Regeneration of uterine horns in rats using collagen scaffolds loaded with human embryonic stem cell-derived endometrium-like cells

Development of regenerative therapies targeting fibrotic endometrium in intrauterine adhesion or thin endometrium to restore uterine function

Intrauterine adhesions (IUA) and thin endometrium (TE) represent significant challenges in human reproduction. The condition arises frequently from damage to the endometrial basal layer, leading to fibrous tissue replacing the functional endometrium and impairing the uterus's ability to accept embryo implantation. Conventional treatments, mainly including hysteroscopic adhesiolysis and estrogen therapies, have shown limited success, particularly in severe cases. Regenerative medicine, with its focus on stem cell-based therapies and biomaterials, offers a promising avenue for restoring endometrial function and structure. This review synthesizes the current landscape of endometrial regeneration, focusing on the therapeutic potential of stem cells, the supportive role of biomaterials, and the importance of understanding molecular mechanisms to develop effective strategies for reconstruction of endometrial functional and fertility restoration.

The application of tissue engineering strategies for uterine regeneration

Uterine injuries, particularly damages to endometrium, are usually associated with abnormal menstruation, recurrent miscarriage, pregnancy complications, and infertility. Tissue engineering using cell-based, biomolecule-based, or biomaterial and scaffold-based strategies has emerged as a novel and promising approach for uterine regeneration. Stem cells, biomolecules, and porous scaffolds used alone or, very often, used in combination as a more effective treatment means have shown great potential in promoting uterine regeneration. The reported preclinical studies have indicated that appropriate tissue engineering strategies could safely and effectively reconstruct not only endometrium but also partial or even the whole uterine structure. However, the progress in the uterine regeneration area is slow in comparison to that of regenerating many other body tissues and hence it still remains a great challenge to apply uterine tissue engineering for clinical applications. In this review, conventional treatments for uterine-related diseases are briefly reviewed and discussed first. Subsequently, tissue engineering strategies (cell-based, biomolecule-based, biomaterial and scaffold-based, or their combinations) for uterine repair in preclinical studies and clinical trials are presented and analyzed. Finally, the challenges and perspectives in uterine regeneration are pointed and discussed. Despite various limitations and obstacles, the tissue engineering approach is viable and holds high promise for uterine regeneration.

Current advances in understanding endometrial epithelial cell biology and therapeutic applications for intrauterine adhesion

The human endometrium is a highly regenerative tissue capable of undergoing scarless repair during the menstruation and postpartum phases. This process is mediated by endometrial adult stem/progenitor cells. During the healing of endometrial injuries, swift reepithelization results in the rapid covering of the wound surface and facilitates subsequent endometrial restoration. The involvement of endogenous endometrial epithelial stem cells, stromal cells, and bone marrow-derived cells in the regeneration of the endometrial epithelium has been a subject of prolonged debate. Increasing evidence suggests that the regeneration of the endometrial epithelium mainly relies on epithelial stem cells rather than stromal cells and bone marrow-derived cells. Currently, no consensus has been established on the identity of epithelial stem cells in the epithelial compartment. Several markers, including stage-specific embryonic antigen-1 (SSEA-1), sex-determining region Y-box 9 (SOX9), neural-cadherin (N-cadherin), leucine-rich-repeat-containing G-protein-coupled receptor 5 (LGR5), CD44, axis inhibition protein 2 (Axin2), and aldehyde dehydrogenase 1A1 (ALDH1A1), have been suggested as potential candidate markers for endometrial epithelial stem cells. The identification of endometrial epithelial stem cells contributes to our understanding of endometrial regeneration and offers new therapeutic insights into diseases characterized by regenerative defects in the endometrium, such as intrauterine adhesion. This review explores different perspectives on the origins of human and mouse endometrial epithelial cells. It summarizes the potential markers, locations, and hierarchies of epithelial stem cells in both human and mouse endometrium. It also discusses epithelial cell-based treatments for intrauterine adhesion, hoping to inspire further research and clinical application of endometrial epithelial stem cells.

Advancements in three-dimensional bioprinting for reproductive medicine: a systematic review

Reproductive failure due to age, genetics and disease necessitates innovative solutions. While reproductive tissue transplantation has advanced, ongoing research seeks superior approaches. Biomaterials, bioengineering and additive manufacturing, such as three-dimensional (3D) bioprinting, are harnessed to restore reproductive function. 3D bioprinting uses materials, cells and growth factors to mimic natural tissues, proving popular for tissue engineering, notably in complex scaffold creation with cell distribution. The versatility which is brought to reproductive medicine by 3D bioprinting allows more accurate and on-site applicability to various problems that are encountered in the field. However, in the literature, there is a lack of studies encompassing the valuable applications of 3D bioprinting in reproductive medicine. This systematic review aims to improve understanding, and focuses on applications in several branches of reproductive medicine. Advancements span the restoration of ovarian function, endometrial regeneration, vaginal reconstruction, and male germ cell bioengineering. 3D bioprinting holds untapped potential in reproductive medicine.

Evolution of biotechnological advances and regenerative therapies for endometrial disorders: a systematic review

BACKGROUND

The establishment and maintenance of pregnancy depend on endometrial competence. Asherman syndrome (AS) and intrauterine adhesions (IUA), or endometrial atrophy (EA) and thin endometrium (TE), can either originate autonomously or arise as a result from conditions (i.e. endometritis or congenital hypoplasia), or medical interventions (e.g. surgeries, hormonal therapies, uterine curettage or radiotherapy). Affected patients may present an altered or inadequate endometrial lining that hinders embryo implantation and increases the risk of poor pregnancy outcomes and miscarriage. In humans, AS/IUA and EA/TE are mainly treated with surgeries or pharmacotherapy, however the reported efficacy of these therapeutic approaches remains unclear. Thus, novel regenerative techniques utilizing stem cells, growth factors, or tissue engineering have emerged to improve reproductive outcomes.

OBJECTIVE AND RATIONALE

This review comprehensively summarizes the methodologies and outcomes of emerging biotechnologies (cellular, acellular, and bioengineering approaches) to treat human endometrial pathologies. Regenerative therapies derived from human tissues or blood which were studied in preclinical models (in vitro and in vivo) and clinical trials are discussed.

SEARCH METHODS

A systematic search of full-text articles available in PubMed and Embase was conducted to identify original peer-reviewed studies published in English between January 2000 and September 2023. The search terms included: human, uterus, endometrium, Asherman syndrome, intrauterine adhesions, endometrial atrophy, thin endometrium, endometritis, congenital hypoplasia, curettage, radiotherapy, regenerative therapy, bioengineering, stem cells, vesicles, platelet-rich plasma, biomaterials, microfluidic, bioprinting, organoids, hydrogel, scaffold, sheet, miRNA, sildenafil, nitroglycerine, aspirin, growth hormone, progesterone, and estrogen. Preclinical and clinical studies on cellular, acellular, and bioengineering strategies to repair or regenerate the human endometrium were included. Additional studies were identified through manual searches.

OUTCOMES

From a total of 4366 records identified, 164 studies (3.8%) were included for systematic review. Due to heterogeneity in the study design and measured outcome parameters in both preclinical and clinical studies, the findings were evaluated qualitatively and quantitatively without meta-analysis. Groups using stem cell-based treatments for endometrial pathologies commonly employed mesenchymal stem cells (MSCs) derived from the human bone marrow or umbilical cord. Alternatively, acellular therapies based on platelet-rich plasma (PRP) or extracellular vesicles are gaining popularity. These are accompanied by the emergence of bioengineering strategies based on extracellular matrix (ECM)-derived hydrogels or synthetic biosimilars that sustain local delivery of cells and growth factors, reporting promising results. Combined therapies that target multiple aspects of tissue repair and regeneration remain in preclinical testing but have shown translational value. This review highlights the myriad of therapeutic material sources, administration methods, and carriers that have been tested.

WIDER IMPLICATIONS

Therapies that promote endometrial proliferation, vascular development, and tissue repair may help restore endometrial function and, ultimately, fertility. Based on the existing evidence, cost, accessibility, and availability of the therapies, we propose the development of triple-hit regenerative strategies, potentially combining high-yield MSCs (e.g. from bone marrow or umbilical cord) with acellular treatments (PRP), possibly integrated in ECM hydrogels. Advances in biotechnologies together with insights from preclinical models will pave the way for developing personalized treatment regimens for patients with infertility-causing endometrial disorders such as AS/IUA, EA/TE, and endometritis.

REGISTRATION NUMBER

https://osf.io/th8yf/.

Bio-distribution and toxicity potential of human umbilical cord mesenchymal stem cells in cynomolgus monkeys

Mesenchymal stem cells (MSCs) have demonstrated promising advantages in the therapies of many diseases, while its multi-directional differentiation potential and immunotoxicity are the major concerns hindered their clinical translation. In this study, human umbilical Mesenchymal stem cell (hUC-MSCs) were labeled with a near-infrared fluorescent dye DiR before infused into cynomolgus monkeys, and the amount of hUC-MSCs in the peripheral blood were dynamically estimated from 5 min to 28 days post a single administration at 3 × 106 cells/kg and 2 × 107 cells/kg intravenously. As results, some hUC-MSCs distributed to the whole body within 5 min, while most of the cells accumulate in the lungs along with the systemic blood circulation, and subsequently released into the blood. The toxicity potentials of hUC-MSCs were investigated in another 30 cynomolgus monkeys, and the cells were repeatedly administrated at doses of 3 × 106 cells/kg and 2 × 107 cells/kg for 5 times on a weekly basis, with a recovery period of 1 months. hUC-MSCs showed no obvious toxic effects in cynomolgus monkeys, except xenogeneic immune rejection to human stem cells. Low levels of the hUC-MSC gene were detected in the peripheral blood of a few animals administered 2 × 107 cells/kg at 30 min subsequent to the first and last administration, and there was no significant difference in the copy number of the hUC-MSC gene in the blood samples compared with the first and last administration, indicating that the hUC-MSC was not significantly amplified in vivo, and it its safe in non-human primates. Our study for the first time verified the safety of long-term use of hUC-MSCs in primates. We have pioneered a technology for the real-time detection of hUC-MSCs in peripheral blood and provide dynamicand rapid monitoring of the distribution characteristics of hUC-MSCs in vivo. Here, we provide data supporting the application of such products for clinical treatment and the application of stem cells in major refractory diseases and regenerative medicine.

New advances in the treatment of thin endometrium

Thin endometrium (TE) is defined as a mid-luteal endometrial thickness ≤7mm. TE can affect endometrial tolerance, leading to lower embryo implantation rates and clinical pregnancy rates, and is also associated with impaired outcomes from assisted reproductive treatment. Herein, we systematically review TE causes, mechanisms, and treatments. TE pathogenesis has multiple causes, with the endometrium becoming thinner with age under hormonal influence. In addition, uterine cavity factors are important, as the inflammatory environment may affect expressions of certain genes thereby inhibiting endometrial stromal cell proliferation and promoting apoptosis. Long-term oral contraceptive use or the use of ovulation-promoting drugs are also definite factors contributing to endometrial thinning. Other patients have primary factors, for which the clinical etiology remains unknown. The main therapeutic strategies available for TE are pharmacological (including hormonal and vasoactive drugs), regenerative medicine, intrauterine infusion of growth factor-granulocyte colony-stimulating factor, autologous platelet-rich plasma, and complementary alternative therapies (including traditional Chinese herbal medicine and acupuncture). However, the associated mechanisms of action are currently unclear. Clinical scholars have proposed various approaches to improve treatment outcomes in patients with TE, and are exploring the principles of efficacy, offering potentials for novel treatments. It is hoped that this will improve TE tolerance, increase embryo implantation rates, and help more couples with infertility with effective treatments.

E2-Loaded Microcapsules and Bone Marrow-Derived Mesenchymal Stem Cells with Injectable Scaffolds for Endometrial Regeneration Application

Bone marrow-derived mesenchymal stem cells (BMSCs) have been recognized as new candidates for the treatment of serious endometrial injuries. However, owing to the local microenvironment of damaged endometrium, transplantation of BMSCs yielded disappointing results. In this study, Pectin-Pluronic® F-127 hydrogel as scaffolds were fabricated to provide three-dimensional architecture for the attachment, growth, and migration of BMSCs. E2 was encapsulated into the W/O/W microspheres to construct pectin-based E2-loaded microcapsules (E2 MPs), which has the potential to serve as a long-term reliable source of E2 for endometrial regeneration. Then, the BMSCs/E2 MPs/scaffolds system was injected into the uterine cavity of mouse endometrial injury model for treatment. At 4 weeks after transplantation, the system increased proliferative abilities of uterine endometrial cells, facilitated microvasculature regeneration, and restored the ability of endometrium to receive an embryo, suggesting that the BMSCs/E2 MPs/scaffolds system is a promising treatment option for endometrial regeneration. Furthermore, the mechanism of E2 in promoting the repair of endometrial injury was also investigated. Exosomes are critical paracrine mediators that act as biochemical cues to direct stem cell differentiation. In this study, it was found that the expression of endometrial epithelial cell (EEC) markers was upregulated in BMSCs treated by exosomes secreted from endometrial stromal cells (ESCs-Exos). Exosomes derived from E2-stimulated ESCs further promoted the expression level of EECs markers in BMSCs, suggesting exosomes released from ESCs by E2 stimulation could enhance the differentiation efficiency of BMSCs. Therefore, exosomes derived from ESCs play paracrine roles in endometrial regeneration stimulated by E2 and provide optimal estrogenic response.

A biomechanical assessment of tissue-engineered polymer neo-uteri after orthotopic implantation

OBJECTIVE

To assess the in vivo biomechanical maturation of tissue-engineered neo-uteri that have previously supported live births in a rabbit model.

DESIGN

Nonclinical animal study.

SETTING

University-based research laboratory.

ANIMALS

Eighteen adult female rabbits.

INTERVENTION

Biodegradable poly-DL-lactide-co-glycolide-coated polyglycolic acid scaffolds seeded with autologous uterine-derived endometrial and myometrial cells. Nonseeded scaffolds and seeded, tissue-engineered neo-uteri were implanted into one uterine horn of rabbits for 1, 3, or 6 months, excised, and biomechanically assessed in comparison to native uterine tissue.

MAIN OUTCOME MEASURES

Tensile stress-relaxation testing, strain-to-failure testing, and viscoelastic modeling.

RESULTS

By evaluating the biomechanical data with several viscoelastic models, it was revealed that tissue-engineered uteri were more mechanically robust than nonseeded scaffolds. For example, the 10% instantaneous stress of the tissue-engineered neo-uteri was 2.1 times higher than the nonseeded scaffolds at the 1-month time point, 1.6 times higher at the 3-month time point, and 1.5 times higher at the 6-month time point. Additionally, as the duration of implantation increased, the engineered constructs became more mechanically robust (e.g., 10% instantaneous stress of the tissue-engineered neo-uteri increased from 22 kPa at 1 month to 42 kPa at 6 months). Compared with native tissue values, tissue-engineered neo-uteri achieved or surpassed native tissue values by the 6-month time point.

CONCLUSION

The present study evaluated the mechanical characteristics of novel tissue-engineered neo-uteri that have previously been reported to support live births in the rabbit model. We demonstrate that the biomechanics of these implants closely resemble those of native tissue, giving further credence to their development as a clinical solution to uterine factor infertility.

Injectable collagen hydrogel combines human umbilical cord mesenchymal stem cells to promote endometrial regeneration in rats with thin endometrium

The regeneration of thin endometrium still remains as a great challenge in the field of reproductive medicine. Stem cells-based therapy has been considered as a promising strategy for the restoration of thin endometrium. However, the low transplantation and retention rate of stem cells and loss of stemness due to in vitro expansion limits the therapeutic efficacy. In our study, we combined collagen hydrogel and human umbilical cord mesenchymal stem cells (uMSCs) for improving the regeneration of thin endometrium, by using the potent pluripotency and low immunogenicity of uMSCs and collagen hydrogel that promotes the anchorage and proliferation of stem cells. Results showed that collagen hydrogel has favorable biocompatibility and the capacity to enhance the cell viability and expression of stemness-associated genes (including organic cation/carnitine transporter4 (Oct-4), Nanog homeobox (Nanog) and SRY-box transcription factor 2 (SOX2)) of uMSCs. The combination of collagen hydrogel and uMSCs prolonged the retention time of the constructs in the uterine cavity and improved endometrial thickness compared with uMSCs alone, leading to increase the fertility of the rats with thin endometrium. These highlighted therapeutic prospects of collagen hydrogel combined with uMSCs for the minimally invasive therapy of thin endometrium in the clinic.

Cyclical endometrial repair and regeneration: Molecular mechanisms, diseases, and therapeutic interventions

The endometrium is a unique human tissue with an extraordinary ability to undergo a hormone-regulated cycle encompassing shedding, bleeding, scarless repair, and regeneration throughout the female reproductive cycle. The cyclical repair and regeneration of the endometrium manifest as changes in endometrial epithelialization, glandular regeneration, and vascularization. The mechanisms encompass inflammation, coagulation, and fibrinolytic system balance. However, specific conditions such as endometriosis or TCRA treatment can disrupt the process of cyclical endometrial repair and regeneration. There is uncertainty about traditional clinical treatments' efficacy and side effects, and finding new therapeutic interventions is essential. Researchers have made substantial progress in the perspective of regenerative medicine toward maintaining cyclical endometrial repair and regeneration in recent years. Such progress encompasses the integration of biomaterials, tissue-engineered scaffolds, stem cell therapies, and 3D printing. This review analyzes the mechanisms, diseases, and interventions associated with cyclical endometrial repair and regeneration. The review discusses the advantages and disadvantages of the regenerative interventions currently employed in clinical practice. Additionally, it highlights the significant advantages of regenerative medicine in this domain. Finally, we review stem cells and biologics among the available interventions in regenerative medicine, providing insights into future therapeutic strategies.

Current status and challenges in uterine myometrial tissue engineering

The uterus undergoes significant modifications throughout pregnancy to support embryo development and fetal growth. However, conditions like fibroids, adenomyosis, cysts, and C-section scarring can cause myometrial damage. The importance of the uterus and the challenges associated with myometrial damage, and the need for alternative approaches are discussed in this review. The review also explores the recent studies in tissue engineering, which involve principles of combining cells, scaffolds, and signaling molecules to create functional uterine tissues. It focuses on two key approaches in uterine tissue engineering: scaffold technique using decellularized, natural, and synthetic polymer and 3D bioprinting. These techniques create supportive structures for cell growth and tissue formation. Current treatment options for myometrial damage have limitations, leading to the exploration of regenerative medicine and integrative therapies. The review emphasizes the potential benefits of tissue engineering, including more effective and less invasive treatment options for myometrial damage. The challenges of developing biocompatible materials and optimizing cell growth and differentiation are discussed. In conclusion, uterine tissue engineering holds promise for myometrial regeneration and the treatment of related conditions. This review highlights the scientific advancements in the field and underscores the potential of tissue engineering as a viable approach. By addressing the limitations of current treatments, tissue engineering offers new possibilities for improving reproductive health and restoring uterine functionality. Future research shall focus on overcoming challenges and refining tissue engineering strategies to advance the field and provide effective solutions for myometrial damage and associated disorders.

Mechanisms of Regeneration and Fibrosis in the Endometrium

The uterine lining (endometrium) regenerates repeatedly over the life span as part of its normal physiology. Substantial portions of the endometrium are shed during childbirth (parturition) and, in some species, menstruation, but the tissue is rapidly rebuilt without scarring, rendering it a powerful model of regeneration in mammals. Nonetheless, following some assaults, including medical procedures and infections, the endometrium fails to regenerate and instead forms scars that may interfere with normal endometrial function and contribute to infertility. Thus, the endometrium provides an exceptional platform to answer a central question of regenerative medicine: Why do some systems regenerate while others scar? Here, we review our current understanding of diverse endometrial disruption events in humans, nonhuman primates, and rodents, and the associated mechanisms of regenerative success and failure. Elucidating the determinants of these disparate repair processes promises insights into fundamental mechanisms of mammalian regeneration with substantial implications for reproductive health.

New Solutions for Old Problems: How Reproductive Tissue Engineering Has Been Revolutionizing Reproductive Medicine

Acquired disorders and congenital defects of the male and female reproductive systems can have profound impacts on patients, causing sexual and endocrine dysfunction and infertility, as well as psychosocial consequences that affect their self-esteem, identity, sexuality, and relationships. Reproductive tissue engineering (REPROTEN) is a promising approach to restore fertility and improve the quality of life of patients with reproductive disorders by developing, replacing, or regenerating cells, tissues, and organs from the reproductive and urinary systems. In this review, we explore the latest advancements in REPROTEN techniques and their applications for addressing degenerative conditions in male and female reproductive organs. We discuss current research and clinical outcomes and highlight the potential of 3D constructs utilizing biomaterials such as scaffolds, cells, and biologically active molecules. Our review offers a comprehensive guide for researchers and clinicians, providing insights into how to reestablish reproductive tissue structure and function using innovative surgical approaches and biomaterials. We highlight the benefits of REPROTEN for patients, including preservation of fertility and hormonal production, reconstruction of uterine and cervical structures, and restoration of sexual and urinary functions. Despite significant progress, REPROTEN still faces ethical and technical challenges that need to be addressed. Our review underscores the importance of continued research in this field to advance the development of effective and safe REPROTEN approaches for patients with reproductive disorders.

Current Status of Tissue Regenerative Engineering for the Treatment of Uterine Infertility

With the recent developments in tissue engineering, scientists have attempted to establish seed cells from different sources, create cell sheets through various technologies, implant them on scaffolds with various spatial structures, or load scaffolds with cytokines. These research results are very optimistic, bringing hope to the treatment of patients with uterine infertility. In this article, we reviewed articles related to the treatment of uterine infertility from the aspects of experimental treatment strategy, seed cells, scaffold application, and repair criteria so as to provide a basis for future research.

Development of cell-laden multimodular Lego-like customizable endometrial tissue assembly for successful tissue regeneration

BACKGROUND

The endometrium, the inner lining of the uterine cavity, plays essential roles in embryo implantation and its subsequent development. Although some positive results were preliminarily archived, the regeneration of damaged endometrial tissues by administrating stem cells only is very challenging due to the lack of specific microenvironments and their low attachment rates at the sites of injury. In this context, various biomaterial-based scaffolds have been used to overcome these limitations by providing simple structural support for cell attachment. However, these scaffold-based strategies also cannot properly reflect patient tissue-specific structural complexity and thus show only limited therapeutic effects.

METHOD

Therefore, in the present study, we developed a customizable Lego-like multimodular endometrial tissue architecture by assembling individually fabricated tissue blocks.

RESULTS

Each tissue block was fabricated by incorporating biodegradable biomaterials and certain endometrial constituent cells. Each small tissue block was effectively fabricated by integrating conventional mold casting and 3D printing techniques. The fabricated individual tissue blocks were properly assembled into a larger customized tissue architecture. This structure not only properly mimics the patient-specific multicellular microenvironment of the endometrial tissue but also properly responds to key reproductive hormones in a manner similar to the physiological functions.

CONCLUSION

This customizable modular tissue assembly allows easy and scalable configuration of a complex patient-specific tissue microenvironment, thus accelerating various tissue regeneration procedures.

A Decellularized Uterine Endometrial Scaffold Enhances Regeneration of the Endometrium in Rats

Partial or whole regeneration of the uterine endometrium using extracellular matrix (ECM)-based scaffolds is a therapeutic strategy for uterine infertility due to functional and/or structural endometrial defects. Here, we examined whether the entire endometrium can be regenerated circumferentially using an acellular ECM scaffold (decellularized endometrial scaffold, DES) prepared from rat endometrium. We placed a silicone tube alone to prevent adhesions or a DES loaded with a silicone tube into a recipient uterus in which the endometrium had been surgically removed circumferentially. Histological and immunofluorescent analyses of the uteri one month after tube placement revealed more abundant regenerated endometrial stroma in the uterine horns treated with tube-loaded DES compared to those treated with a tube alone. Luminal and glandular epithelia, however, were not fully recapitulated. These results suggest that DES can enhance the regeneration of endometrial stroma but additional intervention(s) are needed to induce epithelization. Furthermore, the prevention of adhesions alone allowed the endometrial stroma to regenerate circumferentially even without a DES, but to a lesser degree than that with a DES. The use of a DES together with the prevention of adhesions may be beneficial for efficient endometrial regeneration in the uterus that is largely deficient of endometrium.

3D bio-printed endometrial construct restores the full-thickness morphology and fertility of injured uterine endometrium

Severe damage to the uterine endometrium, which results in scar formation and endometrial dysfunction, eventually leads to infertility or pregnancy-related complications. No effective therapeutic treatment is currently available for such injuries owing to the structural complexity, internal environment, and function of the uterus. Three-dimensional (3D) bio-printing to engineer biomimetic structural constructs provides a unique opportunity for tissue regeneration. Herein, using 3D extrusion-based bioprinting (EBB), we constructed a bilayer endometrial construct (EC) based on a sodium alginate-hyaluronic acid (Alg-HA) hydrogel for functional regeneration of the endometrium. The upper layer of the 3D bio-printed EC is a monolayer of endometrial epithelial cells (EECs), while the lower layer has a grid-like microstructure loaded with endometrial stromal cells (ESCs). In a partial full-thickness uterine excision rat model, our bilayer EC not only restored the morphology and structure of the endometrial wall (including organized luminal/ glandular epithelium, stroma, vasculature and the smooth muscle layer), but also significantly improved the reproductive outcome in the surgical area after implantation (75%, 12/16, p < 0.01). Therefore, repair of the uterine endometrium using the developed 3D bio-printed bilayer EC may represent an effective regenerative treatment for severe endometrial injury. STATEMENT OF SIGNIFICANCE: Achieving structural and functional recovery of the endometrium following severe injury is still a challenge. Here, we designed a 3D bio-printed endometrial construct (EC) to mimic the native bilayer structure and cellular components of the endometrium. The bio-printed EC consists of a dense upper layer with endometrial epithelial cells and a lower layer with endometrial stromal cells. In particular, the 3D bio-printed EC significantly improved the reproductive outcome in the surgical area (75%, 12/16) compared to that of the cell-loaded non-printed group (12.5%, 2/16). This study demonstrates that a biomimetic bilayer construct can facilitate endometrial repair and regeneration. Therefore, an endometrial cells-loaded 3D-bioprinted EC is a promising therapeutic option for patients suffering from severe endometrial damage.

Human amniotic mesenchymal stem cells promote endometrium regeneration in a rat model of intrauterine adhesion

Human amniotic transplantation has been proposed to improve the therapeutic efficacy of intrauterine adhesions (IUAs). Human amniotic mesenchymal stem stromal cells (hAMSCs) can differentiate into multiple tissue types. This study aimed to investigate the mechanism by which hAMSCs transplantation promotes endometrial regeneration. The rat models with IUA were established through mechanical and infective methods, and PKH26-labeled hAMSCs were transplanted through the tail vein (combined with/without estrogen). Under three different conditions, hAMSCs differentiated into endometrium-like cells. HE and Mason staining assays, and immunohistochemistry were used to compare the changes in rat models treated with hAMSCs and/or estrogen transplantation. To define the induction of hAMSCs to endometrium-like cells in vitro, an induction medium (cytokines, estrogen) was used to investigate the differentiation of hAMSCs into endometrium-like cells. qRT-polymerase chain reaction (PCR) and western blotting were performed to detect the differentiation of hAMSCs into endometrium-like cells. A greater number of glands, fewer endometrial fibrotic areas, and stronger expression of vascular endothelial growth factor and cytokeratin in the combined group (hAMSCs transplantation combined with estrogen) than in the other treatment groups were observed. hAMSCs could be induced into endometrium-like cells by cytokine treatment (TGF-β1, EGF, and PDGF-BB). Transplantation of hAMSCs is an effective alternative for endometrial regeneration after injury in rats. The differentiation protocol for hAMSCs will be useful for further studies on human endometrial regeneration.

CRC Therapy Identifies Indian Hedgehog Signaling in Mouse Endometrial Epithelial Cells and Inhibition of Ihh-KLF9 as a Novel Strategy for Treating IUA

Intrauterine adhesion (IUA) causes menstrual disturbance and infertility. There is no effective treatment available for moderate to severe IUA cases. Stem cell-based therapy has been investigated for treating IUA but is limited in clinical applications due to issues including the precise induction of differentiation, tumorigenesis, and unclear molecular mechanisms. In our recent study, we isolated and expanded the long-term cultures of conditional reprogrammed (CR) mouse endometrial epithelial cells. Treating IUA mice with these CR cells (CRCs) restored the morphology and structure of the endometrium and significantly improved the pregnancy rate. In this study, our data with high-throughput sequencing, CRISPR knockout Ihh^-/-CRCs, and transplantation identified for the first time that the Indian hedgehog (Ihh) gene plays a critical role in the regulation of endometrial epithelial cell proliferation. We also found that aberrant activated Ihh-krüppel-like factor 9 (KLF9) signaling contributes to the inhibition of normal progesterone receptor (PR) function in IUA mice. Thus, we hypothesized that inhibition of the Ihh-KLF9 pathway may be a novel strategy to treat IUA. Our data demonstrated that treatment with the hedgehog signaling inhibitor Vismodegib restored the morphology, structure, and microenvironment of the endometrium, and greatly improved the pregnancy rate in IUA mice. This study suggests a promising application of hedgehog inhibitors as a targeted drug in the IUA clinic.

Bi-potential hPSC-derived Müllerian duct-like cells for full-thickness and functional endometrium regeneration

Stem cell-based tissue regeneration strategies are promising treatments for severe endometrial injuries. However, there are few appropriate seed cells for regenerating a full-thickness endometrium, which mainly consists of epithelia and stroma. Müllerian ducts in female embryonic development develop into endometrial epithelia and stroma. Hence, we first generated human pluripotent stem cells (hPSC)-derived Müllerian duct-like cells (MDLCs) using a defined and effective protocol. The MDLCs are bi-potent, can gradually differentiate into endometrial epithelial and stromal cells, and reconstitute full-thickness endometrium in vitro and in vivo. Furthermore, MDLCs showed the in situ repair capabilities of reconstructing endometrial structure and recovering pregnancy function in full-thickness endometrial injury rats, and their differentiation fate was revealed by single-cell RNA sequencing (scRNA-seq). Our study provides a strategy for hPSC differentiation into endometrial lineages and an alternative seed cell for injured endometrial regeneration.

GWAS of Reproductive Traits in Large White Pigs on Chip and Imputed Whole-Genome Sequencing Data

Total number born (TNB), number of stillborn (NSB), and gestation length (GL) are economically important traits in pig production, and disentangling the molecular mechanisms associated with traits can provide valuable insights into their genetic structure. Genotype imputation can be used as a practical tool to improve the marker density of single-nucleotide polymorphism (SNP) chips based on sequence data, thereby dramatically improving the power of genome-wide association studies (GWAS). In this study, we applied Beagle software to impute the 50 K chip data to the whole-genome sequencing (WGS) data with average imputation accuracy (R2) of 0.876. The target pigs, 2655 Large White pigs introduced from Canadian and French lines, were genotyped by a GeneSeek Porcine 50K chip. The 30 Large White reference pigs were the key ancestral individuals sequenced by whole-genome resequencing. To avoid population stratification, we identified genetic variants associated with reproductive traits by performing within-population GWAS and cross-population meta-analyses with data before and after imputation. Finally, several genes were detected and regarded as potential candidate genes for each of the traits: for the TNB trait: NOTCH2, KLF3, PLXDC2, NDUFV1, TLR10, CDC14A, EPC2, ORC4, ACVR2A, and GSC; for the NSB trait: NUB1, TGFBR3, ZDHHC14, FGF14, BAIAP2L1, EVI5, TAF1B, and BCAR3; for the GL trait: PPP2R2B, AMBP, MALRD1, HOXA11, and BICC1. In conclusion, expanding the size of the reference population and finding an optimal imputation strategy to ensure that more loci are obtained for GWAS under high imputation accuracy will contribute to the identification of causal mutations in pig breeding.

A New Horizon in Reproductive Research with Pluripotent Stem Cells: Successful In Vitro Gametogenesis in Rodents, Its Application to Large Animals, and Future In Vitro Reconstitution of Reproductive Organs Such as “Uteroid” and “Oviductoid”

Simple Summary

Functional gametes, such as oocytes and spermatozoa, have been derived from rodent pluripotent stem cells, which can be applied to large animals and ultimately, to humans. In addition to summarizing these topics, we also review additional approaches for in vitro reconstitution of reproductive organs. This review illustrates intensive past efforts and future challenges on stem cell research for in vitro biogenesis in various mammalian models.

Abstract

Recent success in derivation of functional gametes (oocytes and spermatozoa) from pluripotent stem cells (PSCs) of rodents has made it feasible for future application to large animals including endangered species and to ultimately humans. Here, we summarize backgrounds and recent studies on in vitro gametogenesis from rodent PSCs, and similar approaches using PSCs from large animals, including livestock, nonhuman primates (NHPs), and humans. We also describe additional developing approaches for in vitro reconstitution of reproductive organs, such as the ovary (ovarioid), testis (testisoid), and future challenges in the uterus (uteroid) and oviduct (oviductoid), all of which may be derived from PSCs. Once established, these in vitro systems may serve as a robust platform for elucidating the pathology of infertility-related disorders and ectopic pregnancy, principle of reproduction, and artificial biogenesis. Therefore, these possibilities, especially when using human cells, require consideration of ethical issues, and international agreements and guidelines need to be raised before opening “Pandora’s Box”.

Treating intrauterine adhesion using conditionally reprogrammed physiological endometrial epithelial cells

Background

There is unmet need for effective therapies of intrauterine adhesions (IUAs) that are common cause of menstrual disturbance and infertility, since current clinical procedures do not improve prognosis for patients with moderate to severe IUA, with a recurrence rate of 23–50%. Stem cell-based therapy has emerged as a therapeutic option with unsolved issues for IUA patients in the past few years. Primary endometrial epithelial cells for cell therapy are largely hampered with the extremely limited proliferation capacity of uterine epithelial cells. This study was to evaluate whether IUA is curable with conditionally reprogrammed (CR) endometrial epithelial cells.

Methods

Mouse endometrial epithelial cells (MEECs) were isolated from C57BL female mice, and long-term cultures of MEECs were established and maintained with conditional reprogramming (CR) method. DNA damage response analysis, soft agar assay, and matrigel 3D culture were carried out to determine the normal biological characteristics of CR-MEECs. The tissue-specific differentiation potential of MEECs was analyzed with air–liquid interface (ALI) 3D culture, hematoxylin and eosin (H&E) staining, Masson’s trichrome and DAB staining, immunofluorescence assay. IUA mice were constructed and transplanted with CR-MEECs. Repair and mechanisms of MEECs transplantation in IUA mice were measured with qRT-PCR, Masson’s trichrome, and DAB staining.

Results

We first successfully established long-term cultures of MEECs using CR approach. CR-MEECs maintained a rapid and stable proliferation in this co-culture system. Our data confirmed that CR-MEECs retained normal biological characteristics and endometrium tissue-specific differentiation potential. CR-MEECs also expressed estrogen and progesterone receptors and maintained the exquisite sensitivity to sex hormones in vitro. Most importantly, allogeneic transplantation of CR-MEECs successfully repaired the injured endometrium and significantly improved the pregnancy rate of IUA mice.

Conclusions

Conditionally reprogrammed physiological endometrial epithelial cells provide a novel strategy in IUA clinics in a personalized or generalized manner and also serve as a physiological model to explore biology of endometrial epithelial cells and mechanisms of IUA.

Cell-based therapy in thin endometrium and Asherman syndrome

Numerous treatment strategies have so far been proposed for treating refractory thin endometrium either without or with the Asherman syndrome. Inconsistency in the improvement of endometrial thickness is a common limitation of such therapies including tamoxifen citrate as an ovulation induction agent, acupuncture, long-term pentoxifylline and tocopherol or tocopherol only, low-dose human chorionic gonadotropin during endometrial preparation, aspirin, luteal gonadotropin-releasing hormone agonist supplementation, and extended estrogen therapy. Recently, cell therapy has been proposed as an ideal alternative for endometrium regeneration, including the employment of stem cells, platelet-rich plasma, and growth factors as therapeutic agents. The mechanisms of action of cell therapy include the cytokine induction, growth factor production, natural killer cell activity reduction, Th17 and Th1 decrease, and Treg cell and Th2 increase. Since cell therapy is personalized, dynamic, interactive, and specific and could be an effective strategy. Despite its promising nature, further research is required for improving the procedure and the safety of this strategy. These methods and their results are discussed in this article.

OUP accepted manuscript

BACKGROUND

To provide the optimal milieu for implantation and fetal development, the female reproductive system must orchestrate uterine dynamics with the appropriate hormones produced by the ovaries. Mature oocytes may be fertilized in the fallopian tubes, and the resulting zygote is transported toward the uterus, where it can implant and continue developing. The cervix acts as a physical barrier to protect the fetus throughout pregnancy, and the vagina acts as a birth canal (involving uterine and cervix mechanisms) and facilitates copulation. Fertility can be compromised by pathologies that affect any of these organs or processes, and therefore, being able to accurately model them or restore their function is of paramount importance in applied and translational research. However, innate differences in human and animal model reproductive tracts, and the static nature of 2D cell/tissue culture techniques, necessitate continued research and development of dynamic and more complex in vitro platforms, ex vivo approaches and in vivo therapies to study and support reproductive biology. To meet this need, bioengineering is propelling the research on female reproduction into a new dimension through a wide range of potential applications and preclinical models, and the burgeoning number and variety of studies makes for a rapidly changing state of the field.

OBJECTIVE AND RATIONALE

This review aims to summarize the mounting evidence on bioengineering strategies, platforms and therapies currently available and under development in the context of female reproductive medicine, in order to further understand female reproductive biology and provide new options for fertility restoration. Specifically, techniques used in, or for, the uterus (endometrium and myometrium), ovary, fallopian tubes, cervix and vagina will be discussed.

SEARCH METHODS

A systematic search of full-text articles available in PubMed and Embase databases was conducted to identify relevant studies published between January 2000 and September 2021. The search terms included: bioengineering, reproduction, artificial, biomaterial, microfluidic, bioprinting, organoid, hydrogel, scaffold, uterus, endometrium, ovary, fallopian tubes, oviduct, cervix, vagina, endometriosis, adenomyosis, uterine fibroids, chlamydia, Asherman’s syndrome, intrauterine adhesions, uterine polyps, polycystic ovary syndrome and primary ovarian insufficiency. Additional studies were identified by manually searching the references of the selected articles and of complementary reviews. Eligibility criteria included original, rigorous and accessible peer-reviewed work, published in English, on female reproductive bioengineering techniques in preclinical (in vitro/in vivo/ex vivo) and/or clinical testing phases.

OUTCOMES

Out of the 10 390 records identified, 312 studies were included for systematic review. Owing to inconsistencies in the study measurements and designs, the findings were assessed qualitatively rather than by meta-analysis. Hydrogels and scaffolds were commonly applied in various bioengineering-related studies of the female reproductive tract. Emerging technologies, such as organoids and bioprinting, offered personalized diagnoses and alternative treatment options, respectively. Promising microfluidic systems combining various bioengineering approaches have also shown translational value.

WIDER IMPLICATIONS

The complexity of the molecular, endocrine and tissue-level interactions regulating female reproduction present challenges for bioengineering approaches to replace female reproductive organs. However, interdisciplinary work is providing valuable insight into the physicochemical properties necessary for reproductive biological processes to occur. Defining the landscape of reproductive bioengineering technologies currently available and under development for women can provide alternative models for toxicology/drug testing, ex vivo fertility options, clinical therapies and a basis for future organ regeneration studies.

Recent trends in therapeutic strategies for repairing endometrial tissue in intrauterine adhesion

Intrauterine adhesion (IUA) is a common gynaecological disease that develops from infection or trauma. IUA disease may seriously affect the physical and mental health of women of childbearing age, which may lead to symptoms such as hypomenorrhea or infertility. Presently, hysteroscopic transcervical resection of adhesion (TCRA) is the principal therapy for IUAs, although its function in preventing the recurrence of adhesion and preserving fertility is limited. Pharmaceuticals such as hormones and vasoactive agents and the placement of nondegradable stents are the most common postoperative adjuvant therapy methods. However, the repair of injured endometrium is relatively restricted due to the different anatomical structures of the endometrium. Recently, the treatment outcome of IUAs has improved with the advancement of hysteroscopic techniques. In particular, the application of bioactive scaffolds combined with tissue engineering technology has proven to have high therapeutic potential or endometrial repair in IUA treatment. Herein, this review has summarized past therapeutic strategies, including postoperative adjuvant therapy, cell or therapeutic molecular delivery therapy methods and bioactive scaffold-based tissue engineering methods. Therefore, this review presented the recent therapeutic strategies for repairing endometrium treatment and pointed out the issues of clinical concern to provide alternative methods for the management of IUAs.

Stem cell-based therapy for ameliorating intrauterine adhesion and endometrium injury

Intrauterine adhesion refers to endometrial repair disorders which are usually caused by uterine injury and may lead to a series of complications such as abnormal menstrual bleeding, recurrent abortion and secondary infertility. At present, therapeutic approaches to intrauterine adhesion are limited due to the lack of effective methods to promote regeneration following severe endometrial injury. Therefore, to develop new methods to prevent endometrial injury and intrauterine adhesion has become an urgent need. For severely damaged endometrium, the loss of stem cells in the endometrium may affect its regeneration. This article aimed to discuss the characteristics of various stem cells and their applications for uterine tissue regeneration.

Design and Characterization of Maltose-Conjugated Polycaprolactone Nanofibrous Scaffolds for Uterine Tissue Engineering

Purpose

Uterine anomalies are prevalent in women, and the major treatment assisted to them is hysterectomy as donor availability is extremely low. To overcome this, engineering uterine myometrium smooth muscle tissue has become very important. Several studies have shown that polycaprolactone (PCL) nanofibers are very effective in engineering smooth muscles, as this type of scaffold has structural similarities to the extracellular matrices of the cells. Here, we hypothesize that by electrospinning PCL nanofibers, they form a suitable scaffold for uterine tissue engineering.

Methods

Polycaprolactone nanofibrous scaffolds were fabricated, and surface modification was performed following two step wet chemistry method. First step is aminolysis which introduces the primary amine groups on the PCL scaffolds following which maltose is conjugated on the scaffolds. This was confirmed by the ninhydrin assay for the presence of amine groups. This was followed by ELLA assay where the presence of maltose on the scaffold was quantified. Modified scaffolds were further characterized by scanning electron microscope (SEM), contact angle analysis and Fourier transform infrared spectroscopy (FTIR). MTT assay, live-dead assay and actin staining were performed on the maltose immobilization to study the improvement of the cell attachment and proliferation rates on the modified scaffolds.

Results

Human uterine fibroblast (HUF) cells displayed significant proliferation on the maltose-modified PCL scaffolds, and they also exhibited appropriate morphology indicating that these modified fibers are highly suitable for uterine cell growth.

Conclusion

Our results indicate that the fabricated maltose PCL (MPCL) scaffolds would be a potential biomaterial to treat uterine injuries and promote regeneration.

Lay Summary and Future Work

Uterine anomalies are prevalent in women, and the major treatment is hysterectomy as donor availability is extremely low. Over the past few years, considerable efforts have been directed towards uterine tissue regeneration. This study is to design a tissue engineered scaffold that could act as a human uterine myometrial patch. We propose to create uterine fibroblast-based synthetic scaffolds that act in a condition similar to the intrauterine microenvironment where the embryos are embedded in the uterine wall. For understanding of the efficiency of the myometrial patch, functional characterization will be performed to study the effects of estrogen and prostaglandins on myometrial activity of the designed patch. Results from these experiments will assist a deeper understanding of how to construct a total bioengineered uterus which can substitute the uterus transplantation procedure, which nonetheless is in its initial stages of development.

Graphical Abstract

Uterine Tissue Engineering: Where We Stand and the Challenges Ahead

Tissue engineering is an innovative approach to develop allogeneic tissues and organs. The uterus is a very sensitive and complex organ, which requires refined techniques to properly regenerate and even, to rebuild itself. Many therapies were developed in 20th century to solve reproductive issues related to uterus failure and, more recently, tissue engineering techniques provided a significant evolution in this issue. Herein we aim to provide a broad overview and highlights of the general concepts involved in bioengineering to reconstruct the uterus and its tissues, focusing on strategies for tissue repair, production of uterine scaffolds, biomaterials and reproductive animal models, highlighting the most recent and effective tissue engineering protocols in literature and their application in regenerative medicine. In addition, we provide a discussion about what was achieved in uterine tissue engineering, the main limitations, the challenges to overcome and future perspectives in this research field.

3D stem cell-laden artificial endometrium: successful endometrial regeneration and pregnancy

Thin endometrium lining or severe endometrial injury which may occur during artificial abortion can cause defective endometrial receptivity and subsequent infertility. Therefore, much effort has been devoted toward regenerating thin or damaged endometrial lining by applying multiple types of stem cells. Even though there are some positive preliminary outcomes, repairing the injured endometrium with stem cells is considerably challenging, due to the lack of an adequate microenvironment for the administrated stem cells within the tissues and subsequent poor therapeutic efficiency. In this context, as an alternative, we fabricated a 3D stem cell-laden artificial endometrium by incorporating several biodegradable biomaterials (collagen and hyaluronic acid) and multiple cellular components of endometrium (endometrial stem cells, stromal cells, and vessel cells) to properly recapitulate the multicellular microenvironment and multilayered structure. Agarose was used as an inert filler substrate to enhance the mechanical integrity of the three-layered artificial endometrium. Various mechanical characteristics, such as morphology, compression properties, swelling, and viscosity, have been evaluated. Various biological features, such as steroid hormone responsiveness, specific endometrial cell-surface marker expressions, and the secretion of multiple growth factors and steroid hormones, as well as the viability of encapsulated endometrial cells are relatively well maintained within the artificial endometrium. More importantly, severe tissue injuries were significantly relieved by transplanting our 3D artificial endometrium into endometrial ablation mice. Remarkably, artificial endometrium transplantation resulted in a successful pregnancy with subsequent live birth without any morphological or chromosomal abnormalities.

Strategies for managing asherman's syndrome and endometrial atrophy: Since the classical experimental models to the new bioengineering approach

Endometrial function is essential for embryo implantation and pregnancy, but managing endometrial thickness that is too thin to support pregnancy or an endometrium of compromised functionality due to intrauterine adhesions is an ongoing challenge in reproductive medicine. Here, we review current and emerging therapeutic and experimental options for endometrial regeneration with a focus on animal models used to study solutions for Asherman's syndrome and endometrial atrophy, which both involve a damaged endometrium. A review of existing literature was performed that confirmed the lack of consensus on endometrial therapeutic options, though promising new alternatives have emerged in recent years (platelet-rich plasma, exosomes derived from stem cells, bioengineering-based techniques, endometrial organoids, among others). In the future, basic research using established experimental models of endometrial pathologies (combined with new high-tech solutions) and human clinical trials with large population sizes are needed to evaluate these emerging and new endometrial therapies.

Effects of bone marrow mesenchymal stem cells on repair and receptivity of damaged endometrium in rats

AIM

This study was to investigate whether bone marrow mesenchymal stem cells (BMSCs) can repair damaged endometrium in rats and its effect on endometrial receptivity.

METHODS

A rat model of endometrial damage was established by heat injury. BMSCs were labeled with PKH26 and were transplanted into the right uterine cavity. The endometrial thickness and fertility testing were examined to assess the repair of damaged endometrium. The mass on trichrome staining was used to assess the endometrium fibrosis. The expression of integrin avβ3 and leukemia inhibitory factor (LIF) in rat endometrium was used to evaluate the endometrial receptivity.

RESULTS

After transplantation of BMSCs, the distribution of PKH26 positive cells was mainly on the damaged side in the endometrial tissues. Compared to control group, the endometrial tissue structure recovered after treatment with BMSCs. BMSCs transplantation improved the fertility of endometrial injury model rats. BMSCs decreased the area of endometrial fibrosis. The expression of integrin avβ3 and LIF in endometrium was the stronger in BMSCs treatment group than control group.

CONCLUSION

BMSCs can migrate to the endometrium and repair damaged endometrium and improve endometrium receptivity.

Transplantation of Human Adipose Stem Cells Using Acellular Human Amniotic Membrane Improves Angiogenesis in Injured Endometrial Tissue in a Rat Intrauterine Adhesion Model

Endometrial injury resulting in intrauterine adhesion is associated with extensive damage to the regenerative basal layer of the endometrium and represents a major therapeutic challenge. Human adipose stem cells (hASCs) hold promise for future clinical use in the individualized therapy of injured endometrial tissue. Here, we observed that the use of the acellular human amniotic membrane (AHAM) significantly increased the expression of angiogenic factors, including angiogenin (ANG) and vascular endothelial growth factor (VEGF), in hASCs in vitro. The three-dimensional engineered hASC-AHAM grafts significantly increased the endometrial receptivity, as increased endometrial thickness, greater numbers of endometrial glands, and higher protein levels of leukemia inhibitory factor were observed in injured endometrial tissue that was treated with these grafts compared to those detected in injured endometrial tissue that was treated with AHAM alone. In addition, the hASC-AHAM grafts significantly increased the vascular density in the injured endometrial tissue in rats, when transplanted into an injured uterine cavity. Using the EGFP⁺-hASC-AHAM grafts for transplantation, we confirmed that the hASCs maintained higher protein levels of ANG and VEGF in the injured uterine cavity in vivo. The results of this study suggest that the ability of the engineered hASC-AHAM grafts to repair injured endometrial tissue may be associated with their ability to promote angiogenesis through the upregulated expression of angiogenic factors in hASCs. These findings may support individualized stem cell–based therapy for endometrial disease using bioartificial grafts.

Building a stem cell-based primate uterus

The uterus is the organ for embryo implantation and fetal development. Most current models of the uterus are centred around capturing its function during later stages of pregnancy to increase the survival in pre-term births. However, in vitro models focusing on the uterine tissue itself would allow modelling of pathologies including endometriosis and uterine cancers, and open new avenues to investigate embryo implantation and human development. Motivated by these key questions, we discuss how stem cell-based uteri may be engineered from constituent cell parts, either as advanced self-organising cultures, or by controlled assembly through microfluidic and print-based technologies.

Cell-based endometrial regeneration: current status and future perspectives

Endometrial-related disorders including Asherman's syndrome, thin endometrium, pelvic organ prolapse, and cesarean scar pregnancies can be accompanied by different symptoms such as amenorrhea, infertility, abnormal placental implantation and recurrent miscarriage. Different methods have been introduced to overcome these problems such as surgery and hormonal therapy but none of them has shown promising outcomes. On the other hand, the development of novel regenerative therapeutic strategies has opened new avenues for the treatment of endometrial-related deficiencies. In this regard, different types of scaffolds, acellular matrices and also cell therapy with adult or stem cells have been investigated for the treatment of endometrial-related deficiencies. In this paper, we review the current status of cell-based endometrium regeneration using scaffold dependent and scaffold-free methods and future perspectives in this field. Moreover, we discuss the endometrial diseases that can be candidates for cell-based treatments. Also, the cells with the potential for endometrial regeneration are explained.

Regenerative Medicine Approaches in Bioengineering Female Reproductive Tissues

Diseases, disorders, and dysfunctions of the female reproductive tract tissues can result in either infertility and/or hormonal imbalance. Current treatment options are limited and often do not result in tissue function restoration, requiring alternative therapeutic approaches. Regenerative medicine offers potential new therapies through the bioengineering of female reproductive tissues. This review focuses on some of the current technologies that could address the restoration of functional female reproductive tissues, including the use of stem cells, biomaterial scaffolds, bio-printing, and bio-fabrication of tissues or organoids. The use of these approaches could also be used to address issues in infertility. Strategies such as cell-based hormone replacement therapy could provide a more natural means of restoring normal ovarian physiology. Engineering of reproductive tissues and organs could serve as a powerful tool for correcting developmental anomalies. Organ-on-a-chip technologies could be used to perform drug screening for personalized medicine approaches and scientific investigations of the complex physiological interactions between the female reproductive tissues and other organ systems. While some of these technologies have already been developed, others have not been translated for clinical application. The continuous evolution of biomaterials and techniques, advances in bioprinting, along with emerging ideas for new approaches, shows a promising future for treating female reproductive tract-related disorders and dysfunctions.

Bioengineering of the Uterus

Impairment of uterine structure and function causes infertility, pregnancy loss, and perinatal complications in humans. Some types of uterine impairments such as Asherman’s syndrome, also known as uterine synechiae, can be treated medically and surgically in a standard clinical setting, but absolute defects of uterine function or structure cannot be cured by conventional approaches. To overcome such hurdles, partial or whole regeneration and reconstruction of the uterus have recently emerged as new therapeutic strategies. Transplantation of the whole uterus into patients with uterine agenesis results in the successful birth of children. However, it remains an experimental treatment with numerous difficulties such as the need for continuous and long-term use of immunosuppressive drugs until a live birth is achieved. Thus, the generation of the uterus by tissue engineering technologies has become an alternative but indispensable therapeutic strategy to treat patients without a functional or well-structured uterus. For the past 20 years, the bioengineering of the uterus has been studied intensively in animal models, providing the basis for clinical applications. A variety of templates and scaffolds made from natural biomaterials, synthetic materials, or decellularized matrices have been characterized to efficiently generate the uterus in a manner similar to the bioengineering of other organs and tissues. The goal of this review is to provide a comprehensive overview and perspectives of uterine bioengineering focusing on the type, preparation, and characteristics of the currently available scaffolds.

Reconstruction of functional uterine tissues through recellularizing the decellularized rat uterine scaffolds by MSCs in vivo and in vitro

Infertile people who suffered from loss of uterine structures and/or functions can be treated through gestational surrogacy or uterus transplantation, which remains challenging due to the ethical and social issues, the lack of donor organs as well as technical and safety risks. One promising solution is to regenerate and reconstruct a bioartificial uterus for transplantation through the engineering of uterine architecture and appropriate cellular constituents. Here, we developed a well-defined system to regenerate a functional rat uterine through recellularization of the decellularized uterine matrix (DUM) patches reseeded with human mesenchymal stem cells (hMSCs). Engraftment of the recellularized DUMs on the partially excised uteri yielded a functional rat uterus with a pregnancy rate and number of fetuses per uterine horn comparable to that of the control group with an intact uterus. Particularly, the recellularized DUMs enhanced the regeneration of traumatic uterine in vivo because of MSC regulation. The established system here will shed light on the treatment of uterine infertility with heterogeneous DUMs/cell resources through tissue engineering in the future.

Recent developments in bio-scaffold materials as delivery strategies for therapeutics for endometrium regeneration

Intrauterine adhesions (IUAs) refer to the repair disorder after endometrial injury and may lead to uterine infertility, recurrent miscarriage, abnormal menstrual bleeding, and other obstetric complications. It is a pressing public health issue among women of childbearing age. Presently, there are limited clinical treatments for IUA, and there is no sufficient evidence that these treatment modalities can effectively promote regeneration after severe endometrial injury or improve pregnancy outcome. The inhibitory pathological micro-environment is the main factor hindering the repair of endometrial damaged tissues. To address this, tissue engineering and regenerative medicine have been achieving promising developments. Particularly, biomaterials have been used to load stem cells or therapeutic factors or construct an in situ delivery system as a treatment strategy for endometrial injury repair. This article comprehensively discusses the characteristics of various bio-scaffold materials and their application as stem cell or therapeutic factor delivery systems constructed for uterine tissue regeneration.

Decellularization Methods of Uterus in Tissue Engineering

A new field of investigation which aims to design tissues and organs similar to their native origin has been developed recently, named as regenerative medicine (tissue engineering and bio-engineering). Uterus is the main organ for regeneration and contributes in the fertility. At an ultimate level, the uterus plays a role in embryo implantation, sperm migration and fetal nutrition. Uterine congenital anomalies, attained uterine lesions and immune system disorders may affect such uterine functions preventing successful pregnancy. Due to following reasons, it is essential to consider regenerative medicine as a new approach for the treatment of uterine dysfunctions to overcome the failures that cannot be treated with clinical medication.

Application of Hydrogel-Based Delivery System in Endometrial Repair

A receptive endometrium with proper thickness is essential for successful embryo implantation. However, endometrial injury caused by intrauterine procedures often leads to pathophysiological changes in its environment, resulting in subsequent female infertility. Among diverse treatment methods of endometrial injury, hydrogels are a class of hydrophilic three-dimensional polymeric network with biocompatibility as well as the capability of absorbing water and encapsulation, which have potential applications as a promising intrauterine controlled-release delivery system. This review summarizes recent advances in the approaches of endometrial repair and further focuses on the application of a hydrogel-based delivery system in endometrial repair, including its preparation, therapeutic loading considerations, clinical applications, as well as working mechanisms.

3D Bioprinting a human iPSC-derived MSC-loaded scaffold for repair of the uterine endometrium

Common events in the clinic, such as uterine curettage or inflammation, may lead to irreversible endometrial damage, often resulting in infertility in women of childbearing age. Currently, tissue engineering has the potential to achieve tissue manipulation, regeneration, and growth, but personalization and precision remain challenges. The application of "3D cell printing" is more in line with the clinical requirements of tissue repair. In this study, a porous grid-type human induced pluripotent stem cell-derived mesenchymal stem cell (hiMSC)-loaded hydrogel scaffold was constructed using a 3D bioprinting device. The 3D-printed hydrogel scaffold provided a permissive in vitro living environment for hiMSCs and significantly increased the survival duration of transplanted hiMSCs when compared with hiMSCs administered locally in vivo. Using an endometrial injury model, we found that hiMSC transplantation can cause early host immune responses (the serological immune response continued for more than 1 month, and the local immune response continued for approximately 1 week). Compared with the sham group, although the regenerative endometrium failed to show full restoration of the normal structure and function of the lining, implantation of the 3D-printed hiMSC-loaded scaffold not only promoted the recovery of the endometrial histomorphology (endometrial tissue and gland regeneration) and the regeneration of endometrial cells (stromal cells and epithelial cells) and endothelial cells but also improved endometrial receptivity functional indicators, namely, pinopode formation and leukemia inhibitory factor and αvβ3 expression, which partly restored the embryo implantation and pregnancy maintenance functions of the injured endometrium. These indicators were significantly better in the 3D-printed hiMSC-loaded scaffold group than in the unrepaired (empty) group, the hiMSCs alone group and the 3D scaffold group, and the empty group showed the worst repair results. Our study confirm that the 3D-printed hiMSC-loaded hydrogel scaffold may be a promising material for endometrial repair.

Mini-Review of the New Therapeutic Possibilities in Asherman Syndrome-Where Are We after One Hundred and Twenty-Six Years?

Asherman syndrome is a multifaceted condition describing the partial or complete removal of the uterine cavity and/or cervical canal. It is a highly debatable topic because of its pronounced influence on both reproductive outcomes and gynaecologic symptoms. The latest reports demonstrated that trauma to the endometrium is the main cause of intrauterine adhesion formation. Left untreated, such adhesions gradually lead to a range of repercussions ranging from mild to severe. Considering the lack of non-invasive approaches, the advent of hysteroscopy has revolutionized the entire field, being otherwise considered the most efficient tool offering new directions and amplifying the chances of treating the Asherman syndrome.

The role of extracellular matrix in normal and pathological pregnancy: Future applications of microphysiological systems in reproductive medicine

IMPACT STATEMENT

Extracellular matrix in the womb regulates the initiation, progression, and completion of a healthy pregnancy. The composition and physical properties of extracellular matrix in the uterus and at the maternal-fetal interface are remodeled at each gestational stage, while maladaptive matrix remodeling results in obstetric disease. As in vitro models of uterine and placental tissues, including micro-and milli-scale versions of these organs on chips, are developed to overcome the inherent limitations of studying human development in vivo, we can isolate the influence of cellular and extracellular components in healthy and pathological pregnancies. By understanding and recreating key aspects of the extracellular microenvironment at the maternal-fetal interface, we can engineer microphysiological systems to improve assisted reproduction, obstetric disease treatment, and prenatal drug safety.

A tissue-engineered uterus supports live births in rabbits

Bioengineered uterine tissue could provide a treatment option for women with uterine factor infertility. In large-animal models, reconstruction of the uterus has been demonstrated only with xenogeneic tissue grafts. Here we use biodegradable polymer scaffolds seeded with autologous cells to restore uterine structure and function in rabbits. Rabbits underwent a subtotal uterine excision and were reconstructed either with autologous cell-seeded constructs, with non-seeded scaffolds, or by suturing. At 6 months post-implantation, only the cell-seeded engineered uteri developed native tissue-like structures, including organized luminal/glandular epithelium, stroma, vascularized mucosa, and two-layered myometrium. Only rabbits with cell-seeded constructs had normal pregnancies (4/10) within the reconstructed segment of the uterus and supported fetal development to term and live birth. With further development, this approach may provide a regenerative medicine solution to uterine factor infertility.

The orientation of a decellularized uterine scaffold determines the tissue topology and architecture of the regenerated uterus in rats†

A decellularized uterine scaffold (DUS) prepared from rats permits recellularization and regeneration of uterine tissues when placed onto a partially excised uterus and supports pregnancy in a fashion comparable to the intact uterus. The underlying extracellular matrix (ECM) together with an acellular, perfusable vascular architecture preserved in DUS is thought to be responsible for appropriate regeneration of the uterus. To investigate this concept, we examined the effect of the orientation of the DUS-preserving ECM and the vascular architecture on uterine regeneration through placement of a DUS onto a partially defective uterine area in the reversed orientation such that the luminal face of the DUS was outside and the serosal face was inside. We characterized the tissue structure and function of the regenerated uterus, comparing the outcome to that when the DUS was placed in the correct orientation. Histological analysis revealed that aberrant structures including ectopic location of glands and an abnormal lining of smooth muscle layers were observed significantly more frequently in the reversed group than in the correct group (70% vs. 30%, P < 0.05). Despite the changes in tissue topology, the uteri regenerated with an incorrectly oriented DUS could achieve pregnancy in a way similar to uteri regenerated with a correctly oriented DUS. These results suggest that DUS-driven ECM orientation determines the regenerated uterus structure. Using DUS in the correct orientation is preferable when clinically applied. The disoriented DUS may deteriorate the tissue topology leading to structural disease of the uterus even though the fertility potential is not immediately affected.

Microphysiological Modeling of the Human Endometrium

Since the beginning of clinical medicine, the human uterus has held the fascination of clinicians and researchers, given its critical role in the reproduction of our species. The endometrial lining provides residence for the embryo; however, this symbiotic interaction can be disrupted if the timing is not correct and the endometrium is not receptive. Diseases associated with the endometrium interfere with the reproductive process and cause a life-altering burden of pain and even death. With the advancement of technologies and new insights into the biology of the endometrium, much has been uncovered about the dynamic and essential changes that need to occur for normal endometrial function, as well as aberrations that lead to endometrial diseases. As expected, the more that is uncovered, the more the complexity of the endometrium is made evident. In this study, we bring together three areas of scientific advancement that remain in their infancy, but which together have the potential to mirror this complexity and enable understanding. Studies on induced pluripotent stem cells, three-dimensional tissue mimics, and microfluidic culture platforms will be reviewed with a focus on the endometrium. These unconventional approaches will provide new perspectives and appreciation for the elegance and complexity of the endometrium. Impact statement The ability of the human endometrium to regenerate on a monthly basis for ∼4 decades of reproductive years exemplifies its complexity as well as its susceptibility to disease. Restrictions on the types of research that can be done in the human endometrium motivate the development of new technologies and model systems. The three areas of technological advancement reviewed here-induced pluripotent stem cells, three-dimensional model systems, and microfluidic culture systems-will highlight some of the tools that can be applied to studying the human endometrium in ways that have not been done before.

Advances in the Application of Biomimetic Endometrium Interfaces for Uterine Bioengineering in Female Infertility

The Asherman’s syndrome, also known as intrauterine adhesion, often follows endometrium injuries resulting from dilation and curettage, hysteroscopic resection, and myomectomy as well as infection. It often leads to scarring formation and female infertility. Pathological changes mainly include gland atrophy, lack of vascular stromal tissues and hypoxia and anemia microenvironment in the adhesion areas. Surgical intervention, hormone therapy and intrauterine device implantation are the present clinical treatments for Asherman’s syndrome. However, they do not result in functional endometrium recovery or pregnancy rate improvement. Instead, an increasing number of researches have paid attention to the reconstruction of biomimetic endometrium interfaces with advanced tissue engineering technology in recent decades. From micro-scale cell sheet engineering and cell-seeded biological scaffolds to nano-scale extracellular vesicles and bioactive molecule delivery, biomimetic endometrium interfaces not only recreate physiological multi-layered structures but also restore an appropriate nutritional microenvironment by increasing vascularization and reducing immune responses. This review comprehensively discusses the advances in the application of novel biocompatible functionalized endometrium interface scaffolds for uterine tissue regeneration in female infertility.