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.

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.

Menstrual irregularity as a biological limit to early pregnancy awareness.

US state legislatures have proposed laws to prohibit abortion once the earliest embryonic electrical activity is detectable (fetal "heartbeat"). On average, this occurs roughly 6 wk after the last menstrual period. To be eligible for abortion, people must recognize pregnancy very early in gestation. The earliest symptom of pregnancy is a missed period, and irregular menstrual cycles-which occur frequently-can delay pregnancy detection past the point of fetal cardiac activity. In our analysis of 1.6 million prospectively recorded menstrual cycles, cycle irregularity was more common among young women, Hispanic women, and women with common health conditions, such as diabetes and polycystic ovary syndrome. These groups face physiological limitations in detecting pregnancy before fetal cardiac activity. Restriction of abortion this early in gestation differentially affects specific population subgroups, for reasons outside of individual control.

Cyclical endometrial repair and regeneration.

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

Association of heavy menstrual bleeding with cardiovascular disease in US female hospitalizations.

BACKGROUND: Heavy menstrual bleeding (HMB) is a common menstrual disorder associated with multiple risk factors of cardiovascular disease (CVD) in women. In addition, HMB is often present with irregular menstruation (IM) which is a risk factor for CVD outcomes. However, the relationship between HMB and CVD outcomes is unexplored in the presence or absence of IM. We determined the association of HMB with multiple CVD outcomes using a nationally representative sample of female hospitalizations in the US. METHODS: All hospitalizations of females with HMB diagnosis and normal menstrual cycles from ages of 18 to 70 years were extracted from the National Inpatient Sample Database, 2017. The HMB was defined using the International Classification of Diseases (ICD)-10 for excessive and frequent menstruation bleeding and included any current or history of HMB diagnosis. Outcomes including major adverse cardiovascular events (MACE), coronary heart disease (CHD), stroke, heart failure (HF), atrial fibrillation (AF) or arrhythmia, myocardial infarction (MI), and diabetes (DM) were defined using ICD-10 codes. Adjusted logistic regression and prosperity scores-matched logistic regression analyses were conducted to summarize adjusted associations with an odds ratio (OR) and a 95% confidence interval (CI). RESULTS: Among 2,430,851 hospitalizations, HMB was observed in 7762 (0.68%) females with age ≤ 40 years and 11,164 (0.86%) females with age > 40 years. Among hospitalizations with age ≤ 40 years, HMB was significantly associated with increased odds of CVD outcomes including MACE (OR = 1.61; 95% CI: 1.25, 2.08), CHD (OR = 1.72; 95% CI: 1.10, 2.71), stroke (OR = 1.95; 95% CI: 1.12, 3.40), HF (OR = 1.53; 95% CI: 1.15, 2.03), and AF/arrhythmia (OR = 1.84; 95% CI: 1.34, 2.54). These associations were confirmed in multiple sensitivity analyses. In contrast, HMB was not robustly associated with CVD events among hospitalizations of women with age > 40 years. HMB without IM was strongly associated with DM, HF, AF, and MACE outcomes while HMB with IM was strongly associated with CHD and AF outcomes in hospitalizations of young women. CONCLUSIONS: HMB is associated with CVD events among US hospitalizations of young women. A routine investigation and screening of menstrual disorders, especially HMB, is useful for CVD risk stratification and management in young women.

A treatment algorithm for familial Mediterranean fever patients with menstruation-associated attacks.

OBJECTIVES: Familial Mediterranean fever (FMF) is characterized by febrile polyserositis attacks. Menstruation could be a trigger for attacks. We aimed to analyze the features of adolescent FMF patients with menstruation-associated attacks and propose a management algorithm. METHODS: All female FMF patients who had menarche and visited the Pediatric Rheumatology Unit between January-December 2022, were included into this study. Demographics, general characteristics, and the features of menstrual cycle and FMF attacks were noted. RESULTS: A total of 151 female FMF patients were included. Thirty-five (23.2%) had menstruation-associated attacks. Fever and arthritis were less frequent during the menstruation-associated attacks than the attacks not associated with menstruation in these patients (65.7% vs 88.6%, p= 0.01 and 2.9% vs 20%, p= 0.04; respectively). Patients with menstruation-associated FMF attacks were younger at symptom onset and diagnosis (2.5 vs 5 years, p= 0.004 and 4 vs 7 years, p= 0.01; respectively), had a higher rate of dysmenorrhea (74.3% vs 38.8%, p< 0.001, respectively) and higher pre- and post-menarche attack frequency (4 vs 2 and 10 vs 0, respectively; p< 0.001 for both) than patients whose attacks were not associated with menstruation. The interventions for menstruation-associated attacks included initiating colchicine, increasing the dose of colchicine, switching from coated to compressed colchicine tablets or anti-interleukin 1 drugs, and on-demand non-steroidal anti-inflammatory drugs, on-demand glucocorticoids, and on-demand anakinra. On-demand therapies were beneficial in controlling menstruation-associated attacks. CONCLUSIONS: This is the largest cohort of adolescent FMF patients with menstruation-associated attacks. Severe FMF may cause tendency to this association. On-demand therapies could be preferred in the management.

Proposal for targeted, neo-evolutionary-oriented, secondary prevention of early-onset endometriosis and adenomyosis. Part I: pathogenic aspects.

The potential for repeated ovulation and menstruation is thought to have provided a Darwinian advantage during the Palaeolithic. Reproductive conditions remained relatively stable until the pre-industrial era, characterized by late menarche, very young age at first birth, multiple pregnancies, and prolonged periods of lactational amenorrhoea. For hundreds of thousands of years, menstruators experienced few ovulatory cycles, even though they were genetically adapted to ovulate and menstruate every month. In the post-industrial era, the age at menarche gradually declined, the age at first birth progressively increased, and breastfeeding became optional and often of short duration. This created a mismatch between genetic adaptation and socio-environmental evolution, so that what was initially a probable reproductive advantage subsequently contributed to increased susceptibility to diseases associated with lifetime oestrogen exposure, such as ovarian, endometrial and breast cancer and, hypothetically, also those associated with the number of ovulatory menstruations, such as endometriosis and adenomyosis. The incidence of endometriosis shows a steep and progressive increase around the age of 25 years, but given the consistently reported delay in diagnosis, the actual incidence curve should be shifted to the left, supporting the possibility that the disease has its roots in adolescence. This raises the question of whether, from an evolutionary point of view, anovulation and amenorrhoea should not still be considered the physiological state, especially in the postmenarchal period. However, an increase in the frequency of endometriosis in recent decades has not been demonstrated, although this deserves further epidemiological investigation. In addition, as endometriosis occurs in a minority of individuals exposed to retrograde menstruation, other important pathogenic factors should be scrutinised. Research should be resumed to explore in more detail the transtubal reflux of not only blood, but also endometrial cells, and whether they are systematically present in the peritoneal fluid after menstruation. If repetitive ovulatory menstruation during the early reproductive years is shown to increase the risk of endometriosis and adenomyosis development and progression in susceptible individuals, hormonal interventions could be used as secondary prevention in symptomatic adolescents.

Proposal for targeted, neo-evolutionary-oriented secondary prevention of early-onset endometriosis and adenomyosis. Part II: medical interventions.

According to consistent epidemiological data, the slope of the incidence curve of endometriosis rises rapidly and sharply around the age of 25 years. The delay in diagnosis is generally reported to be between 5 and 8 years in adult women, but it appears to be over 10 years in adolescents. If this is true, the actual onset of endometriosis in many young women would be chronologically placed in the early postmenarchal years. Ovulation and menstruation are inflammatory events that, when occurring repeatedly for years, may theoretically favour the early development of endometriosis and adenomyosis. Moreover, repeated acute dysmenorrhoea episodes after menarche may not only be an indicator of ensuing endometriosis or adenomyosis, but may also promote the transition from acute to chronic pelvic pain through central sensitization mechanisms, as well as the onset of chronic overlapping pain conditions. Therefore, secondary prevention aimed at reducing suffering, limiting lesion progression, and preserving future reproductive potential should be focused on the age group that could benefit most from the intervention, i.e. severely symptomatic adolescents. Early-onset endometriosis and adenomyosis should be promptly suspected even when physical and ultrasound findings are negative, and long-term ovulatory suppression may be established until conception seeking. As nowadays this could mean using hormonal therapies for several years, drug safety evaluation is crucial. In adolescents without recognized major contraindications to oestrogens, the use of very low-dose combined oral contraceptives is associated with a marginal increase in the individual absolute risk of thromboembolic events. Oral contraceptives containing oestradiol instead of ethinyl oestradiol may further limit such risk. Oral, subcutaneous, and intramuscular progestogens do not increase the thromboembolic risk, but may interfere with attainment of peak bone mass in young women. Levonorgestrel-releasing intra-uterine devices may be a safe alternative for adolescents, as amenorrhoea is frequently induced without suppression of the ovarian activity. With regard to oncological risk, the net effect of long-term oestrogen-progestogen combinations use is a small reduction in overall cancer risk. Whether surgery should be considered the first-line approach in young women with chronic pelvic pain symptoms seems questionable. Especially when large endometriomas or infiltrating lesions are not detected at pelvic imaging, laparoscopy should be reserved to adolescents who refuse hormonal treatments or in whom first-line medications are not effective, not tolerated, or contraindicated. Diagnostic and therapeutic algorithms, including self-reported outcome measures, for young individuals with a clinical suspicion of early-onset endometriosis or adenomyosis are proposed.

Müllerian anomalies and endometriosis as potential explanatory models for the retrograde menstruation/implantation and the embryonic remnants/celomic metaplasia pathogenic theories: a systematic review and meta-analysis.

STUDY QUESTION: Does endometriosis prevalence differ in patients with obstructive Müllerian anomalies (OMA) versus those with nonobstructive Müllerian anomalies (NOMA), and in patients with NOMA versus those without Müllerian anomalies? SUMMARY ANSWER: The quantitative synthesis of published data demonstrates a substantially increased prevalence of endometriosis in patients with OMA compared with those with NOMA, and a similar prevalence in patients with NOMA and those without Müllerian anomalies. WHAT IS KNOWN ALREADY: The pathogenesis of endometriosis has not been definitively clarified yet. A higher prevalence of endometriosis in patients with OMA than in those with NOMA would support the retrograde menstruation (RM)/implantation theory, whereas a higher prevalence of endometriosis in the NOMA group than in the group without Müllerian anomalies would support the embryonic remnants/celomic metaplasia hypothesis. STUDY DESIGN, SIZE, DURATION: This systematic review with meta-analysis was restricted to full-length, English-language articles published in peer-reviewed journals between 1980 and 2023. The PubMed and EMBASE databases were searched using the keyword 'endometriosis' in combination with 'Müllerian anomalies', 'obstructive Müllerian anomalies', 'female genital malformations', 'retrograde menstruation', 'infertility', 'pelvic pain', and 'classification'. References from relevant publications were screened, and PubMed's 'similar articles' and 'cited by' functions were used. PARTICIPANTS/MATERIALS, SETTING, METHODS: Studies were selected if they reported the prevalence of surgically confirmed endometriosis in either individuals with OMA compared to those with NOMA, or patients with NOMA compared to those without Müllerian anomalies. Cohort and case-control studies and case series were deemed eligible for inclusion. Noncomparative studies, studies not reporting both the number of individuals with endometriosis and the total number of those with Müllerian anomalies or with other gynecological conditions, those including exclusively data on patients with absent or uncertain menstrual function (e.g. complete Müllerian agenesis category), or with imperforate hymen were excluded. Two reviewers independently abstracted data. The risk of bias was assessed with the Risk of Bias In Non-randomized Studies of Exposures tool. The overall certainty of the evidence was graded according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) guidelines. MAIN RESULTS AND THE ROLE OF CHANCE: Seven retrospective studies were included. The overall mean estimate of endometriosis prevalence was 47% (95% CI, 36-58%) in patients with OMA, and 19% (95% CI, 15-24%) in patients with NOMA, with a common odds ratio (OR) of 4.72 (95% CI, 2.54-8.77). The overall mean estimate of endometriosis prevalence in patients with NOMA was 23% (95% CI, 20-27%), and that in patients without Müllerian anomalies was 21% (95% CI, 20-22%), with a common OR of 0.95 (95% CI, 0.57-1.58). The overall certainty of the evidence according to GRADE guidelines was judged as low for both comparisons. LIMITATIONS, REASON FOR CAUTION: Some NOMA subtypes may create a partial obstacle to menstrual efflux and/or generate dysfunctional myometrial contractions that favor transtubal reflux, thus increasing the risk of endometriosis and limiting the difference between OMA and NOMA. As infertility and pelvic pain are strongly associated with endometriosis, women with these symptoms are inappropriate controls. Confounding by indication could explain the lack of difference in endometriosis prevalence between patients with NOMA and those without Müllerian anomalies. WIDER IMPLICATIONS OF THE FINDINGS: The results of this meta-analysis support the validity of the RM theory but do not definitively rule out alternative hypotheses. Thus, RM may be considered the initiator for the development of endometriotic lesions, while not excluding the contribution of both inheritable and tissue-specific genetic and epigenetic modifications as disease-promoting factors. STUDY FUNDING/COMPETING INTEREST(S): No funding was received for this review. P.Ve. is a member of the Editorial Board of Human Reproduction Open, the Journal of Obstetrics and Gynaecology Canada, and the International Editorial Board of Acta Obstetricia et Gynecologica Scandinavica; has received royalties from Wolters Kluwer for chapters on endometriosis management in the clinical decision support resource UpToDate; and maintains both a public and private gynecological practice. E.S. discloses payments from Ferring for research grants and honoraria from Merck-Serono for lectures. All other authors declare they have no conflict of interest. REGISTRATION NUMBER: N/A.

COVID-19 and abnormal uterine bleeding: potential associations and mechanisms.

The impact of COVID-19 on menstruation has received a high level of public and media interest. Despite this, uncertainty exists about the advice that women and people who menstruate should receive in relation to the expected impact of SARS-CoV-2 infection, long COVID or COVID-19 vaccination on menstruation. Furthermore, the mechanisms leading to these reported menstrual changes are poorly understood. This review evaluates the published literature on COVID-19 and its impact on menstrual bleeding, discussing the strengths and limitations of these studies. We present evidence consistent with SARS-CoV-2 infection and long COVID having an association with changes in menstrual bleeding parameters and that the impact of COVID vaccination on menstruation appears less significant. An overview of menstrual physiology and known causes of abnormal uterine bleeding (AUB) is provided before discussing potential mechanisms which may underpin the menstrual disturbance reported with COVID-19, highlighting areas for future scientific study. Finally, consideration is given to the effect that menstruation may have on COVID-19, including the impact of the ovarian sex hormones on acute COVID-19 severity and susceptibility and reported variation in long COVID symptoms across the menstrual cycle. Understanding the current evidence and addressing gaps in our knowledge in this area are essential to inform public health policy, direct the treatment of menstrual disturbance and facilitate development of new therapies, which may reduce the severity of COVID-19 and improve quality of life for those experiencing long COVID.