Periostin's role in uterine leiomyoma development: a mini-review on the potential periostin poses as a pharmacological intervention for uterine leiomyoma.

Uterine leiomyomas, also known as fibroids or myomas, occur in an estimated 70-80% of reproductive aged women. Many experience debilitating symptoms including pelvic pain, abnormal uterine bleeding (AUB), dyspareunia, dysmenorrhea, and infertility. Current treatment options are limited in preserving fertility, with many opting for sterilizing hysterectomy as a form of treatment. Currently, surgical interventions include hysterectomy, myomectomy, and uterine artery embolization in addition to endometrial ablation to control AUB. Non-surgical hormonal interventions, including GnRH agonists, are connotated with negative side effects and are unacceptable for women desiring fertility. Periostin, a regulatory extra cellular matrix (ECM) protein, has been found to be expressed in various gynecological diseases including leiomyomas. We previously determined that periostin over-expression in immortalized myometrial cells led to the development of a leiomyoma-like cellular phenotype. Periostin is induced by TGF-ß, signals through the PI3K/AKT pathway, induces collagen production, and mediates wound repair and fibrosis, all of which are implicated in leiomyoma pathology. Periostin has been linked to other gynecological diseases including ovarian cancer and endometriosis and is being investigated as pharmacological target for treating ovarian cancer, post-surgical scarring, and numerous other fibrotic conditions. In this review, we provide discussion linking pathological inflammation and wound repair, with a TGF-ß-periostin-collagen signaling in the pathogenesis of leiomyomas, and ultimately the potential of periostin as a druggable target to treat leiomyomas.

Endometrial hyperplasia with loss of APC in a novel population of Lyz2-expressing mouse endometrial epithelial cells.

Loss of heterozygosity and promoter hypermethylation of APC is frequently observed in human endometrial cancer, which is the most common gynecological cancer in the USA, but its carcinogenic driver status in the endometrial epithelium has not been confirmed. We have identified a novel population of progenitor endometrial epithelial cells (EECs) in mice that express lysozyme M (LysM) and give rise to approximately 15% of all EECs in adult mice. LysM is a glycoside hydrolase that is encoded by Lyz2 and functions to protect cells from bacteria as part of the innate immune system. Its expression has been shown in a subset of hematopoietic stem cells and in specialized lung and small intestinal epithelial cells. Conditional deletion of Apc in LysM + EECs results in significantly more epithelial cells compared to wild-type mice. At 5 months of age, the ApccKO mice have enlarged uterine horns with pathology that is consistent with endometrial hyperplasia with cystic endometrial glands, non-villous luminal papillae and nuclear atypia. Nuclear accumulation of ß-catenin and ERα, both of which are known to induce endometrial hyperplasia, was observed in the EECs of the ApccKO mice. These results confirm that loss of APC in EECs can result in a phenotype similar to endometrial hyperplasia.

A re-appraisal of mesenchymal-epithelial transition (MET) in endometrial epithelial remodeling.

Mesenchymal-epithelial transition (MET) is a mechanism of endometrial epithelial regeneration. It is also implicated in adenocarcinoma and endometriosis. Little is known about this process in normal uterine physiology. Previously, using pregnancy and menses-like mouse models, MET occurred only as an epithelial damage/repair mechanism. Here, we hypothesized that MET also occurs in other physiological endometrial remodeling events, outside of damage/repair, such as during the estrous cycle and adenogenesis (gland development). To investigate this, Amhr2-Cre-YFP/GFP mesenchyme-specific reporter mice were used to track the fate of mesenchymal-derived (MD) cells. Using EpCAM (epithelial marker), EpCAM+YFP+ MD-epithelial cells were identified in all stages of the estrous cycle except diestrus, in both postpartum and virgin mice. EpCAM+YFP+ MD-epithelial cells comprised up to 80% of the epithelia during estrogen-dominant proestrus and significantly declined to indistinguishable from control uteri in diestrus, suggesting MET is hormonally regulated. MD-epithelial cells were also identified during postnatal epithelial remodeling. MET occurred immediately after birth at postnatal day (P) 0.5 with EpCAM+GFP+ cells ranging from negligible (0.21%) to 82% of the epithelia. EpCAM+GFP+ MD-epithelial cells declined during initiation of adenogenesis (P8, avg. 1.75%) and then increased during gland morphogenesis (P14, avg. 10%). MD-epithelial cells expressed markers in common with non-MD-epithelial cells (e.g., EpCAM, FOXA2, ESR1, PGR). However, MD-epithelial cells were differentially regulated postnatally and in adults, suggesting a functional distinction in the two populations. We conclude that MET occurs not only as an epithelial damage/repair mechanism but also during other epithelial remodeling events, which to our knowledge has not been demonstrated in other tissues.

The role of stem cells in uterine involution.

Uterine remodeling during pregnancy and repair postpartum are fundamental to the successful propagation of eutherian species. The most drastic remodeling occurs in species with invasively implanting embryos, including humans and mice. During embryo implantation, embryonic trophoblasts breach the epithelium, penetrating into the stroma. Stromal cell decidualization, which is critical for the establishment and maintenance of early pregnancy, occurs throughout the implantation site. Trophoblasts further invade into and remodel uterine spiral arteries, which is necessary for placental formation. The uterus increases in size up to 24-fold, which is largely attributed to myometrial expansion. Uterine changes that occur during pregnancy must then be resolved postpartum. Following parturition, the uterus repairs the remodeled tissue in the process of uterine involution. During involution, the majority of the endometrium is regenerated to replace the tissue that is shed postpartum. The myometrium returns to the pre-gravid state which is thought to occur through apoptosis and autophagy of smooth muscle cells. Although we understand the general process of postpartum uterine involution, the detailed mechanisms, particularly the role of putative stem cells, are poorly understood. This review discusses the evidence for the existence of epithelial, stromal and myometrial stem cells and their role in uterine involution. Gaps in knowledge and areas for future research are also considered. Studies of both postpartum and menstrual uterine repair, which likely involve similar mechanisms, are described under the broad definition of uterine involution. Although the primary focus of this review is human, mouse models are discussed to provide additional information.

Putative human myometrial and fibroid stem-like cells have mesenchymal stem cell and endometrial stromal cell properties.

STUDY QUESTION: Can endometrial stromal stem/progenitor cell markers, SUSD2 and CD146/CD140b, enrich for human myometrial and fibroid stem/progenitor cells? SUMMARY ANSWER: SUSD2 enriches for myometrial and fibroid cells that have mesenchymal stem cell (MSC) characteristics and can also be induced to decidualise. WHAT IS KNOWN ALREADY: Mesenchymal stem-like cells have been separately characterised in the endometrial stroma and myometrium and may contribute to diseases in their respective tissues. STUDY DESIGN, SIZE, DURATION: Normal myometrium, fibroids and endometrium were collected from hysterectomies with informed consent. Primary cells or tissues were used from at least three patient samples for each experiment. PARTICIPANTS/MATERIALS, SETTING, METHODS: Flow cytometry, immunohistochemistry and immunofluorescence were used to characterise tissues. In vitro colony formation in normoxic and hypoxic conditions, MSC lineage differentiation (osteogenic and adipogenic) and decidualisation were used to assess stem cell activity. Xenotransplantation into immunocompromised mice was used to determine in vivo stem-like activity. Endpoint measures included quantitative PCR, colony formation, trichrome, Oil Red O and alkaline phosphatase activity staining. MAIN RESULTS AND THE ROLE OF CHANCE: CD146+CD140b+ and/or SUSD2+ myometrial and fibroid cells were located in the perivascular region and formed more colonies in vitro compared to control cells and differentiated down adipogenic and osteogenic mesenchymal lineages in vitro. SUSD2+ myometrial cells had greater in vitro decidualisation potential, and SUSD2+ fibroid cells formed larger tumours in vivo compared to control cells. LARGE-SCALE DATA: N/A. LIMITATIONS, REASONS FOR CAUTION: Markers used in this study enrich for cells with stem/progenitor cell activity; however, they do not distinguish stem from progenitor cells. SUSD2+ myometrial cells express markers of decidualisation when treated in vitro, but in vivo assays are needed to fully demonstration their ability to decidualise. WIDER IMPLICATIONS OF THE FINDINGS: These results suggest a possible common MSC for the endometrial stroma and myometrium, which could be the tumour-initiating cell for uterine fibroids. STUDY FUNDING/COMPETING INTEREST(S): These studies were supported by NIH grants to JMT (R01OD012206) and to ALP (F32HD081856). The authors certify that we have no conflicts of interest to disclose.

Specific deletion of LKB1/Stk11 in the Müllerian duct mesenchyme drives hyperplasia of the periurethral stroma and tumorigenesis in male mice.

Nearly all older men will experience lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH), the etiology of which is not well understood. We have generated Stk11CKO mice by conditional deletion of the liver kinase B1 (LKB1) tumor suppressor gene, Stk11 (serine threonine kinase 11), in the fetal Müllerian duct mesenchyme (MDM), the caudal remnant of which is thought to be assimilated by the urogenital sinus primordial mesenchyme in males during fetal development. We show that MDM cells contribute to the postnatal stromal cells at the dorsal aspect of the prostatic urethra by lineage tracing. The Stk11CKO mice develop prostatic hyperplasia with bladder outlet obstruction, most likely because of stromal expansion. The stromal areas from prostates of Stk11CKO mice, with or without significant expansion, were estrogen receptor positive, which is consistent with both MD mesenchyme-derived cells and the purported importance of estrogen receptors in BPH development and/or progression. In some cases, stromal hyperplasia was admixed with epithelial metaplasia, sometimes with keratin pearls, consistent with squamous cell carcinomas. Mice with conditional deletion of both Stk11 and Pten developed similar features as the Stk11CKO mice, but at a highly accelerated rate, often within the first few months after birth. Western blot analyses showed that the loss of LKB1 and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) induces activation of the phospho-5' adenosine monophosphate-activated protein kinase and phospho-AKT serine/threonine kinase 1 signaling pathways, as well as increased total and active ß-catenin. These results suggest that activation of these signaling pathways can induce hyperplasia of the MD stroma, which could play a significant role in the etiology of human BPH.

Hyperplasia and fibrosis in mice with conditional loss of the TSC2 tumor suppressor in Müllerian duct mesenchyme-derived myometria.

Uterine leiomyomata are the most common tumors found in the female reproductive tract. Despite the high prevalence and associated morbidities of these benign tumors, little is known about the molecular basis of uterine leiomyoma development and progression. Loss of the Tuberous Sclerosis 2 (TSC2) tumor suppressor has been proposed as a mechanism important for the etiology of uterine leiomyomata based on the Eker rat model. However, conflicting evidence showing increased TSC2 expression has been reported in human uterine leiomyomata, suggesting that TSC2 might not be involved in the pathogenesis of this disorder. We have produced mice with conditional deletion of the Tsc2 gene in the myometria to determine whether loss of TSC2 leads to leiomyoma development in murine uteri. Myometrial hyperplasia and increased collagen deposition was observed in Tsc2(cKO) mice compared with control mice, but no leiomyomata were detected by post-natal week 24. Increased signaling activity of mammalian target of rapamycin complex 1, which is normally repressed by TSC2, was also detected in the myometria of Tsc2(cKO) mice. Treatment of the mutant mice with rapamycin significantly inhibited myometrial expansion, but treatment with the progesterone receptor modulator, mifepristone, did not. The ovaries of the Tsc2(cKO) mice appeared normal, but half the mice were infertile and most of the other half became infertile after a single litter, which was likely due to oviductal blockage. Our study shows that although TSC2 loss alone does not lead to leiomyoma development, it does lead to myometrial hyperplasia and fibrosis.

The Effects of Periostin Expression on Fibroid-Like Transition of Myometrial Cells.

Fibroids, benign tumors of the myometrium, are the most common tumors in women and are associated with spontaneous abortion, preterm birth, placenta abruption, and infertility, among others. The incidence of fibroids in reproductive aged women is 20-89%. Fibroids are characterized by high production of extracellular matrix (ECM), particularly collagens, which play a role in their growth. However, their pathogenesis is poorly understood. Recently, we and others have found periostin (POSTN), a regulatory ECM protein, to be overexpressed in the majority of fibroids analyzed. Periostin is an ECM protein that is a critical regulator and well-established biomarker for fibrosis in tissues such as the lung, skin, and kidney. Our hypothesis was that periostin plays a role in the fibrotic transition of myometrial cells to fibroid cells. To test this, we evaluated the effects of POSTN overexpression in myometrial cells. Telomerase-immortalized myometrial cells were transduced with control or POSTN-overexpression lentivirus particles, generating one control (dCas9-Mock) and two overexpression (dCas9-POSTN-01, dCas9-POSTN-02) cell lines. Overexpression of POSTN in immortalized myometrial cells resulted in a change in phenotype consistent with fibroid cells. They upregulated expression of key fibroid genes and had increased proliferation, adhesion, and migration in vitro. Here, we show a potential role for periostin in the transition of myometrial cells to fibroid cells, giving rationale for future investigation into the role of periostin in fibroid pathogenesis and its potential as a therapeutic target.

Gene profiling of inflammatory genes in day 18 endometria from pregnant and non-pregnant mares.

Maternal recognition of pregnancy is a physiological process that primarily describes endometrial responses to a conceptus. Recognition of a conceptus prevents the release of prostaglandin F(2α) , thereby ensuring survival of the corpus luteum and continued progesterone production. Exactly how this occurs in the mare is poorly understood. Because prostaglandin F(2α) is a pro-inflammatory hormone, we hypothesized that differential gene expression in the endometrium at the time of maternal recognition reflects an anti-inflammatory event leading to decreased prostaglandin F(2α) secretion. Mares were inseminated, and endometrial biopsies were recovered from pregnant mares on Day 18 post-ovulation. In subsequent estrous cycles, mares were not inseminated and Day 18 post-ovulation endometrial biopsies were collected (non-pregnant control, matched per individual). Endometrial gene expression profiles were examined by screening an Affymetrix equine GeneChip containing probes specific for genes related to inflammatory processes. Microarray analysis revealed 118 genes that were up-regulated and 93 genes that were down-regulated (P < 0.001) at least 1.5-fold in the endometrium of pregnant versus non-pregnant mares. Quantitative, real-time RT-PCR confirmed the microarray results for three up-regulated genes homologous to TSC22D3, PPAPDC2, and KLF6, and three down-regulated genes homologous to ESR1, MARCKSL1, and EPSTI1 (P < 0.05). It is concluded that the presence of the equine embryo induces differential gene expression in the endometrium of Day 18 pregnant mares, and that these genes are associated with inflammatory processes and pathways involving cellular growth and proliferation. The results from this study provide important new insights into endometrial gene expression in response to early equine pregnancy.

Endometrial stromal beta-catenin is required for steroid-dependent mesenchymal-epithelial cross talk and decidualization.

BACKGROUND: Beta-catenin is part of a protein complex associated with adherens junctions. When allowed to accumulate to sufficient levels in its dephosphorylated form, beta-catenin serves as a transcriptional co-activator associated with a number of signaling pathways, including steroid hormone signaling pathways. METHODS: To investigate the role of beta-catenin in progesterone (P4) signaling and female reproductive physiology, conditional ablation of Ctnnb1 from the endometrial mesenchymal (i.e. stromal and myometrial), but not epithelial, compartment was accomplished using the Amhr2-Cre mice. Experiments were conducted to assess the ability of mutant female mice to undergo pregnancy and pseudopregnancy by or through oil-induced decidualization. The ability of uteri from mutant female mice to respond to estrogen (E2) and P4 was also determined. RESULTS: Conditional deletion of Ctnnb1 from the mesenchymal compartment of the uterus resulted in infertility stemming, in part, from complete failure of the uterus to decidualize. E2-stimulated epithelial cell mitosis and edematization were not altered in mutant uteri indicating that the mesenchyme is capable of responding to E2. However, exposure of ovariectomized mutant female mice to a combined E2 and P4 hormone regimen consistent with early pregnancy revealed that mesenchymal beta-catenin is essential for indirectly opposing E2-induced epithelial proliferation by P4 and in some mice resulted in development of endometrial metaplasia. Lastly, beta-catenin is also required for the induced expression of genes that are known to play a fundamental role in decidualization such as Ihh, Ptch1, Gli1 and Muc1 CONCLUSIONS: Three salient points derive from these studies. First, the findings demonstrate a mechanistic linkage between the P4 and beta-catenin signaling pathways. Second, they highlight an under appreciated role for the mesenchymal compartment in indirectly mediating P4 signaling to the epithelium, a process that intimately involves mesenchymal beta-catenin. Third, the technical feasibility of deleting genes in the mesenchymal compartment of the uterus in an effort to understand decidualization and post-natal interactions with the overlying epithelium has been demonstrated. It is concluded that beta-catenin plays an integral role in selective P4-directed epithelial-mesenchymal communication in both the estrous cycling and gravid uterus.

ARID1A and PI3-kinase pathway mutations in the endometrium drive epithelial transdifferentiation and collective invasion.

ARID1A and PI3-Kinase (PI3K) pathway alterations are common in neoplasms originating from the uterine endometrium. Here we show that monoallelic loss of ARID1A in the mouse endometrial epithelium is sufficient for vaginal bleeding when combined with PI3K activation. Sorted mutant epithelial cells display gene expression and promoter chromatin signatures associated with epithelial-to-mesenchymal transition (EMT). We further show that ARID1A is bound to promoters with open chromatin, but ARID1A loss leads to increased promoter chromatin accessibility and the expression of EMT genes. PI3K activation partially rescues the mesenchymal phenotypes driven by ARID1A loss through antagonism of ARID1A target gene expression, resulting in partial EMT and invasion. We propose that ARID1A normally maintains endometrial epithelial cell identity by repressing mesenchymal cell fates, and that coexistent ARID1A and PI3K mutations promote epithelial transdifferentiation and collective invasion. Broadly, our findings support a role for collective epithelial invasion in the spread of abnormal endometrial tissue.

Label-Retaining, Putative Mesenchymal Stem Cells Contribute to Murine Myometrial Repair During Uterine Involution.

Uterine remodeling during pregnancy is a fundamental, dynamic process required for successful propagation of eutherian species. The uterus can increase in size up to 40-fold during pregnancy, which is largely attributed to expansion of the myometrium by hyperplasia and hypertrophy. After pregnancy, the uterus repairs the remodeled or "damaged" tissue during uterine involution (INV). Little is known about this repair process, particularly the role of mesenchymal stem/progenitor cells. The objective of this study was to identify and characterize putative mesenchymal stem/progenitor cells in the murine myometrium using a combination of label retention and mesenchymal stem cell (MSC) marker expression and a pregnancy and uterine INV model. Tet-off transgenic mice with the Cre-lox system were used to specifically label mesenchymal cells (ie, myometrial and endometrial stromal cells) within the uterus while avoiding other cell types (eg, epithelial, immune, and endothelial cells) to identify slowly dividing cells and assess their stem cell qualities. We identified myometrial label-retaining cells (LRCs) that persisted for at least 3 months, expressed CD146 and CD140b (MSC markers), and proliferated at a higher rate during uterine INV compared with nonlabeled cells. The LRCs did not appear to express either estrogen receptor alpha or progesterone receptor, nor did the number of LRCs change at different estrous stages or in response to exogenous estradiol or progesterone administration, suggesting that LRCs were not involved in normal estrous cycling. The results from this study provide important insight into putative stem/progenitor cells in the myometrium and their possible role in uterine physiology.

Nuclear PTEN Localization Contributes to DNA Damage Response in Endometrial Adenocarcinoma and Could Have a Diagnostic Benefit for Therapeutic Management of the Disease.

Endometrial adenocarcinoma (EndoCA) is the most common gynecologic cancer type in the United States, and its incidence is increasing. The majority of patients are disease-free after surgical resection of stage I tumors, which is often followed by radiotherapy, but most patients with advanced disease recur and have a poor prognosis, largely because the tumors become refractory to cytotoxic chemotherapies. PTEN, a commonly mutated tumor suppressor in EndoCAs, is well known for its ability to inhibit the AKT/mTOR signaling pathway. Nuclear functions for PTEN have been proposed as well, but whether those affect EndoCA development, progression, or outcomes is not well understood. Using immunohistochemistry, nuclear PTEN expression was observed in approximately half of EndoCA patient tumors, independent of grade and cytoplasmic PTEN expression. Higher levels of the DNA damage response (DDR) marker, γH2AX, were observed by immunohistochemistry and immunofluorescence in human EndoCA tumor sections that were PTEN-negative, in murine EndoCA tissues that were genetically modified to be PTEN-null, and in Ishikawa EndoCA cells, which do not express endogenous PTEN. Overexpression of exogenous PTEN-WT or PTEN-NLS, a modified PTEN with an added nuclear localization signal, significantly improved both DDR and G2-M transition in Ishikawa cells treated with a DNA-damaging agent. Whereas PARP inhibition with Olaparib was not as effective in Ishikawa cells expressing native or PTEN-NLS, inhibition with Talazoparib was not affected by PTEN overexpression. These results suggest that nuclear PTEN subcellular localization in human EndoCA could be diagnostic when considering DDR therapeutic intervention. Mol Cancer Ther; 17(9); 1995-2003. ©2018 AACR.

Gain-of-function ß-catenin in the uterine mesenchyme leads to impaired implantation and decidualization.

Embryo implantation and endometrial decidualization are critical events that occur during early pregnancy in humans and mice, and perturbation in either can result in infertility. WNT signaling through the canonical ß-catenin pathway plays a pivotal role in embryonic Müllerian duct development, postnatal uterine maturation and establishment of pregnancy. Loss of ß-catenin in the Müllerian duct mesenchyme (MDM)-derived stroma and myometrium results in impaired decidualization and infertility, whereas gain-of-function (GOF) results in the formation of mesenchymal tumors and sub-fertility attributed to malformed oviducts. We hypothesized that GOF ß-catenin further contributes to sub-fertility through improper stromal and epithelial cell signaling during embryo implantation and decidualization. We show that mice with GOF ß-catenin in MDM-derived stroma and myometrium have reduced implantation sites after embryo transfer and decreased decidualization. On day 4.5 of pseudopregnancy or in mice treated with progesterone and estrogen to mimic early pregnancy, the estrogen-LIF-ERK and progesterone-IHH pathways remain predominantly intact in GOF ß-catenin mice; however, JAK/STAT signaling is altered. pSTAT3 is significantly reduced in GOF ß-catenin mice and expression of downstream epithelial junctional complex factors, Ctnna1 and Cldn1, is increased. We also show that purified stromal cells from GOF ß-catenin uteri, when removed from epithelial cell influence and provided with the appropriate hormonal stimuli, are able to decidualize in vitro indicating that the cells are intrinsically capable of decidualization. Taken together, these results suggest that dysregulated ß-catenin activity in the stroma affects epithelial cell STAT3 signaling and ultimately embryo implantation and stromal decidualization.

Long-term label retaining cells localize to distinct regions within the female reproductive epithelium.

The uterus is an extremely plastic organ that undergoes cyclical remodeling including endometrial regeneration during the menstrual cycle. Endometrial remodeling and regeneration also occur during pregnancy and following parturition, particularly in hemochorial implanting species. The mechanisms of endometrial regeneration are not well understood. Endometrial stem/progenitor cells are proposed to contribute to endometrial regeneration in both humans and mice. BrdU label retention has been used to identify potential stem/progenitor cells in mouse endometrium. However, methods are not available to isolate BrdU label-retaining cells (LRC) for functional analyses. Therefore, we employed a transgenic mouse model to identify H2B-GFP LRCs throughout the female reproductive tract with particular interest on the endometrium. We hypothesized that the female reproductive tract contains a population of long-term LRCs that persist even following pregnancy and endometrial regeneration. Endometrial cells were labeled (pulsed) either transplacentally/translactationally or peripubertally. When mice were pulsed transplacentally/translactationally, the label was not retained in the uterus. However, LRCs were concentrated to the distal oviduct and endocervical transition zone (TZ) following natural (i.e., pregnancy/parturition induced) and mechanically induced endometrial regeneration. LRCs in the distal oviduct and endocervical TZ expressed stem cell markers and did not express ERα or PGR, implying the undifferentiated phenotype of these cells. Oviduct and endocervical TZ LRCs did not proliferate during endometrial re-epithelialization, suggesting that they do not contribute to the endometrium in a stem/progenitor cell capacity. In contrast, when mice were pulsed peripubertally long-term LRCs were identified in the endometrial glandular compartment in mice as far out as 9 months post-pulse. These findings suggest that epithelial tissue of the female reproductive tract contains 3 distinct populations of epithelial cells that exhibit stem/progenitor cell qualities. Distinct stem/progenitor-like cells localize to the oviduct, endometrium, and cervix.

Mesenchymal-to-epithelial transition contributes to endometrial regeneration following natural and artificial decidualization.

Despite being a histologically dynamic organ, mechanisms coordinating uterine regeneration during the menstrual/estrous cycle and following parturition are poorly understood. In the current study, we hypothesized that endometrial epithelial tissue regeneration is accomplished, in part, by mesenchymal-to-epithelial transition (MET). To test this hypothesis, fate mapping studies were completed using a double transgenic (Tg) reporter strain, Amhr2-Cre; Rosa26-Stop(fl/fl-EYFP) (i.e., flox-stop EYFP reporter). EYFP expression was observed in Müllerian duct mesenchyme-derived stroma and myometrium, but not epithelia in young and peripubertal double Tg female mice. However, mosaic EYFP expression was observed in epithelia of double Tg mice after parturition. To ensure the observed epithelial EYFP expression was not due to leaky Amhr2 promoter activity, resulting in aberrant Cre expression, transgenic mice expressing LacZ under the control of the Amhr2 promoter (Amhr2-LacZ) were used to monitor ß-galactosidase (ß-Gal) activity within the uterus. ß-Gal activity was not detected in luminal or glandular epithelia regardless of age, reproductive status, or degree of damage incurred within the uterus. Lastly, a unique population of transitional cells was identified that expressed the epithelial cell marker, pan-cytokeratin, and the stromal cell marker, vimentin. These cells localized predominantly to the regeneration zone in the mesometrial region of the endometrium. These findings suggest a previously unappreciated role for MET in endometrial regeneration and have important implications for proliferative diseases of the endometrium such as endometriosis.