Mixed origin of neovascularization of human endometrial grafts in immunodeficient mouse models

Rodent Models of Experimental Endometriosis: Identifying Mechanisms of Disease and Therapeutic Targets

BACKGROUND

Although it has been more than a century since endometriosis was initially described in the literature, understanding the etiology and natural history of the disease has been challenging. However, the broad utility of murine and rat models of experimental endometriosis has enabled the elucidation of a number of potentially targetable processes which may otherwise promote this disease.

OBJECTIVE

To review a variety of studies utilizing rodent models of endometriosis to illustrate their utility in examining mechanisms associated with development and progression of this disease.

RESULTS

Use of rodent models of endometriosis has provided a much broader understanding of the risk factors for the initial development of endometriosis, the cellular pathology of the disease and the identification of potential therapeutic targets.

CONCLUSION

Although there are limitations with any animal model, the variety of experimental endometriosis models that have been developed has enabled investigation into numerous aspects of this disease. Thanks to these models, our under-standing of the early processes of disease development, the role of steroid responsiveness, inflammatory processes and the peritoneal environment has been advanced. More recent models have begun to shed light on how epigenetic alterations con-tribute to the molecular basis of this disease as well as the multiple comorbidities which plague many patients. Continued de-velopments of animal models which aid in unraveling the mechanisms of endometriosis development provide the best oppor-tunity to identify therapeutic strategies to prevent or regress this enigmatic disease.

Animal models of endometriosis: Replicating the aetiology and symptoms of the human disorder

Endometriosis is a chronic incurable disorder that affects 1 in 10 women of reproductive age: associated symptoms include chronic pain and infertility. The aetiology of endometriosis remains poorly understood but patients, clinicians and researchers are all in agreement that new non-surgical therapies are urgently needed to reduce the severity of symptoms. Preclinical testing of drugs requires the development and validation of models that recapitulate the key features of the disorder. In this review we describe the best-validated animal models (primate, rodent, xenograft) and their contributions to our understanding of the factors underpinning the development of symptoms. We consider the evidence that these models have provided the platform for identification of new therapeutic interventions and reflect on future directions for research and drug validation.

Acute Drug Effects on the Human Placental Tissue: The Development of a Placental Murine Xenograft Model

OBJECTIVE

A pilot study was conducted to establish a human placental xenograft, which could serve as a model to evaluate the effect of toxic exposures during pregnancy.

STUDY DESIGN

The protocol consisted of engraftment of third-trimester human placental tissue in immunocompromised mice, after induction of a pseudo-pregnancy state by ovariectomy and progesterone supplementation. To validate the model, the placental tissue before and after engraftment was examined by immunohistochemistry, fluorescence-activated cell sorting (FACS), single-nucleotide polymorphism (SNP) genotyping, and whole transcriptome sequencing (WTSS). The human chorion gonadotropin (hCG) production in serum and urine was examined by enzyme-linked immunosorbent assay.

RESULTS

Microscopic evaluation of the placental tissue before and after engraftment revealed a stable morphology and preserved histological structure of the human tissue. Viable trophoblast was present after engraftment and remained stable over time. Vascularization and hormonal secretion (hCG) were present till 3 weeks after engraftment. Thirty-one SNPs were equally present, and there was a stable expression level for 56 451 genes evaluated by whole transcriptome sequencing.

CONCLUSION

Although this human placental xenograft model cannot copy the unique uterine environment in which the placenta develops and interacts between the mother and the fetus, it could be a suitable tool to evaluate the acute impact and adaptive processes of the placental tissue to environmental changes.

Endothelin-1 supports clonal derivation and expansion of cardiovascular progenitors derived from human embryonic stem cells

Coronary arteriogenesis is a central step in cardiogenesis, requiring coordinated generation and integration of endothelial cell and vascular smooth muscle cells. At present, it is unclear whether the cell fate programme of cardiac progenitors to generate complex muscular or vascular structures is entirely cell autonomous. Here we demonstrate the intrinsic ability of vascular progenitors to develop and self-organize into cardiac tissues by clonally isolating and expanding second heart field cardiovascular progenitors using WNT3A and endothelin-1 (EDN1) human recombinant proteins. Progenitor clones undergo long-term expansion and differentiate primarily into endothelial and smooth muscle cell lineages in vitro, and contribute extensively to coronary-like vessels in vivo, forming a functional human–mouse chimeric circulatory system. Our study identifies EDN1 as a key factor towards the generation and clonal derivation of ISL1⁺ vascular intermediates, and demonstrates the intrinsic cell-autonomous nature of these progenitors to differentiate and self-organize into functional vasculatures in vivo.

Understanding coronary vessels development provides basis for regenerative strategies. Here, Soh et al. identify endothelin-1 as a key molecule driving long-term expansion of ISL1⁺ bipotent vascular progenitors derived from human embryonic stem cells, and show that these cells can regenerate coronary vessels in mice.

Induction of post-menstrual regeneration by ovarian steroid withdrawal in the functionalis of xenografted human endometrium

STUDY QUESTION

Does the endometrial functionalis have the potential to undergo self-renewal after menstruation and how is this process controlled by ovarian steroids?

SUMMARY ANSWER

Endometrial xenografts subjected to withdrawal of estradiol and progesterone shrink but also show signs of proliferation and tissue repair; new estradiol supply prevents atrophy but is not sufficient to increase graft volume.

WHAT IS KNOWN ALREADY

Menstruation, i.e. cyclic proteolysis of the extracellular matrix of endometrial functionalis, is induced by a fall in estrogen and progesterone concentration and is followed by tissue regeneration. However, there is debate about whether regenerating cells must originate from the basalis or from stem cells and whether new estrogen supply is required for the early repair concomitant with menstruation.

STUDY DESIGN, SIZE, DURATION

Fragments from human endometrial functionalis (from 24 hysterectomy specimens) were xenografted in ovariectomized SCID mice and submitted to a 4-day estradiol and progesterone withdrawal (to mimic menstruation) followed by re-exposure to estradiol (to mimic the proliferative phase). We measured signs of proliferation and changes in graft volume.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Endometrium was collected from spontaneously cycling women. Cell proliferation was examined by immunolabeling Ki-67, cyclin D1 and phosphorylated-histone H3. Xenograft volume was measured by magnetic resonance imaging. Xenograft histomorphometry was performed to determine how the different tissue compartments contributed to volume change.

MAIN RESULTS AND THE ROLE OF CHANCE

Hormone withdrawal induced a rapid decrease in graft volume mainly attributable to stroma condensation and breakdown, concomitant with an increase of proliferation markers. Reinsertion of estradiol pellets after induced menstruation blocked volume decrease and stimulated epithelial and stromal growth, but, surprisingly, did not induce graft enlargement. Reinsertion of both estradiol and progesterone pellets blocked apoptosis.

LIMITATIONS, REASONS FOR CAUTION

Mechanisms of endometrial remodeling are different in women and mice and the contribution of circulating inflammatory cells in both species remains to be clarified. Moreover, during human menstruation, endometrial fragments resulting from tissue proteolysis can be expelled by the menstrual flow, unlike in this model.

WIDER IMPLICATIONS OF THE FINDINGS

Menstruation is a multifocal event within the functionalis. This is the first evidence that endometrial fragments that are not shed after menstrual tissue breakdown can support endometrial regeneration. Endometriosis is commonly thought to result from the retrograde migration of menstrual fragments of the degraded functionalis into the peritoneal cavity. Our study supports their potential to regenerate as ectopic endometrium.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by the Fonds de la Recherche Scientifique Médicale, Concerted Research Actions, Communauté Française de Belgique, Région wallonne, Région bruxelloise and Loterie nationale. P.H. and B.F.J. are research associates of the Belgian Fonds de la Recherche Scientifique (F.R.S.-F.N.R.S.). E.M. is Associate Editor at Human Reproduction. There is no conflict of interest to declare.

Evidence from a mouse model that epithelial cell migration and mesenchymal-epithelial transition contribute to rapid restoration of uterine tissue integrity during menstruation

BACKGROUND

In women dynamic changes in uterine tissue architecture occur during each menstrual cycle. Menses, characterised by the shedding of the upper functional layer of the endometrium, is the culmination of a cascade of irreversible changes in tissue function including stromal decidualisation, inflammation and production of degradative enzymes. The molecular mechanisms that contribute to the rapid restoration of tissue homeostasis at time of menses are poorly understood.

METHODOLOGY

A modified mouse model of menses was developed to focus on the events occurring within the uterine lining during endometrial shedding/repair. Decidualisation, vaginal bleeding, tissue architecture and cell proliferation were evaluated at 4, 8, 12, and 24 hours after progesterone (P4) withdrawal; mice received a single injection of bromodeoxyuridine (BrdU) 90 mins before culling. Expression of genes implicated in the regulation of mesenchymal to epithelial transition (MET) was determined using a RT2 PCR profiler array, qRTPCR and bioinformatic analysis.

PRINCIPAL FINDINGS

Mice exhibited vaginal bleeding between 4 and 12 hours after P4 withdrawal, concomitant with detachment of the decidualised cell mass from the basal portion of the endometrial lining. Immunostaining for BrdU and pan cytokeratin revealed evidence of epithelial cell proliferation and migration. Cells that appeared to be in transition from a mesenchymal to an epithelial cell identity were identified within the stromal compartment. Analysis of mRNAs encoding genes expressed exclusively in the epithelial or stromal compartments, or implicated in MET, revealed dynamic changes in expression, consistent with a role for reprogramming of mesenchymal cells so that they could contribute to re-epithelialisation.

CONCLUSIONS/SIGNIFICANCE

These studies have provided novel insights into the cellular processes that contribute to re-epithelialisation post-menses implicating both epithelial cell migration and mesenchymal cell differentiation in restoration of an intact epithelial cell layer. These insights may inform development of new therapies to induce rapid healing in the endometrium and other tissues and offer hope to women who suffer from heavy menstrual bleeding.

Vascular perfusion with fluorescent labeled lectin to study ovarian functions

The aim of this study was to optimize a method to visualize tissue vascularity by perfusing the local vascular bed with a fluorescently labeled lectin, combined with immunofluorescent labeling of selected vascular/tissue markers. Ovaries with the pedicle were obtained from adult non-pregnant ewes. Immediately after collection, the ovarian artery was perfused with phosphate buffered saline (PBS) to remove blood cells, followed by perfusion with PBS containing fluorescently labeled Griffonia (Bandeiraea) simplicifolia (BS1) lectin. Then, half of ovary was fixed in formalin and another half in Carnoy's fixative. BS1 was detected in blood vessels in ovaries fixed in formalin, but not in Carnoy's fixative. Formalin fixed tissue was used for immunofluorescence staining of two markers of tissue function and/or structure, Ki67 and smooth muscle cell actin (SMCA). Ki67 was detected in granulosa and theca cells, luteal and stromal tissue, and a portion of Ki67 staining was co-localized with blood vessels. SMCA was detected in pericytes within the capillary system, in blood vessels in all ovarian compartments, and in the stroma. Thus, blood vessel perfusion with fluorescently labeled lectin combined with immunohistochemistry, microscopy, and imaging techniques provide an excellent tool to study angiogenesis, vascular architecture, and organ structures and function in physiological and pathological conditions.

Dilated thin-walled blood and lymphatic vessels in human endometrium: a potential role for VEGF-D in progestin-induced break-through bleeding

Progestins provide safe, effective and cheap options for contraception as well as the treatment of a variety of gynaecological disorders. Episodes of irregular endometrial bleeding or breakthrough bleeding (BTB) are a major unwanted side effect of progestin treatment, such that BTB is the leading cause for discontinued use of an otherwise effective and popular medication. The cellular mechanisms leading to BTB are poorly understood. In this study, we make the novel finding that the large, dilated, thin walled vessels characteristic of human progestin-treated endometrium include both blood and lymphatic vessels. Increased blood and lymphatic vessel diameter are features of VEGF-D action in other tissues and we show by immunolocalisation and Western blotting that stromal cell decidualisation results in a significant increase in VEGF-D protein production, particularly of the proteolytically processed 21 kD form. Using a NOD/scid mouse model with xenografted human endometrium we were able to show that progestin treatment causes decidualisation, VEGF-D production and endometrial vessel dilation. Our results lead to a novel hypothesis to explain BTB, with stromal cell decidualisation rather than progestin treatment per se being the proposed causative event, and VEGF-D being the proposed effector agent.

Cellular exchange in an endometriosis-adhesion model using GFP transgenic mice

BACKGROUND

Endometriosis is a debilitating disease that affects women of reproductive age and may lead to impaired fertility. Cell attachment, invasion of the underlying tissue, and vascular ingrowth are important processes in endometrial lesion development. However, the degree of cellular exchange between host peritoneum and endometrial tissue is unclear.

METHODS

An experimental endometriosis model was employed whereby uterine horn fragments from wild-type mice were implanted into genetically identical eGFP (enhanced green fluorescent protein) host mice and vice versa. Hormone sensitivity of the ectopic lesions was assessed and cellular exchange determined histologically.

RESULTS

White cyst-like lesions developed from implanted fibrin-rich fragments by day 7. Lesions consisted of a well-developed stroma with glandular and luminal epithelium. Both ovariectomy and treatment with a GnRH agonist, leuprorelin, resulted in the suppression of ectopic lesion growth, whereas estradiol treatment increased the size of the ectopic lesion (4 mice per group on day 14). Ingrowth and outgrowth of blood vessels was apparent as well as the exchange of cells between host peritoneum and lesion.

CONCLUSION

These findings support the proposal that there is a close cellular interplay between host peritoneum and ectopic tissue and the suitability of this mouse model to study these interactions.

Human uterine stem/progenitor cells: their possible role in uterine physiology and pathology

The human uterus mainly consists of the endometrium and the outer smooth muscle layer termed the myometrium. The uterus harbours the exceptional and remarkable regenerative ability responsible for cyclical regeneration and remodelling throughout the reproductive life. The uterus must swiftly and cooperatively enlarge to hold the growing foetus during pregnancy. Furthermore, the endometrium, in particular the functionalis layer, must also regenerate, differentiate and regress with each menstrual cycle under hormonal control. Endometrial regeneration from the basal layer is thought to contribute to replacement of the functionalis layer followed by its slough off during menses and parturition. These morphological and functional features of human endometrium can be reproduced in murine models in which severely immunodeficient mice are xenotransplanted with dispersed human endometrial cells under the kidney capsule. The uterine myometrium possesses the similar plasticity of the endometrium. This is demonstrated by multiple cycles of pregnancy-induced enlargement and regression after parturition. It is likely that regeneration and remodelling in the female reproductive tract are achieved presumably through endometrial and myometrial stem cell systems. Recent evidence now supports the existence of these stem cell systems in humans. Here, we will review our current understanding of uterine stem/progenitor cells. We also propose a novel hypothetical model in which stem cell activities explain the physiological remodelling and regeneration of the human uterus and the pathogenesis of gynaecological diseases such as endometriosis.