Tissue engineering the small intestine

3D printed human organoids: High throughput system for drug screening and testing in current COVID-19 pandemic

In the current pandemic, scenario the world is facing a huge shortage of effective drugs and other prophylactic medicine to treat patients which created havoc in several countries with poor resources. With limited demand and supply of effective drugs, researchers rushed to repurpose the existing approved drugs for the treatment of COVID-19. The process of drug screening and testing is very costly and requires several steps for validation and treatment efficacy evaluation ranging from in-vitro to in-vivo setups. After these steps, a clinical trial is mandatory for the evaluation of treatment efficacy and side effects in humans. These processes enhance the overall cost and sometimes the lead molecule show adverse effects in humans and the trial ends up in the final stages. Recently with the advent of 3D organoid culture which mimics the human tissue exactly the process of drug screening and testing can be done in a faster and cost-effective manner. Further 3D organoids prepared from stems cells taken from individuals can be beneficial for personalized drug therapy which could save millions of lives. This review discussed approaches and techniques for the synthesis of 3D-printed human organoids for drug screening. The key findings of the usage of organoids for personalized medicine for the treatment of COVID-19 have been discussed. In the end, the key challenges for the wide applicability of human organoids for drug screening with prospects of future orientation have been included. This article is protected by copyright. All rights reserved.

Tissue engineering of the gastrointestinal tract: the historic path to translation

The gastrointestinal (GI) tract is imperative for multiple functions including digestion, nutrient absorption, and timely waste disposal. The central feature of the gut is peristalsis, intestinal motility, which facilitates all of its functions. Disruptions in GI motility lead to sub-optimal GI function, resulting in a lower quality of life in many functional GI disorders. Over the last two decades, tissue engineering research directed towards the intestine has progressed rapidly due to advances in cell and stem-cell biology, integrative physiology, bioengineering and biomaterials. Newer biomedical tools (including optical tools, machine learning, and nuanced regenerative engineering approaches) have expanded our understanding of the complex cellular communication within the GI tract that lead to its orchestrated physiological function. Bioengineering therefore can be utilized towards several translational aspects: (i) regenerative medicine to remedy/restore GI physiological function; (ii) in vitro model building to mimic the complex physiology for drug and pharmacology testing; (iii) tool development to continue to unravel multi-cell communication networks to integrate cell and organ-level physiology. Despite the significant strides made historically in GI tissue engineering, fundamental challenges remain including the quest for identifying autologous human cell sources, enhanced scaffolding biomaterials to increase biocompatibility while matching viscoelastic properties of the underlying tissue, and overall biomanufacturing. This review provides historic perspectives for how bioengineering has advanced over time, highlights newer advances in bioengineering strategies, and provides a realistic perspective on the path to translation.

An organoid-based organ-repurposing approach to treat short bowel syndrome

The small intestine is the main organ for nutrient absorption, and its extensive resection leads to malabsorption and wasting conditions referred to as short bowel syndrome (SBS). Organoid technology enables an efficient expansion of intestinal epithelium tissue in vitro¹, but reconstruction of the whole small intestine, including the complex lymphovascular system, has remained challenging². Here we generate a functional small intestinalized colon (SIC) by replacing the native colonic epithelium with ileum-derived organoids. We first find that xenotransplanted human ileum organoids maintain their regional identity and form nascent villus structures in the mouse colon. In vitro culture of an organoid monolayer further reveals an essential role for luminal mechanistic flow in the formation of villi. We then develop a rat SIC model by repositioning the SIC at the ileocaecal junction, where the epithelium is exposed to a constant luminal stream of intestinal juice. This anatomical relocation provides the SIC with organ structures of the small intestine, including intact vasculature and innervation, villous structures, and the lacteal (a fat-absorbing lymphatic structure specific to the small intestine). The SIC has absorptive functions and markedly ameliorates intestinal failure in a rat model of SBS, whereas transplantation of colon organoids instead of ileum organoids invariably leads to mortality. These data provide a proof of principle for the use of intestinal organoids for regenerative purposes, and offer a feasible strategy for SBS treatment.

New methodologies for old problems: tridimensional gastrointestinal organoids and guts-on-a-chip

Objectives The present review intended to present a critical overview of the methodological and experimental advances concerning tridimensional cell culture models within the scope of gastrointestinal research.

Methods A literature review was performed and some of the main published articles in the area were mentioned.

Main results Classic studies and high impact results were presented, starting from the pioneer works with gastrointestinal organoids, with a small gut organoid, to the achievement of guts-on-a-chip and multi-organ-chips. It was also discussed which implications the construction of such co-cultures bring, as well as future applications arising from these new methodologies.

Conclusions Despite the still discrete number of publications, in quantitative terms, there are qualitative promising and consistent results addressing physiopathological aspects and new therapeutic perspectives of tridimensional in vitro cultures in the gastroenterology field. It is expected, thus, that such new methodological approaches, including organoids and guts-on-a-chip, may contribute decisively to the advance in knowledge on basic aspects, as well as on the translation to new therapeutic approaches in gastrointestinal diseases.

Hydrogel-based 3D bioprints repair rat small intestine injuries and integrate into native intestinal tissue

3D Printing has become a mainstay of industry, with several applications in the medical field. One area that could benefit from 3D printing is intestinal failure due to injury or genetic malformations. We bioprinted cylindrical tubes from rat vascular cells that were sized to form biopatches. 2 mm enterotomies were made in the small intestine of male Sprague-Dawley rats, and sealed with biopatches. These intestinal segments were connected to an ex vivo perfusion device that provided independent extraluminal and intraluminal perfusion. The fluorescence signal of fluorescein isothiocyanate (FITC)-inulin in the intraluminal perfusate, a non-absorbable fluorescent marker of intestinal integrity, was measured every 15 min over 90 min, and used to assess the integrity of the segments under both continuous perfusion and alternate-flow perfusion. Enterotomies were made an inch away from the ileocecal junction in male Wistar rats and sealed with biopatches. The animals were monitored daily and euthanized at post-operative days 7, 14, 21, and 30. Blinded histopathological analysis was conducted to compare the patch segments to native intestine. Biopatch-sealed intestinal segments withstood both continuous and pulsatile flow rates without leakage of FITC-inulin above the control baseline. 21 of 26 animals survived with normal activity, weight gain, and stool output. Histopathology of the explanted segments showed progressive villi and crypt formation over the enterotomies, with complete restoration of the epithelium by 30 days. This study presents a novel application of 3D bioprinting to develop a universal repair patch that can seal lesions in vivo, and fully integrate into the native intestine.

R-Spondin 1 (RSPO1) Increases Mouse Intestinal Organoid Unit Size and Survival in vitro and Improves Tissue-Engineered Small Intestine Formation in vivo

Introduction: Cell therapy and tissue engineering has recently emerged as a new option for short bowel syndrome (SBS) treatment, generating tissue engineered small intestine (TESI) from organoid units (OU) and biodegradable scaffolds. The recombinant human R-Spondin 1 (rhRSPO1) protein may be a key player in this process due to its mitogenic activity in intestinal stem cells.

Objective: Aiming at optimizing the TESI formation process and advancing this technology closer to the clinic, we evaluated the effects of rhRSPO1 protein on OU culture and TESI formation.

Methods: Intestinal OU were isolated from C57BL/6 mice and cultured in Matrigel in the presence or absence of recombinant human rhRSPO1. Throughout the culture, OU growth and survival rates were evaluated, and cells were harvested on day 3. OU were seeded onto biodegradable scaffolds, in the presence or absence of 5 μg of rhRSPO1 and implanted into the omentum of NOD/SCID mice in order to generate TESI. The explants were harvested after 30 days, weighed, fixed in formalin and embedded in paraffin for histological analysis and immunofluorescence for different cell markers.

Results: After 3 days, rhRSPO1-treated OU attained a larger size, when compared to the control group, becoming 5.7 times larger on day 6. Increased survival was observed from the second day in culture, with a 2-fold increase in OU survival between days 3 and 6. A 4.8-fold increase of non-phosphorylated β-catenin and increased relative expression of Lgr5 mRNA in the rhRSPO1-treated group confirms activation of the canonical Wnt pathway and suggests maintenance of the OU stem cell niche and associated stemness. After 30 days of in vivo maturation, rhRSPO1-treated TESI presented a larger mass than constructs treated with saline, developing a more mature intestinal epithelium with well-formed villi and crypts. In addition, the efficiency of OU-loaded rhRSPO1-treated scaffolds significantly increased, forming TESI in 100% of the samples (N = 8), of which 40% presented maximum degree of development, as compared to 66.6% in the control group (N = 9).

Conclusion: rhRSPO1 treatment improves the culture of mouse intestinal OU, increasing its size and survival in vitro, and TESI formation in vivo, increasing its mass, degree of development and engraftment.

Repair and regeneration of small intestine: A review of current engineering approaches

The small intestine (SI) is difficult to regenerate or reconstruct due to its complex structure and functions. Recent developments in stem cell research, advanced engineering technologies, and regenerative medicine strategies bring new hope of solving clinical problems of the SI. This review will first summarize the structure, function, development, cell types, and matrix components of the SI. Then, the major cell sources for SI regeneration are introduced, and state-of-the-art biofabrication technologies for generating engineered SI tissues or models are overviewed. Furthermore, in vitro models and in vivo transplantation, based on intestinal organoids and tissue engineering, are highlighted. Finally, current challenges and future perspectives are discussed to help direct future applications for SI repair and regeneration.

Development of Intestinal Scaffolds that Mimic Native Mammalian Intestinal Tissue

IMPACT STATEMENT

This study is significant because it demonstrates an attempt to design a scaffold specifically for small intestine using a novel fabrication method, resulting in an architecture that resembles intestinal villi. In addition, we use the versatile polymer poly(glycerol sebacate) (PGS) for artificial intestine, which has tunable mechanical and degradation properties that can be harnessed for further fine-tuning of scaffold design. Moreover, the utilization of PGS allows for future development of growth factor and drug delivery from the scaffolds to promote artificial intestine formation.

Gastrointestinal Dysmotility in MNGIE: from thymidine phosphorylase enzyme deficiency to altered interstitial cells of Cajal

Background

MNGIE is a rare and fatal disease in which absence of the enzyme thymidine phosphorylase induces systemic accumulation of thymidine and deoxyuridine and secondary mitochondrial DNA alterations. Gastrointestinal (GI) symptoms are frequently reported in MNGIE patients, however, they are not resolved with the current treatment interventions.

Recently, our understanding of the GI pathology has increased, which rationalizes the pursuit of more targeted therapeutic strategies. In particular, interstitial cells of Cajal (ICC) play key roles in GI physiology and are involved in the pathogenesis of the GI dysmotility. However, understanding of the triggers of ICC deficits in MNGIE is lacking. Herein, we review the current knowledge about the pathology of GI dysmotility in MNGIE, discuss potential mechanisms in relation to ICC loss/dysfunction, remark on the limited contribution of the current treatments, and propose intervention strategies to overcome ICC deficits. Finally, we address the advances and new research avenues offered by organoids and tissue engineering technologies, and propose schemes to implement to further our understanding of the GI pathology and utility in regenerative and personalized medicine in MNGIE.

Conclusion

Interstitial cells of Cajal play key roles in the physiology of the gastrointestinal motility. Evaluation of their status in the GI dysmotility related to MNGIE would be valuable for diagnosis of MNGIE. Understanding the underlying pathological and molecular mechanisms affecting ICC is an asset for the development of targeted prevention and treatment strategies for the GI dysmotility related to MNGIE.

Identifying the Growth Factors for Improving Neointestinal Regeneration in Rats through Transcriptome Analysis Using RNA-Seq Data

Using our novel surgical model of simultaneous intestinal adaptation "A" and neointestinal regeneration "N" conditions in individual rats to determine feasibility for research and clinical application, we further utilized next generation RNA sequencing (RNA-Seq) here in normal control tissue and both conditions ("A" and "N") across time to decipher transcriptome changes in neoregeneration and adaptation of intestinal tissue at weeks 1, 4, and 12. We also performed bioinformatics analyses to identify key growth factors for improving intestinal adaptation and neointestinal regeneration. Our analyses indicate several interesting phenomena. First, Gene Ontology and pathway analyses indicate that cell cycle and DNA replication processes are enhanced in week 1 "A"; however, in week 1 "N", many immune-related processes are involved. Second, we found some growth factors upregulated or downregulated especially in week 1 "N" versus "A". Third, based on each condition and time point versus normal control tissue, we found in week 1 "N" BMP2, BMP3, and NTF3 are significantly and specifically downregulated, indicating that the regenerative process may be inhibited in the absence of these growth factors. This study reveals complex growth factor regulation in small neointestinal regeneration and intestinal adaptation and provides potential applications in tissue engineering by introducing key growth factors identified here into the injury site.

Short-term and long-term human or mouse organoid units generate tissue-engineered small intestine without added signalling molecules

NEW FINDINGS

What is the central question of this study? Tissue-engineered small intestine was previously generated in vivo by immediate implantation of organoid units derived from both mouse and human donor intestine. Although immediate transplantation of organoid units into patients shows promise as a potential future therapy, some critically ill patients might require delayed transplantation. What is the main finding and its importance? Unlike enteroids, which consist of isolated intestinal crypts, short- and long-term cultured organoid units are composed of epithelial and mesenchymal cells derived from mouse or human intestine. Organoid units do not require added signalling molecules and can generate tissue-engineered intestine in vivo.

ABSTRACT

Mouse and human postnatal and fetal organoid units (OUs) maintained in either short-term culture (2 weeks) or long-term culture (from 4 weeks up to 3 months) without adding exogenous growth factors were implanted in immunocompromised mice to form tissue-engineered small intestine (TESI) in vivo. Intestinal epithelial stem and neuronal progenitor cells were maintained in long-term OU cultures from both humans and mice without exogenous growth factors, and these cultures were successfully used to form TESI. This was enhanced with OUs derived from human fetal tissues. Organoid unit culture is different from enteroid culture, which is limited to epithelial cell growth and requires supplementation with R-Spondin, noggin and epidermal growth factor. Organoid units contain multiple cell types, including epithelial, mesenchymal and enteric nervous system cells. Short- and long-term cultured OUs derived from mouse and human intestine develop into TESI in vivo, which contains key components of the small intestine similar to native intestine.

Reinforced Electrospun Polycaprolactone Nanofibers for Tracheal Repair in an In Vivo Ovine Model

Tracheal stenosis caused by congenital anomalies, tumors, trauma, or intubation-related damage can cause severe breathing issues, diminishing the quality of life, and potentially becoming fatal. Current treatment methods include laryngotracheal reconstruction or slide tracheoplasty. Laryngotracheal reconstruction utilizes rib cartilage harvested from the patient, requiring a second surgical site. Slide tracheoplasty involves a complex surgical procedure to splay open the trachea and reconnect both segments to widen the lumen. A clear need exists for new and innovative approaches that can be easily adopted by surgeons, and to avoid harvesting autologous tissue from the patient. This study evaluated the use of an electrospun patch, consisting of randomly layered polycaprolactone (PCL) nanofibers enveloping 3D-printed PCL rings, to create a mechanically robust, suturable, air-tight, and bioresorbable graft for the treatment of tracheal defects. The study design incorporated two distinct uses of PCL: electrospun fibers to promote tissue integration, while remaining air-tight when wet, and 3D-printed rings to hold the airway open and provide external support and protection during the healing process. Electrospun, reinforced tracheal patches were evaluated in an ovine model, in which all sheep survived for 10 weeks, although an overgrowth of fibrous tissue surrounding the patch was observed to significantly narrow the airway. Minimal tissue integration of the surrounding tissue and the electrospun fibers suggested the need for further improvement. Potential areas for further improvement include a faster degradation rate, agents to increase cellular adhesion, and/or an antibacterial coating to reduce the initial bacterial load.

Organoids for Drug Discovery and Personalized Medicine

A wide variety of organs are in a dynamic state, continuously undergoing renewal as a result of constant growth and differentiation. Stem cells are required during these dynamic events for continuous tissue maintenance within the organs. In a steady state of production and loss of cells within these tissues, new cells are constantly formed by differentiation from stem cells. Today, organoids derived from either adult stem cells or pluripotent stem cells can be grown to resemble various organs. As they are similar to their original organs, organoids hold great promise for use in medical research and the development of new treatments. Furthermore, they have already been utilized in the clinic, enabling personalized medicine for inflammatory bowel disease. In this review, I provide an update on current organoid technology and summarize the application of organoids in basic research, disease modeling, drug development, personalized treatment, and regenerative medicine.

Comparison of Different In Vivo Incubation Sites to Produce Tissue-Engineered Small Intestine

OBJECTIVE

The objective of this study was to compare the impact of different in vivo incubation sites on the production of tissue-engineered small intestine (TESI).

MATERIALS AND METHODS

Green fluorescent protein transgenic rat pups (3-5 days) were used as donors of intestinal organoids. Harvested intestine was exposed to enzymatic digestion to release intestinal stem cell-containing organoids. Organoids were purified, concentrated, and seeded onto tubular polyglycolic acid scaffolds. Seeded scaffolds were implanted in each of five locations in recipient female nude rats: wrapped with omentum, wrapped with intestinal mesentery, wrapped with uterine horn membrane, attached to the abdominal wall, and inserted into the subcutaneous space. After 4 weeks of in vivo incubation, specimens from each site were explanted for evaluation.

RESULTS

Wrapping seeded scaffolds with vascularized membranes produced TESI with variable lengths of vascularized pedicles, with the longest pedicle length from uterine horn membrane, the shortest pedicle length from intestinal mesentery, and intermediate length from omentum. The quantity of TESI, as expressed by volume and neomucosal length, was identical in TESI produced by wrapping with any of the three membranes. The smallest quantity of TESI was found in TESI produced from insertion into the subcutaneous space, with an intermediate quantity of TESI produced from attachment to the abdominal wall. Periodic acid-Schiff and immunofluorescence (IF) staining confirmed the presence of all intestinal epithelial cell lineages in TESI produced at all incubation sites. Additional IF staining demonstrated the presence of enteric nervous system components and blood vessels. Wrapping of seeded scaffolds with vascularized membranes significantly increased the density of blood vessels in the TESI produced.

CONCLUSION

Wrapping of seeded scaffolds in vascularized membranes produced the largest quantity and highest quality of TESI. Attaching seeded scaffolds to the abdominal wall produced an intermediate quantity of TESI, but the quality was still comparable to TESI produced in vascularized membranes. Insertion of seeded scaffolds into the subcutaneous space produced the smallest quantity and lowest quality of TESI. In summary, wrapping seeded scaffolds with vascularized membranes is favorable for the production of TESI, and wrapping with omentum may produce TESI that is most easily anastomosed with host intestine.

Critical intestinal cells originate from the host in enteroid-derived tissue-engineered intestine

BACKGROUND

Enteroid-derived tissue-engineered intestine (TEI) contains intestinal subepithelial myofibroblasts (ISEMFs) and smooth muscle cells (SMCs). However, these cell types are not present in the donor enteroids. We sought to determine the origin of these cell types and to quantify their importance in TEI development.

MATERIALS AND METHODS

Crypts from pan-EGFP or LGR5-EGFP mice were used for enteroid culture and subsequent implantation for the production of TEI. TEI from pan-EGFP enteroids was labeled for smooth muscle alpha actin (SMA) to identify ISEMFs and SMCs and green fluorescent protein (GFP) to identify cells from pan-EGFP enteroids. Fluorescence in situ hybridization (FISH) for the Y chromosome was applied to TEI from male LGR5-EGFP enteroids implanted into female nonobese diabetic/severe combined immunodeficiency mice. To identify chemotactic effects of intestinal epithelium on ISEMFs, a Boyden chamber assay was performed.

RESULTS

Immunofluorescence of TEI from pan-EGFP enteroids revealed GFP-positive epithelium with surrounding SMA positivity and no colocalization of the two. FISH of TEI from male LGR5-EGFP enteroids implanted into female nonobese diabetic/severe combined immunodeficiency mice revealed that only the epithelium was Y chromosome positive. Chemotactic assays demonstrated increased ISEMF migration in the presence of enteroids (983 ± 133) compared to that in the presence of either Matrigel alone (357 ± 36) or media alone (339 ± 24; P ≤ 0.05).

CONCLUSIONS

Lack of GFP/SMA colocalization suggests that ISEMFs and SMCs are derived from host animals. This was confirmed by FISH which identified only epithelial cells as being male. All other cell types originated from host animals. The mechanism by which these cells are recruited is unknown; however, Boyden chamber assays indicate a direct chemotactic effect of intestinal epithelium on ISEMFs.

Tissue engineering for the treatment of short bowel syndrome in children

Short bowel syndrome is a major cause of morbidity and mortality in children. Despite decades of experience in the management of short bowel syndrome, current therapy is primarily supportive. Definitive treatment often requires intestinal transplantation, which is associated with significant morbidity and mortality. In order to develop novel approaches to the treatment of short bowel syndrome, we and others have focused on the development of an artificial intestine, by placing intestinal stem cells on a bioscaffold that has an absorptive surface resembling native intestine, and taking advantage of neovascularization to develop a blood supply. This review will explore recent advances in biomaterials, vascularization, and progress toward development of a functional epithelium and mesenchymal niche, highlighting both success and ongoing challenges in the field.

Regenerative medicine for the esophagus

Advances in tissue engineering techniques have made it possible to use human cells as biological material. This has enabled pharmacological studies to be conducted to investigate drug effects and toxicity, to clarify the mechanisms underlying diseases, and to elucidate how they compensate for impaired organ function. Many researchers have tried to construct artificial organs using these techniques, but none has succeeded in growing a whole organ. Unlike other digestive organs with complicated functions, such as the processing and absorption of nutrients, the esophagus has the relatively simple function of transporting content, which can be replicated easily by a substitute. In regenerative medicine, various combinations of materials have been applied, including scaffolding, cell sources, and bioreactors. Exciting results of tissue engineering techniques for the esophagus have been reported. In animal models, replacing full-thickness and full-circumferential defects remains challenging because of stenosis and leakage after implantation. Although many reports have manipulated various scaffolds, most have emphasized the importance of both epithelial and mesenchymal cells for the prevention of stenosis. However, the results of repair of partial full-thickness defects and mucosal defects have been promising. Two successful approaches for the replacement of mucosal defects in a clinical setting have been reported, although in contrast to the many animal models, there are few pilot studies in humans. We review the recent results and evaluate the future of regenerative medicine for the esophagus.

In vitro enteroid-derived three-dimensional tissue model of human small intestinal epithelium with innate immune responses

There is a need for functional in vitro 3D human intestine systems that can bridge the gap between conventional cell culture studies and human trials. The successful engineering in vitro of human intestinal tissues relies on the use of the appropriate cell sources, biomimetic scaffolds, and 3D culture conditions to support vital organ functions. We previously established a compartmentalized scaffold consisting of a hollow space within a porous bulk matrix, in which a functional and physiologically relevant intestinal epithelium system was generated using intestinal cell lines. In this study, we adopt the 3D scaffold system for the cultivation of stem cell-derived human small intestinal enteriods (HIEs) to engineer an in vitro 3D model of a nonstransformed human small intestinal epithelium. Characterization of tissue properties revealed a mature HIE-derived epithelium displaying four major terminally differentiated epithelial cell types (enterocytes, Goblet cells, Paneth cells, enteroendocrine cells), with tight junction formation, microvilli polarization, digestive enzyme secretion, and low oxygen tension in the lumen. Moreover, the tissue model demonstrates significant antibacterial responses to E. coli infection, as evidenced by the significant upregulation of genes involved in the innate immune response. Importantly, many of these genes are activated in human patients with inflammatory bowel disease (IBD), implicating the potential application of the 3D stem-cell derived epithelium for the in vitro study of host-microbe-pathogen interplay and IBD pathogenesis.

Tissue Engineering Functional Gastrointestinal Regions: The Importance of Stem and Progenitor Cells

The intestine shows extraordinary regenerative potential that might be harnessed to alleviate numerous morbid and lethal human diseases. The intestinal stem cells regenerate the epithelium every 5 days throughout an individual's lifetime. Understanding stem-cell signaling affords power to influence the niche environment for growing intestine. The manifold approaches to tissue engineering may be organized by variations of three basic components required for the transplantation and growth of stem/progenitor cells: (1) cell delivery materials or scaffolds; (2) donor cells including adult stem cells, induced pluripotent stem cells, and in vitro expansion of isolated or cocultured epithelial, smooth muscle, myofibroblasts, or nerve cells; and (3) environmental modulators or biopharmaceuticals. Tissue engineering has been applied to the regeneration of every major region of the gastrointestinal tract from esophagus to colon, with scientists around the world aiming to carry these techniques into human therapy.

In Vivo Model of Small Intestine

The utilization of human pluripotent stem cells (hPSCs) offers new avenues in the generation of organs and opportunities to understand development and diseases. The hPSC-derived human intestinal organoids (HIOs) provide a new tool to gain insights in small intestinal development, physiology, and associated diseases. Herein, we provide a method for orthotropic transplantation of HIOs in immunocompromised mice. This method highlights the specific steps to successful engraftment and provides insight into the study of bioengineered human small intestine.

Intestinal Organoids-Current and Future Applications

Recent technical advances in the stem cell field have enabled the in vitro generation of complex structures resembling whole organs termed organoids. Most of these approaches employ culture systems that allow stem cell-derived or tissue progenitor cells to self-organize into three-dimensional (3D)-structures. Since organoids can be grown from different species (human, mouse, cat, dog), organs (intestine, kidney, brain, liver), and from patient-derived induced pluripotent stem cells, they create significant prospects for modelling development and diseases, for toxicology and drug discovery studies, and in the field of regenerative medicine. Here, we report on intestinal stem cells, organoid culture, organoid disease modeling, transplantation, specifically covering the current and future uses of this exciting new insight model to the field of veterinary medicine.

Drug Discovery via Human-Derived Stem Cell Organoids

Patient-derived cell lines and animal models have proven invaluable for the understanding of human intestinal diseases and for drug development although both inherently comprise disadvantages and caveats. Many genetically determined intestinal diseases occur in specific tissue microenvironments that are not adequately modeled by monolayer cell culture. Likewise, animal models incompletely recapitulate the complex pathologies of intestinal diseases of humans and fall short in predicting the effects of candidate drugs. Patient-derived stem cell organoids are new and effective models for the development of novel targeted therapies. With the use of intestinal organoids from patients with inherited diseases, the potency and toxicity of drug candidates can be evaluated better. Moreover, owing to the novel clustered regularly interspaced short palindromic repeats/CRISPR-associated protein-9 genome-editing technologies, researchers can use organoids to precisely modulate human genetic status and identify pathogenesis-related genes of intestinal diseases. Therefore, here we discuss how patient-derived organoids should be grown and how advanced genome-editing tools may be applied to research on modeling of cancer and infectious diseases. We also highlight practical applications of organoids ranging from basic studies to drug screening and precision medicine.

Generation of an artificial intestine for the management of short bowel syndrome

PURPOSE OF REVIEW

This article discusses the current state of the art in artificial intestine generation in the treatment of short bowel syndrome.

RECENT FINDINGS

Short bowel syndrome defines the condition in which patients lack sufficient intestinal length to allow for adequate absorption of nutrition and fluids, and thus need parenteral support. Advances toward the development of an artificial intestine have improved dramatically since the first attempts in the 1980s, and the last decade has seen significant advances in understanding the intestinal stem cell niche, the growth of complex primary intestinal stem cells in culture, and fabrication of the biomaterials that can support the growth and differentiation of these stem cells. There has also been recent progress in understanding the role of the microbiota and the immune cells on the growth of intestinal cultures on scaffolds in animal models. Despite recent progress, there is much work to be done before the development of a functional artificial intestine for short bowel syndrome is successfully achieved.

SUMMARY

Continued concerted efforts by cell biologists, bioengineers, and clinician-scientists will be required for the development of an artificial intestine as a clinical treatment modality for short bowel syndrome.

Three-dimensional Printing in the Intestine

Intestinal transplantation remains a life-saving option for patients with severe intestinal failure. With the advent of advanced tissue engineering techniques, great strides have been made toward manufacturing replacement tissues and organs, including the intestine, which aim to avoid transplant-related complications. The current paradigm is to seed a biocompatible support material (scaffold) with a desired cell population to generate viable replacement tissue. Although this technique has now been extended by the three-dimensional (3D) printing of geometrically complex scaffolds, the overall approach is hindered by relatively slow turnover and negative effects of residual scaffold material, which affects final clinical outcome. Methods recently developed for scaffold-free 3D bioprinting may overcome such obstacles and should allow for rapid manufacture and deployment of "bioprinted organs." Much work remains before 3D bioprinted tissues can enter clinical use. In this brief review we examine the present state and future perspectives of this nascent technology before full clinical implementation.

Generation of intestinal surface: an absorbing tale

The vertebrate small intestine requires an enormous surface area to effectively absorb nutrients from food. Morphological adaptations required to establish this extensive surface include generation of an extremely long tube and convolution of the absorptive surface of the tube into villi and microvilli. In this Review, we discuss recent findings regarding the morphogenetic and molecular processes required for intestinal tube elongation and surface convolution, examine shared and unique aspects of these processes in different species, relate these processes to known human maladies that compromise absorptive function and highlight important questions for future research.

GI stem cells - new insights into roles in physiology and pathophysiology

This overview gives a brief historical summary of key discoveries regarding stem cells of the small intestine. The current concept is that there are two pools of intestinal stem cells (ISCs): an actively cycling pool that is marked by Lgr5, is relatively homogeneous and is responsible for daily turnover of the epithelium; and a slowly cycling or quiescent pool that functions as reserve ISCs. The latter pool appears to be quite heterogeneous and may include partially differentiated epithelial lineages that can reacquire stem cell characteristics following injury to the intestine. Markers and methods of isolation for active and quiescent ISC populations are described as well as the numerous important advances that have been made in approaches to the in vitro culture of ISCs and crypts. Factors regulating ISC biology are briefly summarized and both known and unknown aspects of the ISC niche are discussed. Although most of our current knowledge regarding ISC physiology and pathophysiology has come from studies with mice, recent work with human tissue highlights the potential translational applications arising from this field of research. Many of these topics are further elaborated in the following articles.

Production of tissue-engineered intestine from expanded enteroids

BACKGROUND

Short bowel syndrome is a life-threatening condition with few solutions. Tissue-engineered intestine (TEI) is a potential treatment, but donor intestine is a limiting factor. Expanded epithelial surrogates termed enteroids may serve as a potential donor source.

MATERIALS AND METHODS

To produce TEI from enteroids, crypts were harvested from mice and enteroid cultures established. Enteroids were seeded onto polymer scaffolds using Matrigel or culture medium and implanted in immunosuppressed mice for 4 wk. Histology was analyzed using Periodic acid-Schiff staining and immunofluorescence. Neomucosa was quantified using ImageJ software. To determine whether TEI could be produced from enteroids established from small intestinal biopsies, 2 × 2-mm pieces of jejunum were processed for enteroid culture, enteroids were expanded and seeded onto scaffolds, and scaffolds implanted for 4 wk.

RESULTS

Enteroids in Matrigel produced TEI in 15 of 15 scaffolds, whereas enteroids in medium produced TEI in 9 of 15 scaffolds. Use of Matrigel led to more neomucosal surface area compared to media (10,520 ± 2905 μm versus 450 ± 127 μm, P < 0.05). Histologic examination confirmed the presence of crypts and blunted villi, normal intestinal epithelial lineages, intestinal subepithelial myofibroblasts, and smooth muscle cells. Crypts obtained from biopsies produced an average of 192 ± 71 enteroids. A single passage produced 685 ± 58 enteroids, which was adequate for scaffold seeding. TEI was produced in 8 of 9 scaffolds seeded with expanded enteroids.

CONCLUSIONS

Enteroids can be obtained from minimal starting material, expanded ex vivo, and implanted to produce TEI. This method shows promise as a solution to the limited donor intestine available for TEI production in patients with short bowel syndrome.

Evidence of Absorptive Function in vivo in a Neo-Formed Bio-Artificial Intestinal Segment Using a Rodent Model

A promising therapeutic approach for intestinal failure consists in elongating the intestine with a bio-engineered segment of neo-formed autologous intestine. Using an acellular biologic scaffold (ABS), we, and others, have previously developed an autologous bio-artificial intestinal segment (BIS) that is morphologically similar to normal bowel in rodents. This neo-formed BIS is constructed with the intervention of naïve stem cells that repopulate the scaffold in vivo, and over a period of time, are transformed in different cell populations typical of normal intestinal mucosa. However, no studies are available to demonstrate that such BIS possesses functional absorptive characteristics necessary to render this strategy a possible therapeutic application. The aim of this study was to demonstrate that the BIS generated has functional absorptive capacity. Twenty male August × Copenhagen-Irish (ACI) rats were used for the study. Two-centimeter sections of ABS were transplanted in the anti-mesenteric border of the small bowel. Animals were studied at 4, 8, and 12 weeks post-engraftment. Segments of intestine with preserved vascular supply and containing the BIS were isolated and compared to intestinal segments of same length in sham control animals (n = 10). D-Xylose solution was introduced in the lumen of the intestinal segments and after 2 h, urine and blood were collected to evaluate D-Xylose levels. Quantitative analysis was performed using ELISA. Morphologic, ultrastructural, and indirect functional absorption analyses were also performed. We observed neo-formed intestinal tissue with near-normal mucosa post-implantation as expected from our previously developed model. Functional characteristics such as morphologically normal enterocytes (and other cell types) with presence of brush borders and preserved microvilli by electron microscopy, preserved water, and ion transporters/channels (by aquaporin and cystic fibrosis transmembrane conductance regulator (CFTR)) were also observed. The capacity of BIS containing neo-formed mucosa to increase absorption of d-Xylose in the blood compared to normal intestine was also confirmed. With this study, we demonstrated for the first time that BIS obtained from ABS has functional characteristics of absorption confirming its potential for therapeutic interventions.

Human Enteroids/Colonoids and Intestinal Organoids Functionally Recapitulate Normal Intestinal Physiology and Pathophysiology

Identification of Lgr5 as the intestinal stem cell marker as well as the growth factors necessary to replicate adult intestinal stem cell division has led to the establishment of the methods to generate "indefinite" ex vivo primary intestinal epithelial cultures, termed "mini-intestines." Primary cultures developed from isolated intestinal crypts or stem cells (termed enteroids/colonoids) and from inducible pluripotent stem cells (termed intestinal organoids) are being applied to study human intestinal physiology and pathophysiology with great expectations for translational applications, including regenerative medicine. Here we discuss the physiologic properties of these cultures, their current use in understanding diarrhea-causing host-pathogen interactions, and potential future applications.

Applications of regenerative medicine in organ transplantation

A worldwide shortage of organs for clinical implantation establishes the need to bring forward and test new technologies that will help in solving the problem. The concepts of regenerative medicine hold the potential for augmenting organ function or repairing damaged organ or allowing regeneration of deteriorated organs and tissue. Researchers are exploring possible regenerative medicine applications in organ transplantation so that coming together of the two fields can benefit each other. The present review discusses the strategies that are being implemented to regenerate or bio-engineer human organs for clinical purposes. It also highlights the limitations of the regenerative medicine that needs to be addressed to explore full potential of the field. A web-based research on MEDLINE was done using keywords “regenerative medicine,” “tissue-engineering,” “bio-engineered organs,” “decellularized scaffold” and “three-dimensional printing.” This review screened about 170 articles to get the desired knowledge update.

Protein-engineered scaffolds for in vitro 3D culture of primary adult intestinal organoids

Though in vitro culture of primary intestinal organoids has gained significant momentum in recent years, little has been done to investigate the impact of microenvironmental cues provided by the encapsulating matrix on the growth and development of these fragile cultures. In this work, the impact of various in vitro culture parameters on primary adult murine organoid formation and growth are analyzed with a focus on matrix properties and geometric culture configuration. The air-liquid interface culture configuration was found to result in enhanced organoid formation relative to a traditional submerged configuration. Additionally, through use of a recombinantly engineered extracellular matrix (eECM), the effects of biochemical and biomechanical cues were independently studied. Decreasing mechanical stiffness and increasing cell adhesivity were found to increase organoid yield. Tuning of eECM properties was used to obtain organoid formation efficiency values identical to those observed in naturally harvested collagen I matrices but within a stiffer construct with improved ease of physical manipulation. Increased ability to remodel the surrounding matrix through mechanical or enzymatic means was also shown to enhance organoid formation. As the engineering and tunability of recombinant matrices is essentially limitless, continued property optimization may result in further improved matrix performance and may help to identify additional microenvironmental cues that directly impact organoid formation, development, differentiation, and functional behavior. Continued culture of primary organoids in recombinant matrices could therefore prove to be largely advantageous in the field of intestinal tissue engineering for applications in regenerative medicine and in vitro tissue mimics.

Epithelial-Mesenchymal Interactions in Urinary Bladder and Small Intestine and How to Apply Them in Tissue Engineering

Reciprocal interactions between the epithelium and mesenchyme are essential for the establishment of proper tissue morphology during organogenesis and tissue regeneration as well as for the maintenance of cell differentiation. With this review, we highlight the importance of epithelial-mesenchymal cross talk in healthy tissue and further discuss its significance in engineering functional tissues in vitro. We focus on the urinary bladder and small intestine, organs that are often compromised by disease and are as such in need of research that would advance effective treatment or tissue replacement. To date, the understanding of epithelial-mesenchymal reciprocal interactions has enabled the development of in vitro biomimetic tissue equivalents that have provided many possibilities in treating defective, damaged, or even cancerous tissues. Although research of the past several years has advanced the field of bladder and small intestine tissue engineering, one must be aware of its current limitations in successfully and above all safely introducing tissue-engineered constructs into clinical practice. Special attention is in particular needed when treating cancerous tissues, as initially successful tumor excision and tissue reconstruction may later on result in cancer recurrence due to oncogenic signals originating from an altered stroma. Recent rather poor outcomes in pioneering clinical trials of bladder reconstructions should serve as a reminder that recreating a functional organ to replace a dysfunctional one is an objective far more difficult to reach than initially foreseen. When considering effective tissue engineering approaches for diseased tissues in humans, it is imperative to introduce animal models with dysfunctional or, even more importantly, cancerous organs, which would greatly contribute to predicting possible complications and, hence, reducing risks when translating to the clinic.

Porcine models of digestive disease: the future of large animal translational research

There is increasing interest in nonrodent translational models for the study of human disease. The pig, in particular, serves as a useful animal model for the study of pathophysiological conditions relevant to the human intestine. This review assesses currently used porcine models of gastrointestinal physiology and disease and provides a rationale for the use of these models for future translational studies. The pig has proven its utility for the study of fundamental disease conditions such as ischemia-reperfusion injury, stress-induced intestinal dysfunction, and short bowel syndrome. Pigs have also shown great promise for the study of intestinal barrier function, surgical tissue manipulation and intervention, as well as biomaterial implantation and tissue transplantation. Advantages of pig models highlighted by these studies include the physiological similarity to human intestine and mechanisms of human disease. Emerging future directions for porcine models of human disease include the fields of transgenics and stem cell biology, with exciting implications for regenerative medicine.

Enriched Intestinal Stem Cell Seeding Improves the Architecture of Tissue-Engineered Intestine

OBJECTIVE

To develop a methodology to separate intestinal stem cell (ISC)-enriched crypts from differentiated epithelial cell (DEC)-containing villi to improve the morphology of tissue-engineered intestine (TEI).

METHODS

Small intestinal tissues from 5- to 7-day-old transgenic Lgr5-EGFP mice (with fluorescently labeled ISCs) were used to measure the height of villi and the depth of crypts. Based on the significant size difference between crypts and villi, a novel cell filtration system was developed. Filtration of mixed organoid units from full-thickness intestine of transgenic Lgr5-EGFP mice allowed determination of the percentage of ISCs in the different size-based filtration fractions obtained. In vivo, 5-7-day-old Lewis rat pups were used as cell donors to obtain purified crypts and villi, and the dams of the pups served as recipients. Flat and tubular polyglycolic acid (PGA) scaffolds were seeded with either ISC-enriched crypts or DEC-containing villi and implanted intra-abdominally on the anterior abdominal wall. After 1, 3, 7, 14, 21, and 28 days of in vivo incubation, explants were processed for histologic evaluation.

RESULTS

Small intestine from transgenic Lgr5-EGFP mice contained villi with an average height of 134.89±41.91 μm and crypts with an average depth of 49.59±8.95 μm. After filtration, we found that the 100-200 μm fractions contained relatively pure villi in which DECs were located, whereas the 25-70 μm range fractions contained concentrated crypts in which ISCs were located. In vivo, flat PGA scaffolds implanted with purified crypts formed well-developed mucosa by day 14 postimplantation, whereas flat scaffolds seeded with villi were replaced with fibrous tissue. Tubular scaffolds seeded with the crypt fraction developed a well-formed mucosal layer on the interior surface, with 80.9% circumferential mucosal engraftment and an average villous height of 478±65 μm, which was very close to native intestine (512±98 μm), whereas tubular scaffolds seeded with the villous fraction only had 21.7% circumferential mucosal engraftment and an average villous height of 243±78 μm.

CONCLUSION

The novel filtration system described can effectively and efficiently isolate ISC-containing crypts. TEI produced from ISC-containing crypts has an improved morphology that is similar to native intestine.

Current aspects and future prospects of total anorectal reconstruction--a critical and comprehensive review of the literature

PURPOSE

Many rectal cancer patients undergo abdominoperineal excision worldwide every year. Various procedures to restore perineal (pseudo-) continence, referred to as total anorectal reconstruction, have been proposed. The best technique, however, has not yet been defined. In this study, the different reconstruction techniques with regard to morbidity, functional outcome and quality of life were analysed. Technical and timing issues (i.e. whether the definitive procedure should be performed synchronously or be delayed), oncological safety, economical aspects as well as possible future improvements are further discussed.

METHODS

A MEDLINE and EMBASE search was conducted to identify the pertinent multilingual literature between 1989 and 2013. All publications meeting the defined inclusion/exclusion criteria were eligible for analysis.

RESULTS

Dynamic graciloplasty, artificial bowel sphincter, circular smooth muscle cuff or gluteoplasty result in median resting and squeezing neo-anal pressures that equate to the measurements found in incontinent patients. However, quality of life was generally stated to be good by patients who had undergone the procedures, despite imperfect continence, faecal evacuation problems and a considerable associated morbidity. Many patients developed an alternative perception for the urge to defecate that decisively improved functional outcome. Theoretical calculations suggested cost-effectiveness of total anorectal reconstruction compared well to life with a permanent colostomy.

CONCLUSIONS

Many patients would be highly motivated to have their abdominal replaced by a functional perineal colostomy. Given the considerable morbidity and questionable functional outcome of current reconstruction technique improvements are required. Tissue engineering might be an option to design an anatomically and physiologically matured, and customised continence organ.

Animal models of ischemia-reperfusion-induced intestinal injury: progress and promise for translational research

Research in the field of ischemia-reperfusion injury continues to be plagued by the inability to translate research findings to clinically useful therapies. This may in part relate to the complexity of disease processes that result in intestinal ischemia but may also result from inappropriate research model selection. Research animal models have been integral to the study of ischemia-reperfusion-induced intestinal injury. However, the clinical conditions that compromise intestinal blood flow in clinical patients ranges widely from primary intestinal disease to processes secondary to distant organ failure and generalized systemic disease. Thus models that closely resemble human pathology in clinical conditions as disparate as volvulus, shock, and necrotizing enterocolitis are likely to give the greatest opportunity to understand mechanisms of ischemia that may ultimately translate to patient care. Furthermore, conditions that result in varying levels of ischemia may be further complicated by the reperfusion of blood to tissues that, in some cases, further exacerbates injury. This review assesses animal models of ischemia-reperfusion injury as well as the knowledge that has been derived from each to aid selection of appropriate research models. In addition, a discussion of the future of intestinal ischemia-reperfusion research is provided to place some context on the areas likely to provide the greatest benefit from continued research of ischemia-reperfusion injury.

Animal models of gastrointestinal and liver diseases. Animal models of infant short bowel syndrome: translational relevance and challenges

Intestinal failure (IF), due to short bowel syndrome (SBS), results from surgical resection of a major portion of the intestine, leading to reduced nutrient absorption and need for parenteral nutrition (PN). The incidence is highest in infants and relates to preterm birth, necrotizing enterocolitis, atresia, gastroschisis, volvulus, and aganglionosis. Patient outcomes have improved, but there is a need to develop new therapies for SBS and to understand intestinal adaptation after different diseases, resection types, and nutritional and pharmacological interventions. Animal studies are needed to carefully evaluate the cellular mechanisms, safety, and translational relevance of new procedures. Distal intestinal resection, without a functioning colon, results in the most severe complications and adaptation may depend on the age at resection (preterm, term, young, adult). Clinically relevant therapies have recently been suggested from studies in preterm and term PN-dependent SBS piglets, with or without a functional colon. Studies in rats and mice have specifically addressed the fundamental physiological processes underlying adaptation at the cellular level, such as regulation of mucosal proliferation, apoptosis, transport, and digestive enzyme expression, and easily allow exogenous or genetic manipulation of growth factors and their receptors (e.g., glucagon-like peptide 2, growth hormone, insulin-like growth factor 1, epidermal growth factor, keratinocyte growth factor). The greater size of rats, and especially young pigs, is an advantage for testing surgical procedures and nutritional interventions (e.g., PN, milk diets, long-/short-chain lipids, pre- and probiotics). Conversely, newborn pigs (preterm or term) and weanling rats provide better insights into the developmental aspects of treatment for SBS in infants owing to their immature intestines. The review shows that a balance among practical, economical, experimental, and ethical constraints will determine the choice of SBS model for each clinical or basic research question.

Citrulline levels following proximal versus distal small bowel resection

PURPOSE

Citrulline, a nonprotein amino acid synthesized by enterocytes, is a biomarker of bowel length and the capacity to wean from parenteral nutrition. However, the potentially variant effect of jejunal versus ileal excision on plasma citrulline concentration [CIT] has not been studied. This investigation compared serial serum [CIT] and mucosal adaptive potential after proximal versus distal small bowel resection.

METHODS

Enterally fed Sprague-Dawley rats underwent sham operation or 50% small bowel resection, either proximal (PR) or distal (DR). [CIT] was measured at operation and weekly for 8 weeks. At necropsy, histologic features reflecting bowel adaptation were evaluated.

RESULTS

By weeks 6-7, [CIT] in both resection groups significantly decreased from baseline (P<0.05) and was significantly lower than the concentration in sham animals (P<0.05). There was no difference in [CIT] between PR and DR at any point. Villus height and crypt density were higher in the PR than in the DR group (P≤0.02).

CONCLUSION

[CIT] effectively differentiates animals undergoing major bowel resection from those with preserved intestinal length. The region of intestinal resection was not a determinant of [CIT]. The remaining bowel in the PR group demonstrated greater adaptive potential histologically. [CIT] is a robust biomarker for intestinal length, irrespective of location of small intestine lost.

Synthetic small intestinal scaffolds for improved studies of intestinal differentiation

In vitro intestinal models can provide new insights into small intestinal function, including cellular growth and proliferation mechanisms, drug absorption capabilities, and host-microbial interactions. These models are typically formed with cells cultured on 2D scaffolds or transwell inserts, but it is widely understood that epithelial cells cultured in 3D environments exhibit different phenotypes that are more reflective of native tissue. Our focus was to develop a porous, synthetic 3D tissue scaffold with villous features that could support the culture of epithelial cell types to mimic the natural microenvironment of the small intestine. We demonstrated that our scaffold could support the co-culture of Caco-2 cells with a mucus-producing cell line, HT29-MTX, as well as small intestinal crypts from mice for extended periods. By recreating the surface topography with accurately sized intestinal villi, we enable cellular differentiation along the villous axis in a similar manner to native intestines. In addition, we show that the biochemical microenvironments of the intestine can be further simulated via a combination of apical and basolateral feeding of intestinal cell types cultured on the 3D models.