Generation of bladder urothelium from human pluripotent stem cells under chemically defined serum- and feeder-free system

Human Pluripotent Stem Cell Derived Organoids Reveal a Role for WNT Signaling in Dorsal-Ventral Patterning of the Hindgut

The cloaca is a transient structure that forms in the terminal hindgut giving rise to the rectum dorsally and the urogenital sinus ventrally. Similarly, human hindgut cultures derived from human pluripotent stem cells generate human colonic organoids (HCOs) which also contain co-developing urothelial tissue. In this study, our goal was to identify pathways involved in cloacal patterning and apply this to human hindgut cultures. RNA-seq data comparing dorsal versus ventral cloaca in e10.5 mice revealed that WNT signaling was elevated in the ventral versus dorsal cloaca. Inhibition of WNT signaling in hindgut cultures biased their differentiation towards a colorectal fate. WNT activation biased differentiation towards a urothelial fate, giving rise to human urothelial organoids (HUOs). HUOs contained cell types present in human urothelial tissue. Based on our results, we propose a mechanism whereby WNT signaling patterns the ventral cloaca, prior to cloacal septation, to give rise to the urogenital sinus.

Sources, Selection, and Microenvironmental Preconditioning of Cells for Urethral Tissue Engineering

Urethral stricture is a common urinary tract disorder in men that can be caused by iatrogenic causes, trauma, inflammation, or infection and often requires reconstructive surgery. The current therapeutic approach for complex urethral strictures usually involves reconstruction with autologous tissue from the oral mucosa. With the goal of overcoming the lack of sufficient autologous tissue and donor site morbidity, research over the past two decades has focused on cell-based tissue-engineered substitutes. While the main focus has been on autologous cells from the penile tissue, bladder, and oral cavity, stem cells from sources such as adipose tissue and urine are competing candidates for future urethral regeneration due to their ease of collection, high proliferative capacity, maturation potential, and paracrine function. This review addresses the sources, advantages, and limitations of cells for tissue engineering in the urethra and discusses recent approaches to improve cell survival, growth, and differentiation by mimicking the mechanical and biophysical properties of the extracellular environment.

Establishment of an experimental model of normal dog bladder organoid using a three-dimensional culture method

Dog bladder cancer (BC) is mostly muscle-invasive (MI) with poor prognosis, and its pathogenesis is close to human MIBC. Three-dimensional (3D) organoid culture ensures novel knowledge on cancer diseases including BC. Recently, we have established dog BC organoids (BCO) using their urine samples. BCO recapitulated the epithelial structures, characteristics, and drug sensitivity of BC-diseased dogs. However, organoids from dog normal bladder epithelium are not established yet. Therefore, the present study aimed to establish dog normal bladder organoids (NBO) for further understanding the pathogenesis of dog BC and human MIBC. The established NBO underwent various analyzes including cell marker expressions, histopathological structures, cancer-related gene expression patterns, and drug sensitivity. NBO could be produced non-invasively with a continuous culturing and recapitulated the structures and characteristics of the dog's normal bladder mucosal tissues. Different drug sensitivities were observed in each NBO. The analysis of RNA sequencing revealed that several novel genes were changed in NBO compared with BCO. NBO showed a higher expression of p53 and E-cadherin, but a lower expression of MDM2 and Twist1 compared with BCO. These results suggest that NBO could be a promising experimental 3D model for studying the developmental mechanisms of dog BC and human MIBC.

The urothelium: a multi-faceted barrier against a harsh environment

All mucosal surfaces must deal with the challenge of exposure to the outside world. The urothelium is a highly specialized layer of stratified epithelial cells lining the inner surface of the urinary bladder, a gruelling environment involving significant stretch forces, osmotic and hydrostatic pressures, toxic substances, and microbial invasion. The urinary bladder plays an important barrier role and allows the accommodation and expulsion of large volumes of urine without permitting urine components to diffuse across. The urothelium is made up of three cell types, basal, intermediate, and umbrella cells, whose specialized functions aid in the bladder's mission. In this review, we summarize the recent insights into urothelial structure, function, development, regeneration, and in particular the role of umbrella cells in barrier formation and maintenance. We briefly review diseases which involve the bladder and discuss current human urothelial in vitro models as a complement to traditional animal studies.

Current status of organoid culture in urological malignancy

Urological cancers are common malignancies worldwide. Several conventional models, for example, two-dimensional cell culture and animal models have been used for decades to study tumor genetics. Nonetheless, these methods have limitations in reflecting the real tumor microenvironment in vivo, thereby hindering the development of anti-cancer therapeutic agents. Recently, three-dimensional culture models have gained attention because they can overcome the drawbacks of traditional methods. Above all, three-dimensional organoid models are able to mimic the tumor microenvironment in human bodies more closely as they are able to demonstrate the interactions between cells and extracellular matrix. This type of model has therefore extended our understanding of urological cancers. Tumor cells in organoid models can also be co-cultured with other cellular components, such as peripheral blood lymphocytes, and allow further understanding of the effect of tumor microenvironments on tumor growth. Furthermore, organoid models allow a prolonged culturing period, therefore, tumor evolution, progression and maintenance can also be assessed. Organoid models can be derived from each specific patient, and this facilitates investigation of individual cancer-specific mutations and their subtypes. As a result, the development of personalized medication targeting the signaling pathways or biomolecules of interest will be possible. In the present review, we summarize the development and applications of three-dimensional organoid cultures in urological cancers, mainly focusing on prostate, urinary bladder and kidney cancers, and assess the future prospects of this model.

Pluripotent stem cell differentiation as an emerging model to study human prostate development

Prostate development is a complex process, and knowledge about this process is increasingly required for both basic developmental biology studies and clinical prostate cancer research, as prostate tumorigenesis can be regarded as the restoration of development in the adult prostate. Using rodent animal models, scientists have revealed that the development of the prostate is mainly mediated by androgen receptor (AR) signaling and that some other signaling pathways also play indispensable roles. However, there are still many unknowns in human prostate biology, mainly due to the limited availability of proper fetal materials. Here, we first briefly review prostate development with a focus on the AR, WNT, and BMP signaling pathways is necessary for prostate budding/BMP signaling pathways. Based on the current progress in in vitro prostatic differentiation and organoid techniques, we propose human pluripotent stem cells as an emerging model to study human prostate development.

Current and innovative approaches in the treatment of non-muscle invasive bladder cancer: the role of transurethral resection of bladder tumor and organoids

Background Bladder cancer is the 7th most common cancer in men. About 75% of all bladder cancer are non-muscle invasive (NMIBC). The golden standard for definite diagnosis and first-line treatment of NMIBC is transurethral resection of bladder tumour (TURB). Historically, the monopolar current was used first, today bipolar current is preferred by most urologists. Following TURB, depending on the tumour grade, additional intravesical chemo- or/and immunotherapy is indicated, in order to prevent recurrence and need for surgical resection. Development of new technologies, molecular and cell biology, enabled scientists to develop organoids - systems of human cells that are cultivated in the laboratory and have characteristics of the tissue from which they were harvested. In the field of urologic cancers, the organoids are used mainly for studying the course of different diseases, however, in the field of bladder cancer the data are scarce. Conclusions Different currents - monopolar and bipolar, have different effect on urothelium, that is important for oncological results and pathohistological interpretation. Specimens of bladder cancer can be used for preparation of organoids that are further used for studying carcinogenesis. Bladder organoids are step towards personalised medicine, especially for testing effectiveness of chemo-/immunotherapeutics.

Application of Adult and Pluripotent Stem Cells in Interstitial Cystitis/Bladder Pain Syndrome Therapy: Methods and Perspectives

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a multifactorial, chronic disease without definite etiology characterized by bladder-related pelvic pain. IC/BPS is associated with pain that negatively affects the quality of life. There are various therapeutic approaches against IC/BPS. However, no efficient therapeutic agent against IC/BPS has been discovered yet. Urothelium dysfunction is one of the key factors of IC/BPS-related pathogenicity. Stem cells, including adult stem cells (ASCs) and pluripotent stem cells (PSCs), such as embryonic stem cells (ESCs) and induced PSCs (iPSCs), possess the abilities of self-renewal, proliferation, and differentiation into various cell types, including urothelial and other bladder cells. Therefore, stem cells are considered robust candidates for bladder regeneration. This review provides a brief overview of the etiology, pathophysiology, diagnosis, and treatment of IC/BPS as well as a summary of ASCs and PSCs. The potential of ASCs and PSCs in bladder regeneration via differentiation into bladder cells or direct transplantation into the bladder and the possible applications in IC/BPS therapy are described in detail. A better understanding of current studies on stem cells and bladder regeneration will allow further improvement in the approaches of stem cell applications for highly efficient IC/BPS therapy.

Urine as a Main Effector in Urological Tissue Engineering-A Double-Edged Sword

In order to reconstruct injured urinary tract tissues, biodegradable scaffolds with autologous seeded cells are explored in this work. However, when cells are obtained via biopsy from individuals who have damaged organs due to infection, congenital disorders, or cancer, this can result in unhealthy engineered cells and donor site morbidity. Thus, neo-organ construction through an alternative cell source might be useful. Significant advancements in the isolation and utilization of urine-derived stem cells have provided opportunities for this less invasive, limitless, and versatile source of cells to be employed in urologic tissue-engineered replacement. These cells have a high potential to differentiate into urothelial and smooth muscle cells. However, urinary tract reconstruction via tissue engineering is peculiar as it takes place in a milieu of urine that imposes certain risks on the implanted cells and scaffolds as a result of the highly cytotoxic nature of urine and its detrimental effect on both growth and differentiation of these cells. Both of these projections should be tackled thoughtfully when designing a suitable approach for repairing urinary tract defects and applying the needful precautions is vital.

Tissue Engineering and Stem Cell Therapy in Pediatric Urology

The rapidly expanding field of tissue engineering along with stem cell therapy has a promising future in pediatric urological conditions. The initial struggle seemed difficult in renal regeneration but a functional biounit has been developed. Urine excretion has been demonstrated successfully from stem cell-generated embryonic kidneys. Three-dimensional (3D) stem cell-derived organoids are the new paradigm in research. Techniques to regenerate bladder tissue have reached the clinic, and the urethra is close behind. 3D bioprinted urethras would soon be available. Artificial germ cells produced from mouse pluripotent stem cells have been shown to give rise to live progeny. Myoblast and fibroblast therapy has been safely and effectively used for urinary incontinence. Stress urinary incontinence has been clinically treated with muscle-derived stem cells. Skeletal muscle-derived stem cells have been shown to get converted into smooth muscle cells when implanted into the corpora cavernosa in animal models. This review encompasses the various experimental and clinical developments in this field that can benefit pediatric urological conditions with the contemporary developments in the field.

Direct conversion of fibroblasts into urothelial cells that may be recruited to regenerating mucosa of injured urinary bladder

Urothelial cells play essential roles in protection of urine exudation and bacterial invasion at the urothelial mucosa, so that defect or damage of urothelial cells associated with urinary tract diseases may cause serious problems. If a sufficient number of functional urothelial cells are prepared in culture and transplanted into the damaged urothelial lesions, such technology may provide beneficial effects to patients with diseases of the urinary tract. Here we found that human adult dermal fibroblasts were converted into urothelial cells by transducing genes for four transcription factors, FOXA1, TP63, MYCL and KLF4 (FTLK). The directly converted urothelial cells (dUCs) formed cobblestone-like colonies and expressed urothelium-specific markers. dUCs were successfully expanded and enriched after serial passages using a specific medium that we optimized for the cells. The passaged dUCs showed similar genome-wide gene expression profiles to normal urothelial cells and had a barrier function. The FTLK-transduced fibroblasts were also converted into urothelial cells in vivo and recruited to the regenerating urothelial tissue after they were transplanted into the bladder of mice with interstitial cystitis. Our technology may provide a promising solution for a number of patients with urinary tract disorders.

Urethroplasty performed with an autologous urothelium-vegetated collagen fleece to treat urethral stricture in the minipig model

INTRODUCTION AND OBJECTIVE

Tissue-engineered materials in urethral reconstructive surgeries are a promising field for innovative therapy. Collagen matrices increase stability of cell-based implants and can promote viability and proliferation of urothelial cells. In this study, a collagen type I-based cell carrier (CCC) with stratified multi-layer autologous urothelium was used for urethroplasty after induction of urethral stricture in eight minipigs.

MATERIALS AND METHODS

Minipigs underwent surgical procedures to induce urethral stricture by thermocoagulation. Simultaneously, bladder tissue was harvested. Urothelial cells were expanded, labeled with PKH26 and seeded onto CCC in high density. 3 weeks after strictures were induced and verified by urethrography, minipigs underwent urethroplasty using the seeded CCC. Two animals were euthanized after 1, 2, 4, and 24 weeks. Urethras were histologically examined for integration and survival of seeded CCC. In vivo phenotype of multi-layered urothelium matrix constructs was characterized via immunofluorescence staining with pancytokeratin, CK20, p63, E-cadherin and ZO-1.

RESULTS

Seeded CCCs showed excellent stability and suturability after manipulation and application. Transplanted cells were detected using positive PKH26 fluorescence up to 6 months after labeling. Urothelium matrix implants integrated well into the host tissue without sign of inflammation. Animals showed no sign of rejection or stricture recurrence (urethrography) at any time during experimental period. Immunofluorescence analysis confirmed epithelial phenotype, junction formation and differentiation after 2 weeks.

CONCLUSION

CCC can be suitable for urologic reconstructive surgeries and represents a promising option for clinical application. Longer follow-up results are required to exclude re-occurrence of stricture reformation.

Directed differentiation of human induced pluripotent stem cells into mature stratified bladder urothelium

For augmentation or reconstruction of urinary bladder after cystectomy, bladder urothelium derived from human induced pluripotent stem cells (hiPSCs) has recently received focus. However, previous studies have only shown the emergence of cells expressing some urothelial markers among derivatives of hiPSCs, and no report has demonstrated the stratified structure, which is a particularly important attribute of the barrier function of mature bladder urothelium. In present study, we developed a method for the directed differentiation of hiPSCs into mature stratified bladder urothelium. The caudal hindgut, from which the bladder urothelium develops, was predominantly induced via the high-dose administration of CHIR99021 during definitive endoderm induction, and this treatment subsequently increased the expressions of uroplakins. Terminal differentiation, characterized by the increased expression of uroplakins, CK13, and CK20, was induced with the combination of Troglitazone + PD153035. FGF10 enhanced the expression of uroplakins and the stratification of the epithelium, and the transwell culture system further enhanced such stratification. Furthermore, the barrier function of our urothelium was demonstrated by a permeability assay using FITC-dextran. According to an immunohistological analysis, the stratified uroplakin II-positive epithelium was observed in the transwells. This method might be useful in the field of regenerative medicine of the bladder.

Growing a new human kidney

There are 3 reasons to generate a new human kidney. The first is to learn more about the biology of the developing and mature organ. The second is to generate tissues with which to model congenital and acquired kidney diseases. In particular, growing human kidneys in this manner ultimately should help us understand the mechanisms of common chronic kidney diseases such as diabetic nephropathy and others featuring fibrosis, as well as nephrotoxicity. The third reason is to provide functional kidney tissues that can be used directly in regenerative medicine therapies. The second and third reasons to grow new human kidneys are especially compelling given the millions of persons worldwide whose lives depend on a functioning kidney transplant or long-term dialysis, as well as those with end-stage renal disease who die prematurely because they are unable to access these treatments. As shown in this review, the aim to create healthy human kidney tissues has been partially realized. Moreover, the technology shows promise in terms of modeling genetic disease. In contrast, barely the first steps have been taken toward modeling nongenetic chronic kidney diseases or using newly grown human kidney tissue for regenerative medicine therapies.

Differentiation of Urothelium from Mouse Embryonic Stem Cells in Chemically Defined Conditions

The urothelium of the bladder and urethra are derived from the definitive endoderm during development. Cellular signaling molecules important to the developmental specification of the urothelium are also implicated in the dysregulation of the tissue repair mechanism characteristic of bladder disease. Hence, a complete understanding of the regulation of urothelium development is central to understanding the processes of bladder disease, and in development of simple chemically defined methods for use in regenerative medicine. Key to this is a suitable in vitro model that readily allows for the prosecution of biologically pertinent questions. Here a method for differentiating urothelium from mouse embryonic stem cells in chemically defined conditions is described. The method includes a description of flow cytometry and RT-PCR analysis of definitive endoderm markers Cxcr4, c-Kit, and FoxA2, and of terminally differentiated urothelial cell markers Upk1b and Upk2.

Urothelium with barrier function differentiated from human urine-derived stem cells for potential use in urinary tract reconstruction

Background

Autologous urothelial cells are often obtained via bladder biopsy to generate the bio-engineered urethra or bladder, while urine-derived stem cells (USC) can be obtained by a non-invasive approach. The objective of this study is to develop an optimal strategy for urothelium with permeability barrier properties using human USC which could be used for tissue repair in the urinary tract system.

Methods

USC were harvested from six healthy adult individuals. To optimize urothelial differentiation, five different differentiation methods were studied. The induced cells were assessed for gene and protein expression markers of urothelial cells via RT-PCR, Western blotting, and immunofluorescent staining. Barrier function and ultrastructure of the tight junction were assessed with permeability assays and transmission electron microscopy (TEM). Induced cells were both cultured on trans-well membranes and small intestinal submucosa, then investigated under histology analysis.

Results

Differentiated USC expressed significantly higher levels of urothelial-specific transcripts and proteins (Uroplakin III and Ia), epithelial cell markers (CK20 and AE1/AE3), and tight junction markers (ZO-1, ZO-2, E-cadherin, and Cingulin) in a time-dependent manner, compared to non-induced USC. In vitro assays using fluorescent dye demonstrated a significant reduction in permeability of differentiated USC. In addition, transmission electron microscopy confirmed appropriate ultrastructure of urothelium differentiated from USC, including tight junction formation between neighboring cells, which was similar to positive controls. Furthermore, multilayered urothelial tissues formed 2 weeks after USC were differentiated on intestine submucosal matrix.

Conclusion

The present study illustrates an optimal strategy for the generation of differentiated urothelium from stem cells isolated from the urine. The induced urothelium is phenotypically and functionally like native urothelium and has proposed uses in in vivo urological tissue repair or in vitro urethra or bladder modeling.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1035-6) contains supplementary material, which is available to authorized users.

Differential transcription factor expression by human epithelial cells of buccal and urothelial derivation

Identification of transcription factors expressed by differentiated cells is informative not only of tissue-specific pathways, but to help identify master regulators for cellular reprogramming. If applied, such an approach could generate healthy autologous tissue-specific cells for clinical use where cells from the homologous tissue are unavailable due to disease. Normal human epithelial cells of buccal and urothelial derivation maintained in identical culture conditions that lacked significant instructive or permissive signaling cues were found to display inherent similarities and differences of phenotype. Investigation of transcription factors implicated in driving urothelial-type differentiation revealed buccal epithelial cells to have minimal or absent expression of PPARG, GATA3 and FOXA1 genes. Retroviral overexpression of protein coding sequences for GATA3 or PPARy1 in buccal epithelial cells resulted in nuclear immunolocalisation of the respective proteins, with both transductions also inducing expression of the urothelial differentiation-associated claudin 3 tight junction protein. PPARG1 overexpression alone entrained expression of nuclear FOXA1 and GATA3 proteins, providing objective evidence of its upstream positioning in a transcription factor network and identifying it as a candidate factor for urothelial-type transdifferentiation or reprogramming.

Tissue engineering in pediatric urology - a critical appraisal

Tissue engineering is defined as the combination of biomaterials and bioengineering principles together with cell transplantation or directed growth of host cells to develop a biological replacement tissue or organ that can be a substitute for normal tissue both in structure and function. Despite early promising preclinical studies, clinical translation of tissue engineering in pediatric urology into humans has been unsuccessful both for cell-seeded and acellular scaffolds. This can be ascribed to various factors, including the use of only non-diseased models that inaccurately describe the structural and functional modifications of diseased tissue. The paper addresses potential future strategies to overcome the limitations experienced in clinical applications so far. This includes the use of stem cells of various origins (mesenchymal stem cells, hematopoietic stem/progenitor cells, urine-derived stem cells, and progenitor cells of the urothelium) as well as the need for a deeper understanding of signaling pathways and directing tissue ingrowth and differentiation through the concept of dynamic reciprocity. The development of smart scaffolds that release trophic factors in a set and timely manner will probably improve regeneration. Modulation of innate immune response as a major contributor to tissue regeneration outcome is also addressed. It is unlikely that only one of these strategies alone will lead to clinically applicable tissue engineering strategies in pediatric urology. In the meanwhile, the fundamental new insights into regenerative processes already obtained in the attempts of tissue engineering of the lower urogenital tract remain our greatest gain.

The utility of stem cells in pediatric urinary bladder regeneration

Pediatric patients with a neurogenic urinary bladder, caused by developmental abnormalities including spina bifida, exhibit chronic urological problems. Surgical management in the form of enterocystoplasty is used to enlarge the bladder, but is associated with significant clinical complications. Thus, alternative methods to enterocystoplasty have been explored through the incorporation of stem cells with tissue engineering strategies. Within the context of this review, we will examine the use of bone marrow stem cells and induced pluripotent stem cells (iPSCs), as they relate to bladder regeneration at the anatomic and molecular levels. The use of bone marrow stem cells has demonstrated significant advances in bladder tissue regeneration as multiple aspects of bladder tissue have been recapitulated including the urothelium, bladder smooth muscle, vasculature, and peripheral nerves. iPSCs, on the other hand, have been well characterized and used in multiple tissue-regenerative settings, yet iPSC research is still in its infancy with regards to bladder tissue regeneration with recent studies describing the differentiation of iPSCs to the bladder urothelium. Finally, we examine the role of the Sonic Hedgehog signaling cascade that mediates the proliferative response during regeneration between bladder smooth muscle and urothelium. Taken together, this review provides a current, comprehensive perspective on bladder regeneration.

In Vitro Differentiation and Propagation of Urothelium from Pluripotent Stem Cell Lines

Bioengineering of bladder tissue, particularly for those patients who have advanced bladder disease, requires a source of urothelium that is healthy, capable of significant proliferation in vitro and immunologically tolerated upon transplant. As pluripotent stem cells have the potential to fulfill such criteria, they provide a critical cell source from which urothelium might be derived in vitro and used clinically. Herein, we describe the in vitro differentiation of urothelium from the H9 human embryonic stem cell (hESC) line through the definitive endoderm (DE) phase via selective culture techniques. The protocol can be used to derive urothelium from other hESCs or human-induced pluripotent stem cells.

The potential of organoids in urological cancer research

Technical advances in the development of organoid systems enable cell lines, primary adult cells, or stem or progenitor cells to develop into diverse, multicellular entities, which can self-renew, self-organize, and differentiate. These 3D organoid cultures have proven to be of value in increasing our understanding of the biology of disease and offer the potential of regenerative and genetic therapies. The successful application of 3D organoids derived from adult tissue into urological cancer research can further our understanding of these diseases and could also provide preclinical cancer models to realize the precision medicine paradigm by therapeutic screening of individual patient samples ex vivo. Kidney organoids derived from induced pluripotent stem cells provide personalized biomarkers, which can be correlated with genetic and clinical information. Organoid models can also improve our comprehension of aspects of particular diseases; for example, in prostate cancer, 3D organoids can aid in the identification of tumour-initiating cells from an epithelial cell lineage. Furthermore, kidney organoid differentiation from human pluripotent stem cells enables gene editing to model disease in kidney tubular epithelial cells. State-of-the-art human organoid cultures have potential as tools in basic and clinical research in renal, bladder, and prostatic diseases.

The Current Use of Stem Cells in Bladder Tissue Regeneration and Bioengineering

Many pathological processes including neurogenic bladder and malignancy necessitate bladder reconstruction, which is currently performed using intestinal tissue. The use of intestinal tissue, however, subjects patients to metabolic abnormalities, bladder stones, and other long-term sequelae, raising the need for a source of safe and reliable bladder tissue. Advancements in stem cell biology have catapulted stem cells to the center of many current tissue regeneration and bioengineering strategies. This review presents the recent advancements in the use of stem cells in bladder tissue bioengineering.

Systematic Review to Compare Urothelium Differentiation with Urethral Epithelium Differentiation in Fetal Development, as a Basis for Tissue Engineering of the Male Urethra

BACKGROUND

Tissue-engineered (TE) urethra is desirable in men with urethral disease (stricture or hypospadias) and shortage of local tissue. Although ideally a TE graft would contain urethral epithelium cells, currently, bladder epithelium (urothelium) is widely used, but morphologically different. Understanding the differences and similarities of urothelium and urethral epithelium could help design a protocol for in vitro generation of urethral epithelium to be used in TE grafts for the urethra.

PURPOSE

To understand the development toward urethral epithelium or urothelium to improve TE of the urethra.

METHODS

A literature search was done following PRISMA guidelines. Articles describing urethral epithelium and bladder urothelium development in laboratory animals and humans were selected.

RESULTS

Twenty-nine studies on development of urethral epithelium and 29 studies on development of urothelium were included. Both tissue linings derive from endoderm and although adult urothelium and urethral epithelium are characterized by different gene expression profiles, the signaling pathways underlying their development are similar, including Shh, BMP, Wnt, and FGF. The progenitor of the urothelium and the urethral epithelium is the early fetal urogenital sinus (UGS). The urethral plate and the urothelium are both formed from the p63+ cells of the UGS. Keratin 20 and uroplakins are exclusively expressed in urothelium, not in the urethral epithelium. Further research has to be done on unique markers for the urethral epithelium.

CONCLUSION

This review has summarized the current knowledge about embryonic development of urothelium versus urethral epithelium and especially focuses on the influencing factors that are potentially specific for the eventual morphological differences of both cell linings, to be a basis for developmental or tissue engineering of urethral tissue.

Production of urothelium from pluripotent stem cells for regenerative applications

As bladder reconstruction strategies evolve, a feasible and safe source of transplantable urothelium becomes a major consideration for patients with advanced bladder disease, particularly cancer. Pluripotent stem cells, such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), are attractive candidates from which to derive urothelium as they renew and proliferate indefinitely in vitro and fulfill the non-autologous and/or non-urologic criteria, respectively, that is required for many patients. This review presents the latest advancements in differentiating urothelium from pluripotent stem cells in vitro in the context of current bladder tissue engineering strategies.

Tissue engineering in urothelium regeneration

The development of therapeutic treatments to regenerate urothelium, manufacture tissue equivalents or neourethras for in-vivo application is a significant challenge in the field of tissue engineering. Many studies have focused on urethral defects that, in most cases, inadequately address current therapies. This article reviews the primary tissue engineering strategies aimed at the clinical requirements for urothelium regeneration while concentrating on promising investigations in the use of grafts, cellular preparations, as well as seeded or unseeded natural and synthetic materials. Despite significant progress being made in the development of scaffolds and matrices, buccal mucosa transplants have not been replaced. Recently, graft tissues appear to have an advantage over the use of matrices. These therapies depend on cell isolation and propagation in vitro that require, not only substantial laboratory resources, but also subsequent surgical implant procedures. The choice of the correct cell source is crucial when determining an in-vivo application because of the risks of tissue changes and abnormalities that may result in donor site morbidity. Addressing an appropriately-designed animal model and relevant regulatory issues is of fundamental importance for the principal investigators when a therapy using cellular components has been developed for clinical use.