Insights from imaging the implanting embryo and the uterine environment in three dimensions

Aging-Associated Vacuolation of Multi-Ciliated Cells in the Distal Mouse Oviduct Reflects Unique Cell Identity and Luminal Microenvironment

The female reproductive organs present with the earliest aging characteristics, such as a decline in fertility and estrous cyclicity. While age-related changes in the ovary are well documented, it is unclear if any age-associated changes occur in the other female reproductive organs, such as the oviduct/Fallopian tube. At the distal end of aged oviducts in mice, we found vacuolated multi-ciliated cells (MCCs) with a severely apically displaced and deformed nucleus. This phenotype was unique to the distal oviduct epithelium-the infundibulum (INF) and ampulla (AMP). Ovariectomy did not affect the timeline of MCC vacuolation, suggesting little involvement of ovulation and hormonal regulation. MCC vacuolation was induced in hypoxia or hydroxyurea treatments in in vitro organotypic culture of all oviduct regions, not limited to the INF/AMP epithelium. This suggests a high oxygen demand in MCCs, compared to other cell types, and a uniquely stressed INF/AMP epithelial microenvironment in vivo. We found that the blood circulation of INF/AMP depended on the ovarian artery, different from the rest of the oviduct epithelium, and its circulation declined along with ovarian activities. We conclude that a decline in local blood circulation and distinct cellular identity of the INF/AMP epithelium caused age-associated MCC vacuolation, reflecting its mild, chronically stressed microenvironment.

Analysis pipeline to quantify uterine gland structural variations

BACKGROUND

Technical advances in whole tissue imaging and clearing have allowed 3D reconstruction of exocrine uterine glands deep-seated in the endometrium. However, there are limited gland structure analysis platforms to analyze these imaging data sets. Here, we present a pipeline for segmenting and analyzing uterine gland shape.

RESULTS

Using our segmentation methodology, we derive metrics to describe gland length, shape, and branching patterns. We then quantify gland behavior with respect to organization around the embryo and proximity of each gland to the uterine lumen. We apply this image analysis pipeline to uterine glands at the peri-implantation time points of a mouse pregnancy. Our analysis reveals that at the time of embryo or egg entry into the uterus, glands show changes in length, tortuosity, and proximity to the uterine lumen while gland branch number stays the same. Eventually, these shape changes aid in reorganization of the glands around the embryo implantation site. We further apply our analysis pipeline to human and guinea pig uterine glands, extending feasibility to other mammalian species.

CONCLUSION

This work serves as a resource for researchers to extract quantitative and reproducible morphological features from three-dimensional uterine gland images to reveal insights about functional and structural patterns.

The role of serendipity in our investigation of embryo implantation

Serendipity plays a huge role in science, and having a prepared mind that can seize upon a chance observation or occurrence can drive a project forward. This happened in my lab with a project centered on the regulation of trophoblast cell behavior at implantation. We discovered that amino acids regulate the onset of trophoblast motility through the activation of the kinase complex mTORC1, and that this acts as a checkpoint to trophoblast differentiation. This finding not only broadened our understanding of the mechanisms underlying embryo implantation, but also provided new ways of thinking about the regulation of diapause, a state of suspended embryonic development that occurs in many species. I should say that we re-discovered the fact that amino acids regulate the onset of trophoblast motility, as reading the literature showed us that others had made this same observation some 30 years previously and we were fortuitously able to build upon those findings. This project confirmed to me how valuable it is to read the literature widely, both historical papers and those in fields outside one's area of research, and to go to seminars on topics outside one's area.

Loss of PRICKLE1 leads to abnormal endometrial epithelial architecture, decreased embryo implantation, and reduced fertility in mice

Successful embryo implantation requires coordinated changes in the uterine luminal epithelium, including structural adaptations, apical-basal polarity shifts, intrauterine fluid resorption, and cellular communication. Planar cell polarity proteins, essential for cell organization, are understudied in the context of uterine physiology and implantation. PRICKLE proteins, components of PCP, are suggested to play critical roles in epithelial polarization and tissue morphogenesis. However, their function in the polarized unicellular layer of endometrial epithelium, which supports embryo implantation, is unknown. We developed an endometrial epithelial-specific knockout of mouse Prickle1 using Lactoferrin-iCre to investigate its role in uterine physiology. Prickle1 ablation in the endometrial epithelium of mice resulted in decreased embryo implantation by gestational day 4.5, leading to lower fertility. 3D imaging of the uterus revealed abnormal luminal folding, impaired luminal closure, and altered glandular length in mutant uteri. Additionally, we observed decreased aquaporin-2 expression, disrupted cellular architecture, and altered E-cadherin expression and localization in the mutant uterine epithelium. Evidence of epithelial-mesenchymal transition was found within luminal epithelial cells, further linking PRICKLE1 loss to uterine pathologies. Furthermore, altered polarity of cell division leading to incomplete cytokinesis and increase in binuclear or multinucleated cells suggests a crucial role for PRICKLE1 in the maintenance of epithelial architecture. Our findings highlight PRICKLE1's critical role in the planar cell polarity pathway within the uterus, revealing its importance in the molecular and cellular responses essential for successful pregnancy and fertility.

Signaling via retinoic acid receptors mediates decidual angiogenesis in mice and human stromal cell decidualization

At the maternal-fetal interface, tightly regulated levels of retinoic acid (RA), the physiologically active metabolite of vitamin A, are required for embryo implantation and pregnancy success. Herein, we utilize mouse models, primary human cells, and pharmacological tools to demonstrate how depletion of RA signaling via RA receptor (RAR) disrupts implantation and progression of early pregnancy. To inhibit RAR signaling during early pregnancy, BMS493, an inverse pan-RAR agonist that prevents RA-induced differentiation, was administered to pregnant mice during the peri-implantation period. Attenuation of RA/RAR signaling prior to embryo implantation results in implantation failure, whereas attenuation of RA/RAR signaling after embryo implantation disrupts the post-implantation decidual vasculature and results in pregnancy failure by mid-gestation. To inhibit RAR signaling during human endometrial stromal cell (HESC) decidualization, primary HESCs and decidualized primary HESCs were transfected with silencing RNA specific for human RARA. Inhibition of RA/RARA signaling prevents initiation of HESC decidualization, but not maintenance of the decidualized HESC phenotype. These data show that RA/RAR signaling is required for maintenance of the decidual vasculature that supports early pregnancy in mice, and distinct RAR signaling is required for initiation, but not maintenance of primary HESC decidualization in vitro.

YAP1 and WWTR1 are required for murine pregnancy initiation

In brief

The HIPPO signaling effectors YAP1 and WWTR1 are required for murine pregnancy initiation, and mutation of these factors compromises the decidualization response and overall pregnancy success.

Abstract

Endometrial stromal cell decidualization is required for pregnancy success. Although this process is integral to fertility, many of the intricate molecular mechanisms contributing to decidualization remain undefined. One pathway that has been implicated in endometrial stromal cell decidualization in humans in vitro is the HIPPO signaling pathway. Two previously conducted studies showed that the effectors of the HIPPO signaling pathway YAP1 and WWTR1 are required for decidualization of primary endometrial stromal cells in vitro. To investigate the in vivo role of YAP1 and WWTR1 in decidualization and pregnancy initiation, we generated progesterone receptor Cre-mediated mutation of a combination of Yap1 and Wwtr1 alleles. Female Yap1 and Wwtr1 triple allele mutants exhibited subfertility, a compromised decidualization response, decreased endometrial receptivity, delayed embryonic development and a unique transcriptional profile at 7.5 days post-coitus (dpc). Bulk mRNA sequencing revealed aberrant maternal remodeling evidenced by significant alterations in extracellular matrix-encoding genes at 7.5 dpc in mutant dams and enrichment for terms associated with fertility-compromising diseases such as pre-eclampsia and endometriosis. In addition, differentially expressed genes overlapped directionally with estrogen receptor- and epidermal growth factor receptor-regulated genes as identified by microarray. Our results indicate that Yap1 and Wwtr1 are necessary for successful mammalian pregnancy initiation.

Spatiotemporal functions of leukemia inhibitory factor in embryo attachment and implantation chamber formation

Embryo implantation is crucial for successful pregnancy, requiring appropriate uterine responses to implantation-competent blastocysts. Molecular communication at the maternal-fetal junction governs this process. Leukemia inhibitory factor (Lif) plays a pivotal role in implantation across species. Lif is abundantly expressed in the glandular epithelium during blastocyst-receptive phase and is induced in the stroma surrounding attached blastocysts. While diminished Lif expression leads to infertility, its influence on peri-implantation uteri remains unclear. Therefore, we investigated the role of Lif in uterine physiology using its uterine-specific knockout (uKO) and uterine epithelial-specific KO (eKO) in mice. Lif eKO and uKO mice displayed infertility owing to failed embryo attachment. Recombinant Lif supplementation rescued the reproductive phenotype of Lif eKO mice, but not Lif uKO mice; however, recombinant Lif injection rescued embryo attachment in Lif uKO mice. RNA-seq analysis indicated that Lif governs uterine epithelial genes, but not embryonic genes, to facilitate embryo attachment via activating nuclear Stat3. Concordantly, three-dimensional imaging of the uterine epithelium revealed that luminal closure and crypt formation are regulated by the uterine Lif-Stat3 axis as well as the presence of blastocysts. Collectively, our findings shed light on previously unknown mechanism on how Lif influences uterine functions molecularly and physiologically during early pregnancy.

Uterine stromal but not epithelial PTGS2 is critical for murine pregnancy success

Use of non-steroidal anti-inflammatory drugs that target prostaglandin synthase (PTGS) enzymes have been implicated in miscarriage. Further, PTGS2-derived prostaglandins are reduced in the endometrium of patients with a history of implantation failure. However, in the mouse model of pregnancy, peri-implantation PTGS2 function is controversial. Some studies suggest that Ptgs2-/- mice display deficits in ovulation, fertilization, and implantation, while other studies suggest a role for PTGS2 only in ovulation but not implantation. Further, the uterine cell type responsible for PTGS2 function and role of PTGS2 in regulating implantation chamber formation is not known. To address this we generated tissue-specific deletion models of Ptgs2. We observed that PTGS2 ablation from the epithelium alone in Ltfcre/+; Ptgs2f/f mice and in both the epithelium and endothelium of the Pax2cre/+; Ptgs2f/f mice does not affect embryo implantation. Further, deletion of PTGS2 in the ovary, oviduct, and the uterus using Pgrcre/+; Ptgs2f/f does not disrupt pre-implantation events but instead interferes with post-implantation chamber formation, vascular remodeling and decidualization. While all embryos initiate chamber formation, more than half of the embryos fail to transition from blastocyst to epiblast stage, resulting in embryo death and resorbing decidual sites at mid-gestation. Thus, our results suggest no role for uterine epithelial PTGS2 in early pregnancy but instead highlight a role for uterine stromal PTGS2 in modulating post-implantation embryo and implantation chamber growth. Overall, our study provides clarity on the compartment-specific role of PTGS2 and provides a valuable model for further investigating the role of stromal PTGS2 in post-implantation embryo development.

Spatial and molecular anatomy of the endometrium during embryo implantation: a current overview of key regulators of blastocyst invasion

Embryo implantation is composed of three steps: blastocyst apposition, adhesion/attachment and invasion. Blastocyst invasion has been studied less extensively than the other two events. Historically, studies conducted using electron microscopy have shown the removal of epithelial cells in the vicinity of the attached blastocysts in rodents, although the underlying mechanisms have remained unclear. Here, we describe recent studies using mice with uterine-specific gene deletion that demonstrated important roles for nuclear proteins such as progesterone receptor, hypoxia inducible factor and retinoblastoma in the regulation of embryo invasion. In these mouse models, the detachment of the endometrial luminal epithelium, decidualization in the stroma, and the activation of trophoblasts have been found to be important in ensuring embryo invasion. This review summarizes the molecular signaling associated with these cellular events, mainly evidenced by mouse models.

Spatiotemporally distinct roles of cyclooxygenase-1 and cyclooxygenase-2 at fetomaternal interface in mice

Embryo implantation is crucial for ensuring a successful pregnancy outcome and subsequent child health. The intrauterine environment during the peri-implantation period shows drastic changes in gene expression and cellular metabolism in response to hormonal stimuli and reciprocal communication with embryos. Here, we performed spatial transcriptomic analysis to elucidate the mechanisms underlying embryo implantation. Transcriptome data revealed that lipid metabolism pathways, especially arachidonic acid-related (AA-related) ones, were enriched in the embryo-receptive luminal epithelia. Cyclooxygenases (COXs), rate-limiting enzymes involved in prostaglandin production by AA, were spatiotemporally regulated in the vicinity of embryos during implantation, but the role of each COX isozyme in the uterus for successful pregnancy was unclear. We established uterine-specific COX2-knockout (uKO) and COX1/uterine COX2-double-KO (COX1/COX2-DKO) mice. COX2 uKO caused deferred implantation with failed trophoblast invasion, resulting in subfertility with reduced pregnancy rates and litter sizes. COX1/COX2 DKO induced complete infertility, owing to abrogated embryo attachment. These results demonstrate that both isozymes have distinct roles during embryo implantation. Spatial transcriptome and lipidome analyses revealed unique profiles of prostaglandin synthesis by each COX isozyme and spatiotemporal expression patterns of downstream receptors throughout the endometrium. Our findings reveal previously unappreciated roles of COXs at the fetomaternal interface to establish early pregnancy.

Loss of PRICKLE1 leads to abnormal endometrial epithelial architecture, decreased embryo implantation, and reduced fertility in mice

Successful embryo implantation requires coordinated changes in the uterine luminal epithelium, including structural adaptations, apical-basal polarity shifts, intrauterine fluid resorption, and cellular communication. Planar cell polarity (PCP) proteins, essential for cell organization, are understudied in the context of uterine physiology and implantation. PRICKLE proteins, components of PCP, are suggested to play critical roles in epithelial polarization and tissue morphogenesis. However, their function in the polarized unicellular layer of endometrial epithelium, which supports embryo implantation, is unknown. We developed an endometrial epithelial-specific knockout (cKO) of mouse Prickle1 using Lactoferrin-iCre to investigate its's role in uterine physiology. Prickle1 ablation in the endometrial epithelium of mice resulted in decreased embryo implantation by gestational day 4.5 leading to lower fertility. Three-dimensional imaging of the uterus revealed abnormal luminal folding, impaired luminal closure, and altered glandular length in mutant uteri. Additionally, we observed decreased aquaporin-2 expression, disrupted cellular architecture, and altered E-Cadherin expression and localization in the mutant uterine epithelium. Evidence of epithelial-mesenchymal transition (EMT) was found within luminal epithelial cells, further linking PRICKLE1 loss to uterine pathologies. Furthermore, altered polarity of cell division leading to incomplete cytokinesis and increase in binuclear or multinucleated cells suggests a crucial role for PRICKLE1 in the maintenance of epithelial architecture. Our findings highlight PRICKLE1's critical role in the PCP pathway within the uterus, revealing its importance in the molecular and cellular responses essential for successful pregnancy and fertility.

Significance Statement

Conservative cell division is essential to maintain apical-basal polarity and proper epithelial function in the uterus. Wnt/ Planar cell polarity signaling molecules are hypothesized to provide the spatial cues to organize unicellular, 2-dimensional sheet of epithelium in a plane orthogonal to the apical-basal polarity. Conditional ablation of Prickle1 , a crucial Wnt/ PCP gene, in mouse uterine epithelium results in aberrant expression of epithelial cadherin, altered plane of cell division, incomplete cytokinesis leading to binucleated/ multinucleated cells, epithelial - mesenchymal transition, and defective implantation. Role of Prickle1 in maintaining symmetric uterine epithelial cell division and tissue architecture is unique among Wnt/PCP genes, including previously described mouse models for Vangl2, Ror2, and Wnt5a . Classification: Biological Sciences (Major) Cell Biology (Minor), Physiology (Minor).

3DMOUSEneST: a volumetric label-free imaging method evaluating embryo-uterine interaction and decidualization efficacy

Effective interplay between the uterus and the embryo is essential for pregnancy establishment; however, convenient methods to screen embryo implantation success and maternal uterine response in experimental mouse models are currently lacking. Here, we report 3DMOUSEneST, a groundbreaking method for analyzing mouse implantation sites based on label-free higher harmonic generation microscopy, providing unprecedented insights into the embryo-uterine dynamics during early pregnancy. The 3DMOUSEneST method incorporates second-harmonic generation microscopy to image the three-dimensional structure formed by decidual fibrillar collagen, named 'decidual nest', and third-harmonic generation microscopy to evaluate early conceptus (defined as the embryo and extra-embryonic tissues) growth. We demonstrate that decidual nest volume is a measurable indicator of decidualization efficacy and correlates with the probability of early pregnancy progression based on a logistic regression analysis using Smad1/5 and Smad2/3 conditional knockout mice with known implantation defects. 3DMOUSEneST has great potential to become a principal method for studying decidual fibrillar collagen and characterizing mouse models associated with early embryonic lethality and fertility issues.

Imaging the dynamics of murine uterine contractions in early pregnancy†

Uterine muscle contractility is essential for reproductive processes including sperm and embryo transport, and during the uterine cycle to remove menstrual effluent. Even still, uterine contractions have primarily been studied in the context of preterm labor. This is partly due to a lack of methods for studying the uterine muscle contractility in the intact organ. Here, we describe an imaging-based method to evaluate mouse uterine contractility of both the longitudinal and circular muscles in the cycling stages and in early pregnancy. By transforming the image-based data into three-dimensional spatiotemporal contractility maps, we calculate waveform characteristics of muscle contractions, including amplitude, frequency, wavelength, and velocity. We report that the native organ is highly contractile during the progesterone-dominant diestrus stage of the cycle when compared to the estrogen-dominant proestrus and estrus stages. We also observed that during the first phase of uterine embryo movement when clustered embryos move toward the middle of the uterine horn, contractions are dynamic and non-uniform between different segments of the uterine horn. In the second phase of embryo movement, contractions are more uniform and rhythmic throughout the uterine horn. Finally, in Lpar3-/- uteri, which display faster embryo movement, we observe global and regional increases in contractility. Our method provides a means to understand the wave characteristics of uterine smooth muscle in response to modulators and in genetic mutants. Better understanding uterine contractility in the early pregnancy stages is critical for the advancement of artificial reproductive technologies and a possibility of modulating embryo movement during clinical embryo transfers.

Murine uterine gland branching is necessary for gland function in implantation

Uterine glands are branched, tubular structures whose secretions are essential for pregnancy success. It is known that pre-implantation glandular expression of leukemia inhibitory factor (LIF) is crucial for embryo implantation; however, the contribution of uterine gland structure to gland secretions, such as LIF, is not known. Here, we use mice deficient in estrogen receptor 1 (ESR1) signaling to uncover the role of ESR1 signaling in gland branching and the role of a branched structure in LIF secretion and embryo implantation. We observed that deletion of ESR1 in neonatal uterine epithelium, stroma, and muscle using the progesterone receptor PgrCre causes a block in uterine gland development at the gland bud stage. Embryonic epithelial deletion of ESR1 using a Müllerian duct Cre line, Pax2Cre, displays gland bud elongation but a failure in gland branching. Reduction of ESR1 in adult uterine epithelium using the lactoferrin-Cre (LtfCre) displays normally branched uterine glands. Unbranched glands from Pax2Cre Esr1flox/flox uteri fail to express glandular pre-implantation Lif, preventing implantation chamber formation and embryo alignment along the uterine mesometrial-antimesometrial axis. In contrast, branched glands from LtfCre Esr1flox/flox uteri display reduced expression of ESR1 and glandular Lif resulting in delayed implantation chamber formation and embryo-uterine axes alignment but mice deliver a normal number of pups. Finally, pre-pubertal unbranched glands in control mice express Lif in the luminal epithelium but fail to express Lif in the glandular epithelium, even in the presence of estrogen. These data strongly suggest that branched glands are necessary for pre-implantation glandular Lif expression for implantation success. Our study is the first to identify a relationship between the branched structure and secretory function of uterine glands and provides a framework for understanding how uterine gland structure-function contributes to pregnancy success.

Yes Associated Transcriptional Regulator 1 (YAP1) and WW Domain Containing Transcription Regulator (WWTR1) are required for murine pregnancy initiation

Endometrial stromal cell decidualization is required for pregnancy success. Although this process is integral to fertility, many of the intricate molecular mechanisms contributing to decidualization remain undefined. One pathway that has been implicated in endometrial stromal cell decidualization in humans in vitro is the Hippo signaling pathway. Two previously conducted studies showed that the effectors of the Hippo signaling pathway, YAP1 and WWTR1, were required for decidualization of primary stromal cells in culture. To investigate the in vivo role of YAP1 and WWTR1 in decidualization and pregnancy initiation, we generated a Progesterone Cre mediated partial double knockout (pdKO) of Yap1 and Wwtr1. Female pdKOs exhibited subfertility, a compromised decidualization response, partial interruption in embryo transport, blunted endometrial receptivity, delayed implantation and subsequent embryonic development, and a unique transcriptional profile. Bulk mRNA sequencing revealed aberrant maternal remodeling evidenced by significant alterations in extracellular matrix proteins at 7.5 days post-coitus in pdKO dams and enrichment for terms associated with fertility-compromising diseases like pre-eclampsia and endometriosis. Our results indicate a required role for YAP1 and WWTR1 for successful mammalian uterine function and pregnancy success.

Three-dimensional particle streak velocimetry based on optical coherence tomography for assessing preimplantation embryo movement in mouse oviduct in vivo

The mammalian oviduct (or fallopian tube) is a tubular organ hosting reproductive events leading to pregnancy. Dynamic 3D imaging of the mouse oviduct with optical coherence tomography (OCT) has recently emerged as a promising approach to study the hidden processes vital to elucidate the role of oviduct in mammalian reproduction and reproductive disorders. In particular, with an intravital window, in vivo OCT imaging is a powerful solution to studying how the oviduct transports preimplantation embryos towards the uterus for pregnancy, a long-standing question that is critical for uncovering the functional cause of tubal ectopic pregnancy. However, simultaneously tracking embryo movement and acquiring large-field-of-view images of oviduct activity in 3D has been challenging due to the generally limited volumetric imaging rate of OCT. A lack of OCT-based 3D velocimetry method for large, sparse particles acts as a technical hurdle for analyzing the mechanistic process of the embryo transport. Here, we report a new particle streak velocimetry method to address this hurdle. The method relies on the 3D streak of a moving particle formed during the acquisition of a single OCT volume, where double B-scans are acquired at each B-scan location to resolve ambiguity in assessing the movement of particle. We validated this method with the gold-standard, direct volumetric particle tracking in a flow phantom, and we demonstrated its in vivo applications for simultaneous velocimetry of embryos and imaging of oviduct. This work sets the stage for quantitative understanding of the oviduct transport function in vivo, and the method fills in a gap in OCT-based velocimetry, providing the potential to enable new applications in 3D flow imaging.

Analysis of Mid- to Late-Gestation Phenotypes in Mice

Mid- to late gestation is characterized by tissue differentiation, maturation, organogenesis, and growth, and many mutant genes have detrimental effects during this phase of development. The outcome may be lethal before birth or may be compatible with life but result in birth defects. Some of the common causes of death during late gestation are hematopoietic defects, cardiovascular problems, and placental insufficiency. Many morphological abnormalities, lethal or not, can be investigated with gross and histological analyses or by visualization of the developing skeleton. Molecular characterization of mutant phenotypes, guided by the expression pattern of the mutant gene, can reveal disruptions in gene expression patterns of known developmental genes. Cell proliferation and cell death assays will reveal disruptions in cellular dynamics. Various modalities of 3D imaging of intact embryos can provide volumetric information about mutant phenotypes.

Mechanisms of Regeneration and Fibrosis in the Endometrium

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

CD133/Prom1 marks proximal mouse oviduct epithelial progenitors and adult epithelial cells with a low generative capacity

The epithelium lining the oviduct or fallopian tube consists of multiciliated and secretory cells, which support fertilization and preimplantation development, however, its homeostasis remains poorly understood. CD133/Prom1 expression has been used as a marker to identify adult stem cell populations in various organs and often associated with cancer cells that have stem-like properties. Using an antibody targeted to CD133 and a Cre recombinase-based lineage tracing strategy, we found that CD133/Prom1 expression is not associated with a stem/progenitor population in the oviduct but marked predominantly multiciliated cells with a low generative capacity. Additionally, we have shown that CD133 is disparately localised along the oviduct during neonatal development, and that Prom1 expressing secretory cells in the ampulla rapidly transitioned to multiciliated cells and progressively migrated to the ridge of epithelial folds.

Cysteine-rich intestinal protein 1 is a novel surface marker for human myometrial stem/progenitor cells

Myometrial stem/progenitor cells (MyoSPCs) have been proposed as the cells of origin for uterine fibroids, but the identity of the MyoSPC has not been well established. We previously identified SUSD2 as a possible MyoSPC marker, but the relatively poor enrichment in stem cell characteristics of SUSD2+ over SUSD2- cells compelled us to find better markers. We combined bulk RNA-seq of SUSD2+/- cells with single cell RNA-seq to identify markers for MyoSPCs. We observed seven distinct cell clusters within the myometrium, with the vascular myocyte cluster most highly enriched for MyoSPC characteristics and markers. CRIP1 expression was found highly upregulated by both techniques and was used as a marker to sort CRIP1+/PECAM1- cells that were both enriched for colony forming potential and able to differentiate into mesenchymal lineages, suggesting that CRIP1+/PECAM1- cells could be used to better study the etiology of uterine fibroids.

Aberrant activation of Notch1 signaling in the mouse uterine epithelium promotes hyper-proliferation by increasing estrogen sensitivity

In mammals, the endometrium undergoes dynamic changes in response to estrogen and progesterone to prepare for blastocyst implantation. Two distinct types of endometrial epithelial cells, the luminal (LE) and glandular (GE) epithelial cells play different functional roles during this physiological process. Previously, we have reported that Notch signaling plays multiple roles in embryo implantation, decidualization, and postpartum repair. Here, using the uterine epithelial-specific Ltf-iCre, we showed that Notch1 signaling over-activation in the endometrial epithelium caused dysfunction of the epithelium during the estrous cycle, resulting in hyper-proliferation. During pregnancy, it further led to dysregulation of estrogen and progesterone signaling, resulting in infertility in these animals. Using 3D organoids, we showed that over-activation of Notch1 signaling increased the proliferative potential of both LE and GE cells and reduced the difference in transcription profiles between them, suggesting disrupted differentiation of the uterine epithelium. In addition, we demonstrated that both canonical and non-canonical Notch signaling contributed to the hyper-proliferation of GE cells, but only the non-canonical pathway was involved with estrogen sensitivity in the GE cells. These findings provided insights into the effects of Notch1 signaling on the proliferation, differentiation, and function of the uterine epithelium. This study demonstrated the important roles of Notch1 signaling in regulating hormone response and differentiation of endometrial epithelial cells and provides an opportunity for future studies in estrogen-dependent diseases, such as endometriosis.

The EZH2-PRC2-H3K27me3 axis governs the endometrial cell cycle and differentiation for blastocyst invasion

Infertility occurs in 15% of couples worldwide. Recurrent implantation failure (RIF) is one of the major problems in in vitro fertilization and embryo transfer (IVF-ET) programs, and how to manage patients with RIF to achieve successful pregnancy outcomes remains unresolved. Here, a uterine polycomb repressive complex 2 (PRC2)-regulated gene network was found to control embryo implantation. Our RNA-seq analyses of the human peri-implantation endometrium obtained from patients with RIF and fertile controls revealed that PRC2 components, including its core enzyme enhancer of zeste homolog 2 (EZH2)-catalyzing H3K27 trimethylation (H3K27me3) and their target genes are dysregulated in the RIF group. Although fertility of uterine epithelium-specific knockout mice of Ezh2 (eKO mice) was normal, Ezh2-deleted mice in the uterine epithelium and stroma (uKO mice) exhibited severe subfertility, suggesting that stromal Ezh2 plays a key role in female fertility. The RNA-seq and ChIP-seq analyses revealed that H3K27me3-related dynamic gene silencing is canceled, and the gene expression of cell-cycle regulators is dysregulated in Ezh2-deleted uteri, causing severe epithelial and stromal differentiation defects and failed embryo invasion. Thus, our findings indicate that the EZH2-PRC2-H3K27me3 axis is critical to preparing the endometrium for the blastocyst invasion into the stroma in mice and humans.

Cysteine-Rich Intestinal Protein 1 is a Novel Surface Marker for Myometrial Stem/Progenitor Cells

Myometrial stem/progenitor cells (MyoSPCs) have been proposed as the cells of origin for uterine fibroids, which are benign tumors that develop in the myometrium of most reproductive age women, but the identity of the MyoSPC has not been well established. We previously identified SUSD2 as a possible MyoSPC marker, but the relatively poor enrichment in stem cell characteristics of SUSD2+ over SUSD2- cells compelled us to find better discerning markers for more rigorous downstream analyses. We combined bulk RNA-seq of SUSD2+/- cells with single cell RNA-seq to identify markers capable of further enriching for MyoSPCs. We observed seven distinct cell clusters within the myometrium, with the vascular myocyte cluster most highly enriched for MyoSPC characteristics and markers, including SUSD2. CRIP1 expression was found highly upregulated in both techniques and was used as a marker to sort CRIP1+/PECAM1- cells that were both enriched for colony forming potential and able to differentiate into mesenchymal lineages, suggesting that CRIP1+/PECAM1- cells could be used to better study the etiology of uterine fibroids.

Maternal IL-33 critically regulates tissue remodeling and type 2 immune responses in the uterus during early pregnancy in mice

The pregnant uterus is an immunologically rich organ, with dynamic changes in the inflammatory milieu and immune cell function underlying key stages of pregnancy. Recent studies have implicated dysregulated expression of the interleukin-1 (IL-1) family cytokine, IL-33, and its receptor, ST2, in poor pregnancy outcomes in women, including recurrent pregnancy loss, preeclampsia, and preterm labor. How IL-33 supports pregnancy progression in vivo is not well understood. Here, we demonstrate that maternal IL-33 signaling critically regulates uterine tissue remodeling and immune cell function during early pregnancy in mice. IL-33-deficient dams exhibit defects in implantation chamber formation and decidualization, and abnormal vascular remodeling during early pregnancy. These defects coincide with delays in early embryogenesis, increased resorptions, and impaired fetal and placental growth by late pregnancy. At a cellular level, myometrial fibroblasts, and decidual endothelial and stromal cells, are the main IL-33⁺ cell types in the uterus during decidualization and early placentation, whereas ST2 is expressed by uterine immune populations associated with type 2 immune responses, including ILC2s, Tregs, CD4⁺ T cells, M2- and cDC2-like myeloid cells, and mast cells. Early pregnancy defects in IL-33-deficient dams are associated with impaired type 2 cytokine responses by uterine lymphocytes and fewer Arginase-1⁺ macrophages in the uterine microenvironment. Collectively, our data highlight a regulatory network, involving crosstalk between IL-33-producing nonimmune cells and ST2⁺ immune cells at the maternal-fetal interface, that critically supports pregnancy progression in mice. This work has the potential to advance our understanding of how IL-33 signaling may support optimal pregnancy outcomes in women.

Three-Dimensional Visualization of Mouse Endometrial Remodeling After Superovulation

Uterine status determines pregnancy success. Although it is well known that superovulation operations can disrupt uterine function, our understanding of the morphological changes in the uterine endometrium at the three-dimensional (3D) level is limited. Here, combining the tissue clearing with 3D deep imaging, we reveal an increase in epithelial density and angiogenesis after ovarian stimulation, which is accompanied by a circulating surge in P4 levels. Using an ovariectomized mouse model, we further detected the separate regulatory effects of P4 and E2 on the uterine endometrium, with P4 promoting endothelial cell growth and E2 inducing epithelial proliferation. Additionally, we observed that the effects of E2 can be partially neutralized by P4, and vice versa. By analyzing the 3D uterine imaging, we discovered an interesting phenomenon in which the growing blood vessels closely surround the remodeling uterine epithelium, indicating a close relationship between angiogenesis and epithelial growth. These findings provide new insight into the uterine epithelial changes and angiogenesis at the 3D level, and explain a potential reason for endometrial changes due to the low implantation rate in patients undergoing clinic super-ovulation.

Aberrant uterine folding in mice disrupts implantation chamber formation and alignment of embryo-uterine axes

The uterine luminal epithelium folds characteristically in mammals, including humans, horses and rodents. Improper uterine folding in horses results in pregnancy failure, but the precise function of folds remains unknown. Here, we uncover dynamic changes in the 3D uterine folding pattern during early pregnancy with the entire lumen forming pre-implantation transverse folds along the mesometrial-antimesometrial axis. Using a time course, we show that transverse folds are formed before embryo spacing, whereas implantation chambers form as the embryo begins attachment. Thus, folds and chambers are two distinct structures. Transverse folds resolve to form a flat implantation region, after which an embryo arrives at its center to attach and form the post-implantation chamber. Our data also suggest that the implantation chamber facilitates embryo rotation and its alignment along the uterine mesometrial-antimesometrial axis. Using WNT5A- and RBPJ-deficient mice that display aberrant folds, we show that embryos trapped in longitudinal folds display misalignment of the embryo-uterine axes, abnormal chamber formation and defective post-implantation morphogenesis. These mouse models with disrupted uterine folding provide an opportunity to understand uterine structure-based mechanisms that are crucial for implantation and pregnancy success.

This article has an associated ‘The people behind the papers’ interview.

Intraductal Delivery and X-ray Visualization of Ethanol-Based Ablative Solution for Prevention and Local Treatment of Breast Cancer in Mouse Models

Breast cancer is the most prevalent cancer and the second-leading cause of cancer-related death for women in the USA. For high-risk women, prophylactic mastectomy is the most effective primary prevention strategy. Prophylactic mastectomy is an aggressive surgical procedure that completely removes the mammary epithelial cells from which breast cancer arises along with the surrounding tissue. We seek to develop a minimally invasive intraductal procedure as an alternative to prophylactic mastectomy to locally ablate the mammary epithelial cells before they can become malignant. We and others have developed an intraductal delivery procedure to reach and treat these epithelial cells in rodent models of breast cancer. While the mouse mammary gland with a single non-anastomosed ductal tree opening at the nipple has a much less complex and tortuous architecture than the human breast, chemically induced and genetically engineered mouse models of breast cancer are valuable to produce proof-of-concept studies of new preventative strategies. Here, we describe a procedure for intraductal delivery of an ethanol-based ablative solution containing micro-CT/X-ray tantalum-based contrast agent within the mouse mammary ductal tree for the therapeutic purpose of primary prevention of breast cancer. Intraductal delivery of aqueous reagents (e.g., cytotoxic compounds, siRNAs, AdCre) has been previously described in mouse models. Thus, we focus our protocol description on methodological modifications and unique experimental considerations for optimizing delivery of ethanol, for minimizing local and systemic side effects of ethanol administration, and for in vivo visualization of ductal tree filling via micro-CT/fluoroscopy imaging. Visualization of the ductal tree immediately after injection of a contrast-containing solution allows for confirmation of complete filling or unsuccessful outcomes such as underfilling or overfilling. This procedure can be applied for delivery and imaging of other ablative compounds aimed at either preventing tumor formation or locally treating early-stage tumors accessible via the ductal tree.

Spatiotemporal dynamics of clonal selection and diversification in normal endometrial epithelium

It has become evident that somatic mutations in cancer-associated genes accumulate in the normal endometrium, but spatiotemporal understanding of the evolution and expansion of mutant clones is limited. To elucidate the timing and mechanism of the clonal expansion of somatic mutations in cancer-associated genes in the normal endometrium, we sequence 1311 endometrial glands from 37 women. By collecting endometrial glands from different parts of the endometrium, we show that multiple glands with the same somatic mutations occupy substantial areas of the endometrium. We demonstrate that “rhizome structures”, in which the basal glands run horizontally along the muscular layer and multiple vertical glands rise from the basal gland, originate from the same ancestral clone. Moreover, mutant clones detected in the vertical glands diversify by acquiring additional mutations. These results suggest that clonal expansions through the rhizome structures are involved in the mechanism by which mutant clones extend their territories. Furthermore, we show clonal expansions and copy neutral loss-of-heterozygosity events occur early in life, suggesting such events can be tolerated many years in the normal endometrium. Our results of the evolutionary dynamics of mutant clones in the human endometrium will lead to a better understanding of the mechanisms of endometrial regeneration during the menstrual cycle and the development of therapies for the prevention and treatment of endometrium-related diseases.

Through regeneration, the endometrium accumulates somatic mutations that can lead to diseases like endometriosis and cancer. Here, the authors use genomics to analyse normal endometrial glands from different patient cohorts, detect rhizome structures with common clonal ancestors and infer clonal expansion dynamics.

Exocrine gland structure-function relationships

Fluid secretion by exocrine glandular organs is essential to the survival of mammals. Each glandular unit within the body is uniquely organized to carry out its own specific functions, with failure to establish these specialized structures resulting in impaired organ function. Here, we review glandular organs in terms of shared and divergent architecture. We first describe the structural organization of the diverse glandular secretory units (the end-pieces) and their fluid transporting systems (the ducts) within the mammalian system, focusing on how tissue architecture corresponds to functional output. We then highlight how defects in development of end-piece and ductal architecture impacts secretory function. Finally, we discuss how knowledge of exocrine gland structure-function relationships can be applied to the development of new diagnostics, regenerative approaches and tissue regeneration.

The role of epithelial progesterone receptor isoforms in embryo implantation

The loss of uterine epithelial progesterone receptor (PGR) is crucial for successful embryo implantation in both humans and mice. The two major isoforms PGRA and PGRB have divergent functions under both physiological and pathological conditions. The present study compares phenotypes and gene signatures of PGRA and PGRB in uterine epithelium using uterine epithelial-specific constitutively expressed PGRA or PGRB mouse models. The cistrome and transcriptome analysis reveals substantial overlap between epithelial PGRA and PGRB, and both disrupt embryo implantation through FOXO1 pathways. Constitutive epithelial PGRA and PGRB expression impairs ESR1 occupancy at the promoter of Lif leading to reduced Lif transcription and further exaggerates SGK1 expression leading to enhanced PI3K-SGK1 activities, and both contribute to the decline of nuclear FOXO1 expression. Our study demonstrates that PGRA and PGRB in the uterine epithelium act on a similar set of target genes and commonly regulate the LIF-SGK1-FOXO1 signaling pathway for embryo implantation.

Uterine glands impact embryo survival and stromal cell decidualization in mice

Uterine glands are essential for the establishment of pregnancy and have critical roles in endometrial receptivity to blastocyst implantation, stromal cell decidualization, and placentation. Uterine gland dysfunction is considered a major contributing factor to pregnancy loss, however our understanding of how glands impact embryo survival and stromal cell decidualization is incomplete. Forkhead box A2 (FOXA2) is expressed only in the glandular epithelium and regulates its development and function. Mice with a conditional deletion of FOXA2 in the uterus are infertile due to defective embryo implantation arising from a lack of leukemia inhibitory factor (LIF), a critical factor of uterine gland origin. Here, a glandless FOXA2-deficient mouse model, coupled with LIF repletion to rescue the implantation defect, was used to investigate the roles of uterine glands in embryo survival and decidualization. Studies found that embryo survival and decidualization were compromised in glandless FOXA2-deficient mice on gestational day 6.5, resulting in abrupt pregnancy loss by day 7.5. These findings strongly support the hypothesis that uterine glands secrete factors other than LIF that impact embryo survival and stromal cell decidualization for pregnancy success.

The New Era of Three-Dimensional Histoarchitecture of the Human Endometrium

The histology of the endometrium has traditionally been established by observation of two-dimensional (2D) pathological sections. However, because human endometrial glands exhibit coiling and branching morphology, it is extremely difficult to obtain an entire image of the glands by 2D observation. In recent years, the development of three-dimensional (3D) reconstruction of serial pathological sections by computer and whole-mount imaging technology using tissue clearing methods with high-resolution fluorescence microscopy has enabled us to observe the 3D histoarchitecture of tissues. As a result, 3D imaging has revealed that human endometrial glands form a plexus network in the basalis, similar to the rhizome of grass, whereas mouse uterine glands are single branched tubular glands. This review summarizes the relevant literature on the 3D structure of mouse and human endometrium and discusses the significance of the rhizome structure in the human endometrium and the expected role of understanding the 3D tissue structure in future applications to systems biology.

Generation and analysis of Prss28 and Prss29 deficient mice using CRISPR-Cas9 genome-editing

Glands of the uterus are essential for the establishment of pregnancy in mice and their products regulate embryo implantation and stromal cell decidualization critical for pregnancy establishment. Forkhead box A2 (FOXA2) is expressed specifically in the glands and a critical regulator of their differentiation, development and function. Progesterone and FOXA2 regulate members of a serine proteinase gene family (Prss28 and Prss29). Here, CRISPR-Cas9 genome-editing was used to create mice with a heterozygous or homozygous deletion of Prss28 or/and Prss29 to determine their biological roles in uterine function. Female mice lacking Prss28 and Prss29 or both developed normally and were fertile without alterations in uterine histoarchitecture, uterine gland number, or and gene expression. Thus, Prss28 and Prss29 are dispensable for female fertility and do not impact endometrial gland development or uterine function mice.

Three-dimensional understanding of the morphological complexity of the human uterine endometrium

The fundamental morphology of the endometrial glands is not sufficiently understood by 2D observation because these glands have complicated winding and branching patterns. To construct a large picture of the endometrial gland structure, we performed tissue-clearing-based 3D imaging of human uterine endometrial tissue. Our 3D immunohistochemistry and layer analyses revealed that the endometrial glands form a plexus network in the stratum basalis and expand horizontally along the muscular layer, similar to the rhizome of grass. We then extended our method to assess the 3D morphology of tissue affected by adenomyosis, a representative "endometrium-related disease," and observed its 3D morphological features, including the direct invasion of endometrial glands into the myometrium and an ant colony-like network of ectopic endometrial glands within the myometrium. Thus, further understanding of the morphology of the human endometrium based on 3D analysis will lead to the identification of the pathogenesis of endometrium-related diseases.

Anatomical and cellular heterogeneity in the mouse oviduct-its potential roles in reproduction and preimplantation development†

The oviduct/fallopian tube is a tube-like structure that extends from the uterus to the ovary. It is an essential reproductive organ that provides an environment for internal fertilization and preimplantation development. However, our knowledge of its regional and cellular heterogeneity is still limited. Here, we examined the anatomical complexity of mouse oviducts using modern imaging techniques and fluorescence reporter lines. We found that there are consistent coiling patterns and turning points in the coiled mouse oviduct that serve as reliable landmarks for luminal morphological regionalities. We also found previously unrecognized anatomical structures in the isthmus and uterotubal junction, which likely play roles in reproduction. Furthermore, we demarcated the ampulla-isthmus junction as a distinct region. Taken together, the oviduct mucosal epithelium has highly diverse structures with distinct epithelial cell populations, reflecting its complex functions in reproduction.

Endometrial epithelial ARID1A is critical for uterine gland function in early pregnancy establishment

Though endometriosis and infertility are clearly associated, the pathophysiological mechanism remains unclear. Previous work has linked endometrial ARID1A loss to endometriosis-related endometrial non-receptivity. Here, we show in mice that ARID1A binds and regulates transcription of the Foxa2 gene required for endometrial gland function. Uterine-specific deletion of Arid1a compromises gland development and diminishes Foxa2 and Lif expression. Deletion of Arid1a with Ltf-iCre in the adult mouse endometrial epithelium preserves the gland development while still compromising the gland function. Mice lacking endometrial epithelial Arid1a are severely sub-fertile due to defects in implantation, decidualization, and endometrial receptivity from disruption of the LIF-STAT3-EGR1 pathway. FOXA2 is also reduced in the endometrium of women with endometriosis in correlation with diminished ARID1A, and both ARID1A and FOXA2 are reduced in nonhuman primates induced with endometriosis. Our findings describe a role for ARID1A in the endometrial epithelium supporting early pregnancy establishment through the maintenance of gland function.

Mechanical and signaling mechanisms that guide pre-implantation embryo movement

How a mammalian embryo determines and arrives at its attachment site has been studied for decades, but our understanding of this process is far from complete. Using confocal imaging and image analysis, we evaluate embryo location along the longitudinal oviductal-cervical axis of murine uteri. Our analysis reveals three distinct pre-implantation phases: embryo entry, unidirectional movement of embryo clusters and bidirectional scattering and spacing of embryos. We show that unidirectional clustered movement is facilitated by a mechanical stimulus of the embryo and is regulated by adrenergic uterine smooth muscle contractions. Embryo scattering, on the other hand, depends on embryo-uterine communication reliant on the LPAR3 signaling pathway and is independent of adrenergic muscle contractions. Finally, we demonstrate that uterine implantation sites in mice are neither random nor predetermined but are guided by the number of embryos entering the uterine lumen. These studies have implications for understanding how embryo-uterine communication is key to determining an optimal implantation site necessary for the success of a pregnancy.

WNK1 regulates uterine homeostasis and its ability to support pregnancy

WNK1 (with no lysine [K] kinase 1) is an atypical kinase protein ubiquitously expressed in humans and mice. A mutation in its encoding gene causes hypertension in humans, which is associated with abnormal ion homeostasis. WNK1 is critical for in vitro decidualization in human endometrial stromal cells, thereby demonstrating its importance in female reproduction. Using a mouse model, WNK1 was ablated in the female reproductive tract to define its in vivo role in uterine biology. Loss of WNK1 altered uterine morphology, causing endometrial epithelial hyperplasia, adenomyotic features, and a delay in embryo implantation, ultimately resulting in compromised fertility. Combining transcriptomic, proteomic, and interactomic analyses revealed a potentially novel regulatory pathway whereby WNK1 represses AKT phosphorylation through protein phosphatase 2A (PP2A) in endometrial cells from both humans and mice. We show that WNK1 interacted with PPP2R1A, the alpha isoform of the PP2A scaffold subunit. This maintained the levels of PP2A subunits and stabilized its activity, which then dephosphorylated AKT. Therefore, loss of WNK1 reduced PP2A activity, causing AKT hypersignaling. Using FOXO1 as a readout of AKT activity, we demonstrate that there was escalated FOXO1 phosphorylation and nuclear exclusion, leading to a disruption in the expression of genes that are crucial for embryo implantation.

Epithelial morphogenesis in the perinatal mouse uterus

BACKGROUND

The uterus is the location where multiple events occur that are required for the start of new life in mammals. The adult uterus contains endometrial or uterine glands that are essential for female fertility. In the mouse, uterine glands are located in the lateral and antimesometrial regions of the uterine horn. Previous three-dimensional (3D)-imaging of the adult uterus, its glands, and implanting embryos has been performed by multiple groups, using fluorescent microscopy. Adenogenesis, the formation of uterine glands, initiates after birth. Recently, we created a 3D-staging system of mouse uterine gland development at postnatal time points, using light sheet fluorescent microscopy. Here, using a similar approach, we examine the morphological changes in the epithelium of the perinatal mouse uterus.

RESULTS

The uterine epithelium exhibits dorsoventral (mesometrial-antimesometrial) patterning as early as 3 days after birth (P3), marked by the presence of the dorsally positioned developing uterine rail. Uterine gland buds are present beginning at P4. Novel morphological epithelial structures, including a ventral ridge and uterine segments were identified.

CONCLUSIONS

The perinatal mouse uterine luminal epithelium develops dorsal-ventral morphologies at 3 to 4 days postpartum. Between 5 and 6 days postpartum uterine epithelial folds form, defining alternating left-right segments.

Differential roles of uterine epithelial and stromal STAT3 coordinate uterine receptivity and embryo attachment

Although it has been reported that uterine signal transducer and activator of transcription 3 (STAT3) is essential for embryo implantation, the exact roles of uterine epithelial and stromal STAT3 on embryo implantation have not been elucidated. To address this issue, we generated Stat3-floxed/Ltf-iCre (Stat3-eKO), Stat3-floxed/Amhr2-Cre (Stat3-sKO), and Stat3-floxed/Pgr-Cre (Stat3-uKO) mice to delete Stat3 in uterine epithelium, uterine stroma, and whole uterine layers, respectively. We found that both epithelial and stromal STAT3 have critical roles in embryo attachment because all the Stat3-eKO and Stat3-sKO female mice were infertile due to implantation failure without any embryo attachment sites. Stat3-eKO uteri showed indented structure of uterine lumen, indicating the role of epithelial STAT3 in slit-like lumen formation in the peri-implantation uterus. Stat3-sKO uteri exhibited hyper-estrogenic responses and persistent cell proliferation of the epithelium in the peri-implantation uterus, suggesting the role of stromal STAT3 in uterine receptivity. In addition, Stat3-uKO female mice possessed not only the characteristic of persistent epithelial proliferation but also that of indented structure of uterine lumen. These findings indicate that epithelial STAT3 controls the formation of slit-like structure in uterine lumen and stromal STAT3 suppresses epithelial estrogenic responses and cell proliferation. Thus, epithelial and stromal STAT3 cooperatively controls uterine receptivity and embryo attachment through their different pathways.

Sexually dimorphic effects of forkhead box a2 (FOXA2) and uterine glands on decidualization and fetoplacental development

Glands of the uterus are essential for pregnancy establishment. Forkhead box A2 (FOXA2) is expressed specifically in the glands of the uterus and a critical regulator of glandular epithelium (GE) differentiation, development, and function. Mice with a conditional deletion of FOXA2 in the adult uterus, created using the lactotransferrin iCre (Ltf-iCre) model, have a morphologically normal uterus with glands, but lack FOXA2-dependent GE-expressed genes, such as leukemia inhibitory factor (LIF). Adult FOXA2 conditional knockout (cKO; Ltf ^iCre/+ Foxa2 ^f/f ) mice are infertile due to defective embryo implantation arising from a lack of LIF, a critical implantation factor of uterine gland origin. However, intraperitoneal injections of LIF can initiate embryo implantation in the uterus of adult FOXA2 cKO mice with pregnancies maintained to term. Here, we tested the hypothesis that FOXA2-regulated genes in the uterine glands impact development of the decidua, placenta, and fetus. On gestational day 8.5, the antimesometrial and mesometrial decidua transcriptome was noticeably altered in LIF-replaced FOXA2 cKO mice. Viable fetuses were reduced in FOXA2 cKO mice on gestational days 12.5 and 17.5. Sex-dependent differences in fetal weight, placenta histoarchitecture, and the placenta and metrial gland transcriptome were observed between control and FOXA2 cKO mice. The transcriptome of the placenta with a female fetus was considerably more altered than the placenta with a male fetus in FOXA2 cKO dams. These studies reveal previously unrecognized sexually dimorphic effects of FOXA2 and uterine glands on fetoplacental development with potential impacts on offspring health into adulthood.

Histological 3D reconstruction and in vivo lineage tracing of the human endometrium

Regular menstrual shedding and repair of the endometrial functionalis is unique to humans and higher-order primates. The current consensus postulates endometrial glands to have a single-tubular architecture, where multi-potential stem cells reside in the blind-ending glandular-bases. Utilising fixed samples from patients, we have studied the three-dimensional (3D) micro-architecture of the human endometrium. We demonstrate that some non-branching, single, vertical functionalis glands originate from a complex horizontally interconnecting network of basalis glands. The existence of a multipotent endometrial epithelial stem cell capable of regenerating the entire complement of glandular lineages was demonstrated by in vivo lineage tracing, using naturally occurring somatic mitochondrial DNA mutations as clonal markers. Vertical tracking of mutated clones showed that at least one stem-cell population resides in the basalis glands. These novel findings provide insight into the efficient and scar-less regenerative potential of the human endometrium. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Uterine Patterning, Endometrial Gland Development, and Implantation Failure in Mice Exposed Neonatally to Genistein

BACKGROUND

Embryo implantation relies on precise hormonal regulation, associated gene expression changes, and appropriate female reproductive tract tissue architecture. Female mice exposed neonatally to the phytoestrogen genistein (GEN) at doses similar to those in infants consuming soy-based infant formulas are infertile due in part to uterine implantation defects.

OBJECTIVES

Our goal was to determine the mechanisms by which neonatal GEN exposure causes implantation defects.

METHODS

Female mice were exposed to GEN on postnatal days (PND)1-5 and uterine tissues collected on PND5, PND22-26, and during pregnancy. Analysis of tissue weights, morphology, and gene expression was performed using standard histology, confocal imaging with three-dimensional analysis, real-time reverse transcription polymerase chain reaction (real-time RT-PCR), and microarrays. The response of ovariectomized adults to 17 β -estradiol (E2) and artificial decidualization were measured. Leukemia inhibitory factor (LIF) injections were given intraperitoneally and implantation sites visualized. Gene expression patterns were compared with curated data sets to identify upstream regulators.

RESULTS

GEN-exposed mice exhibited reduced uterine weight gain in response to E2 treatment or artificial decidualization compared with controls; however, expression of select hormone responsive genes remained similar between the two groups. Uteri from pregnant GEN-exposed mice were posteriorized and had reduced glandular epithelium. Implantation failure was not rescued by LIF administration. Microarray analysis of GEN-exposed uteri during early pregnancy revealed significant overlap with several conditional uterine knockout mouse models, including Foxa2, Wnt4, and Sox17. These models exhibit reduced endometrial glands, features of posteriorization and implantation failure. Expression of Foxa2, Wnt4, and Sox17, as well as genes important for neonatal uterine differentiation (Wnt7a, Hoxa10, and Msx2), were severely disrupted on PND5 in GEN-exposed mice.

DISCUSSION

Our findings suggest that neonatal GEN exposure in mice disrupts expression of genes important for uterine development, causing posteriorization and diminished gland function during pregnancy that contribute to implantation failure. These findings could have implications for women who consumed soy-based formulas as infants. https://doi.org/10.1289/EHP6336.

Estradiol-17β regulates proliferation and apoptosis of sheep endometrial epithelial cells by regulating the relative abundance of YAP1

Yes-associated protein 1 (YAP1) transcription regulator of the Hippo protein kinase pathway, serves as a key regulator of tissue growth and organ size by regulating cell proliferation and apoptosis. Effects of YAP1 on proliferation and apoptosis of sheep endometrial epithelial cells (EEC) as a result of estradiol-17β (E₂) treatment, however, remain unclear. In the present study, the abundance of YAP1 protein in the uterine horn was greater than that in the uterine body or cervix. The YAP1 protein was primarily localized in the endometrial luminal and glandular epithelial cells of the uterine horn of ewes on day 2 of the estrous cycle. Compared with control samples, there was a lesser abundance of YAP1 mRNA transcript that was associated with a lesser proliferation and greater apoptosis of EEC. There were also lesser concentrations of epidermal growth factor and insulin-like growth factor 1 in the spent culture medium when there was a lesser abundance of YAP1 mRNA in EEC compared with those in the control group. When there was a greater abundance of YAP1 mRNA transcript, there were greater concentrations of epidermal growth factor and insulin-like growth factor 1 in the spent media. Furthermore, with estradiol-17β treatment the abundance of YAP1 mRNA transcript was similar to that of the control samples. Taken together, estradiol-17β may function as an essential regulator of EEC proliferation and apoptosis by modulation of concentrations of YAP1 protein in the sheep uterus. These results indicate there are molecular mechanisms of estradiol-17β and YAP1 in EEC proliferation and apoptosis of ewes.

Uterine Luminal Epithelium as the Transient Gateway for Embryo Implantation

The uterine luminal epithelium (LE) is the first maternal contact for an implanting embryo. Intrauterine fluid resorption, cessation of LE proliferation and apoptosis, and LE structural changes are prerequisites for establishing transient uterine receptivity for embryo implantation. Vesicle trafficking in the LE and receptor-mediated paracrine and autocrine mechanisms are crucial both for LE preparation and LE communications with the embryo and stroma during the initiation of embryo implantation. This review mainly covers recent in vivo studies in LE of mouse models from 0.5 days post-coitus (D0.5) to ∼D4 20 h when the trophoblasts pass through the LE layer for embryo implantation. The review is organized into three interconnected sections: preimplantation LE preparation for embryo attachment, embryo-LE communications, and LE-stroma communications.

Micro-Engineered Models of Development Using Induced Pluripotent Stem Cells

During fetal development, embryonic cells are coaxed through a series of lineage choices which lead to the formation of the three germ layers and subsequently to all the cell types that are required to form an adult human body. Landmark cell fate decisions leading to symmetry breaking, establishment of the primitive streak and first tri-lineage differentiation happen after implantation, and therefore have been attributed to be a function of the embryo's spatiotemporal 3D environment. These mechanical and geometric cues induce a cascade of signaling pathways leading to cell differentiation and orientation. Due to the physiological, ethical, and legal limitations of accessing an intact human embryo for functional studies, multiple in-vitro models have been developed to try and recapitulate the key milestones of mammalian embryogenesis using mouse embryos, or mouse and human embryonic stem cells. More recently, the development of induced pluripotent stem cells represents a cell source which is being explored to prepare a developmental model, owing to their genetic and functional similarities to embryonic stem cells. Here we review the use of micro-engineered cell culture materials as platforms to define the physical and geometric contributions during the cell fate defining process and to study the underlying pathways. This information has applications in various biomedical contexts including tissue engineering, stem cell therapy, and organoid cultures for disease modeling.

Uterine Glands: Developmental Biology and Functional Roles in Pregnancy

All mammalian uteri contain glands in the endometrium that develop only or primarily after birth. Gland development or adenogenesis in the postnatal uterus is intrinsically regulated by proliferation, cell-cell interactions, growth factors and their inhibitors, as well as transcription factors, including forkhead box A2 (FOXA2) and estrogen receptor α (ESR1). Extrinsic factors regulating adenogenesis originate from other organs, including the ovary, pituitary, and mammary gland. The infertility and recurrent pregnancy loss observed in uterine gland knockout sheep and mouse models support a primary role for secretions and products of the glands in pregnancy success. Recent studies in mice revealed that uterine glandular epithelia govern postimplantation pregnancy establishment through effects on stromal cell decidualization and placental development. In humans, uterine glands and, by inference, their secretions and products are hypothesized to be critical for blastocyst survival and implantation as well as embryo and placental development during the first trimester before the onset of fetal-maternal circulation. A variety of hormones and other factors from the ovary, placenta, and stromal cells impact secretory function of the uterine glands during pregnancy. This review summarizes new information related to the developmental biology of uterine glands and discusses novel perspectives on their functional roles in pregnancy establishment and success.

Understanding the Mechanobiology of Early Mammalian Development through Bioengineered Models

Research in developmental biology has been recently enriched by a multitude of in vitro models recapitulating key milestones of mammalian embryogenesis. These models obviate the challenge posed by the inaccessibility of implanted embryos, multiply experimental opportunities, and favor approaches traditionally associated with organoids and tissue engineering. Here, we provide a perspective on how these models can be applied to study the mechano-geometrical contributions to early mammalian development, which still escape direct verification in species that develop in utero. We thus outline new avenues for robust and scalable perturbation of geometry and mechanics in ways traditionally limited to non-implanting developmental models.

A tale of two tracts: history, current advances, and future directions of research on sexual differentiation of reproductive tracts†

Alfred Jost's work in the 1940s laid the foundation of the current paradigm of sexual differentiation of reproductive tracts, which contends that testicular hormones drive the male patterning of reproductive tract system whereas the female phenotype arises by default. Once established, the sex-specific reproductive tracts undergo morphogenesis, giving rise to anatomically and functionally distinct tubular organs along the rostral-caudal axis. Impairment of sexual differentiation of reproductive tracts by genetic alteration and environmental exposure are the main causes of disorders of sex development, and infertility at adulthood. This review covers past and present work on sexual differentiation and morphogenesis of reproductive tracts, associated human disorders, and emerging technologies that have made impacts or could radically expand our knowledge in this field.

Evidence for functional interactions between the placenta and brain in pregnant mice

The placenta plays a pivotal role in the development of the fetal brain and also influences maternal brain function, but our understanding of communication between the placenta and brain remains limited. Using a gene expression and network analysis approach, we provide evidence that the placenta transcriptome is tightly interconnected with the maternal brain and fetal brain in d 15 pregnant C57BL/6J mice. Activation of serotonergic synapse signaling and inhibition of neurotrophin signaling were identified as potential mediators of crosstalk between the placenta and maternal brain and fetal brain, respectively. Genes encoding specific receptors and ligands were predicted to affect functional interactions between the placenta and brain. Paralogous genes, such as sex comb on midleg homolog 1/scm-like with 4 mbt domains 2 and polycomb group ring finger (Pcgf) 2/ Pcgf5, displayed antagonistic regulation between the placenta and brain. Additionally, conditional ablation of forkhead box a2 ( Foxa2) in the glands of the uterus altered the transcriptome of the d 15 placenta, which provides novel evidence of crosstalk between the uterine glands and placenta. Furthermore, expression of cathepsin 6 and monocyte to macrophage differentiation associated 2 was significantly different in the fetal brain of Foxa2 conditional knockout mice compared with control mice. These findings provide a better understanding of the intricacies of uterus-placenta-brain interactions during pregnancy and provide a foundation and model system for their exploration.-Behura, S. K., Kelleher, A. M., Spencer, T. E. Evidence for functional interactions between the placenta and brain in pregnant mice.