Differential expression of Wnt signaling molecules between pre- and postmenopausal endometrial epithelial cells suggests a population of putative epithelial stem/progenitor cells reside in the basalis layer

Transplantation of acellular amniotic membrane seeded with adipose-derived mesenchymal stem cells in a rat model of intrauterine adhesion

Objective

This study aimed to investigate the role of acellular amniotic membrane (AAM) loaded with adipose-derived mesenchymal stem cells (ADSCs) for the treatment of intrauterine adhesion.

Methods

One hundred twenty female Spargue-Dawley rats were randomly divided into four groups: sham operation group (the uterus was picked out and incised without treatment), intrauterine adhesion group, the experimental group treated with AAM, and experimental group treated with AAM loaded with ADSCs. Histological and immunohistochemical analysis were performed on 3, 7, and 14 days after surgery to evaluate the degree of uterine fibrosis and regeneration of injured endometrium. RNA sequencing and real-time PCR were used to explore the potential mechanism by which ADSCs modulated immune response and promoted endometrial regeneration.

Results

On 14 days after surgery, the endometrial thickness, number of glands, and degree of fibrosis reduction in the ADSCs/AAM group was higher than those in the AAM group, and similar to the sham operation group. RNA sequencing analysis showed that ADSCs can modulate local immune responses and promote the formation of functional endometrium. Meanwhile, we found that ADSCs significantly decreased the levels of pro-inflammatory cytokines (TNF-α and IL-1β) and increased the levels of anti-inflammatory cytokines (bFGF and IL-6).

Conclusion

Our results demonstrated that AAM loaded with ADSCs can result in the regeneration of injured endometrium and fibrosis reduction. Meanwhile, ADSCs also regulated the immune microenvironment, which was beneficial to functional endometrial recovery.

Regenerative Medicine in Gynecology

The female reproductive tract undergoes dynamic changes across the life span. Congenital abnormalities, life events, and medical interventions can negatively affect the structure and function of reproductive tract organs, resulting in lifelong sequelae. The objective of regenerative gynecology is to discover and promote endogenous mechanisms by which a healthy tissue maintains overall tissue integrity after injury, after disease, or with age. In this review, we discuss some of the key state-of-the-art cell-based and scaffolding therapies that have been applied to regenerate gynecologic tissues and organs primarily in animal and tissue culture models. We further discuss the limitations of current technologies, problems of implementation and scalability, and future outlook of the field.

Endometrial and placental stem cells in successful and pathological pregnancies

The endometrium is a dynamic tissue that undergoes extensive remodeling during the menstrual cycle and further gets modified during pregnancy. Different kinds of stem cells are reported in the endometrium. These include epithelial stem cells, endometrial mesenchymal stem cells, side population stem cells, and very small embryonic-like stem cells. Stem cells are also reported in the placenta which includes trophoblast stem cells, side population trophoblast stem cells, and placental mesenchymal stem cells. The endometrial and placental stem cells play a pivotal role in endometrial remodeling and placental vasculogenesis during pregnancy. The dysregulation of stem cell function is reported in various pregnancy complications like preeclampsia, fetal growth restriction, and preterm birth. However, the mechanisms by which it does so are yet elusive. Herein, we review the current knowledge of the different type of stem cells involved in pregnancy initiation and also highlight how their improper functionality leads to pathological pregnancy.

The Therapeutic Potential of Multipotent Mesenchymal Stromal Cell-Derived Extracellular Vesicles in Endometrial Regeneration

Disruption of endometrial regeneration, fibrosis formation, and intrauterine adhesions underlie the development of "thin" endometrium and/or Asherman's syndrome (AS) and are a common cause of infertility and a high risk for adverse obstetric outcomes. The methods used (surgical adhesiolysis, anti-adhesive agents, and hormonal therapy) do not allow restoration of the regenerative properties of the endometrium. The experience gained today with cell therapy using multipotent mesenchymal stromal cells (MMSCs) proves their high regenerative and proliferative properties in tissue damage. Their contribution to regenerative processes is still poorly understood. One of these mechanisms is based on the paracrine effects of MMSCs associated with the stimulation of cells of the microenvironment by secreting extracellular vesicles (EVs) into the extracellular space. EVs, whose source is MMSCs, are able to stimulate progenitor cells and stem cells in damaged tissues and exert cytoprotective, antiapoptotic, and angiogenic effects. This review described the regulatory mechanisms of endometrial regeneration, pathological conditions associated with a decrease in endometrial regeneration, and it presented the available data from studies on the effect of MMSCs and their EVs on endometrial repair processes, and the involvement of EVs in human reproductive processes at the level of implantation and embryogenesis.

New concepts on the etiology of endometriosis

Endometriosis is a serious, chronic disorder where endometrial tissue grows outside the uterus, causing severe pelvic pain and infertility. It affects 11% of women. Endometriosis is a multifactorial disorder of unclear etiology, although retrograde menstruation plays a major role. It has a genetic component with over 40 genetic risk factors mapped, although their mechanism of action is still emerging. New evidence suggests a role for retrograde menstruation of endometrial stem/progenitor cells, now that identifying markers of these cells are available. Recent lineage tracing and tissue clearing microscopy and 3D reconstruction has provided new understanding of endometrial glandular structure, particularly the horizontal orientation and interconnection of basalis glands. New sequencing technologies, particularly whole genome DNA sequencing are revealing somatic mutations, including in cancer driver genes, in normal and eutopic endometrium of patients with endometriosis, as well as ectopic endometriotic lesions. Methylome sequencing is offering insight into the regulation of genes and the role of the environmental factors. Single cell RNA sequencing reveals the transcriptome of individual endometrial cells, shedding new light on the diversity and range of cellular subpopulations of the major cell types present in the endometrium and in endometriotic lesions. New endometrial epithelial organoid cultures replicating glandular epithelium are providing tractable models for studying endometriosis. Organoids derived from menstrual fluid offer a non-invasive source of endometrial tissue and a new avenue for testing drugs and developing personalized medicine for treating endometriosis. These new approaches are rapidly advancing our understanding of endometriosis etiology.

Subtle changes in perivascular endometrial mesenchymal stem cells after local endometrial injury in recurrent implantation failure

Improvements in reproductive techniques have resulted in the live birth rates from IVF procedures increasing from 5% to approximately 30% in recent decades but has plateaued since. Emerging preclinical and clinical data implicates endometrial receptivity deficiencies in patients with recurrent implantation failure (RIF) as the predominant factor hindering successful implantation. Mechanisms on how local endometrial injury (LEI) improves implantation rates in patients with RIF are currently unknown. We hypothesized that LEI may influence perivascular endometrial mesenchymal stem/progenitor cells (eMSCs) which are thought to regenerate the stromal vascular component of the functional layer every month. Here, we assessed the effect of LEI on the proportion and function of eMSCs present in consecutive LEI biopsies. Consecutive paired mid-luteal phase endometrial biopsies obtained from patients with RIF were digested to single cells and the proportion of SUSD2-expressing cells determined. Growth kinetics and decidualization were compared between the consecutive LEI samples. A mid-luteal LEI altered the decidualization capacity of SUSD2⁺ eMSCs in women with RIF, but not their proportion or clonogenicity. With the potential of LEI to improve IVF outcomes in women with RIF, additional investigations are needed to understand the impact of the altered decidualization response in eMSCs.

Long-term maintenance of human endometrial epithelial stem cells and their therapeutic effects on intrauterine adhesion

Background

The human endometrium is a highly regenerative tissue that is believed to have two main types of stem cells: endometrial mesenchymal/stromal stem cells (eMSCs) and endometrial epithelial stem cells (eESCs). So far, eMSCs have been extensively studied, whereas the studies of eESCs are constrained by the inability to culture and expand them in vitro. The aim of this study is to establish an efficient method for the production of eESCs from human endometrium for potential clinical application in intrauterine adhesion (IUA).

Results

Here we developed a culture condition with a combination of some small molecules for in vitro culturing and expansion of human SSEA-1⁺ cells. The SSEA-1⁺ cells exhibited stem/progenitor cell activity in vitro, including clonogenicity and differentiation capacity into endometrial epithelial cell-like cells. In addition, the SSEA-1⁺ cells, embedded in extracellular matrix, swiftly self-organized into organoid structures with long-term expansion capacity and histological phenotype of the human endometrial epithelium. Specifically, we found that the SSEA-1⁺ cells showed stronger therapeutic potential than eMSCs for IUA in vitro. In a rat model of IUA, in situ injection of the SSEA-1⁺ cells-laden chitosan could efficiently reduce fibrosis and facilitate endometrial regeneration.

Conclusions

Our work demonstrates an approach for isolation and expansion of human eESCs in vitro, and an appropriate marker, SSEA-1, to identify eESCs. Furthermore, the SSEA-1⁺ cells-laden chitosan might provide a novel cell-based approach for IUA treatment. These findings will advance the understanding of pathophysiology during endometrial restoration which may ultimately lead to more rational clinical practice.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00905-4.

The Role of Endometrial Stem/Progenitor Cells in Recurrent Reproductive Failure

Recurrent implantation failure (RIF) and recurrent pregnancy loss (RPL), collectively referred to as recurrent reproductive failure (RRF), are both challenging conditions with many unanswered questions relating to causes and management options. Both conditions are proposed to be related to an aberrant endometrial microenvironment, with different proposed aetiologies related to a restrictive or permissive endometrium for an invading embryo. The impressive regenerative capacity of the human endometrium has been well-established and has led to the isolation and characterisation of several subtypes of endometrial stem/progenitor cells (eSPCs). eSPCs are known to be involved in the pathogenesis of endometrium-related disorders (such as endometriosis) and have been proposed to be implicated in the pathogenesis of RRF. This review appraises the current knowledge of eSPCs, and their involvement in RRF, highlighting the considerable unknown aspects in this field, and providing avenues for future research to facilitate much-needed advances in the diagnosis and management of millions of women suffering with RRF.

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.

Endometrial Stem/Progenitor Cells–Their Role in Endometrial Repair and Regeneration

The human endometrium is a remarkable tissue, undergoing ~450 cycles of proliferation, differentiation, shedding (menstruation), repair, and regeneration over a woman's reproductive lifespan. Post-menstrual repair is an extremely rapid and scar-free process, with re-epithelialization of the luminal epithelium completed within 48 h of initiation of shedding. Following menstruation, the functionalis grows from the residual basalis layer during the proliferative phase under the influence of rising circulating estrogen levels. The regenerative capacity of the endometrium is attributed to stem/progenitor cells which reside in both the epithelial and stromal cell compartments of the basalis layer. Finding a definitive marker for endometrial epithelial progenitors (eEPCs) has proven difficult. A number of different markers have been suggested as putative progenitor markers including, N-cadherin, SSEA-1, AXIN2, SOX-9 and ALDH1A1, some of which show functional stem cell activity in in vitro assays. Each marker has a unique location(s) in the glandular epithelium, which has led to the suggestion that a differentiation hierarchy exists, from the base of epithelial glands in the basalis to the luminal epithelium lining the functionalis, where epithelial cells express different combinations of markers as they differentiate and move up the gland into the functionalis away from the basalis niche. Perivascular endometrial mesenchymal stem cells (eMSCs) can be identified by co-expression of PDGFRβ and CD146 or by a single marker, SUSD2. This review will detail the known endometrial stem/progenitor markers; their identity, location and known interactions and hierarchy across the menstrual cycle, in particular post-menstrual repair and estrogen-driven regeneration, as well as their possible contributions to menstruation-related disorders such as endometriosis and regeneration-related disorder Asherman's syndrome. We will also highlight new techniques that allow for a greater understanding of stem/progenitor cells' role in repair and regeneration, including 3D organoids, 3D slice cultures and gene sequencing at the single cell level. Since mouse models are commonly used to study menstruation, repair and regeneration we will also detail the mouse stem/progenitor markers that have been investigated in vivo.

Comparison of Organoids from Menstrual Fluid and Hormone-Treated Endometrium: Novel Tools for Gynecological Research

Endometrial organoids (EMO) are an important tool for gynecological research but have been limited by generation from (1) invasively acquired tissues and thus advanced disease states and (2) from women who are not taking hormones, thus excluding 50% of the female reproductive-aged population. We sought to overcome these limitations by generating organoids from (1) menstrual fluid (MF; MFO) using a method that enables the concurrent isolation of menstrual fluid supernatant, stromal cells, and leukocytes and (2) from biopsies and hysterectomy samples from women taking hormonal medication (EMO-H). MF was collected in a menstrual cup for 4–6 h on day 2 of menstruation. Biopsies and hysterectomies were obtained during laparoscopic surgery. Organoids were generated from all sample types, with MFO and EMO-H showing similar cell proliferation rates, proportion and localization of the endometrial basalis epithelial marker, Stage Specific Embryonic Antigen-1 (SSEA-1), and gene expression profiles. Organoids from different disease states showed the moderate clustering of epithelial secretory and androgen receptor signaling genes. Thus, MFO and EMO-H are novel organoids that share similar features to EMO but with the advantage of (1) MFO being obtained non-invasively and (2) EMO-H being obtained from 50% of the women who are not currently being studied through standard methods. Thus, MFO and EMO-H are likely to prove to be invaluable tools for gynecological research, enabling the population-wide assessment of endometrial health and personalized medicine.

Cyclical endometrial repair and regeneration

Uniquely among adult tissues, the human endometrium undergoes cyclical shedding, scar-free repair and regeneration during a woman's reproductive life. Therefore, it presents an outstanding model for study of such processes. This Review examines what is known of endometrial repair and regeneration following menstruation and parturition, including comparisons with wound repair and the influence of menstrual fluid components. We also discuss the contribution of endometrial stem/progenitor cells to endometrial regeneration, including the importance of the stem cell niche and stem cell-derived extracellular vesicles. Finally, we comment on the value of endometrial epithelial organoids to extend our understanding of endometrial development and regeneration, as well as therapeutic applications.

Endometrial stem/progenitor cells and their roles in immunity, clinical application, and endometriosis

Endometrial stem/progenitor cells have been proved to exist in periodically regenerated female endometrium and can be divided into three categories: endometrial epithelial stem/progenitor cells, CD140b+CD146+ or SUSD2+ endometrial mesenchymal stem cells (eMSCs), and side population cells (SPs). Endometrial stem/progenitor cells in the menstruation blood are defined as menstrual stem cells (MenSCs). Due to their abundant sources, excellent proliferation, and autotransplantation capabilities, MenSCs are ideal candidates for cell-based therapy in regenerative medicine, inflammation, and immune-related diseases. Endometrial stem/progenitor cells also participate in the occurrence and development of endometriosis by entering the pelvic cavity from retrograde menstruation and becoming overreactive under certain conditions to form new glands and stroma through clonal expansion. Additionally, the limited bone marrow mesenchymal stem cells (BMDSCs) in blood circulation can be recruited and infiltrated into the lesion sites, leading to the establishment of deep invasive endometriosis. On the other hand, cell derived from endometriosis may also enter the blood circulation to form circulating endometrial cells (CECs) with stem cell-like properties, and to migrate and implant into distant tissues. In this manuscript, by reviewing the available literature, we outlined the characteristics of endometrial stem/progenitor cells and summarized their roles in immunoregulation, regenerative medicine, and endometriosis, through which to provide some novel therapeutic strategies for reproductive and cancerous diseases.

Adult stem cells in endometrial regeneration: Molecular insights and clinical applications

Endometrial damage is an important cause of female reproductive problems, manifested as menstrual abnormalities, infertility, recurrent pregnancy loss, and other complications. These conditions are collectively termed "Asherman syndrome" (AS) and are typically associated with recurrent induced pregnancy terminations, repeated diagnostic curettage and intrauterine infections. Cancer treatment also has unexpected detrimental side effects on endometrial function in survivors independently of ovarian effects. Endometrial stem cells act in the regeneration of the endometrium and in repair through direct differentiation or paracrine effects. Nonendometrial adult stem cells, such as bone marrow-derived mesenchymal stem cells and umbilical cord-derived mesenchymal stem cells, with autologous and allogenic applications, can also repair injured endometrial tissue in animal models of AS and in human studies. However, there remains a lack of research on the repair of the damaged endometrium after the reversal of tumors, especially endometrial cancers. Here, we review the biological mechanisms of endometrial regeneration, and research progress and challenges for adult stem cell therapy for damaged endometrium, and discuss the potential applications of their use for endometrial repair after cancer remission, especially in endometrial cancers. Successful application of such cells will improve reproductive parameters in patients with AS or cancer. Significance: The endometrium is the fertile ground for embryos, but damage to the endometrium will greatly impair female fertility. Adult stem cells combined with tissue engineering scaffold materials or not have made great progress in repairing the injured endometrium due to benign lesions. However, due to the lack of research on the repair of the damaged endometrium caused by malignant tumors or tumor therapies, the safety and effectiveness of such stem cell-based therapies need to be further explored. This review focuses on the molecular insights and clinical application potential of adult stem cells in endometrial regeneration and discusses the possible challenges or difficulties that need to be overcome in stem cell-based therapies for tumor survivors. The development of adult stem cell-related new programs will help repair damaged endometrium safely and effectively and meet fertility needs in tumor survivors.

The Elusive Endometrial Epithelial Stem/Progenitor Cells

The human endometrium undergoes approximately 450 cycles of proliferation, differentiation, shedding and regeneration over a woman's reproductive lifetime. The regenerative capacity of the endometrium is attributed to stem/progenitor cells residing in the basalis layer of the tissue. Mesenchymal stem cells have been extensively studied in the endometrium, whereas endometrial epithelial stem/progenitor cells have remained more elusive. This review details the discovery of human and mouse endometrial epithelial stem/progenitor cells. It highlights recent significant developments identifying putative markers of these epithelial stem/progenitor cells that reveal their in vivo identity, location in both human and mouse endometrium, raising common but also different viewpoints. The review also outlines the techniques used to identify epithelial stem/progenitor cells, specifically in vitro functional assays and in vivo lineage tracing. We will also discuss their known interactions and hierarchy and known roles in endometrial dynamics across the menstrual or estrous cycle including re-epithelialization at menses and regeneration of the tissue during the proliferative phase. We also detail their potential role in endometrial proliferative disorders such as endometriosis.

Cellular Origins of Endometriosis: Towards Novel Diagnostics and Therapeutics

Endometriosis remains an enigmatic disease of unknown etiology, with delayed diagnosis and poor therapeutic options. This review will discuss the cellular, physiological, and genomic evidence of Sampson's hypothesis of retrograde menstruation as a cause of pelvic endometriosis and as the basis of phenotypic heterogeneity of the disease. We postulate that collaborative research at the single cell level focused on unlocking the cellular, physiological, and genomic mechanisms of endometriosis will be accompanied by advances in personalized diagnosis and therapies that target unique subtypes of endometriosis disease. These advances will address the clinical conundrums of endometriosis clinical care—including diagnostic delay, suboptimal treatments, disease recurrence, infertility, chronic pelvic pain, and quality of life. There is an urgent need to improve outcomes for women with endometriosis. To achieve this, it is imperative that we understand which cells form the lesions, how they arrive at distant sites, and what factors govern their ability to survive and invade at ectopic locations. This review proposes new research avenues to address these basic questions of endometriosis pathobiology that will lay the foundations for new diagnostic tools and treatment pathways.

Cells expressing PAX8 are the main source of homeostatic regeneration of adult mouse endometrial epithelium and give rise to serous endometrial carcinoma

Humans and mice have cyclical regeneration of the endometrial epithelium. It is expected that such regeneration is ensured by tissue stem cells, but their location and hierarchy remain debatable. A number of recent studies have suggested the presence of stem cells in the mouse endometrial epithelium. At the same time, it has been reported that this tissue can be regenerated by stem cells of stromal/mesenchymal or bone marrow cell origin. Here, we describe a single-cell transcriptomic atlas of the main cell types of the mouse uterus and epithelial subset transcriptome and evaluate the contribution of epithelial cells expressing the transcription factor PAX8 to the homeostatic regeneration and malignant transformation of adult endometrial epithelium. According to lineage tracing, PAX8+ epithelial cells are responsible for long-term maintenance of both luminal and glandular epithelium. Furthermore, multicolor tracing shows that individual glands and contiguous areas of luminal epithelium are formed by clonal cell expansion. Inactivation of the tumor suppressor genes Trp53 and Rb1 in PAX8+ cells, but not in FOXJ1+ cells, leads to the formation of neoplasms with features of serous endometrial carcinoma, one of the most aggressive types of human endometrial malignancies. Taken together, our results show that the progeny of single PAX8+ cells represents the main source of regeneration of the adult endometrial epithelium. They also provide direct experimental genetic evidence for the key roles of the P53 and RB pathways in the pathogenesis of serous endometrial carcinoma and suggest that PAX8+ cells represent the cell of origin of this neoplasm.

Endometrial and Menstrual Blood Mesenchymal Stem/Stromal Cells: Biological Properties and Clinical Application

A highly proliferative mesenchymal stem/stromal cell (MSC) population was recently discovered in the dynamic, cyclically regenerating human endometrium as clonogenic stromal cells that fulfilled the International Society for Cellular Therapy (ISCT) criteria. Specific surface markers enriching for clonogenic endometrial MSC (eMSC), CD140b and CD146 co-expression, and the single marker SUSD2, showed their perivascular identity in the endometrium, including the layer which sheds during menstruation. Indeed, cells with MSC properties have been identified in menstrual fluid and commonly termed menstrual blood stem/stromal cells (MenSC). MenSC are generally retrieved from menstrual fluid as plastic adherent cells, similar to bone marrow MSC (bmMSC). While eMSC and MenSC share several biological features with bmMSC, they also show some differences in immunophenotype, proliferation and differentiation capacities. Here we review the phenotype and functions of eMSC and MenSC, with a focus on recent studies. Similar to other MSC, eMSC and MenSC exert immunomodulatory and anti-inflammatory impacts on key cells of the innate and adaptive immune system. These include macrophages, T cells and NK cells, both in vitro and in small and large animal models. These properties suggest eMSC and MenSC as additional sources of MSC for cell therapies in regenerative medicine as well as immune-mediated disorders and inflammatory diseases. Their easy acquisition via an office-based biopsy or collected from menstrual effluent makes eMSC and MenSC attractive sources of MSC for clinical applications. In preparation for clinical translation, a serum-free culture protocol was established for eMSC which includes a small molecule TGFβ receptor inhibitor that prevents spontaneous differentiation, apoptosis, senescence, maintains the clonogenic SUSD2+ population and enhances their potency, suggesting potential for cell-therapies and regenerative medicine. However, standardization of MenSC isolation protocols and culture conditions are major issues requiring further research to maximize their potential for clinical application. Future research will also address crucial safety aspects of eMSC and MenSC to ensure these protocols produce cell products free from tumorigenicity and toxicity. Although a wealth of data on the biological properties of eMSC and MenSC has recently been published, it will be important to address their mechanism of action in preclinical models of human disease.

Understanding the regulatory mechanisms of endometrial cells on activities of endometrial mesenchymal stem-like cells during menstruation

BACKGROUND

The identification of endometrial stem/progenitor cells in a high turnover rate tissue suggests that a well-orchestrated underlying network controls the behaviour of these stem cells. The thickness of the endometrium can grow from 0.5-1 mm to 5-7 mm within a week indicating the need of stem cells for self-renewal and differentiation during this period. The cyclical regeneration of the endometrium suggests specific signals can activate the stem cells during or shortly after menstruation.

METHODS

Endometrial mesenchymal stem-like cells (eMSCs) were cocultured with endometrial epithelial or stromal cells from different phases of the menstrual cycle; the clonogenicity and the phenotypic expression of eMSC markers (CD140b and CD146) were assessed. The functional role of WNT/β-catenin signalling on eMSC was determined by western blot analysis, immunofluorescent staining, flow cytometry, quantitative real-time PCR and small interfering RNA. The cytokine levels in the conditioned medium of epithelial or stromal cells cocultured with eMSCs were evaluated by enzyme-linked immunosorbent assays.

RESULTS

Coculture of endometrial cells (epithelial or stromal) from the menstrual phase enhanced the clonogenicity and self-renewal activities of eMSCs. Such phenomenon was not observed in niche cells from the proliferative phase. Coculture with endometrial cells from the menstrual phase confirmed an increase in expression of active β-catenin in the eMSCs. Treatment with IWP-2, a WNT inhibitor, suppressed the observed effects. Anti-R-spondin-1 antibody reduced the stimulatory action of endometrial niche cells on WNT/β-catenin activation in the T cell factor/lymphoid enhancer-binding factor luciferase reporter assay. Moreover, the mRNA level and protein immunoreactivities of leucine-rich repeat-containing G-protein coupled receptor 5 were higher in eMSCs than unfractionated stromal cells. Conditioned media of endometrial niche cells cocultured with eMSCs contained increased levels of C-X-C motif ligand 1 (CXCL1), CXCL5 and interleukin 6. Treatment with these cytokines increased the clonogenic activity and phenotypic expression of eMSCs.

CONCLUSIONS

Our findings indicate a role of WNT/β-catenin signalling in regulating activities of endometrial stem/progenitor cells during menstruation. Certain cytokines at menstruation can stimulate the proliferation and self-renewal activities of eMSCs. Understanding the mechanism in the regulation of eMSCs may contribute to treatments of endometrial proliferative disorders such as Asherman's syndrome.

Myometrial Cells Stimulate Self-Renewal of Endometrial Mesenchymal Stem-Like Cells Through WNT5A/β-Catenin Signaling

Human endometrium undergoes cycles of proliferation and differentiation throughout the reproductive years of women. The endometrial stem/progenitor cells contribute to this regenerative process. They lie in the basalis layer of the endometrium next to the myometrium. We hypothesized that human myometrial cells provide niche signals regulating the activities of endometrial mesenchymal stem-like cells (eMSCs). In vitro coculture of myometrial cells enhanced the colony-forming and self-renewal ability of eMSCs. The cocultured eMSCs retained their multipotent characteristic and exhibited a greater total cell output when compared with medium alone culture. The expression of active β-catenin in eMSCs increased significantly after coculture with myometrial cells, suggesting activation of WNT/β-catenin signaling. Secretory factors in spent medium from myometrial cell culture produced the same stimulatory effects on eMSCs. The involvement of WNT/β-catenin signaling in self-renewal of eMSCs was confirmed with the use of WNT activator (Wnt3A conditioned medium) and WNT inhibitors (XAV939 and inhibitor of Wnt Production-2 [IWP-2]). The myometrial cells expressed more WNT5A than other WNT ligands. Recombinant WNT5A stimulated whereas anti-WNT5A antibody suppressed the colony formation, self-renewal, and T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) transcriptional activities of eMSCs. Moreover, eMSCs expressed FZD4 and LRP5. WNT5A is known to activate the canonical WNT signaling in the presence of these receptor components. WNT antagonist, DKK1, binds to LRP5/6. Consistently, DKK1 treatment nullified the stimulatory effect of myometrial cell coculture. In conclusion, our findings show that the myometrial cells are niche components of eMSCs, modulating the self-renewal activity of eMSCs by WNT5A-dependent activation of WNT/β-catenin signaling. Stem Cells 2019;37:1455-1466.

Does human endometrial LGR5 gene expression suggest the existence of another hormonally regulated epithelial stem cell niche?

STUDY QUESTION

Is human endometrial leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5) gene expression limited to the postulated epithelial stem cell niche, stratum basalis glands, and is it hormonally regulated?

SUMMARY ANSWER

LGR5 expressing cells are not limited to the postulated stem cell niche but LGR5 expression is hormonally regulated.

WHAT IS KNOWN ALREADY

The human endometrium is a highly regenerative tissue; however, endometrial epithelial stem cell markers are yet to be confirmed. LGR5 is a marker of stem cells in various epithelia.

STUDY DESIGN, SIZE, DURATION

The study was conducted at a University Research Institute. Endometrial samples from 50 healthy women undergoing benign gynaecological surgery with no endometrial pathology at the Liverpool Women's hospital were included and analysed in the following six sub-categories; proliferative, secretory phases of menstrual cycle, postmenopausal, those using oral and local progestagens and samples for in vitro explant culture.

PARTICIPANTS/MATERIALS, SETTING, METHODS

In this study, we used the gold standard method, in situ hybridisation (ISH) along with qPCR and a systems biology approach to study the location of LGR5 gene expression in full thickness human endometrium and Fallopian tubes. The progesterone regulation of endometrial LGR5 was examined in vivo and in short-term cultured endometrial tissue explants in vitro. LGR5 expression was correlated with epithelial proliferation (Ki67), and expression of previously reported epithelia progenitor markers (SOX9 and SSEA-1) immunohistochemistry (IHC).

MAIN RESULTS AND THE ROLE OF CHANCE

LGR5 gene expression was significantly higher in the endometrial luminal epithelium than in all other epithelial compartments in the healthy human endometrium, including the endometrial stratum basalis (P < 0.05). The strongest SSEA-1 and SOX9 staining was observed in the stratum basalis glands, but the general trend of SOX9 and SSEA-1 expression followed the same cyclical pattern of expression as LGR5. Stratum functionalis epithelial Ki67-LI and LGR5 expression levels correlated significantly (r = 0.74, P = 0.01), however, they did not correlate in luminal and stratum basalis epithelium (r = 0.5 and 0.13, respectively). Endometrial LGR5 demonstrates a dynamic spatiotemporal expression pattern, suggesting hormonal regulation. Oral and local progestogens significantly reduced endometrial LGR5 mRNA levels compared with women not on hormonal treatment (P < 0.01). Our data were in agreement with in silico analysis of published endometrial microarrays.

LARGE SCALE DATA

We did not generate our own large scale data but interrogated publically available large scale data sets.

LIMITATIONS, REASONS FOR CAUTION

In the absence of reliable antibodies for human LGR5 protein and validated lineage markers for the various epithelial populations that potentially exist within the endometrium, our study does not formally characterise or examine the functional ability of the resident LGR5+ cells as multipotent.

WIDER IMPLICATIONS OF THE FINDINGS

These data will facilitate future lineage tracing studies in the human endometrial epithelium; to identify the location of stem cells and further complement the in vitro functional studies, to confirm if the LGR5 expressing epithelial cells indeed represent the epithelial stem cell population.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by funding from the Wellbeing of Women project grant (RTF510) and Cancer Research UK (A14895). None of the authors have any conflicts of interest to disclose.

Human menstrual blood: a renewable and sustainable source of stem cells for regenerative medicine

Stem cells (SCs) play an important role in autologous and even allogenic applications. Menstrual blood discharge has been identified as a valuable source of SCs which are referred to as menstrual blood-derived stem cells (MenSCs). Compared to SCs from bone marrow and adipose tissues, MenSCs come from body discharge and obtaining them is non-invasive to the body, they are easy to collect, and there are no ethical concerns. There is, hence, a growing interest in the functions of MenSCs and their potential applications in regenerative medicine. This review presents recent progress in research into MenSCs and their potential application. Clinical indications of using MenSCs for various regenerative medicine applications are emphasized, and future research is recommended to accelerate clinical applications of MenSCs.

Endometrial Stem Cell Markers: Current Concepts and Unresolved Questions

The human endometrium is a highly regenerative organ undergoing over 400 cycles of shedding and regeneration over a woman’s lifetime. Menstrual shedding and the subsequent repair of the functional layer of the endometrium is a process unique to humans and higher-order primates. This massive regenerative capacity is thought to have a stem cell basis, with human endometrial stromal stem cells having already been extensively studied. Studies on endometrial epithelial stem cells are sparse, and the current belief is that the endometrial epithelial stem cells reside in the terminal ends of the basalis glands at the endometrial/myometrial interface. Since almost all endometrial pathologies are thought to originate from aberrations in stem cells that regularly regenerate the functionalis layer, expansion of our current understanding of stem cells is necessary in order for curative treatment strategies to be developed. This review critically appraises the postulated markers in order to identify endometrial stem cells. It also examines the current evidence supporting the existence of epithelial stem cells in the human endometrium that are likely to be involved both in glandular regeneration and in the pathogenesis of endometrial proliferative diseases such as endometriosis and endometrial cancer.

N-cadherin identifies human endometrial epithelial progenitor cells by in vitro stem cell assays

STUDY QUESTION

Is there a specific surface marker that identifies human endometrial epithelial progenitor cells with adult stem cell activity using in vitro assays?

SUMMARY ANSWER

N-cadherin isolates clonogenic, self-renewing human endometrial epithelial progenitor cells with high proliferative potential that differentiate into cytokeratin+ gland-like structures in vitro and identifies their location in some cells of gland profiles predominantly in basalis endometrium adjacent to the myometrium.

WHAT IS KNOWN ALREADY

Human endometrium contains a small population of clonogenic, self-renewing epithelial cells with high proliferative potential that differentiate into large gland-like structures, but their identity and location is unknown. Stage-specific embryonic antigen-1 (SSEA-1) distinguishes the epithelium of basalis from functionalis and is a marker of human post-menopausal (Post-M) endometrial epithelium.

STUDY DESIGN, SIZE, DURATION

Prospective observational study of endometrial epithelial cells obtained from hysterectomy samples taken from 50 pre-menopausal (Pre-M) and 24 Post-M women, of which 4 were from women who had taken daily estradiol valerate 2 mg/day for 8 weeks prior.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Gene profiling was used to identify differentially expressed surface markers between fresh EpCAM (Epithelial Cell Adhesion Molecule)-magnetic bead-selected basalis-like epithelial cells from Post-M endometrium compared with predominantly functionalis epithelial cells from Pre-M endometrium and validated by qRT-PCR. In vitro clonogenicity and self-renewal assays were used to assess the stem/progenitor cell properties of magnetic bead-sorted N-cadherin+ and N-cadherin- epithelial cells. The cellular identity, location and phenotype of N-cadherin+ cells was assessed by dual colour immunofluorescence and confocal microscopy for cytokeratin, proliferative status (Ki-67), ERα, SSEA-1, SOX9 and epithelial mesenchymal transition (EMT) markers on full thickness human endometrium.

MAIN RESULTS AND THE ROLE OF CHANCE

CDH2 (N-cadherin gene) was one of 11 surface molecules highly expressed in Post-M compared to Pre-M endometrial epithelial cells. N-cadherin+ cells comprise a median 16.7% (n = 8) and 20.2% (n = 5) of Pre-M endometrial epithelial cells by flow cytometry and magnetic bead sorting, respectively. N-cadherin+ epithelial cells from Pre-M endometrium were more clonogenic than N-cadherin- cells (n = 12, P = 0.003), underwent more population doublings (n = 7), showed greater capacity for serial cloning (n = 7) and differentiated into cytokeratin+ gland-like organoids. N-cadherin immunolocalised to the lateral and apical membrane of epithelial cells in the bases of glands in the basalis of Pre-M endometrium and Post-M gland profiles, co-expressing cytokeratin, ERα but not SSEA-1 or SOX9, which localized on gland profiles proximal to N-cadherin+ cells. N-cadherin+ cells were quiescent (Ki-67-) in the basalis and in Post-M endometrial glands and co-localized with EMT markers vimentin and E-cadherin.

LARGE SCALE DATA

The raw and processed data files from the gene microarray have been deposited in the National Center for Biotechnology Information Gene Expression Omnibus data set with accession number GSE35221.

LIMITATIONS, REASONS FOR CAUTION

This is a descriptive study in human endometrium only using in vitro stem cell assays. The differential ability of N-cadherin+ and N-cadherin-cells to generate endometrial glands in vivo was not determined. A small number of uterine tissues analysed contained adenomyosis for which N-cadherin has been implicated in epithelial-EMT.

WIDER IMPLICATIONS OF THE FINDINGS

A new marker enriching for human endometrial epithelial progenitor cells identifies a different and potentially more primitive cell population than SSEA-1, suggesting a potential hierarchy of epithelial differentiation in the basalis. Using N-cadherin as a marker, the molecular and cellular characteristics of epithelial progenitor cells and their role in endometrial proliferative disorders including endometriosis, adenomyosis and thin dysfunctional endometrium can be investigated.

STUDY FUNDING/COMPETING INTEREST(S)

This research was supported by Cancer Council Victoria grant 491079 (C.E.G.) and Australian National Health and Medical Research Council grants 1021127 (C.E.G.), 1085435 (C.E.G., J.A.D.), 145780 and 288713 (C.N.S.), RD Wright Career Development Award 465121 (C.E.G.), Senior Research Fellowship 1042298 (C.E.G.), the Victorian Government's Operational Infrastructure Support and an Australian Postgraduate Award (HPTN), and China Council Scholarship (L.X.). The authors have nothing to declare.

Oestrogen, progesterone and stem cells: the discordant trio in endometriosis?

Oestrogen–progesterone signalling is highly versatile and critical for the maintenance of healthy endometrium in humans. The genomic and nongenomic signalling cascades initiated by these hormones in differentiated cells of endometrium have been the primary focus of research since 1920s. However, last decade of research has shown a significant role of stem cells in the maintenance of a healthy endometrium and the modulatory effects of hormones on these cells. Endometriosis, the growth of endometrium outside the uterus, is very common in infertile patients and the elusiveness in understanding of disease pathology causes hindrance in selection of treatment approaches to enhance fertility. In endometriosis, the stem cells are dysfunctional as it can confer progesterone resistance to their progenies resulting in disharmony of hormonal orchestration of endometrial homeostasis. The bidirectional communication between stem cell signalling pathways and oestrogen–progesterone signalling is found to be disrupted in endometriosis though it is not clear which precedes the other. In this paper, we review the intricate connection between hormones, stem cells and the cross-talks in their signalling cascades in normal endometrium and discuss how this is deregulated in endometriosis. Re-examination of the oestrogen–progesterone dependency of endometrium with a focus on stem cells is imperative to delineate infertility associated with endometriosis and thereby aid in designing better treatment modalities.

Role of Wnt signalling in early pregnancy

The integration of a complex network of signalling molecules promotes implantation of the blastocyst and development of the placenta. These processes are crucial for a successful pregnancy and fetal growth and development. The signalling network involves both cell-cell and cell-extracellular matrix communication. The family of secreted glycoprotein ligands, the Wnts, plays a major role in regulating a wide range of biological processes, including embryonic development, cell fate, proliferation, migration, stem cell maintenance, tumour suppression, oncogenesis and tissue homeostasis. Recent studies have provided evidence that Wnt signalling pathways play an important role in reproductive tissues and in early pregnancy events. The focus of this review is to summarise our present knowledge of expression, regulation and function of the Wnt signalling pathways in early pregnancy events of human and other model systems, and its association with pathological conditions. Despite our recent progress, much remains to be learned about Wnt signalling in human reproduction. The advancement of knowledge in this area has applications in the reduction of infertility and the incidence and morbidity of gestational diseases.

Menstrual Blood-Derived Stem Cells: In Vitro and In Vivo Characterization of Functional Effects

Accumulating evidence has demonstrated that menstrual blood stands as a viable source of stem cells. Menstrual blood-derived stem cells (MenSCs) are morphologically and functionally similar to cells directly extracted from the endometrium, and present dual expression of mesenchymal and embryonic cell markers, thus becoming interesting tools for regenerative medicine. Functional reports show higher proliferative and self-renewal capacities than bone marrow-derived stem cells, as well as successful differentiation into hepatocyte-like cells, glial-like cells, endometrial stroma-like cells, among others. Moreover, menstrual blood stem cells may be used with increased efficiency in reprogramming techniques for induced Pluripotent Stem cell (iPS) generation. Experimental studies have shown successful treatment of stroke, colitis, limb ischemia, coronary disease, Duchenne's muscular atrophy and streptozotocin-induced type 1 diabetes animal models with MenSCs. As we envision an off-the-shelf product for cell therapy, cryopreserved MenSCs appear as a feasible clinical product. Clinical applications, although still very limited, have great potential and ongoing studies should be disclosed in the near future.

The Importance of the Canonical Wnt Signaling Pathway in the Porcine Endometrial Stromal Stem/Progenitor Cells: Implications for Regeneration

The regenerative ability of the endometrium is strongly associated with the presence of adult stem/progenitor cells. Purposes of the present study were (1) to establish the presence of stem/progenitor cells in porcine endometrial stroma using a clonogenic assay and (2) to investigate whether the canonical Wnt pathway affects the potential of stem/progenitor cells to undergo self-renewal or differentiation. The utility of endometrial stromal clones as a model for stem/progenitor studies was evaluated based on these cells' increased expression of mesenchymal stem cell (MSC) marker genes, including CD29, CD73, CD90, and CD105, compared with primary cultured cells. Small molecules were introduced to activate (BIO) or inhibit (XAV939) the canonical Wnt pathway during stromal clone formation. Cloning efficiency assays revealed that activation of the Wnt/β-catenin pathway promoted formation of more differentiated small clones. Moreover, activation of the Wnt/β-catenin pathway decreased, whereas inhibition of the pathway increased MSC marker expression. Additionally, we confirmed the importance of canonical Wnt pathway stimulation in endometrial stromal cells through observing the appropriate changes in β-catenin cellular localization. These data indicate that modulation of the canonical Wnt pathway effects the process of regeneration in the porcine endometrium during the course of the estrous cycle.

Endometrial stem/progenitor cells: the first 10 years

BACKGROUND

The existence of stem/progenitor cells in the endometrium was postulated many years ago, but the first functional evidence was only published in 2004. The identification of rare epithelial and stromal populations of clonogenic cells in human endometrium has opened an active area of research on endometrial stem/progenitor cells in the subsequent 10 years.

METHODS

The published literature was searched using the PubMed database with the search terms ‘endometrial stem cells and menstrual blood stem cells' until December 2014.

RESULTS

Endometrial epithelial stem/progenitor cells have been identified as clonogenic cells in human and as label-retaining or CD44⁺ cells in mouse endometrium, but their characterization has been modest. In contrast, endometrial mesenchymal stem/stromal cells (MSCs) have been well characterized and show similar properties to bone marrow MSCs. Specific markers for their enrichment have been identified, CD146⁺PDGFRβ⁺ (platelet-derived growth factor receptor beta) and SUSD2⁺ (sushi domain containing-2), which detected their perivascular location and likely pericyte identity in endometrial basalis and functionalis vessels. Transcriptomics and secretomics of SUSD2⁺ cells confirm their perivascular phenotype. Stromal fibroblasts cultured from endometrial tissue or menstrual blood also have some MSC characteristics and demonstrate broad multilineage differentiation potential for mesodermal, endodermal and ectodermal lineages, indicating their plasticity. Side population (SP) cells are a mixed population, although predominantly vascular cells, which exhibit adult stem cell properties, including tissue reconstitution. There is some evidence that bone marrow cells contribute a small population of endometrial epithelial and stromal cells. The discovery of specific markers for endometrial stem/progenitor cells has enabled the examination of their role in endometrial proliferative disorders, including endometriosis, adenomyosis and Asherman's syndrome. Endometrial MSCs (eMSCs) and menstrual blood stromal fibroblasts are an attractive source of MSCs for regenerative medicine because of their relative ease of acquisition with minimal morbidity. Their homologous and non-homologous use as autologous and allogeneic cells for therapeutic purposes is currently being assessed in preclinical animal models of pelvic organ prolapse and phase I/II clinical trials for cardiac failure. eMSCs and stromal fibroblasts also exhibit non-stem cell-associated immunomodulatory and anti-inflammatory properties, further emphasizing their desirable properties for cell-based therapies.

CONCLUSIONS

Much has been learnt about endometrial stem/progenitor cells in the 10 years since their discovery, although several unresolved issues remain. These include rationalizing the terminology and diagnostic characteristics used for distinguishing perivascular stem/progenitor cells from stromal fibroblasts, which also have considerable differentiation potential. The hierarchical relationship between clonogenic epithelial progenitor cells, endometrial and decidual SP cells, CD146⁺PDGFR-β⁺ and SUSD2⁺ cells and menstrual blood stromal fibroblasts still needs to be resolved. Developing more genetic animal models for investigating the role of endometrial stem/progenitor cells in endometrial disorders is required, as well as elucidating which bone marrow cells contribute to endometrial tissue. Deep sequencing and epigenetic profiling of enriched populations of endometrial stem/progenitor cells and their differentiated progeny at the population and single-cell level will shed new light on the regulation and function of endometrial stem/progenitor cells.

Mesenchymal stem/stromal cells in post-menopausal endometrium

STUDY QUESTION

Does post-menopausal endometrium contain mesenchymal stem/stromal cells (MSC) that have adult stem cell properties and can be prospectively isolated from a biopsy?

SUMMARY ANSWER

Perivascular W5C5(+) cells isolated from post-menopausal endometrial biopsies displayed characteristic MSC properties of clonogenicity, multipotency and surface phenotype irrespective of whether the women were or were not pre-treated with estrogen to regenerate the endometrium.

WHAT IS KNOWN ALREADY

Recently MSCs have been identified in human premenopausal endometrium, and can be prospectively isolated using a single marker, W5C5/SUSD2.

STUDY DESIGN, SIZE, DURATION

Endometrial tissue of both the functional and basal layers, from 17 premenopausal (pre-MP) women, 19 post-menopausal (post-MP) women without hormonal treatment and 15 post-menopausal women on estrogen replacement therapy (post-MP+ E2), was collected through a prospective phase IV clinical trial over 2 years.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Post-menopausal women <65 years of age were treated with or without E2 for 6-8 weeks prior to tissue collection. Serum E2 levels were determined by estradiol immunoenzymatic assay. Endometrial tissue was obtained from women by biopsy (curettage) just prior to the hysterectomy. The effect of E2 on endometrial thickness and glandular and luminal epithelial height was determined using image analysis. Endometrial tissue was dissociated into single cell suspensions and MSC properties were examined in freshly isolated and short-term cultured, magnetic bead-purified W5C5(+) cells. MSC properties were assessed using clonogenicity, serial cloning, mesodermal differentiation in adipogenic, chondrogenic, osteogenic and myogenic induction culture media, and surface phenotype analysis by flow cytometry. Estrogen receptor α expression in W5C5(+) cells was examined using dual colour immunofluorescence. Vascularity was analysed using CD34 and alpha smooth muscle actin immunostaining and subsequent image analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

A small population of stromal cells with MSC properties was purified with the W5C5 antibody from post-menopausal endometrium, whether atrophic from low circulating estrogen or regenerated from systemic estrogen treatment, similar to premenopausal endometrium. The MSC derived from post-menopausal endometrium treated with or without E2 fulfilled the minimum MSC criteria: clonogenicity, surface phenotype (CD29(+), CD44(+), CD73(+), CD105(+), CD140b(+), CD146(+)) and multipotency. The post-menopausal endometrial MSCs also showed comparable properties to premenopausal eMSC with respect to self-renewal in vitro and W5C5 expression. The W5C5(+) cells were located perivascularly as expected and did not express estrogen receptor α.

LIMITATIONS, REASONS FOR CAUTION

The properties of the MSC derived from post-menopausal endometrium were evaluated in vitro and their in vivo tissue reconstitution capacity has not been established as it has for premenopausal endometrial MSC.

WIDER IMPLICATIONS OF THE FINDINGS

The endometrium is an accessible source of MSC obtainable with minimum morbidity that could be used for future clinical applications as a cell-based therapy. This study shows that menopausal women can access their endometrial MSC by a simple biopsy for use in autologous therapies, particularly if their endometrium has been regenerated by short-term E2 treatment, provided they have an intact uterus and are not contraindicated for short-term E2 treatment. Endometrial MSC in post-menopausal women possess key MSC properties and are a promising source of MSC independent of a woman's age.

STUDY FUNDING/COMPETING INTERESTS

This study was supported by the National Health and Medical Research Council (NHMRC) of Australia grant (1021126) (C.E.G., A.R.) and Senior Research Fellowship (1042298) (C.E.G.), Australian Gynaecological Endoscopic Society grant (A.R.) , Monash International Postgraduate Research Scholarship (DU), Australian Stem Cell Centre, South East Melbourne Alliance for Regenerative Therapies and Australian Stem Cell Centre top up scholarships (DU) and Victorian Government's Operational Infrastructure Support Program. Competing interests: AR receives Preceptorship fees from AMS, advisory board fees and sponsored study from Astellas, and conducts investigator led studies sponsored by AMS and Boston Scientific for other projects.

TRIAL REGISTRATION NUMBER

CTNRN12610000563066.

DNA microarray-based gene expression profiling of estrogenic chemicals

We summarize updated information about DNA microarray-based gene expression profiling by focusing on its application to estrogenic chemicals. First, estrogenic chemicals, including natural/industrial estrogens and phytoestrogens, and the methods for detection and evaluation of estrogenic chemicals were overviewed along with a comprehensive list of estrogenic chemicals of natural or industrial origin. Second, gene expression profiling of chemicals using a focused microarray containing estrogen-responsive genes is summarized. Third, silent estrogens, a new type of estrogenic chemicals characterized by their estrogenic gene expression profiles without growth stimulative or inhibitory effects, have been identified so far exclusively by DNA microarray assay. Lastly, the prospect of a microarray assay is discussed, including issues such as commercialization, future directions of applications and quality control methods.

Transcriptome profiling of the theca interna in transition from small to large antral ovarian follicles

The theca interna layer of the ovarian follicle forms during the antral stage of follicle development and lies adjacent to and directly outside the follicular basal lamina. It supplies androgens and communicates with the granulosa cells and the oocyte by extracellular signaling. To better understand developmental changes in the theca interna, we undertook transcriptome profiling of the theca interna from small (3-5 mm, n = 10) and large (9-12 mm, n = 5) healthy antral bovine follicles, representing a calculated >7-fold increase in the amount of thecal tissue. Principal Component Analysis and hierarchical classification of the signal intensity plots for the arrays showed no clustering of the theca interna samples into groups depending on follicle size or subcategories of small follicles. From the over 23,000 probe sets analysed, only 76 were differentially expressed between large and small healthy follicles. Some of the differentially expressed genes were associated with processes such as myoblast differentiation, protein ubiquitination, nitric oxide and transforming growth factor β signaling. The most significant pathway affected from our analyses was found to be Wnt signaling, which was suppressed in large follicles via down-regulation of WNT2B and up-regulation of the inhibitor FRZB. These changes in the transcriptional profile could have been due to changes in cellular function or alternatively since the theca interna is composed of a number of different cell types it could have been due to any systematic change in the volume density of any particular cell type. However, our study suggests that the transcriptional profile of the theca interna is relatively stable during antral follicle development unlike that of granulosa cells observed previously. Thus both the cellular composition and cellular behavior of the theca interna and its contribution to follicular development appear to be relatively constant throughout the follicle growth phase examined.

Regenerating endometrium from stem/progenitor cells: is it abnormal in endometriosis, Asherman's syndrome and infertility?

PURPOSE OF REVIEW

Stem/progenitor cells are present in human and rodent endometrium and have a key role in endometrial regeneration in normal cycling and after parturition. We review emerging evidence of multiple types of endometrial stem/progenitor cells, and that abnormalities in their location and function may contribute to endometriosis.

RECENT FINDINGS

Candidate human endometrial stem/progenitors have been identified as clonogenic, Side Population and possessing tissue reconstitution activity. Markers have been identified for human endometrial mesenchymal stem cells, showing their perivascular location in functionalis and basalis endometrium. Human embryonic stem cells can be induced to develop endometrial epithelium, recapitulating endometrial development. In rodent studies, endometrial stem/progenitor cells were identified as label-retaining cells and their role in endometrial repair and regeneration revealed, perhaps via mesenchymal to epithelial transition. Studies of Wnt signalling in the regulation of endometrial stem/progenitor cells may yield insights into their function in endometrial regeneration. Stem/progenitor cells can be isolated from endometrial biopsy or menstrual blood and may be used autologously to regenerate endometrium in Asherman's syndrome.

SUMMARY

There is much to be learnt about endometrial stem/progenitor cell biology and their role in endometriosis. Endometrial stem/progenitor cells hold great promise for new treatments for infertility associated disorders, including thin dysfunctional endometrium and Asherman's syndrome.

SSEA-1 isolates human endometrial basal glandular epithelial cells: phenotypic and functional characterization and implications in the pathogenesis of endometriosis

STUDY QUESTION

Can the basal epithelial compartment of the human endometrium be defined by specific markers?

SUMMARY ANSWER

Human endometrial epithelial cells from the basalis express nuclear SOX9 and the cell-surface marker SSEA-1, with some cells expressing nuclear β-catenin. In vitro, primary endometrial epithelial cells enriched for SSEA-1+ show some features expected of the basalis epithelium.

WHAT IS KNOWN ALREADY

The endometrial glands of the functionalis regenerate from the basalis gland stumps following menstruation. Endometriosis is thought to originate from abnormal dislocation of the basalis endometrium. In the highly regenerative intestinal epithelium, SOX9 and nuclear β-catenin are more highly expressed in the intestinal crypt, the stem/progenitor cell region.

STUDY DESIGN, SIZE, DURATION

A large prospective observational study analysing full-thickness human endometrial hysterectomy samples from 115 premenopausal women, 15 post-menopausal women and ectopic endometriotic lesions from 20 women with endometriosis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Full-thickness endometrium from hysterectomy tissues was analysed by immunohistochemistry for SSEA-1, SOX9 and β-catenin. Primary human endometrial epithelial cells from short-term cultures were sorted into SSEA1+/- fractions with a cell sorter or magnetic beads and analysed for markers of differentiation and pluripotency and telomere lengths (TLs) using qPCR, telomerase activity [telomere repeat amplification protocol (TRAP)] and growth in 3D culture.

MAIN RESULTS AND THE ROLE OF CHANCE

Similar to the intestinal crypt epithelium, human endometrial basal glandular epithelial cells expressed nuclear SOX9 and contained a rare subpopulation of cells with nuclear β-catenin suggestive of an activated Wnt pathway. The embryonic stem cell-surface marker, SSEA-1, also marked the human endometrial basal glandular epithelial cells, and isolated SSEA-1(+) epithelial cells grown in monolayer showed significantly higher expression of telomerase activity, longer mean TLs, lower expression of genes for steroid receptors and produced a significantly higher number of endometrial gland-like spheroids in 3D culture compared with SSEA-1(-) epithelial cells (P = 0.009). Cells in ectopic endometriosis lesions also expressed SSEA-1 and nuclear SOX9, suggesting that the basalis contributes to ectopic lesion formation in endometriosis following retrograde menstruation.

LIMITATIONS, REASONS FOR CAUTION

This is a descriptive study with only short-term culture of the primary human epithelial cells in vitro.

WIDER IMPLICATIONS OF THE FINDINGS

The surface marker SSEA1 enriches for an endometrial epithelial cell subpopulation from the basalis. Since the functional endometrium originates from these cells, it is now possible to study basalis epithelium for stem/progenitor cell activity to extend our current understanding of endometrial biology in health and diseases.

STUDY FUNDING/COMPETING INTEREST(S)

The work included in this manuscript was funded by Wellbeing of Women project grant RG1073 (D.K.H. and C.G.). We also acknowledge the support by National Health and Medical Research Council, RD Wright Career Development Award 465121 and Senior Research Fellowship 1042298, and the Victorian Government's Operation Infrastructure Support Program to C.G. and MRC G0601333 to T.V.Z. All authors have no conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5) as a putative human endometrial stem cell marker

The endometrium is recognized for its remarkable regenerative and remodeling capacity. Every month this hormonally regulated organ undergoes cycles of growth (from 0.5-2 to 7 mm), regression and shedding of two-third of the tissue, leading to its monthly renewal that occurs ∼400 times in a woman's reproductive lifetime. Several groups have suggested the existence of a human endometrial somatic stem cell (SSC) population located around the spiral arterioles of the basalis. Different groups have isolated, identified and characterized putative endometrial SSC populations in human endometrium based on the general features of undifferentiated cells, such as slow cycling detected using the 5-bromo-2-deoxyuridine technique or identification of a side population using the Hoechst efflux dye technique. Nevertheless, specific markers to isolate these endometrial SSC have not yet been consistently elucidated. Accumulated evidence based on lineage tracing studies indicates that a surface protein named Leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5) is a marker that can identify SSC in several tissues such as small intestine mucosa (endodermal origin), hair follicles (ectodermal origin) or mature kidney nephrons (mesodermal origin). This protein plays a crucial role in the Wnt/β-catenin signaling system by acting on the self-renewal and maintenance of the SSC population. In this work, we present novel data suggestive of Lgr5 as a putative human endometrial SSC marker, and since this is a mesoderm-derived tissue, these findings reinforce the concept that Lgr5 can be considered a universal SSC marker.

Endometrial regeneration and endometrial stem/progenitor cells

The functional layer of the human endometrium is a highly regenerative tissue undergoing monthly cycles of growth, differentiation and shedding during a woman's reproductive years. Fluctuating levels of circulating estrogen and progesterone orchestrate this dramatic remodeling of human endometrium. The thin inactive endometrium of postmenopausal women which resembles the permanent basal layer of cycling endometrium retains the capacity to respond to exogenous sex steroid hormones to regenerate into a thick functional endometrium capable of supporting pregnancy. Endometrial regeneration also follows parturition and endometrial resection. In non menstruating rodents, endometrial epithelium undergoes rounds of proliferation and apoptosis during estrus cycles. The recent identification of adult stem cells in both human and mouse endometrium suggests that epithelial progenitor cells and the mesenchymal stem/stromal cells have key roles in the cyclical regeneration of endometrial epithelium and stroma. This review will summarize the evidence for endometrial stem/progenitor cells, examine their role in mouse models of endometrial epithelial repair and estrogen-induced endometrial regeneration, and also describe the generation of endometrial-like epithelium from human embryonic stem cells. With markers now available for identifying endometrial mesenchymal stem/stromal cells, their possible role in gynecological diseases associated with abnormal endometrial proliferation and their potential application in cell-based therapies to regenerate reproductive and other tissues will be discussed.