Intestinal tissue engineering: from regenerative medicine to model systems

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.

In Vitro Intestinal and Liver Models for Toxicity Testing

The human body is exposed to hundreds of chemicals every day. Many of these toxicants have unknown effects on the body that can be deleterious. Furthermore, chemicals can have a synergistic effect, resulting in toxic responses of cocktails at relatively low individual exposure levels. The gastrointestinal (GI) tract and the liver are the first organs to be exposed to ingested pharmaceuticals and environmental chemicals. As a result, these organs often experience extensive damage from xenobiotics and their metabolites. In vitro models offer a promising method for testing toxic effects. Many advanced in vitro models have been developed for GI and liver toxicity. These models strive to recapitulate the in vivo organ architecture to more accurately model chemical toxicity. In this review, we discuss many of these advances, in addition to recent efforts to integrate the GI and the liver in vitro for a more holistic toxicity model.

Intestinal stem cells and stem cell-based therapy for intestinal diseases

Currently, many gastrointestinal diseases are a major reason for the increased mortality rate of children and adults every year. Additionally, these patients may cope with the high cost of the parenteral nutrition (PN), which aids in the long-term survival of the patients. Other treatment options include surgical lengthening, which is not sufficient in many cases, and intestinal transplantation. However, intestinal transplantation is still accompanied by many challenges, including immune rejection and donor availability, which may limit the transplant's success. The development of more safe and promising alternative treatments for intestinal diseases is still ongoing. Stem cell-based therapy (SCT) and tissue engineering (TE) appear to be the next promising choices for the regeneration of the damaged intestine. However, suitable stem cell source is required for the SCT and TE process. Thus, in this review we discuss how intestinal stem cells (ISCs) are a promising cell source for small intestine diseases. We will also discuss the different markers were used to identify ISCs. Moreover, we discuss the dominant Wnt signaling pathway in the ISC niche and its involvement in some intestinal diseases. Additionally, we discuss ISC culture and expansion, which are critical to providing enough cells for SCT and TE. Finally, we conclude and recommend that ISC isolation, culture and expansion should be considered when SCT is a treatment option for intestinal disorders. Therefore, we believe that ISCs should be considered a cell source for SCT for many gastrointestinal diseases and should be highlighted in future clinical applications.

Physiological intestinal oxygen modulates the Caco-2 cell model and increases sensitivity to the phytocannabinoid cannabidiol

The Caco-2 cell model is widely used as a model of colon cancer and small intestinal epithelium but, like most cell models, is cultured in atmospheric oxygen conditions (∼21%). This does not reflect the physiological oxygen range found in the colon. In this study, we investigated the effect of adapting the Caco-2 cell line to routine culturing in a physiological oxygen (5%) environment. Under these conditions, cells maintain a number of key characteristics of the Caco-2 model, such as increased formation of tight junctions and alkaline phosphatase expression over the differentiation period and maintenance of barrier function. However, these cells exhibit differential oxidative metabolism, proliferate less and become larger during differentiation. In addition, these cells were more sensitive to cannabidiol-induced antiproliferative actions through changes in cellular energetics: from a drop of oxygen consumption rate and loss of mitochondrial membrane integrity in cells treated under atmospheric conditions to an increase in reactive oxygen species in intact mitochondria in cells treated under low-oxygen conditions. Inclusion of an additional physiological parameter, sodium butyrate, into the medium revealed a cannabidiol-induced proliferative response at low doses. These effects could impact on its development as an anticancer therapeutic, but overall, the data supports the principle that culturing cells in microenvironments that more closely mimic the in vivo conditions is important for drug screening and mechanism of action studies.

Gastrointestinal tissue engineering

Tissue engineering is an emerging discipline that combines engineering principles and the biological sciences toward the development of functional replacement tissue. Virtually every tissue in the body has been investigated and tremendous advances have been made in many areas. This article focuses on the gastrointestinal tract and reviews the current status of bioengineering gastrointestinal tissues, including the esophagus, stomach, small intestine and colon. Although progress has been achieved, there continues to be significant challenges that need to be addressed.

Organs-on-a-chip for drug discovery

The current drug discovery process is arduous and costly, and a majority of the drug candidates entering clinical trials fail to make it to the marketplace. The standard static well culture approaches, although useful, do not fully capture the intricate in vivo environment. By merging the advances in microfluidics with microfabrication technologies, novel platforms are being introduced that lead to the creation of organ functions on a single chip. Within these platforms, microengineering enables precise control over the cellular microenvironment, whereas microfluidics provides an ability to perfuse the constructs on a chip and to connect individual sections with each other. This approach results in microsystems that may better represent the in vivo environment. These organ-on-a-chip platforms can be utilized for developing disease models as well as for conducting drug testing studies. In this article, we highlight several key developments in these microscale platforms for drug discovery applications.

Experimental study on reconstruction of segmental mandible defects using tissue engineered bone combined bone marrow stromal cells with three-dimensional tricalcium phosphate

Reconstructive procedures of segmental mandible defects often require bone graft harvesting, which results in donor site morbidity; the use of tissue-engineered bone might mitigate this problem. The aim of the present experimental pilot study was to produce three-dimensional (3D) autologous tissue-engineered constructs that combine autogenous cultivated bone marrow stromal cells with beta-tricalcium phosphate to reconstruct segmental mandible defects without donor site morbidity. Bone marrow stromal cells were isolated from a dog's caput femoris. After differentiation and proliferation in vitro, the cells were seeded into a 3D beta-tricalcium phosphate scaffold. The constructs were incubated under osteogenic culture conditions for 5 days. Segmental defects of 30 mm length were created unilaterally in the mandibles of the animals. Reconstruction was performed using the construct in three dogs and the scaffold only in three dogs as a control group. The specimens were retrieved 3 months later, and the reconstructed areas were processed for gross observation, radiographic examination, 3D computed tomographic (CT) imaging, biomechanical evaluations, and histologic observation. The construct implanted group (n = 3) showed an average height of the reconstructed area of 18.54 mm and the control group 9.16 mm (P < 0.05). Higher radiodensity was present in the construct group than in the control group, as shown by radiograph. 3D CT imaging showed nearly two-thirds absorption of the reconstructed area in the control group. The biomechanical examination of the construct and control groups showed a compression strength of 102.77 N and 42.90 N and stress of 3.504 N/mm and 1.930 N/mm, which demonstrates significant difference. Histologic micrographs showed new bone formation in the scaffolds in central sections of the defects in the construct group 3 months later, with osteoblast seams, osteoclastic resorption, and cartilage formation. The construct of morphologic, 3D beta-tricalcium phosphate scaffold seeded, autologous bone marrow stromal cells ensure bone formation and vascularization throughout the procedure of mandible segmental defect reconstruction, closely resembling how tissue engineering would be used to reconstruct a segmental mandible defect in the clinical setting.

Endocrine Cell and Nerve Regeneration in Autologous In Situ Tissue-Engineered Small Intestine

BACKGROUND

The purpose of this study was to regenerate a larger size of small intestinal tissue than that of our previous study and to evaluate the regeneration of the endocrine cells (ECC) and nerve system of autologous tissue-engineered small intestine. The effect of implantation of large numbers of smooth muscle cells (SMC) for the regeneration of small intestine was also investigated.

METHODS

Two types of scaffolds with different cell densities were fabricated: low density (LD) of SMC in the scaffold and high density (HD) of SMC in the scaffold. Both scaffolds were implanted into defects of isolated ileum in a canine model. Animals were sacrificed at 8, 12, 18, and 24 weeks.

RESULTS

The area of engineered small intestine in the HD group was four times larger than that in the LD group, although that was smaller in size than the original size of the defect. There were no significant changes in the thickness of regenerated smooth muscle layer (SML) in the LD and HD groups. The numbers of endocrine cells gradually increased after implantation. At 18 weeks of regeneration, the number of ECC reached levels comparable to that of normal mucosa. The nerve fibers extended to the center of the graft area and were observed in regenerated SML and regenerated villi at 24 weeks.

CONCLUSIONS

The ECC and nerve fibers were regenerated in autologous in situ tissue-engineered small intestine. Seeding a large number of SMC was not sufficient for the regeneration of the small intestine in a tubular configuration.

To Make a New Intestinal Mucosa

A number of clinical conditions are caused by disorders affecting the mucosal lining of the gastrointestinal tract. Some patients suffer from a loss of mucosal surface area due to congenital defects or due to surgical resections ("short bowel syndrome"). Other patients have inborn or acquired defects of certain mucosal functions (e.g., glucose-galactose malabsorption, bile acid malabsorption). Many patients with these mucosal disorders could be more effectively treated if healthy mucosa were available in larger quantities as a replacement or functional supplement. We therefore developed methods to transplant mucosal stem cells from one part of the intestine to another and to make bioengineered intestinal mucosa. We generated an animal model of bile acid malabsorption using rats that underwent resection of the distal 25% of their small intestine (ileum). This resulted in significant losses of bile acids with the fecal excretions in these animals. We subsequently harvested ileal stem cell clusters from neonatal donors, removed the mucosa from a segment of proximal intestine (jejunum), and implanted the stem cell clusters into the debrided segment of jejunum. After four weeks, the animals had developed a functional "neomucosa." We inserted the "neo-ileal" segment into continuity as a substitute ileum. Postoperative measurements of fecal bile acid excretion showed that we were able to reverse the malabsorption syndrome in this model. This was the first reported neo-mucosa-based treatment of a malabsorption syndrome in vivo. We subsequently studied different biodegradable PGA and PLLA scaffoldings to generate bioengineered intestinal mucosa. We implanted these materials into omentum of rats and were able to identify a PGA/PLLA hybrid material on which engraftment rates of 36% of the available surface area could be achieved. Most recently, we developed a novel technique that permits direct observation of cell-biomaterial interactions after implantation into omentum or intestine in vivo. This method will help to optimize engraftment conditions for stem cell clusters on biomaterials.

Long-Term Culture of Human Urothelial Cells – A Qualitative Analysis

Today, in vitro culturing of autologous cells is an established method in the field of tissue reconstruction. It can be applied to urothelial cells and could have many clinical implications in urological reconstructive surgery. This development calls for quality controls concerning cells used for clinical treatment when cells are autotransplanted back to the patient. We have studied cultured cells in order to detect whether genetic or morphologic changes occur. Urothelial cells isolated from bladder lavage were cultured according to different protocols based on the presence or absence of feeder cells. Genetic studies were performed by means of karyotyping with standard G-banding and interphase fluorescent in situ hybridization (FISH) analyses. The morphology of these epithelial cells was judged as well as immunostaining for epithelial cell markers. In addition, to minimize the risk of feeder cell contamination, proliferation studies were performed on cultures including feeder cells that had been pretreated with different doses of mitomycin or radiation. In initial studies, when using feeder cells in each passage according to standard protocols, urothelial cells proliferated unfavourably after the fourth passage with increasing numbers of mouse cells as well as urothelial tetraploid cells. We could also show that urothelial cells from bladder lavage need feeder cells in order to establish primary cultures. Further propagation up to 14 passages was performed without feeder cells and the urothelial cells retained normal karyotypes. We also found that mitomycin treatment had its main effect on feeder cells during the first 2 h. When feeder cells were irradiated, 20 Gy was effective and no feeder cell contamination was seen. In conclusion, we found that a high standard of quality in urothelial cell culturing can be achieved with a careful culturing technique.