Characterization of human-like menstruation in the spiny mouse: comparative studies with the human and induced mouse model

Immune system adaptation during gender-affirming testosterone treatment

Infectious, inflammatory and autoimmune conditions present differently in males and females. SARS-CoV-2 infection in naive males is associated with increased risk of death, whereas females are at increased risk of long COVID1, similar to observations in other infections2. Females respond more strongly to vaccines, and adverse reactions are more frequent3, like most autoimmune diseases4. Immunological sex differences stem from genetic, hormonal and behavioural factors5 but their relative importance is only partially understood6-8. In individuals assigned female sex at birth and undergoing gender-affirming testosterone therapy (trans men), hormone concentrations change markedly but the immunological consequences are poorly understood. Here we performed longitudinal systems-level analyses in 23 trans men and found that testosterone modulates a cross-regulated axis between type-I interferon and tumour necrosis factor. This is mediated by functional attenuation of type-I interferon responses in both plasmacytoid dendritic cells and monocytes. Conversely, testosterone potentiates monocyte responses leading to increased tumour necrosis factor, interleukin-6 and interleukin-15 production and downstream activation of nuclear factor kappa B-regulated genes and potentiation of interferon-γ responses, primarily in natural killer cells. These findings in trans men are corroborated by sex-divergent responses in public datasets and illustrate the dynamic regulation of human immunity by sex hormones, with implications for the health of individuals undergoing hormone therapy and our understanding of sex-divergent immune responses in cisgender individuals.

Bioengineering approaches for the endometrial research and application

The endometrium undergoes a series of precise monthly changes under the regulation of dynamic levels of ovarian hormones that are characterized by repeated shedding and subsequent regeneration without scarring. This provides the potential for wound healing during endometrial injuries. Bioengineering materials highlight the faithful replication of constitutive cells and the extracellular matrix that simulates the physical and biomechanical properties of the endometrium to a larger extent. Significant progress has been made in this field, and functional endometrial tissue bioengineering allows an in-depth investigation of regulatory factors for endometrial and myometrial defects in vitro and provides highly therapeutic methods to alleviate obstetric and gynecological complications. However, much remains to be learned about the latest progress in the application of bioengineering technologies to the human endometrium. Here, we summarize the existing developments in biomaterials and bioengineering models for endometrial regeneration and improving the female reproductive potential.

From grouping and cooperation to menstruation: Spiny mice (Acomys cahirinus) are an emerging mammalian model for sociality and beyond

While spiny mice are primarily used as a model for Type II diabetes and for studying complex tissue regeneration, they are also an emerging model for a variety of studies examining hormones, behavior, and the brain. We began studying the spiny mouse to take advantage of their highly gregarious phenotype to examine how the brain facilitates large group-living. However, this unique rodent can be readily bred and maintained in the lab and can be used to ask a wide variety of scientific questions. In this brief communication we provide an overview of studies that have used spiny mice for exploring physiology and behavior. Additionally, we describe how the spiny mouse can serve as a useful model for researchers interested in studying precocial development, menstruation, cooperation, and various grouping behaviors. With increasingly available technological advancements for non-traditional organisms, spiny mice are well-positioned to become a valuable organism in the behavioral neuroscience community.

Long-term maintenance of human endometrial epithelial organoids and their stem cell properties

The endometrium undergoes dynamic changes throughout the menstrual cycle and pregnancy, which is unique to primates. Endometrium remodeling is essential for the implantation and nutritional support of the conceptus. Despite this, the role of uterine glands in driving endometrial tissue remodeling is still poorly understood. To address this, a 3-dimensional culture system was used to generate endometrial epithelial organoids from human endometrium biopsies. These organoids are genetically stable, long-term expandability. They reproduce some functions of uterine glands in vivo. The epithelial organoids exhibit characteristics of stem cells, with the proportion of stem cells increasing with culture time and passage number. Long-term maintenance of organoids strongly expressed stemness related genes accompanied by a decrease expression in mature epithelial gene, which suggests the organoids had switched from a mature stage to a progenitor stage. Thus we proposed the possible markers for epithelial progenitors. Meanwhile, long-term cultured organoids exhibit an increase in the proportion of luminal epithelial stem cells, accompanied by a decrease of glandular epithelial stem cells. Organoids also show hormone responsiveness, reflecting the various stages of the menstrual cycle and early pregnancy.

Aspects of human uterine creatine metabolism during the menstrual cycle and at term pregnancy†

Creatine metabolism likely contributes to energy homeostasis in the human uterus, but whether this organ synthesizes creatine and whether creatine metabolism is adjusted throughout the menstrual cycle and with pregnancy are largely unknown. This study determined endometrial protein expression of creatine-synthesizing enzymes arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT), creatine kinase (CKBB), and the creatine transporter (SLC6A8) throughout the menstrual cycle in fertile and primary infertile women. It also characterized creatine metabolism at term pregnancy, measuring aspects of creatine metabolism in myometrial and decidual tissue. In endometrial samples, AGAT, GAMT, SLC6A8, and CKBB were expressed in glandular and luminal epithelial cells. Except for SLC6A8, the other proteins were also located in stromal cells. Irrespective of fertility, AGAT, GAMT, and SLC6A8 high-intensity immunohistochemical staining was greatest in the early secretory phase of the menstrual cycle. During the proliferative phase, staining for SLC6A8 protein was greater (P = 0.01) in the primary infertile compared with the fertile group. Both layers of the term pregnant uterus contained creatine, phosphocreatine, guanidinoacetic acid, arginine, glycine, and methionine; detectable gene and protein expression of AGAT, GAMT, CKBB, and ubiquitous mitochondrial CK (uMt-CK); and gene expression of SLC6A8. The proteins AGAT, GAMT, CKBB, and SLC6A8 were uniformly distributed in the myometrium and localized to the decidual glands. In conclusion, endometrial tissue has the capacity to produce creatine and its capacity is highest around the time of fertilization and implantation. Both layers of the term pregnant uterus also contained all the enzymatic machinery and substrates of creatine metabolism.

TIGER: Single-step in vivo genome editing in a non-traditional rodent

Rodents are taxonomically diverse and have evolved a variety of traits. A mechanistic understanding of such traits has remained elusive, however, largely because genome editing in non-traditional model species remains challenging. Here, using the African striped mouse (Rhabdomys pumilio), we describe TIGER (targeted in vivo genome editing in rodents), a method that relies on a simple intraoviductal injecting technique and uses recombinant adeno-associated viruses (rAAVs) as the sole vehicle to deliver reagents into pregnant females. We demonstrate that TIGER generates knockout and knockin (up to 3 kb) lines with high efficiency. Moreover, we engineer a double-cleaving repair rAAV template and find that it significantly increases knockin frequency and germline transmission rates. Lastly, we show that an oversized double-cleaving rAAV template leads to an insertion of 3.8 kb. Thus, TIGER constitutes an attractive alternative to traditional ex vivo genome-editing methods and has the potential to be extended to a broad range of species.

Distribution of vasopressin 1a and oxytocin receptor protein and mRNA in the basal forebrain and midbrain of the spiny mouse (Acomys cahirinus)

The nonapeptide system modulates numerous social behaviors through oxytocin and vasopressin activation of the oxytocin receptor (OXTR) and vasopressin receptor (AVPR1A) in the brain. OXTRs and AVPR1As are widely distributed throughout the brain and binding densities exhibit substantial variation within and across species. Although OXTR and AVPR1A binding distributions have been mapped for several rodents, this system has yet to be characterized in the spiny mouse (Acomys cahirinus). Here we conducted receptor autoradiography and in situ hybridization to map distributions of OXTR and AVPR1A binding and Oxtr and Avpr1a mRNA expression throughout the basal forebrain and midbrain of male and female spiny mice. We found that nonapeptide receptor mRNA is diffuse throughout the forebrain and midbrain and does not always align with OXTR and AVPR1A binding. Analyses of sex differences in brain regions involved in social behavior and reward revealed that males exhibit higher OXTR binding densities in the lateral septum, bed nucleus of the stria terminalis, and anterior hypothalamus. However, no association with gonadal sex was observed for AVPR1A binding. Hierarchical clustering analysis further revealed that co-expression patterns of OXTR and AVPR1A binding across brain regions involved in social behavior and reward differ between males and females. These findings provide mapping distributions and sex differences in nonapeptide receptors in spiny mice. Spiny mice are an excellent organism for studying grouping behaviors such as cooperation and prosociality, and the nonapeptide receptor mapping here can inform the study of nonapeptide-mediated behavior in a highly social, large group-living rodent.

Crosstalk between Extracellular Matrix Stiffness and ROS Drives Endometrial Repair via the HIF-1α/YAP Axis during Menstruation

Although the menstrual cycle driven by sex steroid hormones is an uncomplicated physiological process, it is important for female health, fertility and regenerative biology. However, our understanding of this unique type of tissue homeostasis remains unclear. Here, we examined the biological effects of mechanical force by evaluating the changing trend of extracellular matrix (ECM) stiffness, and the results suggested that ECM stiffness was reduced and that breaking of mechanotransduction delayed endometrium repair in a mouse model of simulated menses. We constructed an ECM stiffness interference model in vitro to explain the mechanical force conduction mechanism during endometrial regeneration. We discovered that ECM stiffness increased the expression and nuclear transfer of YAP, which improved the creation of a microenvironment, in a manner that induced proliferation and angiogenesis for endometrial repair by activating YAP. In addition, we observed that physiological endometrial hypoxia occurs during the menstrual cycle and that the expression of HIF-1α was increased. Mechanistically, in addition to the classical F-actin/YAP pathway, we also found that the ROS/HIF-1α/YAP axis was involved in the transmission of mechanical signals. This study provides novel insights into the essential menstrual cycle and presents an effective, nonhormonal treatment for menstrual disorders.

Pre-clinical models to study abnormal uterine bleeding (AUB)

Abnormal Uterine Bleeding (AUB) is a common debilitating condition that significantly reduces quality of life of women across the reproductive age span. AUB creates significant morbidity, medical, social, and economic problems for women, their families, workplace, and health services. Despite the profoundly negative effects of AUB on public health, advancement in understanding the pathophysiology of AUB and the discovery of novel effective therapies is slow due to lack of reliable pre-clinical models. This review discusses currently available laboratory-based pre-clinical scientific models and how they are used to study AUB. Human and animal in vitro, ex vivo, and in vivo models will be described along with advantages and limitations of each method.

Modeling Endometrium Biology and Disease

The endometrium, lining the uterine lumen, is highly essential for human reproduction. Its exceptional remodeling plasticity, including the transformation process to welcome and nest the embryo, is not well understood. Lack of representative and reliable study models allowing the molecular and cellular mechanisms underlying endometrium development and biology to be deciphered is an important hurdle to progress in the field. Recently, powerful organoid models have been developed that not only recapitulate endometrial biology such as the menstrual cycle, but also faithfully reproduce diseases of the endometrium such as endometriosis. Moreover, single-cell profiling endeavors of the endometrium in health and disease, and of derived organoids, start to provide deeper insight into cellular complexity and expression specificities, and in resulting tissue processes. This granular portrayal will not only help in understanding endometrium biology and disease, but also in pinning down the tissue's stem cells, at present not yet conclusively defined. Here, we provide a general overview of endometrium development and biology, and the efforts of modeling both the healthy tissue, as well as its key diseased form of endometriosis. The future of modeling and deciphering this key tissue, hidden inside the womb, looks bright.

A remarkable rodent: Regeneration and reproduction in spiny mice (Acomys)

Although certain organisms are chosen and employed to better understand a specific problem in biology (so-called model organisms), sometimes an animal model reveals its' biomedical importance by happenstance. In many ways, the advent of spiny mice (Acomys) as an emerging model to study regeneration and menstruation stands as a case study in scientific pseudoserendipity (Diaz de Chumaceiro, 1995). As we recount in this chapter, the discovery of these phenotypes, while not entirely accidental, was nonetheless unexpected. In addition to recounting how we uncovered these unusual mammalian traits, we outline recent work by our groups and others that has begun to outline the cellular and genetic mechanisms underlying bonafide mammalian tissue regeneration and a human-like mode of reproduction in spiny mice.

Mechanisms of Scarless Repair at Time of Menstruation: Insights From Mouse Models

The human endometrium is a remarkable tissue which may experience up to 400 cycles of hormone-driven proliferation, differentiation and breakdown during a woman's reproductive lifetime. During menstruation, when the luminal portion of tissue breaks down, it resembles a bloody wound with piecemeal shedding, exposure of underlying stroma and a strong inflammatory reaction. In the absence of pathology within a few days the integrity of the tissue is restored without formation of a scar and the endometrium is able to respond appropriately to subsequent endocrine signals in preparation for establishment of pregnancy if fertilization occurs. Understanding mechanisms regulating scarless repair of the endometrium is important both for design of therapies which can treat conditions where this is aberrant (heavy menstrual bleeding, fibroids, endometriosis, Asherman's syndrome) as well as to provide new information that might allow us to reduce fibrosis and scar formation in other tissues. Menstruation only occurs naturally in species that exhibit spontaneous stromal cell decidualization during the fertile cycle such as primates (including women) and the Spiny mouse. To take advantage of genetic models and detailed time course analysis, mouse models of endometrial shedding/repair involving hormonal manipulation, artificial induction of decidualization and hormone withdrawal have been developed and refined. These models are useful in modeling dynamic changes across the time course of repair and have recapitulated key features of endometrial repair in women including local hypoxia and immune cell recruitment. In this review we will consider the evidence that scarless repair of endometrial tissue involves changes in stromal cell function including mesenchyme to epithelial transition, epithelial cell proliferation and multiple populations of immune cells. Processes contributing to endometrial fibrosis (Asherman's syndrome) as well as scarless repair of other tissues including skin and oral mucosa are compared to that of menstrual repair.

Menstrual Fluid Factors Mediate Endometrial Repair

Menstruation is a process whereby the outer functionalis layer of the endometrium is shed each month in response to falling progesterone and estrogen levels in a non-conception cycle. Simultaneously with the tissue breakdown, the surface is re-epithelialized, protecting the wound from infection. Once menstruation is complete and estrogen levels start to rise, regeneration progresses throughout the proliferative phase of the cycle, to fully restore endometrial thickness. Endometrial repair is unique compared to tissue repair elsewhere in the adult, in that it is rapid, scar-free and occurs around 400 times during each modern woman's reproductive life. The shedding tissue and that undergoing repair is bathed in menstrual fluid, which contains live cells, cellular debris, fragments of extracellular matrix, activated leukocytes and their products, soluble cellular components and extracellular vesicles. Proteomic and other analyses have revealed some detail of these components. Menstrual fluid, along with a number of individual proteins enhances epithelial cell migration to cover the wound. This is shown in endometrial epithelial and keratinocyte cell culture models, in an ex vivo decellularized skin model and in pig wounds in vivo. Thus, the microenvironment provided by menstrual fluid, is likely responsible for the unique rapid and scar-free repair of this remarkable tissue. Insight gained from analysis of this fluid is likely to be of value not only for treating endometrial bleeding problems but also in providing potential new therapies for poorly repairing wounds such as those seen in the aged and in diabetics.

The Spiny Mouse—A Menstruating Rodent to Build a Bridge From Bench to Bedside

Menstruation, the cyclical breakdown of the uterine lining, is arguably one of evolution's most mysterious reproductive strategies. The complexity and rarity of menstruation within the animal kingdom is undoubtedly a leading contributor to our current lack of understanding about menstrual function and disorders. In particular, the molecular and environmental mechanisms that drive menstrual and fertility dysregulation remain ambiguous, owing to the restricted opportunities to study menstruation and model menstrual disorders in species outside the primates. The recent discovery of naturally occurring menstruation in the Egyptian spiny mouse (Acomys cahirinus) offers a new laboratory model with significant benefits for prospective research in women's health. This review summarises current knowledge of spiny mouse menstruation, with an emphasis on spiral artery formation, inflammation and endocrinology. We offer a new perspective on cycle variation in menstrual bleeding between individual animals, and propose that this is indicative of fertility success. We discuss how we can harness our knowledge of the unique physiology of the spiny mouse to better understand vascular remodelling and its implications for successful implantation, placentation, and foetal development. Our research suggests that the spiny mouse has the potential as a translational research model to bridge the gap between bench to bedside and provide improved reproductive health outcomes for women.

Reproductive aging and menopause-like transition in the menstruating spiny mouse (Acomys cahirinus)

STUDY QUESTION

Does the naturally menstruating spiny mouse go through menopause?

SUMMARY ANSWER

Our study is the first to show a natural and gradual menopausal transition in a rodent.

WHAT IS KNOWN ALREADY

Age-related depletion of the human ovarian reserve (OvR) leads to menopause, the permanent cessation of menstruation and reproduction. Current rodent models of menopause are inappropriate for inferences of the human condition, as reproductive senescence is abrupt or induced through ovariectomy. The spiny mouse is the only confirmed rodent with a naturally occurring menstrual cycle.

STUDY DESIGN, SIZE, DURATION

Histological assessment of virgin spiny mice occurred in females aged 6 months (n = 14), 1 year (n = 7), 2 years (n = 13), 3 years (n = 9) and 4 years (n = 9). Endocrinology was assessed in a further 9 females per age group. Five animals per group were used for ovarian stereology with additional ovaries collected at prenatal Day 35 (n = 3), day of birth (n = 5), postnatal Days 35 (n = 5) and 100 (n = 5) and 15 months (n = 5).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Morphological changes in the reproductive system were examined using hematoxylin and eosin stains. Proliferating cell nuclear antigen immunohistochemistry assessed endometrial proliferation and sex steroids estradiol and testosterone were assayed using commercial ELISA kits.

MAIN RESULTS AND THE ROLE OF CHANCE

The proportion of females actively cycling was 86% at 6 months, 71% at 1 year, 69% at 2 years, 56% at 3 years and 44% at 4 years. Uterine and ovarian weights declined steadily from 1 year in all groups and corresponded with loss of uterine proliferation (P < 0.01). Estradiol was significantly decreased at 1 and 2 years compared to 6-month-old females, before becoming erratic at 3 and 4 years, with no changes in testosterone across any age. Fully formed primordial follicles were observed in prenatal ovaries. Aging impacted on both OvR and growing follicle numbers (P < 0.001-0.0001). After the age of 3 years, the follicle decline rate increased more than 5-fold.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This is a descriptive study in a novel research rodent whereby reagents validated for use in the spiny mouse were limited.

WIDER IMPLICATIONS OF THE FINDINGS

The gradual, rather than sudden, menopausal transition suggests that the spiny mouse is a more appropriate perimenopausal model than the current rodent models in which to examine the neuroendocrine pathways that encompass all hormonal interactions in the hypothalamic-pituitary-gonadal axis. The logistic, ethical and economic advantages of such a model may reduce our reliance on primates in menopause research and enable more thorough and invasive investigation than is possible in humans.

STUDY FUNDING/COMPETING INTEREST(S)

Hudson Institute is supported by the Victorian State Government Operational Infrastructure Scheme. The authors declare no competing interests.

Characteristics of the endometrium in menstruating species: lessons learned from the animal kingdom†

Here we have summarized what is currently known about menstruating animal species with special emphasis on non-primate species: length of their menstrual cycle, ovulation, implantation, placentation, decidualization, and endometrial characteristics. Having an overview of all the possible animal models that can be used to study menstruation and the menstrual cycle could be useful to select the one that better matches the needs of the individual research projects. The most promising species to study menstruation seems to be the spiny mouse Acomys cahirinus. It is a rodent that could be easily held in the existing laboratory facilities for rats and mice but with the great advantage of having spontaneous menstruation and several human-like menstrual cycle characteristics. Among the species of menstruating bats, the black mastiff bat Molossus ater and wild fulvous fruit bat Rousettus leschenaultii are the ones presenting the most human-like characteristics. The elephant shrew seems to be the less suitable species among the ones analyzed. The induced mouse model of menstruation is also presented as an adaptable alternative to study menstruation.

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.

Distribution of Vasopressin and Oxytocin Neurons in the Basal Forebrain and Midbrain of Spiny Mice (Acomys cahirinus)

The nonapeptides vasopressin (VP) and oxytocin (OT) are present in some form in most vertebrates. VP and OT play critical roles in modulating physiology and are well-studied for their influences on a variety of social behaviors, ranging from affiliation to aggression. Their anatomical distributions have been mapped for numerous species across taxa, demonstrating relatively strong evolutionary conservation in distributions throughout the basal forebrain and midbrain. Here we examined the distribution of VP-immunoreactive (-ir) and OT-ir neurons in a gregarious, cooperatively breeding rodent species, the spiny mouse (Acomys cahirinus), for which nonapeptide mapping does not yet exist. Immunohistochemical techniques revealed VP-ir and OT-ir neuronal populations throughout the hypothalamus and amygdala of males and females that are consistent with those of other rodents. However, a novel population of OT-ir neurons was observed in the median preoptic nucleus of both sexes, located dorsally to the anterior commissure. Furthermore, we found widespread sex differences in OT neuronal populations, with males having significantly more OT-ir neurons than females. However, we observed a sex difference in only one VP cell group - that of the bed nucleus of the stria terminalis (BST), a VP neuronal population that exhibits a phylogenetically widespread sexual dimorphism. These findings provide mapping distributions of VP and OT neurons in Acomys cahirinus. Spiny mice lend themselves to the study of mammalian cooperation and sociality, and the nonapeptide neuronal mapping presented here can serve as a basic foundation for the study of nonapeptide-mediated behavior in a group of highly social rodents.

Single-cell RNA sequencing redefines the mesenchymal cell landscape of mouse endometrium

The endometrium is a dynamic tissue that exhibits remarkable resilience to repeated episodes of differentiation, breakdown, regeneration, and remodeling. Endometrial physiology relies on a complex interplay between the stromal and epithelial compartments with the former containing a mixture of fibroblasts, vascular, and immune cells. There is evidence for rare populations of putative mesenchymal progenitor cells located in the perivascular niche of human endometrium, but the existence of an equivalent cell population in mouse is unclear. We used the Pdgfrb‐BAC‐eGFP transgenic reporter mouse in combination with bulk and single‐cell RNA sequencing to redefine the endometrial mesenchyme. In contrast to previous reports we show that CD146 is expressed in both PDGFRβ + perivascular cells and CD31 + endothelial cells. Bulk RNAseq revealed cells in the perivascular niche which express the high levels of Pdgfrb as well as genes previously identified in pericytes and/or vascular smooth muscle cells (Acta2, Myh11, Olfr78, Cspg4, Rgs4, Rgs5, Kcnj8, and Abcc9). scRNA‐seq identified five subpopulations of cells including closely related pericytes/vascular smooth muscle cells and three subpopulations of fibroblasts. All three fibroblast populations were PDGFRα+/CD34 + but were distinct in their expression of Ngfr/Spon2/Angptl7 (F1), Cxcl14/Smoc2/Rgs2 (F2), and Clec3b/Col14a1/Mmp3 (F3), with potential functions in the regulation of immune responses, response to wounding, and organization of extracellular matrix, respectively. Immunohistochemistry was used to investigate the spatial distribution of these populations revealing F1/NGFR + cells in most abundance beside epithelial cells. We provide the first definitive analysis of mesenchymal cells in the adult mouse endometrium identifying five subpopulations providing a platform for comparisons between mesenchymal cells in endometrium and other adult tissues which are prone to fibrosis.

Postpartum ovulation and early pregnancy in the menstruating spiny mouse, Acomys cahirinus

Egyptian spiny mice are the only known species to have human-like menstruation and a postpartum ovulation. Unfortunately, no endocrine or morphological evidence has been provided for a postpartum ovulation in spiny mice, and while later stages of pregnancy have been well studied, early events including embryo implantation and spiral artery remodelling have not been reported. This study compared the sex steroid endocrinology and reproductive tract morphology of dams at eight timepoints (n = 40) postpartum to determine the timing of ovulation and the timing and invasiveness of embryo implantation in A. cahirinus. Reproductive tracts were fixed and stained for histology and immunohistochemistry, and plasma was prepared for enzyme-linked immunosorbent assay. Ovarian histology and estradiol-17B concentrations indicate ovulation within 48 h of parturition and then immediate resumption of follicular growth. Uterine histology and immunohistochemistry revealed progressive epithelial repair, endometrial growth and spiral artery assembly and remodelling in dams postpartum. Blastocysts were seen in the uterine lumen at day 4-5 postpartum and embryos had implanted superficially with minimal stromal invasion by day 5-6. This study provides further evidence for the unique, humanesque reproductive biology of spiny mice and for a postpartum ovulation using endocrine and morphological changes observed during early pregnancy. Taken together, our data suggest that spiny mice may act as appropriate models of human pregnancy disorders such as implantation failure or pre-eclampsia.

Scar-Free Healing of Endometrium: Tissue-Specific Program of Stromal Cells and Its Induction by Soluble Factors Produced After Damage

Besides certain exceptions, healing of most tissues in the human body occurs via formation of scar tissue, rather than restoration of lost structures. After extensive acute injuries, this phenomenon substantially limits the possibility of lost function recovery and, in case of chronic injury, it leads to pathological remodeling of organs affected. Managing outcomes of damaged tissue repair is one of the main objectives of regenerative medicine. The first priority for reaching it is comparative investigation of mechanisms responsible for complete restoration of damaged tissues and mechanisms of scarring. However, human body tissues that undergo complete scar-free healing are scarce. The endometrium is a unique mucous membrane in the human body that heals without scarring after various injuries, as well as during each menstrual cycle (i.e., up to 400 times during a woman's life). We hypothesized that absence of scarring during endometrial healing may be associated with tissue-specific features of its stromal cells (SCs) or their microenvironment, since SCs transform into myofibroblasts-the main effector link of scarring. We found that during healing of the endometrium, soluble factors are formed that inhibit the transition of SCs into myofibroblasts. Without influence of these factors, the SCs of the endometrium undergo transformation into myofibroblasts after transforming growth factor β1 (TGF-β1) treatment as well as the SCs from tissues that heal by scarring-skin or fat. However, unlike the latter, endometrial SCs organize extracellular matrix (ECM) in a specific way and are not prone to formation of bulky connective tissue structures. Thus, we may suggest that tissue-specific features of endometrial SCs along with effects of soluble factors secreted in utero during menstruation ensure scar-free healing of human endometrium.

Immune Tolerance of the Human Decidua

The endometrium is necessary for implantation, complete development of the placenta, and a successful pregnancy. The endometrium undergoes repeated cycles of proliferation, decidualization (differentiation), and shedding during each menstrual cycle. The endometrium—including stromal, epithelial, vascular endothelial, and immune cells—is both functionally and morphologically altered in response to progesterone, causing changes in the number and types of immune cells. Immune cells make up half of the total number of endometrial cells during implantation and menstruation. Surprisingly, immune tolerant cells in the endometrium (uterine natural killer cells, T cells, and macrophages) have two conflicting functions: to protect the body by eliminating pathogenic microorganisms and other pathogens and to foster immunological change to tolerate the embryo during pregnancy. One of the key molecules involved in this control is the cytokine interleukin-15 (IL-15), which is secreted by endometrial stromal cells. Recently, it has been reported that IL-15 is directly regulated by the transcription factor heart- and neural crest derivatives-expressed protein 2 in endometrial stromal cells. In this review, we outline the significance of the endometrium and immune cell population during menstruation and early pregnancy and describe the factors involved in immune tolerance and their involvement in the establishment and maintenance of pregnancy.

Pseudopregnancy and reproductive cycle synchronisation cannot be induced using conventional methods in the spiny mouse (Acomys cahirinus)

The menstruating spiny mouse is the first rodent identified to exhibit natural spontaneous decidualisation, cyclical endometrial shedding and regeneration. While the spiny mouse shares several primate-like characteristics in its reproductive biology, it has not been established whether pseudopregnancy can be induced or if its cycles can be synchronised as in non-human mammals. Here we describe attempts to induce pseudopregnancy and synchronisation of menstrual cycles (i.e. Whitten effect) in spiny mice. Virgin females (n=3-8 per group) underwent one of the following procedures to induce pseudopregnancy: daily vaginal lavage only (control), progesterone injection, mechanical stimulation of the cervix and sterile mating. A separate cohort was also exposed to male-soiled bedding to assess the Whitten effect. Pseudopregnancy was deemed successful if females presented with extended (>12 consecutive days) leukocytic vaginal cytology. No female from any method of induction met this criterion. In addition, the menstrual cycles of a group of six females could not be synchronised, nor immediate ovulation induced via exposure to male-soiled bedding. These responses indicate that the spiny mouse does not behave as a typical rodent. Like higher-order primates, the spiny mouse exhibits a relatively rare reproductive strategy, of failure to show pseudopregnancy or cyclical synchronisation. This is further endorsement of the use of this species as a versatile animal model for translational studies of menstruation and fertility.

Nonhuman primate model in mammary gland biology and neoplasia research

Research on breast cancer pathogenesis, prevention and drug development remains an important field as this disease is still one of the leading causes of cancer death worldwide. Nonhuman primates, particularly macaque species, may serve as a highly translational animal model in breast cancer studies due to their similarity with humans in genetics, anatomy, reproductive and endocrine physiology including mammary gland development profile. The use of nonhuman primates in biomedical research, however, requires high ethical standards and an increasing expectation to improve strategies to replace, reduce and refine their use. Here, we discuss some key features of nonhuman primate mammary gland biology relevant to their strengths and limitations as models in studies of breast development and cancer risk.

Cellular Origins of Endometriosis: Towards Novel Diagnostics and Therapeutics

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

Mouse model of menstruation: An indispensable tool to investigate the mechanisms of menstruation and gynaecological diseases (Review)

Abnormal menstruation may result in several pathological alterations and gynaecological diseases, including endometriosis, menstrual pain and miscarriage. However, the pathogenesis of menstruation remains unclear due to the limited number of animal models available to study the menstrual cycle. In recent years, an effective, reproducible, and highly adaptive mouse model to study menstruation has been developed. In this model, progesterone and oestrogen were administered in cycles following the removal of ovaries. Subsequently, endometrial decidualisation was induced using sesame oil, followed by withdrawal of progesterone administration. Vaginal bleeding in mice is similar to that in humans. Therefore, the use of mice as a model organism to study the mechanism of menstruation and gynaecological diseases may prove to be an important breakthrough. The present review is focussed ond the development and applications of a mouse model of menstruation. Furthermore, various studies have been described to improve this model and the research findings that may aid in the treatment of menstrual disorders in women are presented.

Physiology of the Endometrium and Regulation of Menstruation

The physiological functions of the uterine endometrium (uterine lining) are preparation for implantation, maintenance of pregnancy if implantation occurs, and menstruation in the absence of pregnancy. The endometrium thus plays a pivotal role in reproduction and continuation of our species. Menstruation is a steroid-regulated event, and there are alternatives for a progesterone-primed endometrium, i.e., pregnancy or menstruation. Progesterone withdrawal is the trigger for menstruation. The menstruating endometrium is a physiological example of an injured or “wounded” surface that is required to rapidly repair each month. The physiological events of menstruation and endometrial repair provide an accessible in vivo human model of inflammation and tissue repair. Progress in our understanding of endometrial pathophysiology has been facilitated by modern cellular and molecular discovery tools, along with animal models of simulated menses. Abnormal uterine bleeding (AUB), including heavy menstrual bleeding (HMB), imposes a massive burden on society, affecting one in four women of reproductive age. Understanding structural and nonstructural causes underpinning AUB is essential to optimize and provide precision in patient management. This is facilitated by careful classification of causes of bleeding. We highlight the crucial need for understanding mechanisms underpinning menstruation and its aberrations. The endometrium is a prime target tissue for selective progesterone receptor modulators (SPRMs). This class of compounds has therapeutic potential for the clinical unmet need of HMB. SPRMs reduce menstrual bleeding by mechanisms still largely unknown. Human menstruation remains a taboo topic, and many questions concerning endometrial physiology that pertain to menstrual bleeding are yet to be answered.

Altered exploratory behaviour and increased food intake in the spiny mouse before menstruation: a unique pre-clinical model for examining premenstrual syndrome

STUDY QUESTION

Does the newly discovered menstruating spiny mouse exhibit behavioural and metabolic changes in correlation with premenstrual phases of the menstrual cycle?

SUMMARY ANSWER

This is the first report of cycle variability in the exploratory and interactive behaviour, and food consumption in menstruating spiny mice, and demonstrates that physiological changes are also dependent on within-subject variation.

WHAT IS KNOWN ALREADY

Premenstrual syndrome (PMS) is a prominent cyclic disorder that affects millions of women worldwide. More than 70% of women endure symptoms of impending menstruation, such as bloating, abdominal cramping and nausea to some degree. Consequently, ~8% of women experience recurrent physical and emotional symptoms which are extreme enough to disrupt daily life and seek intervention. Due to a lack of an appropriate animal model, the mechanisms underlying PMS are poorly understood, and subsequently, effective treatments are limited.

STUDY DESIGN, SIZE, DURATION

This study analyses the changes in behavioural responses to the investigator during vaginal lavage (n = 14), exploratory behaviour (n = 11) and metabolism (n = 20) across the menstrual cycle in the spiny mouse (Acomys cahirinus).

PARTICIPANTS/MATERIALS, SETTING, METHODS

We performed vaginal lavages on virgin spiny mice (6-8 months of age) and subjected each cohort of females to repeated measures for vaginal lavage, exploratory behaviour and metabolism. Stages of the menstrual cycle were designated as early follicular, late follicular, early luteal, late luteal, early menstrual and late menstrual, with the late luteal and early menstrual phases considered as premenstrual phases and analysed using generalized estimating equations. For vaginal lavage, the behavioural responses to researcher handling were scored on an increasing scale of severity during the lavage process (e.g. restraint, frequency of vocalizations, total handling time). For exploratory behaviour, exploration, memory and sociability were assessed through subjection to Open Field (OF), Novel Object Recognition (NORT), Social Novelty (SN) and Elevated Plus Maze (EPM) tests. For metabolism, physiological changes were measured over a 24-h period in metabolic cages. Results are mean ± SD with statistical significance set to P < 0.05.

MAIN RESULTS AND THE ROLE OF CHANCE

Qualitative behavioural assessment showed that compared to early follicular controls, during premenstrual phases, cycling females had significantly increased probability of: manifesting difficulties during restraint (4×, P < 0.01), vocalizing (8×, P < 0.01) and exhibiting isolation in the cage (40×, P = 0.041). We saw significant increases in handling time during the premenstrual phase in cycling females (76 ± 16 s) compared to controls (55 ± 7 s, P < 0.001). For exploratory behaviour, cycling females in their early menstrual phase travelled significantly less distance in the outer zone of the OF arena (13.3 ± 9.0 m) than females in their early luteal phase (22.3 ± 9.9 m, P = 0.038) and at significantly reduced velocities (40.2 ± 10.5 mm/s and 78.8 ± 31.0 mm/s, respectively, P = 0.006). These females also had fewer entries into the EPM open arms during the same phases (9.6 ± 6.1 and versus 20.0 ± 7.2, respectively, P = 0.030) and travelled less distance (3.2 ± 2.8 m versus 7.0 ± 5.5 m, respectively, P = 0.026). No differences were observed in NORT or SN across the cycle. In the metabolism studies, spiny mice demonstrated a significant increase in food consumption (percentage of body weight) during the early follicular and late luteal phases (3.9 ± 2.4% and 3.8 ± 2.1%, respectively) compared to the late follicular phase (2.3 ± 2.6%, P = 0.015).

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This is an observational study to determine fundamental changes in behaviour and metabolism in a novel species, and as such, lacks commercially available laboratory reagents and protocols specific to the spiny mouse.

WIDER IMPLICATIONS OF THE FINDINGS

The timing of these behavioural and physiological changes suggests that spiny mice exhibit symptoms analogous to PMS in higher order primates, thus providing a pre-clinical model for testing novel interventions to alleviate premenstrual symptoms and overcoming many limitations associated with this research area.

STUDY FUNDING/COMPETING INTEREST(S)

N.B. is supported by a Research Training Program stipend through Monash University. J.E. is supported by a Fellowship awarded by the Peter Fielding Foundation. The Hudson Institute of Medical Research is supported by the Victorian Government Operational Research Infrastructure Support. The authors declare no conflicts of interest.

Model systems for regeneration: the spiny mouse, Acomys cahirinus

The spiny mouse, Acomys spp., is a recently described model organism for regeneration studies. For a mammal, it displays surprising powers of regeneration because it does not fibrose (i.e. scar) in response to tissue injury as most other mammals, including humans, do. In this Primer article, we review these regenerative abilities, highlighting the phylogenetic position of the spiny mouse relative to other rodents. We also briefly describe the Acomys tissues that have been used for regeneration studies and the common features of their regeneration compared with the typical mammalian response. Finally, we discuss the contribution that Acomys has made in understanding the general principles of regeneration and elaborate hypotheses as to why this mammal is successful at regenerating.

Development of A 3D Tissue Slice Culture Model for the Study of Human Endometrial Repair and Regeneration

The human endometrium undergoes sequential phases of shedding of the upper functionalis zone during menstruation, followed by regeneration of the functionalis zone from the remaining basalis zone cells, and secretory differentiation under the influence of the ovarian steroid hormones estradiol (E2) and progesterone (P4). This massive tissue regeneration after menstruation is believed to arise from endometrial stromal and epithelial stem cells residing in the basal layer of the endometrium. Although many endometrial pathologies are thought to be associated with defects in these stem cells, studies on their identification and regulation are limited, primarily due to lack of easily accessible animal models, as these processes are unique to primates. Here we describe a robust new method to study endometrial regeneration and differentiation processes using human endometrial tissue slice cultures incorporating an air-liquid interface into a 3D matrix scaffold of type I collagen gel, allowing sustained tissue viability over three weeks. The 3D collagen gel-embedded endometrial tissue slices in a double-dish culture system responded to ovarian steroid hormones, mimicking the endometrial changes that occur in vivo during the menstrual cycle. These changes included the E2-induced upregulation of Ki-67, estrogen receptor (ER), and progesterone receptor (PR) in all endometrial compartments and were markedly suppressed by both P4 and E2 plus P4 treatments. There were also distinct changes in endometrial morphology after E2 and P4 treatments, including subnuclear vacuolation and luminal secretions in glands as well as decidualization of stromal cells, typical characteristics of a progestational endometrium in vivo. This long-term slice culture method provides a unique in vivo-like microenvironment for the study of human endometrial functions and remodeling during early pregnancy and experiments on stem cell populations involved in endometrial regeneration and remodeling. Furthermore, this model has the potential to enable studies on several endometrial diseases, including endometrial cancers and pregnancy complications associated with defects in endometrial remodeling.

Monkeys, mice and menses: the bloody anomaly of the spiny mouse

The common spiny mouse (Acomys cahirinus) is the only known rodent to demonstrate a myriad of physiological processes unseen in their murid relatives. The most recently discovered of these uncharacteristic traits: spontaneous decidual transformation of the uterus in virgin females, preceding menstruation. Menstruation occurring without experimental intervention in rodents has not been documented elsewhere to date, and natural menstruation is indeed rare in the animal kingdom outside of higher order primates. This review briefly summarises the current knowledge of spiny mouse biology and taxonomy, and explores their endocrinology which may aid in our understanding of the evolution of menstruation in this species. We propose that DHEA, synthesised by the spiny mouse (but not other rodents), humans and other menstruating primates, is integral in spontaneous decidualisation and therefore menstruation. We discuss both physiological and behavioural attributes across the menstrual cycle in the spiny mouse analogous to those observed in other menstruating species, including premenstrual syndrome. We further encourage the use of the spiny mouse as a small animal model of menstruation and female reproductive biology.