can be identified: besides the physical, chemical, and immune barriers that are inherent to the tissue, the skin microbiota (i.e., the numerous microorganisms living on the skin surface) provides an additional barrier. Studying the skin barrier function or the effects of drugs or cosmetic agents on human skin is a difficult task since snapshot evidence can only be obtained using bioptic samples where dynamic processes cannot properly be followed. To overcome these limitations, many different in vitro models of human skin have been developed that are characterized by diverse levels of complexity in terms of chemical, structural, and cellular composition. The aim of this review is to summarize and discuss the advantages and disadvantages of the different human skin models so far available and to underline how the insertion of a proper microbiota would positively impact an in vitro human skin model in an attempt to better mimic conditions in vivo.

Perfusable Tissue Bioprinted into a 3D-Printed Tailored Bioreactor System

Bioprinting provides a powerful tool for regenerative medicine, as it allows tissue construction with a patient's specific geometry. However, tissue culture and maturation, commonly supported by dynamic bioreactors, are needed. We designed a workflow that creates an implant-specific bioreactor system, which is easily producible and customizable and supports cell cultivation and tissue maturation. First, a bioreactor was designed and different tissue geometries were simulated regarding shear stress and nutrient distribution to match cell culture requirements. These tissues were then directly bioprinted into the 3D-printed bioreactor. To prove the ability of cell maintenance, C2C12 cells in two bioinks were printed into the system and successfully cultured for two weeks. Next, human mesenchymal stem cells (hMSCs) were successfully differentiated toward an adipocyte lineage. As the last step of the presented strategy, we developed a prototype of an automated mobile docking station for the bioreactor. Overall, we present an open-source bioreactor system that is adaptable to a wound-specific geometry and allows cell culture and differentiation. This interdisciplinary roadmap is intended to close the gap between the lab and clinic and to integrate novel 3D-printing technologies for regenerative medicine.

Novel chicken two-dimensional intestinal model comprising all key epithelial cell types and a mesenchymal sub-layer

The intestinal epithelium plays a variety of roles including providing an effective physical barrier and innate immune protection against infection. Two-dimensional models of the intestinal epithelium, 2D enteroids, are a valuable resource to investigate intestinal cell biology and innate immune functions and are suitable for high throughput studies of paracellular transport and epithelial integrity. We have developed a chicken 2D enteroid model that recapitulates all major differentiated cell lineages, including enterocytes, Paneth cells, Goblet cells, enteroendocrine cells and leukocytes, and self-organises into an epithelial and mesenchymal sub-layer. Functional studies demonstrated the 2D enteroids formed a tight cell layer with minimal paracellular flux and a robust epithelial integrity, which was maintained or rescued following damage. The 2D enteroids were also able to demonstrate appropriate innate immune responses following exposure to bacterial endotoxins, from Salmonella enterica serotype Typhimurium and Bacillus subtilis. Frozen 2D enteroids cells when thawed were comparable to freshly isolated cells. The chicken 2D enteroids provide a useful ex vivo model to study intestinal cell biology and innate immune function, and have potential uses in screening of nutritional supplements, pharmaceuticals, and bioactive compounds.

Scale-up of a Composite Cultured Skin Using a Novel Bioreactor Device in a Porcine Wound Model

Extensive deep-burn management with a two-stage strategy can reduce reliance on skin autografts; a biodegradable polyurethane scaffold to actively temporize the wound and later an autologous composite cultured skin (CCS) for definitive closure. The materials fulfilling each stage have undergone in vitro and in vivo pretesting in "small" large animal wounds. For humans, producing multiple, large CCSs requires a specialized bioreactor. This article reports a system used to close large porcine wounds. Three Large White pigs were used, each with two wounds (24.5 cm × 12 cm) into which biodegradable dermal scaffolds were implanted. A sample from discarded tissue allowed isolation/culture of autologous fibroblasts and keratinocytes. CCS production began by presoaking a 1-mm-thick biodegradable polyurethane foam in autologous plasma. In the bioreactor cassette, fibroblasts were seeded into the matrix with thrombin until established, followed by keratinocytes. The CCSs were applied onto integrated dermal scaffolds on day 35, alongside a sheet skin graft (30% of one wound). Serial punch biopsies, trans-epidermal water loss readings (TEWL), and wound measurements indicated epithelialization. During dermal scaffold integration, negligible wound contraction was observed (average 4.5%). After CCS transplantation, the control skin grafts were "taken" by day 11 when visible islands of epithelium were clinically observed on 2/3 CCSs. Closure was confirmed histologically, with complete epithelialization by day 63 post-CCS transplantation (CCS TEWL ~ normal skin average 11.9 g/m2h). Four of six wounds demonstrated closure with robust, stratified epithelium. Generating large pieces of CCS capable of healing large wounds is thus possible using a specialized designed bioreactor.

An overview on the manufacturing of functional and mature cellular skin substitutes

There are different types of skin diseases due to chronic injuries that impede the natural healing process of the skin. Tissue engineering (TE) has focused on the development of bioengineered skin or skin substitutes that cover the wound, providing the necessary care to restore the functionality of injured skin. There are two types of substitutes: acellular skin substitutes (ASSs), which offer a low response of the body, and cellular skin substitutes (CSSs), which incorporate living cells and appear as a great alternative in the treatment of skin injuries due to them presenting a greater interaction and integration with the rest of the body. For the development of a CSS, it is necessary to select the most suitable biomaterials, cell components, and methodology of biofabrication for the wound to be treated. Moreover, these CSSs are immature substitutes that must undergo a maturing process in specific bioreactors, guaranteeing their functionality. The bioreactor simulates the natural state of maturation of the skin by controlling parameters such as temperature, pressure, or humidity, allowing a homogeneous maturation of the CSSs in an aseptic environment. The use of bioreactors not only contributes to the maturation of the CSSs, but also offers a new way of obtaining large sections of skin substitutes or natural skin from small portions acquired from the patient, donor, or substitute. Based on the innovation of this technology and the need to develop efficient CSSs, this work offers an update on bioreactor technology in the field of skin regeneration.

Bioengineered Skin Substitutes: the Role of Extracellular Matrix and Vascularization in the Healing of Deep Wounds

The formation of severe scars still represents the result of the closure process of extended and deep skin wounds. To address this issue, different bioengineered skin substitutes have been developed but a general consensus regarding their effectiveness has not been achieved yet. It will be shown that bioengineered skin substitutes, although representing a valid alternative to autografting, induce skin cells in repairing the wound rather than guiding a regeneration process. Repaired skin differs from regenerated skin, showing high contracture, loss of sensitivity, impaired pigmentation and absence of cutaneous adnexa (i.e., hair follicles and sweat glands). This leads to significant mobility and aesthetic concerns, making the development of more effective bioengineered skin models a current need. The objective of this review is to determine the limitations of either commercially available or investigational bioengineered skin substitutes and how advanced skin tissue engineering strategies can be improved in order to completely restore skin functions after severe wounds.

A new dynamic culture device suitable for rat skin culture

Cultured skin has been used extensively for testing therapeutic drugs because it replicates the physical and biochemical properties of whole skin. However, traditional static culture cannot fully maintain cell viability and skin morphology because of the limitations involved with nutrient transmission. Here, we develop a new dynamic perfusion platform for skin culture and compare it with a static culture device. Rat skins were cultured in either static or dynamic condition for 0, 3, 6, 9 and 12 days. H&E, periodic acid-Schiff (PAS) and picrosirius red (PSR) staining were used for skin morphology detection, immunostaining against cytokeratin 10 (CK10) for differentiation detection, immunostaining against proliferating cell nuclear antigen (PCNA) for cell proliferation detection and TUNEL staining for apoptosis detection. After culturing for 12 days, the epidermis, basement membrane, hair follicles and connective tissue were disrupted in the static group, whereas these features were preserved in the dynamic group. Moreover, compared to the static group, proliferation in the epidermis and hair follicles was significantly improved and apoptosis in dermis was significantly decreased in the dynamic group. These findings suggest that our device is effective for extending the culture period of rat skin to maintain its characteristics and viability in vitro.

The effects of constant flow bioreactor cultivation and keratinocyte seeding densities on prevascularized organotypic skin grafts based on a fibrin scaffold

Organotypic full-thickness skin grafts (OTSG) are already an important technology for treating various skin conditions and are well established for skin research and development. These obvious benefits are often impaired by the need of laborious production, their noncomplete autologous composition, and, most importantly, their lack of included vasculature. Therefore, our study focused on combining a prevascularized dermal layer with an epidermis to cultivate full-thickness skin grafts incorporating capillary-like networks. It has been shown that prevascularization accelerates ingrowth of tissue-engineered grafts, and it is a prerequisite to circumvent diffusion limits due to graft thickness. To obtain such a graft, we chose a dermal layer incorporating human umbilical vein endothelial cells (HuVEC) amid human dermal fibroblasts within a fibrin-based scaffold, seeded apically with human foreskin keratinocytes (hfKC). Our research investigated the used concept's feasibility, as well as the effect of hfKC addition on the development of a well-connected capillary-like network after approximately 21 days. In addition, we evaluated the utilization of a custom-made constant flow bioreactor for simplified cultivation of these grafts, therefore possibly easing graft production and presumably increasing their cost effectiveness. Skin grafts were assessed by conventional two-dimensional histology. In addition, software-assisted three-dimensional evaluation of the capillary-like structure networks was performed by two-photon laser scanning microscopy (TPLSM) and subsequent image processing was done with ImagePro(®) Analyzer 7.0 software, thereby evaluating its platform technology power in the field of prevascularized skin grafts. All samples showed a capillary-like structure network, but we could report a significant reduction of its total length after 14 days of tri-culture with 5×10(5)/cm(2) seeded hfKC, possibly indicating nutritional deficiencies for this particular high cell density experimental setup. Lower concentrations of hfKC did not affect the formation of the capillary-like structures significantly. The developed bioreactor simplified cultivation of prevascularized OTSG. However, a flow-dependent reduction of capillary-like structures in 1 and 5 mL/min flow conditions occurred. We conclude that our technique for creating prevascularized OTSG is feasible. In addition, TPLSM is well suited for analyzing the prevascularization process. We hypothesize that the handling benefits of our bioreactor can be preserved by using considerably lower flow rates while not impairing the forming of capillary-like structure networks.

Development of a novel 3D culture system for screening features of a complex implantable device for CNS repair

Tubular scaffolds which incorporate a variety of micro- and nanotopographies have a wide application potential in tissue engineering especially for the repair of spinal cord injury (SCI). We aim to produce metabolically active differentiated tissues within such tubes, as it is crucially important to evaluate the biological performance of the three-dimensional (3D) scaffold and optimize the bioprocesses for tissue culture. Because of the complex 3D configuration and the presence of various topographies, it is rarely possible to observe and analyze cells within such scaffolds in situ. Thus, we aim to develop scaled down mini-chambers as simplified in vitro simulation systems, to bridge the gap between two-dimensional (2D) cell cultures on structured substrates and three-dimensional (3D) tissue culture. The mini-chambers were manipulated to systematically simulate and evaluate the influences of gravity, topography, fluid flow, and scaffold dimension on three exemplary cell models that play a role in CNS repair (i.e., cortical astrocytes, fibroblasts, and myelinating cultures) within a tubular scaffold created by rolling up a microstructured membrane. Since we use CNS myelinating cultures, we can confirm that the scaffold does not affect neural cell differentiation. It was found that heterogeneous cell distribution within the tubular constructs was caused by a combination of gravity, fluid flow, topography, and scaffold configuration, while cell survival was influenced by scaffold length, porosity, and thickness. This research demonstrates that the mini-chambers represent a viable, novel, scale down approach for the evaluation of complex 3D scaffolds as well as providing a microbioprocessing strategy for tissue engineering and the potential repair of SCI.

Double-chamber rotating bioreactor for dynamic perfusion cell seeding of large-segment tracheal allografts: comparison to conventional static methods

Tracheal transplantation with a long-segment recellularized tracheal allograft has previously been performed without the need for immunosuppressive therapy. Recipients' mesenchymal stromal cells (MSC) and tracheal epithelial cells (TEC) were harvested, cultured, expanded, and seeded on a donor trachea within a bioreactor. Prior techniques used for cellular seeding have involved only static-seeding methods. Here, we describe a novel bioreactor for recellularization of long-segment tracheae. Tracheae were recellularized with epithelial cells on the luminal surface and bone marrow-derived MSC on the external surface. We used dynamic perfusion seeding for both cell types and demonstrate an increase in both cellular counts and homogeneity scores compared with traditional methods. Despite these improvements, orthotopic transplantation of these scaffolds revealed no labeled cells at postoperative day 3 and lack of re-epithelialization within the first 2 weeks. The animals in this study had postoperative respiratory distress and tracheal collapse that was incompatible with life.

The development of a 3D immunocompetent model of human skin

As the first line of defence, skin is regularly exposed to a variety of biological, physical and chemical insults. Therefore, determining the skin sensitization potential of new chemicals is of paramount importance from the safety assessment and regulatory point of view. Given the questionable biological relevance of animal models to human as well as ethical and regulatory pressure to limit or stop the use of animal models for safety testing, there is a need for developing simple yet physiologically relevant models of human skin. Herein, we describe the construction of a novel immunocompetent 3D human skin model comprising of dendritic cells co-cultured with keratinocytes and fibroblasts. This model culture system is simple to assemble with readily-available components and importantly, can be separated into its constitutive individual layers to allow further insight into cell-cell interactions and detailed studies of the mechanisms of skin sensitization. In this study, using non-degradable microfibre scaffolds and a cell-laden gel, we have engineered a multilayer 3D immunocompetent model comprised of keratinocytes and fibroblasts that are interspersed with dendritic cells. We have characterized this model using a combination of confocal microscopy, immuno-histochemistry and scanning electron microscopy and have shown differentiation of the epidermal layer and formation of an epidermal barrier. Crucially the immune cells in the model are able to migrate and remain responsive to stimulation with skin sensitizers even at low concentrations. We therefore suggest this new biologically relevant skin model will prove valuable in investigating the mechanisms of allergic contact dermatitis and other skin pathologies in human. Once fully optimized, this model can also be used as a platform for testing the allergenic potential of new chemicals and drug leads.

A bioreactor system for interfacial culture and physiological perfusion of vascularized tissue equivalents

A pivotal requirement for the generation of vascularized tissue equivalents is the development of culture systems that provide a physiological perfusion of the vasculature and tissue-specific culture conditions. Here, we present a bioreactor system that is suitable to culture vascularized tissue equivalents covered with culture media and at the air-medium interface, which is a vital stimulus for skin tissue. For the perfusion of the vascular system a new method was integrated into the bioreactor system that creates a physiological pulsatile medium flow between 80 and 120 mmHg to the arterial inflow of the equivalent's vascular system. Human dermal microvascular endothelial cells (hDMECs) were injected into the vascular system of a biological vascularized scaffold based on a decellularized porcine jejunal segment and cultured in the bioreactor system for 14 days. Histological analysis and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) staining revealed that the hDMECs were able to recolonize the perfused vascular structures and expressed endothelial cell specific markers such as platelet endothelial cell adhesion molecule and von Willebrand factor. These results indicate that our bioreactor system can serve as a platform technology to generate advanced bioartificial tissues with a functional vasculature for future clinical applications.

Novel strategies to engineering biological tissue in vitro

Tissue engineering creates biological tissues that aim to improve the function of diseased or damaged tissues. In this chapter, we examine the promise and shortcomings of "top-down" and "bottom-up" approaches for creating engineered biological tissues. In top-down approaches, the cells are expected to populate the scaffold and create the appropriate extracellular matrix and microarchitecture often with the aid of a bioreactor that furnish the set of stimuli required for an optimal cellular viability. Specifically, we survey the role of cell material interaction on oxygen metabolism in three-dimensional (3D) in vitro cultures as well as the time and space evolution of the transport and biophysical properties during the development of de novo synthesized tissue-engineered constructs. We show how to monitor and control the evolution of these parameters that is of crucial importance to process biohybrid constructs in vitro as well as to elaborate reliable mathematical model to forecast tissue growth under specific culture conditions. Furthermore, novel strategies such as bottom-up approaches to build tissue constructs in vitro are examined. In this fashion, tissue building blocks with specific microarchitectural features are used as modular units to engineer biological tissues from the bottom up. In particular, the attention will be focused on the use of cell seeded microbeads as functional building blocks to realize 3D complex tissue. Finally, a challenge will be the potential integration of bottom-up techniques with more traditional top-down approaches to create more complex tissues than are currently achievable using either technique alone by optimizing the advantages of each technique.

Cytocentrifugation: a convenient and efficient method for seeding tendon-derived cells into monolayer cultures or 3-D tissue engineering scaffolds

Tendon and ligament injuries are very common, requiring some 200,000 reconstructions per year in the USA. Autografting can be used to repair these but donor tissue is limited and harvesting leads to morbidity at the graft sites. Tissue engineering has been used to grow simple tissues such as skin, cartilage and bone and due to their low vascularity and simple structure, tendons should be ideal candidates for such an approach. Scaffolds are essential for tissue engineering as they provide structure and signals that regulate growth. However, they present a physical barrier to cell seeding with the majority of the cells congregating at the scaffold surface. To address this we used centrifugation to enhance penetration of tendon-derived cells to the centres of 3-D scaffolds. The process had no apparent deleterious effects on the cells and both plating efficiency and cell distribution improved. After attachment the cells continued to proliferate and deposit a collagenous matrix. Scaffold penetration was investigated using layers of Azowipes allowing the separation and examination of individual leaves. At relatively low g-forces, cells penetrated a stack of 6 Azowipes leaving cells attached to each leaf. These data suggest that cytocentrifugation improves the penetration and homogeneity of tendon derived cells in 3-D and monolayer cultures.

The interactions of astrocytes and fibroblasts with defined pore structures in static and perfusion cultures

Open pores to maintain nutrient diffusion and waste removal after cell colonization are crucial for the successful application of constructs based on assembled membranes, in our case tubular scaffolds made of ɛ-polycaprolactone (PCL), for use in tissue engineering. Due to the complex three-dimensional structure and large size of such scaffolds needed for transplantable tissues, it is difficult to investigate the cell–pore interactions in situ. Therefore miniaturized bioreactors inside Petri dishes (30 mm in diameter), containing porous PCL or poly-dimethylsiloxane (PDMS) membranes, were developed to allow the interactions of different cells with defined pores to be investigated in situ during both static and perfusion cultures. Investigation of two different cell types (fibroblasts and cortical astrocytes) and how they interact with a range of pores (100–350 μm in diameter) for up to 50 days indicated that the cells either ‘covered’ or ‘bridged’ the pores. Three distinct behaviors were observed in the way cortical astrocytes interacted with pores, while fibroblasts were able to quickly bridge the pores based on consistent “joint efforts”. Our studies demonstrate that the distinct pore sealing behaviors of both cell types were influenced by pore size, initial cell density and culture period, but not by medium perfusion within the range of shear forces investigated. These findings form important basic data about the usability of pores within scaffolds that could inform the design and fabrication of suitable scaffolds for various applications in tissue engineering.

Growth and stratification of epithelial cells in minimal culture conditions

Biological risk management is required in modern tissue engineering. Particular attention should be paid to the culture medium and the scaffold used. In this perspective, it is important to define minimal culture conditions which allow proper growth and differentiation of epithelial cells in vitro. We propose a simple experimental system which permits the generation of three-dimensional epidermal constructs using a collagen layer as a scaffold mimicking the entire dermal tissue and without the need of any feeder layer. Although showing significant differences compared to natural epidermis, these epidermal constructs appear useful to study keratinocyte differentiation and epidermis histogenesis.

Potential and bottlenecks of bioreactors in 3D cell culture and tissue manufacturing

Over the last decade, we have witnessed an increased recognition of the importance of 3D culture models to study various aspects of cell physiology and pathology, as well as to engineer implantable tissues. As compared to well-established 2D cell-culture systems, cell/tissue culture within 3D porous biomaterials has introduced new scientific and technical challenges associated with complex transport phenomena, physical forces, and cell-microenvironment interactions. While bioreactor-based 3D model systems have begun to play a crucial role in addressing fundamental scientific questions, numerous hurdles currently impede the most efficient utilization of these systems. We describe how computational modeling and innovative sensor technologies, in conjunction with well-defined and controlled bioreactor-based 3D culture systems, will be key to gain further insight into cell behavior and the complexity of tissue development. These model systems will lay a solid foundation to further develop, optimize, and effectively streamline the essential bioprocesses to safely and reproducibly produce appropriately scaled tissue grafts for clinical studies.

Development of a bioreactor for evaluating novel nerve conduits

We describe an experimental closed bioreactor device for studying novel tissue engineered peripheral nerve conduits in vitro. The system integrates a closed loop system consisting of one, two, or three experimental nerve conduits connected in series or parallel, with the ability to study novel scaffolds within guidance conduits. The system was established using aligned synthetic microfiber scaffolds of viscose rayon and electrospun polystyrene. Schwann cells were seeded directly into conduits varying from 10 to 80 mm in length and allowed to adhere under 0 flow for 1 h, before being cultured for 4 days under static or continuous flow conditions. In situ viability measurements showed the distribution of live Schwann cells within each conduit and enabled quantification thereafter. Under static culture viable cells only existed in short conduit scaffolds (10 mm) or at the ends of longer conduits (20-80 mm) with a variation in viable cell distribution. Surface modification of scaffold fibers with type-1 collagen or acrylic acid increased cell number by 17% and 30%, respectively. However, a continuous medium flow of 0.8 mL/h was found to increase total cell number by 2.5-fold verses static culture. Importantly, under these conditions parallel viability measurements revealed a ninefold increase compared to static culture. Fluorescence microscopy of scaffolds showed cellular adhesion and alignment on the longitudinal axis. We suggest that such a system will enable a rigorous and controlled approach for evaluating novel conduits for peripheral nerve repair, in particular using hydrolysable materials for the parallel organization of nerve support cells, prior to in vivo study.

Generation of human epidermal constructs on a collagen layer alone

Because engineered tissues are designed for clinical applications in humans, a major problem is the contamination of cocultures and tissues by allogenic molecules used to grow stem cells in vitro. The protocols that are commonly applied to generate epidermal equivalents in vitro require the use of irradiated murine fibroblasts as a feeder layer for keratinocytes. In this study, we report a simple procedure for growing human keratinocytes, isolated from adult skin, to generate an epidermal construct on a collagen layer alone. In this model, no human or murine feeder layers were used to amplify cell growth, and isolated keratinocytes were seeded directly at high cell density on the collagen-coated flasks or coverslips in an epithelial growth medium containing low calcium concentration. Morphological, immunochemical, and cytokinetic features of epithelial colonies grown on the collagen layer were typical of keratinocytes and were comparable with those reported for keratinocytes grown on a feeder layer. The stratification of keratinocytes generated 3-dimensional synthetic constructs displaying a tissue architecture comparable with that of natural epidermis. Epithelial cells expressed specific markers of keratinocyte terminal differentiation, including involucrin and filaggrin. Nevertheless, the number of cell layers was lower than in natural skin, and electron microscopical analysis revealed that the overall organization of these layers was poor compared with natural epidermis, including the formation of junctional complexes, basement membrane, and keratinization. The lack of epithelial-mesenchymal interactions that occur during skin histogenesis may account for such an incomplete maturation of epidermal constructs.

Development of a 3D cell culture system for investigating cell interactions with electrospun fibers

There are many variables to be considered in studying how cells interact with 3D scaffolds used in tissue engineering. In this study we investigated the influence of the fiber diameter and interfiber spaces of 3D electrospun fiber scaffolds on the behavior of human dermal fibroblasts. Fibers of two dissimilar model materials, polystyrene and poly-L-lactic acid, with a broad range of diameters were constructed in a specifically developed 3D cell culture system. When fibroblasts were introduced to freestanding fibers, and encouraged to "walk the plank," a minimum fiber diameter of 10 microm was observed for cell adhesion and migration, irrespective of fiber material chemistry. A distance between fibers of up to 200 microm was also observed to be the maximum gap that could be bridged by cell aggregates--a behavior not seen in conventional 2D culture. This approach has identified some basic micro-architectural parameters for electrospun scaffold design and some key differences in fibroblast growth in 3D. We suggest the findings will be of value for optimizing the integration of cells in these scaffolds for skin tissue engineering.

Assessment of an Automated Bioreactor to Propagate and Harvest Keratinocytes for Fabrication of Engineered Skin Substitutes

Engineered skin substitutes (ESS) composed of autologous fibroblasts and keratinocytes attached to collagen-glycosaminoglycan (GAG) scaffolds are effective adjuncts in the treatment of massive burns. The Kerator, an automated bioreactor for keratinocyte culture, could hypothetically reduce labor and material requirements, and increase availability of ESS. Human keratinocytes were cultured in the Kerator and also in tissue-culture flasks. It was found that keratinocyte confluence increased exponentially with time in both the Kerator (r2=0.99) and the flasks (r2=0.96). Confluence (mean+/-SEM) of keratinocytes in the flasks (28+/-2.3%) was significantly higher than in the Kerator (18+/-0.93%) at day 4. However, there was no difference in confluence at harvest. The colony forming efficiency (CFE) and population doublings (PD) per day of keratinocytes harvested from the Kerator were 67+/-4.7% and 0.80+/-0.06, respectively, and were not different from the corresponding values for keratinocytes from flasks. ESS fabricated with keratinocytes from the Kerator or from the flasks were comparable in vitro in terms of histological anatomy, cellular viability, and surface hydration. These findings show that there are no differences between keratinocytes from the Kerator and those from the flasks regarding (a) growth to confluence, (b) CFE and growth rate (PD/day), or (c) quality of ESS in vitro, suggesting that the Kerator can automate fabrication of ESS and increase its availability for treatment of skin wounds.

Transgene Expression Level and Inherent Differences in Target Gene Activation Determine the Rate and Fate of Neurogenin3-Mediated Islet Cell DifferentiationIn Vitro

A significant challenge in many areas of tissue engineering is a readily available source of cells. One approach to address this challenge is to direct the differentiation of expandable stem or progenitor cells or the transdifferentiation of an already differentiated cell type to the desired cell type. A variety of methods have been explored for directing cell differentiation, including the ectopic expression of transcriptional factors that are known to influence cell differentiation during development. One such transcription factor, neurogenin3 (Ngn3), plays a critical role in islet cell development in vivo. Ectopic expression of Ngn3 in various cell types has previously been shown to promote differentiation toward islet cell phenotypes, but the overall efficiency of this differentiation and the specific islet cell type produced vary widely between reports. The present work evaluates the hypotheses that cellular response is determined by (1) differentiation status of the starting cell, (2) basal expression of other transcriptional factors, and (3) level of ectopic Ngn3 expression. Retroviral vectors were used to express Ngn3 in primary adult pancreatic ductal epithelial cells (PDEC), embryonic and adult stem cells (ESC and ASC), and transformed mouse pancreatic adenocarcinoma (mPAC) cells in vitro. Changes in phenotypes were assessed using quantitative reverse transcription polymerase chain reaction (qRT-PCR), gene arrays, and immunohistochemistry. When Ngn3 was ectopically expressed in mouse and rat PDEC, downstream transcription factors (e.g., NeuroD, Nkx2.2, Isl-1) and endocrine hormones (most notably, ghrelin and somatostatin) were highly upregulated in a dose-dependent manner. In comparison to mPAC and mouse embryonic stem cells (mESC), PDEC displayed higher expression of most islet markers after normalization to Ngn3 levels. Differences in the basal expression and activation of transcription factors (e.g., Pax4, Pax6, and Nkx6.1) were observed between cell types, suggesting a mechanism by which precursors might preferentially generate different islet cell types.

Culture of skin cells in 3D rather than 2D improves their ability to survive exposure to cytotoxic agents

In this study, we asked the question of whether cells in 3D culture cope more effectively with cytotoxic agents than cells in 2D. The sensitivities of human skin cells (keratinocytes, dermal fibroblasts and endothelial cells) to oxidative stress (hydrogen peroxide) and to a potentially toxic heavy metal (silver) when cultured under 2D and 3D conditions were investigated. The results show a marked resistance of cells to a given dose of hydrogen peroxide or silver nitrate causing a 50% loss of viability in 3D cultures, when compared to the same cells grown in 2D. There was also an improvement in the ability of cells to withstand both stresses when cells were in co-culture rather than in mono-culture. Foetal calf serum was found to have a mild protective effect in 2D culture but this was not extended to findings in 3D culture. This study suggests that dermatotoxicity testing using 3D co-cultures might be more likely to reflect true physiological responses to xenobiotic materials than existing models that rely on 2D mono-cultures.