Bioengineering of physiologically functional intrinsically innervated human internal anal sphincter constructs

Crisis in the gut: navigating gastrointestinal challenges in Gulf War Illness with bioengineering

Gulf War Illness (GWI) is characterized by a wide range of symptoms that manifests largely as gastrointestinal symptoms. Among these gastrointestinal symptoms, motility disorders are highly prevalent, presenting as chronic constipation, stomach pain, indigestion, diarrhea, and other conditions that severely impact the quality of life of GWI veterans. However, despite a high prevalence of gastrointestinal impairments among these veterans, most research attention has focused on neurological disturbances. This perspective provides a comprehensive overview of current in vivo research advancements elucidating the underlying mechanisms contributing to gastrointestinal disorders in GWI. Generally, these in vivo and in vitro models propose that neuroinflammation alters gut motility and drives the gastrointestinal symptoms reported in GWI. Additionally, this perspective highlights the potential and challenges of in vitro bioengineering models, which could be a crucial contributor to understanding and treating the pathology of gastrointestinal related-GWI.

Cell Therapy for Anal Sphincter Incontinence: Where Do We Stand?

Anal sphincter incontinence is a chronic disease, which dramatically impairs quality of life and induces high costs for the society. Surgery, considered as the best curative option, shows a disappointing success rate. Stem/progenitor cell therapy is pledging, for anal sphincter incontinence, a substitute to surgery with higher efficacy. However, the published literature is disparate. Our aim was to perform a review on the development of cell therapy for anal sphincter incontinence with critical analyses of its pitfalls. Animal models for anal sphincter incontinence were varied and tried to reproduce distinct clinical situations (acute injury or healed injury with or without surgical reconstruction) but were limited by anatomical considerations. Cell preparations used for treatment, originated, in order of frequency, from skeletal muscle, bone marrow or fat tissue. The characterization of these preparations was often incomplete and stemness not always addressed. Despite a lack of understanding of sphincter healing processes and the exact mechanism of action of cell preparations, this treatment was evaluated in 83 incontinent patients, reporting encouraging results. However, further development is necessary to establish the correct indications, to determine the most-suited cell type, to standardize the cell preparation method and to validate the route and number of cell delivery.

Gut bioengineering strategies for regenerative medicine

Gastrointestinal disease burden continues to rise in the United States and worldwide. The development of bioengineering strategies to model gut injury or disease and to reestablish functional gut tissue could expand therapeutic options and improve clinical outcomes. Current approaches leverage a rapidly evolving gut bioengineering toolkit aimed at 1) de novo generation of gutlike tissues at multiple scales for microtissue models or implantable grafts and 2) regeneration of functional gut in vivo. Although significant progress has been made in intestinal organoid cultures and engineered tissues, development of predictive in vitro models and effective regenerative therapies remains challenging. In this review, we survey emerging bioengineering tools and recent methodological advances to identify current challenges and future opportunities in gut bioengineering for disease modeling and regenerative medicine.

Design Approaches for Generating Organ Constructs

Organ constructs are organ-like structures grown in vitro or in vivo that harbor the components, architecture, and function of in vivo organs, in part or in toto. The convergence of stem cell biology, bioengineering, and gene editing tools have substantially broadened our ability to generate various types of organ constructs for regenerative medicine as well as to address pressing biomedical questions. In this Review, we highlight prevailing approaches for generating organ constructs, from organoids to chimeric organ engineering. We also discuss design principles of different approaches, their utility and limitations, and propose strategies to resolve existing hurdles.

Functional restoration of ex vivo model of pylorus: Co-injection of neural progenitor cells and interstitial cells of Cajal

Transplantation of neural stem cells is a promising approach in treatment of intestinal dysfunctionality. The interstitial cells of Cajal (ICCs) are also critical in conditions such as pyloric dysfunctionality and gastroparesis. The objective of this study was to replenish neurons and ICCs in a dysfunctional pylorus as cell-based therapy to restore functionality. ICCs and enteric neural progenitor cells (NPCs) were isolated from rat duodenum and transduced with fluorescent proteins. Rat pylorus was harvested, and an ex-vivo neuromuscular dysfunctional model was developed by selective ablation of neurons and ICCs via chemical treatments. Cellular repopulation and restoration of motility were assessed by immunohistochemistry, qPCR, and functional analysis after delivery of fluorescently tagged cells. Chemical treatment of pylorus resulted in significant depletion of ICCs (67%, P = .0024; n = 3) and neural cells (83%, P = .0012; n = 3). Delivered ICCs and NPCs survived and integrated with host muscle layers. Co-injection of ICCs with NPCs exhibited 34.4% (P = .0004; n = 3) and 61.0% (P = .0003; n = 3) upregulation of ANO1 and βIII tubulin, respectively. This regeneration resulted in the restoration of agonist-induced excitatory contraction (82%) and neuron evoked relaxation (83%). The functional studies with specific neuronal nitric oxide (NO) synthase blocker confirmed that restoration of relaxation was NO mediated and neuronally derived. The simultaneous delivery of ICCs observed 35.7% higher neuronal differentiation and functional restoration compared with injection of NPCs alone. Injected NPCs and ICCs integrated into the dysfunctional ex vivo pylorus tissues and restored neuromuscular functionality. The co-transplantation of NPCs and ICCs can be used to treat neurodegenerative disorders of the pylorus.

BioSphincters to treat Fecal Incontinence in Nonhuman Primates

Loss of anorectal resting pressure due to internal anal sphincter (IAS) dysfunctionality causes uncontrolled fecal soiling and leads to passive fecal incontinence (FI). The study is focused on immediate and long-term safety and potential efficacy of bioengineered IAS BioSphincters to treat passive FI in a clinically relevant large animal model of passive FI. Passive FI was successfully developed in Non-Human Primates (NHPs) model. The implantation of autologous intrinsically innervated functional constructs resolved the fecal soiling, restored the resting pressure and Recto Anal Inhibitory Reflex (RAIR) within 1-month. These results were sustained with time, and efficacy was preserved up to 12-months. The histological studies validated manometric results with the regeneration of a well-organized neuro-muscular population in IAS. The control groups (non-treated and sham) remained affected by poor anal hygiene, lower resting pressure, and reduced RAIR throughout the study. The pathological assessment of implants, blood, and the vital organs confirmed biocompatibility without any adverse effect after implantation. This regenerative approach of implanting intrinsically innervated IAS BioSphincters has the potential to offer a better quality of life to the patients suffering from FI.

Stem cell therapy for faecal incontinence: Current state and future perspectives

Faecal continence is a complex function involving different organs and systems. Faecal incontinence is a common disorder with different pathogeneses, disabling consequences and high repercussions for quality of life. Current management modalities are not ideal, and the development of new treatments is needed. Since 2008, stem cell therapies have been validated, 36 publications have appeared (29 in preclinical models and seven in clinical settings), and six registered clinical trials are currently ongoing. Some publications have combined stem cells with bioengineering technologies. The aim of this review is to identify and summarise the existing published knowledge of stem cell utilization as a treatment for faecal incontinence. A narrative or descriptive review is presented. Preclinical studies have demonstrated that cellular therapy, mainly in the form of local injections of muscle-derived (muscle derived stem cells or myoblasts derived from them) or mesenchymal (bone-marrow- or adipose-derived) stem cells, is safe. Cellular therapy has also been shown to stimulate the repair of both acute and subacute anal sphincter injuries, and some encouraging functional results have been obtained. Stem cells combined with normal cells on bioengineered scaffolds have achieved the successful creation and implantation of intrinsically-innervated anal sphincter constructs. The clinical evidence, based on adipose-derived stem cells and myoblasts, is extremely limited yet has yielded some promising results, and appears to be safe. Further investigation in both animal models and clinical settings is necessary to drawing conclusions. Nevertheless, if the preliminary results are confirmed, stem cell therapy for faecal incontinence may well become a clinical reality in the near future.

Successful Treatment of Passive Fecal Incontinence in an Animal Model Using Engineered Biosphincters: A 3-Month Follow-Up Study

Fecal incontinence (FI) is the involuntary passage of fecal material. Current treatments have limited successful outcomes. The objective of this study was to develop a large animal model of passive FI and to demonstrate sustained restoration of fecal continence using anorectal manometry in this model after implantation of engineered autologous internal anal sphincter (IAS) biosphincters. Twenty female rabbits were used in this study. The animals were divided into three groups: (a) Non‐treated group: Rabbits underwent IAS injury by hemi‐sphincterectomy without treatment. (b) Treated group: Rabbits underwent IAS injury by hemi‐sphincterectomy followed by implantation of autologous biosphincters. (c) Sham group: Rabbits underwent IAS injury by hemi‐sphincterectomy followed by re‐accessing the surgical site followed by immediate closure without implantation of biosphincters. Anorectal manometry was used to measure resting anal pressure and recto‐anal inhibitory reflex (RAIR) at baseline, 1 month post‐sphincterectomy, up to 3 months after implantation and post‐sham. Following sphincterectomy, all rabbits had decreased basal tone and loss of RAIR, indicative of FI. Anal hygiene was also lost in the rabbits. Decreases in basal tone and RAIR were sustained more than 3 months in the non‐treated group. Autologous biosphincters were successfully implanted into eight donor rabbits in the treated group. Basal tone and RAIR were restored at 3 months following biosphincter implantation and were significantly higher compared with rabbits in the non‐treated and sham groups. Histologically, smooth muscle reconstruction and continuity was restored in the treated group compared with the non‐treated group. Results in this study provided promising outcomes for treatment of FI. Results demonstrated the feasibility of developing and validating a large animal model of passive FI. This study also showed the efficacy of the engineered biosphincters to restore fecal continence as demonstrated by manometry. Stem Cells Translational Medicine2017;6:1795–1802

Biomechanical properties of an implanted engineered tubular gut-sphincter complex

Neuromuscular diseases of the gut alter the normal motility patterns. Although surgical intervention remains the standard treatment, preservation of the sphincter attached to the rest of the gut is challenging. The present study aimed to evaluate a bioengineered gut-sphincter complex following its subcutaneous implantation for 4 weeks in rats. Engineered innervated human smooth muscle sheets and innervated human sphincters with a predefined alignment were placed around tubular scaffolds to create a gut-sphincter complex. The engineered complex was subcutaneously implanted in the abdomen of the rats for 4 weeks. The implanted tissues were vascularized. In vivo manometry revealed luminal pressure at the gut and the sphincter zone. Tensile strength, elongation at break and Young's modulus of the engineered complexes were similar to those of native rat intestine. Histological and immunofluorescence assays showed maintenance of smooth muscle circular alignment in the engineered tissue, maintenance of smooth muscle contractile phenotype and innervation of the smooth muscle. Electrical field stimulation induced relaxation of the smooth muscle of both the sphincter and the gut parts. Relaxation was partly inhibited by nitric oxide inhibitor indicating nitrergic contribution to relaxation. The present study has demonstrated for the first time a successfully developed and subcutaneously implanted a tubular human-derived gut-sphincter complex. The sphincteric part of Tubular Gut-Sphincter Complex (TGSC) maintained the basal tone characteristic of a native sphincter. The gut part also maintained its specific neuromuscular characteristics. The results of this study provide a promising therapeutic approach to restore gut continuity and motility. Copyright © 2016 John Wiley & Sons, Ltd.

Is bioengineering a possibility in gastrointestinal disorders?

The gastrointestinal (GI) tract is responsible for conducting multiple functions including motility, digestion and absorption. In gastrointestinal disorders, some of those functions are weakened or lost. Excision of the diseased segment of the GI tract is a common treatment; however, patients suffer from complications and low quality of life. Functional replacements are therefore needed to restore, repair or replace damaged parts of the tract. Tissue engineering and regenerative medicine provide an alternative approach to reconstruct different segments of the GI tract. The GI tract is a complex system with multiple cell types and layers. In previous years, bioengineering approaches focused on identifying an optimal cell source and scaffolding material to engineer GI tissues. In this editorial, we address some of our thoughts with regard to the recent discoveries in bioengineering the GI tract.

Bioengineering the gut: future prospects of regenerative medicine

Functions of the gastrointestinal tract include motility, digestion and absorption of nutrients. These functions are mediated by several specialized cell types including smooth muscle cells, neurons, interstitial cells and epithelial cells. In gastrointestinal diseases, some of the cells become degenerated or fail to accomplish their normal functions. Surgical resection of the diseased segments of the gastrointestinal tract is considered the gold-standard treatment in many cases, but patients might have surgical complications and quality of life can remain low. Tissue engineering and regenerative medicine aim to restore, repair, or regenerate the function of the tissues. Gastrointestinal tissue engineering is a challenging process given the specific phenotype and alignment of each cell type that colonizes the tract - these properties are critical for proper functionality. In this Review, we summarize advances in the field of gastrointestinal tissue engineering and regenerative medicine. Although the findings are promising, additional studies and optimizations are needed for translational purposes.

Regenerative medicine provides alternative strategies for the treatment of anal incontinence

INTRODUCTION AND HYPOTHESIS

Anal incontinence is a common disorder but current treatment modalities are not ideal and the development of new treatments is needed. The aim of this review was to identify the existing knowledge of regenerative medicine strategies in the form of cellular therapies or bioengineering as a treatment for anal incontinence caused by anal sphincter defects.

METHODS

PubMed was searched for preclinical and clinical studies in English published from January 2005 to January 2016.

RESULTS

Animal studies have demonstrated that cellular therapy in the form of local injections of culture-expanded skeletal myogenic cells stimulates repair of both acute and 2 - 4-week-old anal sphincter injuries. The results from a small clinical trial with ten patients and a case report support the preclinical findings. Animal studies have also demonstrated that local injections of mesenchymal stem cells stimulate repair of sphincter injuries, and a complex bioengineering strategy for creation and implantation of an intrinsically innervated internal anal sphincter construct has been successfully developed in a series of animal studies.

CONCLUSION

Cellular therapies with myogenic cells and mesenchymal stem cells and the use of bioengineering technology to create an anal sphincter are new potential strategies to treat anal incontinence caused by anal sphincter defects, but the clinical evidence is extremely limited. The use of culture-expanded autologous skeletal myogenic cells has been most intensively investigated and several clinical trials were ongoing at the time of this report. The cost-effectiveness of such a therapy is an issue and muscle fragmentation is suggested as a simple alternative.

Bioengineered Human Pyloric Sphincters Using Autologous Smooth Muscle and Neural Progenitor Cells

Gastroparesis leads to inadequate emptying of the stomach resulting in severe negative health impacts. Appropriate long-term treatments for these diseases may require pyloric sphincter tissue replacements that possess functional smooth muscle cell (SMC) and neural components. This study aims to bioengineer, for the first time, innervated human pylorus constructs utilizing autologous human pyloric sphincter SMCs and human neural progenitor cells (NPCs). Autologous SMCs and NPCs were cocultured in dual-layered hydrogels and formed concentrically aligned pylorus constructs. Innervated autologous human pylorus constructs were characterized through biochemical and physiologic assays to assess the phenotype and functionality of SMCs and neurons. SMCs within bioengineered human pylorus constructs displayed a tonic contractile phenotype and maintained circumferential alignment. Neural differentiation within bioengineered constructs was verified by positive expression of βIII-tubulin, neuronal nitric oxide synthase (nNOS), and choline acetyltransferase (ChAT). Autologous bioengineered innervated human pylorus constructs generated a robust spontaneous basal tone and contracted in response to potassium chloride (KCl). Contraction in response to exogenous neurotransmitter acetylcholine (ACh), relaxation in response to vasoactive intestinal peptide (VIP), and electrical field stimulation (EFS) were also observed. Neural network integrity was demonstrated by inhibition of EFS-induced relaxation in the presence of a neurotoxin or nNOS inhibitors. Partial inhibition of ACh-induced contraction and VIP-induced relaxation following neurotoxin treatment was observed. These studies provide a proof of concept for bioengineering functional innervated autologous human pyloric sphincter constructs that generate a robust basal tone and contain circumferentially aligned SMCs, which display a tonic contractile phenotype and functional differentiated neurons. These autologous constructs have the potential to be used as (1) functional replacement organs and (2) physiologically relevant models to investigate human pyloric sphincter disorders.

Bioengineering functional human sphincteric and non-sphincteric gastrointestinal smooth muscle constructs

Digestion and motility of luminal content through the gastrointestinal (GI) tract are achieved by cooperation between distinct cell types. Much of the 3 dimensional (3D) in vitro modeling used to study the GI physiology and disease focus solely on epithelial cells and not smooth muscle cells (SMCs). SMCs of the gut function either to propel and mix luminal contents (phasic; non-sphincteric) or to act as barriers to prevent the movement of luminal materials (tonic; sphincteric). Motility disorders including pyloric stenosis and chronic intestinal pseudoobstruction (CIPO) affect sphincteric and non-sphincteric SMCs, respectively. Bioengineering offers a useful tool to develop functional GI tissue mimics that possess similar characteristics to native tissue. The objective of this study was to bioengineer 3D human pyloric sphincter and small intestinal (SI) constructs in vitro that recapitulate the contractile phenotypes of sphincteric and non-sphincteric human GI SMCs. Bioengineered 3D human pylorus and circular SI SMC constructs were developed and displayed a contractile phenotype. Constructs composed of human pylorus SMCs displayed tonic SMC characteristics, including generation of basal tone, at higher levels than SI SMC constructs which is similar to what is seen in native tissue. Both constructs contracted in response to potassium chloride (KCl) and acetylcholine (ACh) and relaxed in response to vasoactive intestinal peptide (VIP). These studies provide the first bioengineered human pylorus constructs that maintain a sphincteric phenotype. These bioengineered constructs provide appropriate models to study motility disorders of the gut or replacement tissues for various GI organs.

Successful implantation of an engineered tubular neuromuscular tissue composed of human cells and chitosan scaffold

BACKGROUND

There is an urgent need for gut lengthening secondary to massive resections of the gastrointestinal tract. In this study, we propose to evaluate the remodeling, vascularization, and functionality of a chitosan-based, tubular neuromuscular tissue on subcutaneous implantation in the back of athymic rats.

METHODS

Aligned innervated smooth muscle sheets were bioengineered with the use of human smooth muscle and neural progenitor cells. The innervated sheets were wrapped around tubular chitosan scaffolds. The engineered tubular neuromuscular tissue was implanted subcutaneously in the back of athymic rats. The implant was harvested after 14 days and assessed for morphology, vascularization, and functionality.

RESULTS

Gross examination of the implants showed healthy color with no signs of inflammation. The implanted tissue became vascularized as demonstrated by gross and histologic analysis. Chitosan supported the luminal patency of the tissue. The innervated muscle remodeled around the tubular chitosan scaffold. Smooth muscle maintained its circumferential alignment and contractile phenotype. The functionality of the implant was characterized further by the use of real-time force generation. A cholinergic response was demonstrated by robust contraction in response to acetylcholine. Vasoactive intestinal peptide-, and electrical field stimulation-caused relaxation. In the presence of neurotoxin tetrodotoxin, the magnitude of acetylcholine-induced contraction and vasoactive intestinal peptide-induced relaxation was attenuated whereas electrical field stimulation-induced relaxation was completely abolished, indicating neuronal contribution to the response.

CONCLUSION

Our results indicated the successful subcutaneous implantation of engineered tubular neuromuscular tissues. The tissues became vascularized and maintained their myogenic and neurogenic phenotype and function, which provides potential therapeutic prospects for providing implantable replacement GI segments for treating GI motility disorders.

Stem cell therapy for GI neuromuscular disorders

The enteric nervous system is the intrinsic innervation of the gut. Several neuromuscular disorders affect the neurons and glia of the enteric nervous system adversely, resulting in disruptions in gastrointestinal motility and function. Pharmacological interventions to remedy gastrointestinal function do not address the underlying cause of dysmotility arising from lost, absent, or damaged enteric neuroglial circuitry. Cell-based therapies have gained traction in the past decade, following the discovery of several adult stem cell niches in the human body. Adult neural stem cells can be isolated from the postnatal and adult intestine using minimally invasive biopsies. These stem cells retain the ability to differentiate into several functional classes of enteric neurons and enteric glia. Upon identification of these cells, several groups have also established that transplantation of these cells into aganglionic or dysganglionic intestine rescues gastrointestinal motility and function. This chapter highlights key studies performed in the field of stem cell transplantation therapies that are targeted towards the remedy of gastrointestinal motility and function.

Enteric neural differentiation in innervated, physiologically functional, smooth muscle constructs is modulated by bone morphogenic protein 2 secreted by sphincteric smooth muscle cells

The enteric nervous system (ENS) controls gastrointestinal (GI) functions, including motility and digestion, which are impaired in ENS disorders. Differentiation of enteric neurons is mediated by factors released by the gut mesenchyme, including smooth muscle cells (SMCs). SMC-derived factors involved in adult enteric neural progenitor cells (NPCs) differentiation remain elusive. Furthermore, physiologically relevant in vitro models to investigate the innervations of various regions of the gut, such as the pylorus and lower oesophageal sphincter (LES), are not available. Here, neural differentiation in bioengineered innervated circular constructs composed of SMCs isolated from the internal anal sphincter (IAS), pylorus, LES and colon of rabbits was investigated. Additionally, SMC-derived factors that induce neural differentiation were identified to optimize bioengineered construct innervations. Sphincteric and non-sphincteric bioengineered constructs aligned circumferentially and SMCs maintained contractile phenotypes. Sphincteric constructs generated spontaneous basal tones. Higher levels of excitatory and inhibitory motor neuron differentiation and secretion of bone morphogenic protein 2 (BMP2) were observed in bioengineered, innervated, sphincteric constructs compared to non-sphincteric constructs. The addition of BMP2 to non-sphincteric colonic SMC constructs increased nitrergic innervations, and inhibition of BMP2 with noggin in sphincteric constructs decreased functional relaxation. These studies provide: (a) the first bioengineered innervated pylorus and LES constructs; (b) physiologically relevant models to investigate SMCs and adult NPCs interactions; and (c) evidence of the region-specific effects of SMCs on neural differentiation mediated by BMP2. Furthermore, this study paves the way for the development of innervated bioengineered GI tissue constructs tailored to specific disorders and locations within the gut. Copyright © 2015 John Wiley & Sons, Ltd.

The appendix as a viable source of neural progenitor cells to functionally innervate bioengineered gastrointestinal smooth muscle tissues

Appendix-derived neural progenitor cells (NPCs) have both neurogenic and gliogenic potential, but use of these cells for enteric neural cell therapy has not been addressed. The objective of this study was to determine whether NPCs obtained from the appendix would differentiate into enteric neural subsets capable of inducing neurotransmitter-mediated smooth muscle cell (SMC) contraction and relaxation. NPCs were isolated from the appendix and small intestine (SI) of rabbits. Bioengineered internal anal sphincter constructs were developed using the same source of smooth muscle and innervated with NPCs derived from either the appendix or SI. Innervated constructs were assessed for neuronal differentiation markers through Western blots and immunohistochemistry, and functionality was assessed through force-generation studies. Expression of neural and glial differentiation markers was observed in constructs containing appendix- and SI-derived NPCs. The addition of acetylcholine to both appendix and SI constructs caused a robust contraction that was decreased by pretreatment with the neural inhibitor tetrodotoxin (TTX). Electrical field stimulation caused relaxation of constructs that was completely abolished in the presence of TTX and significantly reduced on pretreatment with nitric oxide synthase inhibitor (Nω-nitro-l-arginine methyl ester hydrochloride [l-NAME]). These data indicate that in the presence of identical soluble factors arising from intestinal SMCs, enteric NPCs derived from the appendix and SI differentiate in a similar manner and are capable of responding to physiological stimuli. This coculture paradigm could be used to explore the nature of the soluble factors derived from SMCs and NPCs in generating specific functional innervations.

SIGNIFICANCE

This study demonstrates the ability of neural stem cells isolated from the appendix to differentiate into mature functional enteric neurons. The differentiation of neural stem cells from the appendix is similar to differentiation of neural stem cells derived from the gastrointestinal tract. The appendix is a vestigial organ that can be removed with minimal clinical consequence through laparoscopy. Results presented in this paper indicate that the appendix is a potential source of autologous neural stem cells required for cell therapy for the gastrointestinal tract.

The influence of extracellular matrix composition on the differentiation of neuronal subtypes in tissue engineered innervated intestinal smooth muscle sheets

Differentiation of enteric neural stem cells into several appropriate neural phenotypes is crucial while considering transplantation as a cellular therapy to treat enteric neuropathies. We describe the formation of tissue engineered innervated sheets, where intestinal smooth muscle and enteric neuronal progenitor cells are brought into close association in extracellular matrix (ECM) based microenvironments. Uniaxial alignment of constituent smooth muscle cells was achieved by substrate microtopography. The smooth muscle component of the tissue engineered sheets maintained a contractile phenotype irrespective of the ECM composition, and generated equivalent contractions in response to potassium chloride stimulation, similar to native intestinal tissue. We provided enteric neuronal progenitor cells with permissive ECM-based compositional and viscoelastic cues to generate excitatory and inhibitory neuronal subtypes. In the presence of the smooth muscle cells, the enteric neuronal progenitor cells differentiated to functionally innervate the smooth muscle. The differentiation of specific neuronal subtypes was influenced by the ECM microenvironment, namely combinations of collagen I, collagen IV, laminin and/or heparan sulfate. The physiology of differentiated neurons within tissue engineered sheets was evaluated. Sheets with composite collagen and laminin had the most similar patterns of Acetylcholine-induced contraction to native intestinal tissue, corresponding to an increased protein expression of choline acetyltransferase. An enriched nitrergic neuronal population, evidenced by an increased expression of neuronal nitric oxide synthase, was obtained in tissue engineered sheets that included collagen IV. These sheets had a significantly increased magnitude of electrical field stimulated relaxation, sensitive maximally to nitric oxide synthase inhibition. Tissue engineered sheets containing laminin and/or heparan sulfate had a balanced expression of contractile and relaxant motor neurons. Our studies demonstrated that neuronal subtype was modulated by varying ECM composition. This observation could be utilized to derive enriched populations of specific enteric neurons in vitro prior to transplantation.

Tissue engineering in the gut: developments in neuromusculature

The complexity of the gastrointestinal (GI) tract lies in its anatomy as well as in its physiology. Several different cell types populate the GI tract, adding to the complexity of cell sourcing for regenerative medicine. Each cell layer has a specialized function in mediating digestion, absorption, secretion, motility, and excretion. Tissue engineering and regenerative medicine aim to regenerate the specific layers mimicking architecture and recapitulating function. Gastrointestinal motility is the underlying program that mediates the diverse functions of the intestines, as an organ. Hence, the first logical step in GI regenerative medicine is the reconstruction of the tubular smooth musculature along with the drivers of their input, the enteric nervous system. Recent advances in the field of GI tissue engineering have focused on the use of scaffolding biomaterials in combination with cells and bioactive factors. The ability to innervate the bioengineered muscle is a critical step to ensure proper functionality. Finally, in vivo studies are essential to evaluate implant integration with host tissue, survival, and functionality. In this review, we focus on the tubular structure of the GI tract, tools for innervation, and, finally, evaluation of in vivo strategies for GI replacements.

Design strategies of biodegradable scaffolds for tissue regeneration

There are numerous available biodegradable materials that can be used as scaffolds in regenerative medicine. Currently, there is a huge emphasis on the designing phase of the scaffolds. Materials can be designed to have different properties in order to match the specific application. Modifying scaffolds enhances their bioactivity and improves the regeneration capacity. Modifications of the scaffolds can be later characterized using several tissue engineering tools. In addition to the material, cell source is an important component of the regeneration process. Modified materials must be able to support survival and growth of different cell types. Together, cells and modified biomaterials contribute to the remodeling of the engineered tissue, which affects its performance. This review focuses on the recent advancements in the designs of the scaffolds including the physical and chemical modifications. The last part of this review also discusses designing processes that involve viability of cells.