The Possibility and Characteristics of Senescence‐Related Changes in Reproductive Tissues Such as the Ovary, Oviduct, and Uterus

ABSTRACT

Background

Senescence is characterized by a functional decline of cells or tissues, whereas aging refers to the passage of time. Aging and senescence have been viewed as potentially controllable processes, and research on their underlying mechanisms and countermeasures has progressed. With the increasing number of women of advanced maternal age, the decline in fertility is thought to be associated not only with reduced oocyte quality but also with aging and senescence of the reproductive organs—that is, age‐related changes in the ovarian, oviduct (fallopian tube), and uterine environments.

Methods and Main Findings

Findings from animal experiments and clinical studies have shown that age‐related changes in the ovaries, oviducts (fallopian tubes), and uterus contribute to a decline in reproductive performance. This effect may be explained by the progression of cellular senescence, chronic inflammation, DNA damage, and mitochondrial dysfunction in these organs, leading to fibrosis and abnormal cellular differentiation. These hallmarks of aging are thought to contribute to reduced oocyte quality, diminished ovarian reserve, and ovulatory disorders. Furthermore, even when fertilization occurs, impaired embryo development may result, ultimately reducing endometrial receptivity and leading to decreased fertility.

Conclusion

This review discusses the potential roles and significance of aging and senescence in reproductive tissues.

1. Introduction

Aging and senescence are common phenomena observed in almost all animal species. As individuals age, the functions of the body and its tissues gradually decline, and their capacity for recovery also weakens. In general, aging refers to the process of growing older—that is, the time elapsed from birth to death—and is a change that progresses at a similar pace in all individuals. In contrast, senescence is a progressive biological process that occurs in all individuals and ultimately contributes to death. As humans age, various physiological changes accumulate, and when homeostatic mechanisms eventually fail, death occurs. Therefore, senescence has long been regarded as an unavoidable phenomenon.

However, a novel concept has recently emerged that conceptualizes senescence as a disease, attracting increasing research interest [1]. This perspective suggests that senescence may be suppressed or delayed under certain conditions rather than being considered solely a natural phenomenon [1]. If new methods are developed to suppress the senescence process, it is anticipated that not only healthy life expectancy and overall survival will be extended, but also age‐related decline in organ function may be alleviated, thereby improving quality of life.

Recently, changes in modern society and lifestyle have led to an increasing number of women conceiving in their late 30s, resulting in a higher average age at conception than in previous decades. In general, advanced maternal age refers to pregnancy around 35 years of age or older. Women who conceive at an advanced maternal age have a higher risk of pregnancy complications, such as preterm birth and pregnancy‐induced hypertension [2, 3]. It is well established that fertility—commonly assessed by pregnancy rate—declines markedly with age because of the qualitative deterioration of oocytes and the reduction in oocyte number, known as diminished ovarian reserve [2, 4].

Although aging‐related dysfunction of oocytes undoubtedly plays a major role in declining fertility, the question remains whether this decline is solely attributable to oocyte factors. Despite advancements in assisted reproductive technologies, such as in vitro fertilization (IVF) and intracytoplasmic sperm injection, the pregnancy success rate remains below 50%, even with the use of high‐quality embryos [5, 6]. These findings suggest that improving oocyte and embryo quality alone is insufficient and that maternal physical condition is also crucial. In particular, maternal aging may promote senescence of the reproductive organs, thereby compromising the environment required to support pregnancy [7].

2. Understanding the Characteristics of Aging

In 2013, Lopez‐Otin et al. [8] proposed the “Hallmarks of Aging” as a conceptual framework for understanding the complex mechanisms of aging and senescence and for developing strategies to mitigate age‐related functional decline and prevent age‐related diseases. This concept identified nine key characteristics to provide a systematic understanding of aging and senescence at the molecular and cellular levels. Over the following decade, substantial scientific evidence accumulated, further demonstrating that aging and senescence are not simple phenomena but rather multilayered and interactive processes involving multiple factors. In 2023, the concept was revised and expanded to include 12 characteristics [9]. Currently, the “Hallmarks of Aging” serve as an essential framework guiding aging and senescence research worldwide. Below is an overview of the 12 “Hallmarks of Aging” believed to drive aging (Figure 1) [9].

FIGURE 1.

The hallmarks of aging. The scheme compiles the 12 hallmarks of aging proposed by Lopez‐Otin et al. [9]. These hallmarks are grouped into three categories as primary (blue hallmarks), antagonistic (green hallmarks), and integrative (gray hallmarks).

2.1. Genomic Instability

DNA is affected and damaged by various deleterious factors, including external influences such as ultraviolet radiation and ionizing radiation, as well as internal factors such as DNA replication errors and oxidative stress. In younger cells, robust mechanisms protect and repair against these deleterious events; however, in older cells, mutations and chromosomal abnormalities accumulate because DNA damage repair mechanisms can no longer function efficiently [10]. As a result, cellular function gradually declines, potentially leading to organ failure and/or cancer development. It has been reported that overexpression of DNA repair–related factors can extend the lifespan of mice [11].

2.2. Telomere Attrition

With each cell division, telomeres that protect the ends of chromosomes gradually shorten. When telomeres become shorter than a critical length, cells can no longer divide, leading to a state known as cellular senescence. Telomerase is the enzyme responsible for maintaining telomere length. Activation of telomerase activity suppresses telomere shortening, thereby potentially slowing the senescence process [12, 13].

2.3. Epigenetic Alterations

Epigenetics is defined as a mechanism of gene functional change in which the DNA sequence remains unchanged and this altered state is maintained after cell division and across generations. Representative epigenetic mechanisms include DNA methylation, histone modification, and changes in chromatin structure, all of which contribute substantially to aging and senescence. When epigenetic regulatory mechanisms become disrupted with aging, abnormalities in gene function may be induced, contributing to the progression of various age‐related diseases.

2.4. Loss of Proteostasis

Proteins produced within cells are folded into their correct conformations and transported to their respective functional sites or organelles to perform their biological functions. Conversely, unnecessary or misfolded proteins are degraded and removed. This coordinated system is referred to as proteostasis, meaning the maintenance of protein homeostasis. If the proteostasis system becomes impaired with aging, misfolded or abnormal proteins accumulate within cells, resulting in a decline in cellular function. Dysfunction of proteostasis can contribute to the development of various age‐related diseases.

2.5. Disabled Macroautophagy

Autophagy, particularly macroautophagy, plays crucial roles in the degradation and recycling of damaged organelles and unnecessary proteins within cells, as well as in the regulation of cellular functions. With advancing age, autophagic activity gradually declines, resulting in the accumulation of waste products and damaged organelles and the deterioration of the intracellular environment. In mice, activation of autophagy has been shown to extend lifespan and improve metabolic function [14, 15].

2.6. Deregulated Nutrient‐Sensing

Cells possess nutrient‐sensing pathways, such as mechanistic target of rapamycin, sirtuin 1, and AMP‐activated protein kinase, that detect changes in nutrient availability. Depending on nutrient conditions, cells regulate their functions, including growth, proliferation, energy conservation, and cellular repair. When the balance of this nutrient‐sensing network is disrupted by aging or dietary factors, cellular and organ dysfunction may be accelerated. Restoration of nutrient‐sensing pathway balance has attracted attention as a potential therapeutic strategy to slow aging and senescence [1, 16].

2.7. Mitochondrial Dysfunction

Mitochondria are intracellular organelles responsible for producing adenosine triphosphate (ATP), the primary cellular energy source. As mitochondrial function declines with age, intracellular energy metabolism becomes impaired, and increased production of reactive oxygen species (ROS) promotes the accumulation of oxidative stress–induced cellular damage, which can subsequently trigger inflammation. As a result, cellular function declines and senescence progresses [17].

2.8. Cellular Senescence

Cellular senescence is a cellular response to damage or stress that results in irreversible cell cycle arrest and permanent cessation of cell division. Senescent cells secrete various bioactive substances, including inflammatory cytokines, chemokines, and growth factors. This phenotype is referred to as the senescence‐associated secretory phenotype (SASP), and SASP factors exert paracrine effects on surrounding cells, thereby exacerbating cellular senescence [18, 19].

Under normal conditions, senescent cells are removed by immune cells. However, when immune function declines with age, senescent cells accumulate in tissues, which is associated with the development of various age‐related diseases. Therefore, research on senolytics—agents that selectively eliminate senescent cells—has progressed. For example, combination therapy with dasatinib and quercetin has been shown to remove senescent cells and extend the healthy lifespan of mice [20].

2.9. Stem Cell Exhaustion

Stem cells present in each tissue play an essential role in repairing and regenerating damaged cells, thereby maintaining tissue homeostasis. With advancing age, the function and number of stem cells gradually decline, impairing the regenerative capacity of tissues and promoting the aging and senescence process. Consequently, research is underway to slow aging and senescence by restoring stem cell function through partial cellular reprogramming [21].

2.10. Altered Intercellular Communication

Cells communicate with one another through various signaling molecules, such as hormones and cytokines, to maintain whole‐body homeostasis. With advancing age, the balance of this intercellular communication becomes disrupted. In particular, excessive increases in inflammatory cytokines and a decline in immune surveillance impair homeostasis, thereby contributing to the progression of aging and senescence.

2.11. Chronic Inflammation

Inflammation is a localized response to stimuli within the body and serves as a biological defense mechanism against tissue damage and infection. In contrast, chronic inflammation refers to a persistent, low‐grade inflammatory state caused by the continuous accumulation of stimuli that function as danger signals. With advancing age, chronic inflammation becomes more systemic, gradually damaging tissues through increased production of SASP factors and oxidative stress, thereby contributing to age‐related diseases. Research has shown that suppressing chronic inflammation may slow the aging and senescence process [22].

2.12. Dysbiosis

The gut microbiota produces various metabolites that regulate the functions of host cells and organs, and it has therefore been described as “another organ [23].” The composition of the gut microbiota changes with age and is thought to be involved in the development of age‐related diseases by promoting chronic inflammation, decreasing immune function, and inducing metabolic abnormalities [24, 25]. Modulation of the gut microbiota toward a more youthful composition may contribute to extending healthy lifespan [25].

With increasing research on the molecular mechanisms of aging, exemplified by the “Hallmarks of Aging,” it has become clear that aging and senescence are not merely the passage of time but dynamic processes directly linked to disease risk and healthy lifespan. In recent years, numerous studies have employed advanced technologies to elucidate systemic aging and senescence.

Recently, Ding et al. [26] reported that, by analyzing 13 different human organs—including blood vessels, heart, liver, and spleen—from individuals aged 14–68 years and quantifying a large number of proteins, they were able to identify which organs and proteins begin to change and at what age these changes occur. Their findings demonstrated that the rate of aging and senescence differs substantially among organs. Specifically, they constructed a multi‐tissue proteomic atlas based on proteins that fluctuate in an age‐dependent manner. They reported that the vascular system, particularly the aorta, exhibited aging‐associated signals at relatively early stages, whereas changes in other organs appeared later. In blood vessels, inflammation‐related proteins, extracellular matrix (ECM)–related factors, and coagulation‐ and immunity‐related factors increased at early stages, leading to the concept that vascular aging may serve as a central hub driving systemic aging. Furthermore, through transcriptome–proteome decoupling analysis, they demonstrated that proteostasis failure marks aged tissues, as the expected correspondence between increased mRNA levels and protein abundance was significantly disrupted in aged tissues [26].

3. Although Oocyte Aging Is a Major Factor in Decreased Fertility, Other Contributing Factors May Also Be Involved

Age‐related decline in oocyte function is well established as a primary cause of reduced fertility. The ovaries are among the organs that exhibit particularly rapid aging. Recently, Wu et al. [4] applied the “Hallmarks of Aging” framework to the ovaries and demonstrated that nine hallmarks are strongly associated with oocyte and ovarian aging, such as Genomic instability, telomere attrition, epigenetic alterations, dysfunction of autophagy, deregulated nutrient‐sensing, mitochondrial dysfunction, and cellular senescence [27, 28, 29, 30, 31, 32, 33, 34]. It is proposed that multiple aging and senescence phenomena interact in a chain reaction, such that aging and senescence progress not in a unidirectional manner but through reinforcing feedback loops, thereby accelerating ovarian aging [4].

4. Atlas of Ovarian Aging

Although the ovaries contain numerous cell types in addition to oocytes, research has advanced using transcriptomic technologies, such as single‐cell RNA sequencing (scRNA‐seq), to identify which cell populations are involved in ovarian aging and senescence.

Wang et al. [35] reported aging‐associated and cell type–specific dysregulation, including alterations in antioxidative pathways, between young and old cynomolgus monkeys using scRNA‐seq analysis. As expected, the number of oocytes was reduced and fibrosis was increased in the ovaries of older individuals. From a perspective beyond oocytes, granulosa cells in particular exhibited aging‐related changes, characterized by enrichment of genes involved in the positive regulation of apoptotic signaling and downregulation of oxidoreductase activity. These findings indicate that age‐dependent apoptosis and oxidative damage in granulosa cells are associated with ovarian functional decline (Figure 2).

FIGURE 2.

The possible hallmarks of ovarian aging. Based on the hallmarks of ovarian aging proposed by Wu et al. [4], this review presents the six hallmarks of ovarian aging discussed herein.

Research has also focused on constructing single‐cell atlases of human ovaries. Zhou et al. [36] reported that, in human ovaries from a young age group (under 33 years) and a middle‐aged group (37 years and older), decreased anti‐Müllerian hormone (AMH) levels and increased ovarian fibrosis were observed, confirming the presence of ovarian aging. In the ovaries of the middle‐aged group, a key characteristic was an increased proportion of stromal cells and a decreased proportion of macrophages and T cells. Multiple stromal cell subtypes are present within the ovary. Some subtypes express high levels of fibrosis‐related genes, whereas others express inflammation‐related genes, suggesting that these stromal cells contribute to age‐dependent ovarian fibrosis (Figure 2). In contrast, ovarian resident macrophages are also heterogeneous and are characterized by a predominance of pyroptotic inflammatory phenotypes regulated by the NLRP3 inflammasome–gasdermin D (GSDMD) pathway in middle‐aged ovaries. These alterations in the ovarian immune microenvironment may promote stromal cell senescence and accelerate reproductive decline (Figure 2). Indeed, Maruyama et al. [37] reported that senescence‐associated β‐galactosidase (SA‐β‐gal) positive senescent cells are accumulated in the stromal area together with the existence of macrophages in the aged mice ovaries (8–10 months old) compared with young ovaries (2–4 months old). Notably, the NLRP3 inflammasome has been implicated in ovarian aging, as ablation of NLRP3 increased AMH levels and follicle numbers and improved pregnancy rates during reproductive aging in mice [38]. In addition, deletion of GSDMD protected against age‐related follicular decline and ovarian fibrosis [36].

Indeed, scRNA‐seq analysis of mouse ovaries has shown that cell death mechanisms change significantly with age [39]. Zhang et al. [39] investigated multiple types of regulated cell death (RCD) pathways in murine ovaries across the aging process and mapped the pan‐RCD landscape, including apoptosis, ferroptosis, pyroptosis, and parthanatos, thereby revealing cell type–specific RCD trajectories (Figure 2). In young ovaries, the overall RCD score was low, and ovarian cells were in a relatively stable state. However, the apoptotic score exhibited the strongest positive correlation with advancing age across all ovarian cell types. Notably, parthanatos—a caspase‐independent, DNA damage–induced form of cell death [40]—showed the steepest age‐associated upregulation in ovarian cells, including granulosa cells, endothelial cells, stromal cells, and macrophages. Furthermore, they demonstrated that parthanatos was enriched in human ovaries with advancing age across five ovarian cell types, including stromal cells, lymphatic endothelial cells, smooth muscle cells, granulosa cells, and immune cells [39]. These findings indicate that RCD pathways, particularly parthanatos, may play a pivotal role in age‐related cellular functional changes and reproductive decline within the ovary.

Although decreased fertility is influenced by the hallmarks of ovarian aging, reproductive decline begins to emerge long before follicular exhaustion. Accordingly, a study modeling premenopausal women presented a single‐cell atlas of reproductive ovaries in mice at reproductive age (9 months old) compared with young mice (3 months old) [41]. Isola et al. [41] reported a doubling of immune cell populations in reproductive‐aged ovaries, with marked increases in T lymphocytes, particularly Type 1 and Type 17 lymphoid cells such as natural killer T (NKT) cells and γδ T cells. These distinctive immune cell subsets may play critical roles in ovarian fibrosis, as well as fibrosis in other organs, including the liver, kidney, and heart.

These comprehensive atlas analyses suggest that ovarian aging is not attributable to a single cause but rather to the interplay of multiple changes (Figure 2). Among these, increased immune cell infiltration is hypothesized to promote inflammation, thereby contributing to the progression of ovarian fibrosis.

5. Ovarian Fibrosis as a Significant Feature of Ovarian Aging

Fibrosis is observed in various organs and tissues and is characterized by excessive fibroblast proliferation and accumulation of ECM [42, 43]. Excessive fibrosis can induce tissue dysfunction. Transforming growth factor‐β (TGF‐β) plays a central role in the development of fibrosis. In particular, exacerbation of ovarian fibrosis represents a critical issue in ovarian aging because it contributes to ovarian functional decline and infertility [42, 43]. Ovarian fibrosis occurs concurrently with activation of TGF‐β signaling, as well as other inflammatory pathways, including the NLRP3 inflammasome and nuclear factor‐kappa B signaling [44].

Numerous recent studies have demonstrated that ovarian fibrosis increases with age in mice, monkeys, and humans, and efforts are underway to elucidate its underlying mechanisms (Figure 2). For example, stromal cell markers are upregulated with aging in both humans and mice, accompanied by increased collagen deposition and inflammation, suggesting that ovarian aging is associated with fibrosis and inflammaging [45]. In addition to ovarian fibroblasts, granulosa and theca cells have been implicated in fibrosis induction due to age‐related upregulation of fibrotic, inflammatory, and stress‐response signaling pathways in mice [41].

Umehara et al. [46] reported that ovarian fibrosis increases in reproductively aged mice, with prominent collagen deposition in stromal areas, resulting in reduced ovulatory function. Furthermore, age‐related mitochondrial dysfunction has been identified as a trigger of the ovarian fibrotic cascade, including oxidative and endoplasmic reticulum stress, inflammatory responses with macrophage accumulation, and dysregulation of ECM degradation, such as altered matrix metalloproteinase 13 activity, ultimately leading to reduced ovulation. Reproductively aged mice also exhibited increased collagen deposition, fewer ovulations, and a higher number of oocytes trapped within the corpus luteum, indicating ovulatory failure associated with ovarian aging [47]. These findings demonstrate that ovulatory dysfunction secondary to ovarian fibrosis is one of the contributing factors to age‐related subfertility.

Treatment with antifibrotic agents has been shown to reverse ovarian fibrosis, improve ovulation, and extend female fertility. Amargant et al. [48] also reported that ovarian fibrosis increases with aging in mice and that antifibrotic treatment improved ovarian collagen levels, follicle numbers, corpus luteum counts (a marker of ovulation), and inflammaging within the ovary. In addition to natural aging models, administration of cyclophosphamide can mimic ovarian aging phenotypes in mice, including decreased follicle numbers and AMH levels, increased expression of senescence markers (p16, p21, etc.), and enhanced fibrosis [49]. Senolytic treatment (dasatinib and quercetin) effectively attenuated cyclophosphamide‐induced ovarian damage by eliminating senescent cells, reducing inflammaging and fibrosis, and subsequently increasing follicle numbers. Moreover, Hense et al. [50] reported that mice with metabolic dysfunction–associated liver disease exhibited increased ovarian senescence, inflammation, and fibrosis, along with reduced pregnancy rates. Treatment with senolytics (dasatinib and quercetin) ameliorated these adverse effects. These findings suggest that ovarian fibrosis and reproductive dysfunction may be reversible through modulation of ovarian senescence and that senolytic therapy has therapeutic potential for improving reproductive function.

6. Vascular Aging as a Key Factor in Ovarian Aging

The vascular system is essential for the physiological regulation of tissue and cellular function [51]. Ding et al. [26] recently conducted comprehensive proteomic and histological analyses of human tissues spanning five decades and reported that blood vessels are among the tissues that age early and are particularly susceptible to aging. In general, vascular endothelial growth factor (VEGF) plays an essential role in vascular function, including angiogenesis, vascular permeability, and maintenance of endothelial barrier integrity. If vascular aging occurs early, it might be assumed that a decline in VEGF levels is the primary cause. However, studies in mice have shown that systemic VEGF levels and VEGF levels in major peripheral organs do not decrease significantly with age. Rather, impairment of the VEGF signaling system appears to underlie decreased vascular function. This impairment is associated with factors such as a significant age‐related increase in soluble VEGF receptor 1 (sFlt1), which inhibits VEGF activity, and a decline in VEGF receptor signaling that regulates vascular homeostasis. Indeed, Grunewald et al. [51] demonstrated that mice with upregulated VEGF signaling exhibited increased lifespan and extended health span, accompanied by improvements in aging‐related phenotypes, including inflammaging, liver dysfunction, endothelial senescence, and sarcopenia.

Until recently, ovarian blood vessels have primarily been examined using tissue histochemical analysis; however, the application of three‐dimensional (3D) imaging techniques has clarified their importance in ovarian aging. Xu et al. [52] developed a high‐resolution, ovary‐scale 3D imaging system to enable detailed visualization and tracing of angiogenesis and vascular remodeling. Using this system, they demonstrated that ovarian vascular density increased significantly with reproductive maturation in female mice. Moreover, they observed a significant decrease in stromal blood vessel density in aged mice (14 months old) compared with young mice (4 months old), indicating that degeneration of ovarian vasculature accompanies ovarian aging [52]. Interestingly, they treated middle‐reproductive‐aged mice (7 months old) with the small‐molecule VEGF receptor inhibitor axitinib to block adult angiogenesis and found that this intervention improved ovarian reserve by increasing follicle numbers, thereby extending reproductive lifespan in later life (12–14 months old). They concluded that temporary suppression of angiogenesis preserves ovarian reserve in the long term, leading to delayed ovarian senescence and extension of the female reproductive lifespan.

Using the same vascular imaging system, they further demonstrated that the decrease in ovarian vascular density is tissue‐specific and represents an early feature of aging (4, 8, and 12 months old) [53, 54]. In contrast, the vascular density of non‐reproductive organs, including the heart, liver, and kidney, did not change significantly with age. The age‐related decline in ovarian vascular density was directly associated with reduced female fertility, as diminished vascularization led to insufficient in vivo follicular development in middle‐aged female mice. Consistent with systemic observations showing increased VEGF levels with aging, ovarian VEGFA levels were also elevated with age. However, the proportion of ovarian capillaries was markedly reduced in middle‐aged ovaries, indicating that ovarian blood vessel decline is not attributable to insufficient VEGF levels. To clarify this discrepancy, they performed scRNA‐seq analysis and demonstrated that oxidative stress–related endothelial aging is a key factor contributing to diminished angiogenesis and reduced blood supply in middle‐aged ovaries (Figure 2).

Notably, administration of the natural compound salidroside to aged female mice (from 12 to 24 months old) reversed vascular aging, enhanced angiogenesis, and restored ovarian blood supply, thereby significantly improving follicular development and fertility [54]. These findings suggest that salidroside exerts dual effects by rejuvenating vascular health and enhancing ovarian function and may represent a potential therapeutic strategy targeting vascular aging in ovarian senescence.

7. Possible Aging of the Oviduct

The animal oviduct and the human fallopian tube are essential sites for the transport of oocytes and sperm and serve as the platform for fertilization and regulation of early embryonic development, thereby influencing pregnancy success [3, 55]. Morphologically and anatomically, the oviduct is divided into the infundibulum, ampulla, isthmus, and uterotubal junction. The oviduct is lined by a single‐layered epithelium composed of several cell types, primarily ciliated epithelial cells and non‐ciliated secretory cells [3, 55].

After fertilization, the oviduct supports early‐stage embryonic development by producing factors required for embryonic cleavage [55]. The blastocyst cleavage rate reached approximately 80% when embryos were co‐cultured with oviductal epithelial cells, compared with a 33% cleavage rate when cultured with fibroblasts [55, 56]. Oviductal epithelial cells secrete various cytokines and growth factors that regulate embryonic transport and development, including epidermal growth factor, fibroblast growth factor, and TGF [55, 57]. Therefore, the presence of oviductal epithelial cells is crucial for optimal blastocyst development. In contrast, ciliated epithelial cells possess multiple motile cilia on their apical surface, and coordinated ciliary beating generates fluid movement within the oviduct, thereby facilitating embryo transport [55]. In general, steroid hormones, particularly progesterone and estrogen, are key regulators of ciliary beat frequency [58, 59, 60, 61].

The morphology and cellular function of the oviduct change with age in both humans and animals (Figure 3). In humans, oviductal length and the degree of convolution decrease with aging [62]. Nassir et al. [63] developed 3D human fallopian tube models and demonstrated that the length of the fallopian tube decreases from the 20s to the 30s and 40s, whereas the oviductal lumen becomes wider and structurally simpler. Using this simulation model, they further showed that aging in women is associated with reductions in the number of motile sperm, swimming distance, and velocity within the fertilization region, attributable to morphological and mechanical changes in the fallopian tube. These alterations may contribute to reduced reproductive performance and infertility.

FIGURE 3.

The possible hallmarks of oviduct aging. This review presents the six hallmarks of oviduct aging discussed herein.

Studies using mouse models have enabled detailed characterization of age‐dependent changes in oviductal cells and structure. Harwalkar et al. [64] reported that, at the distal end of aged mouse oviducts, multiciliated cells exhibited vacuolization, with severely apically displaced and deformed nuclei. The vacuolated structures observed in multiciliated cells were not associated with cellular senescence; rather, hypoxia or hydroxyurea treatment induced vacuolation in these cells, suggesting that a mildly and chronically stressed microenvironment contributes to age‐dependent vacuolation in oviductal multiciliated cells. Winkler et al. [65] reported, using scRNA‐seq analysis in mice, that fibroblasts are the predominant cell type in the oviduct and represent a key source of inflammatory cytokines and ECM components [65]. Indeed, oviductal fibroblasts exhibit higher communication scores for ECM‐associated signaling than fibroblasts in other reproductive tissues, such as the uterus and ovary [65]. Importantly, collagen deposition increases rapidly with age in oviductal tissue, indicating age‐related accumulation of oviductal fibrosis in mice (Figure 3). Moreover, cell–cell communication analysis revealed that the oviduct exhibits the highest rate of fibrosis development during aging in mice, accompanied by the highest levels of TGF signaling to fibroblasts [65]. Similarly, in human fallopian tubes, increased collagen deposition and decreased smooth muscle and elastic fibers have been observed in older women compared with younger women [66]. These findings suggest that oviductal cellular composition and tissue structure undergo age‐dependent alterations, including progressive fibrosis, which may contribute to declining reproductive function (Figure 3).

8. Changes in Oviductal Cells With Aging

It has been proposed that a bovine model may be suitable for studying reproductive function and age‐associated fertility decline in women because of its physiological similarities and relatively longer lifespan [3, 67, 68]. Therefore, we used bovine oviduct epithelial cells to analyze age‐dependent transcriptomic changes. Aged bovine oviduct epithelial cells exhibited enriched expression of pro‐inflammatory cytokine signaling pathways and related SASP factors, including interleukin (IL)‐1β, IL‐1α, IL‐8, S100A8, S100A9, and C‐C motif chemokine ligand 2 (CCL2), along with downregulation of ECM‐related components such as collagens, actin, and biglycan [69]. Moreover, aged bovine oviduct epithelial cells showed SA‐β‐gal staining—a major marker of cellular senescence—than younger cells [70]. Notably, the ciliary beat frequency of aged bovine oviduct epithelial cells was lower than that of younger cells [69]. In addition, intracellular and mitochondrial ROS levels were elevated, whereas proliferative capacity was reduced in aged bovine oviduct epithelial cells [69]. These findings suggest that aged bovine oviduct epithelial cells exhibit features consistent with several hallmarks of aging, including mitochondrial dysfunction, cellular senescence, altered intercellular communication, and chronic inflammation (Figure 3).

In a pro‐inflammatory environment, IL‐1 often triggers cellular stress that activates senescence [71]. Deletion of the Il1a gene resulted in higher pregnancy rates in aged mice compared with wild‐type aged mice [72]. Therefore, we hypothesized that elevated levels of IL‐1α and IL‐1β induce cellular senescence in bovine oviduct epithelial cells. However, treatment with IL‐1α and IL‐1β did not alter SA‐β‐gal staining, whereas these cytokines stimulated expression of SASP factors such as IL‐8 and CCL2 in bovine oviduct epithelial cells [73], suggesting that IL‐1 primarily promotes chronic inflammation rather than directly inducing senescence in the bovine oviduct.

Therefore, we further focused on S100A8 and S100A9, which were highly expressed in aged bovine oviduct epithelial cells. S100A8 and S100A9 have emerged as endogenous damage‐associated molecular patterns capable of regulating inflammatory signaling [74, 75]. In bovine oviduct epithelial cells, S100A8, but not S100A9, stimulated the mRNA expression of senescence markers (p16 and p21) and SASP factors (IL‐8 and tumor necrosis factor‐α), increased SA‐β‐gal staining, elevated intracellular ROS levels, disrupted mitochondrial distribution, and induced dysregulation of ECM‐related factors, characterized by decreased collagen expression and increased matrix metalloproteinase expression [70]. In contrast, direct exposure to S100A9 decreased sperm motility and viability and exerted adverse effects on embryo development in bovine models [76]. Recently, Zhang et al. [77] proposed S100A8/S100A9 as a novel aging biomarker termed the “SenoIndex”. Through comprehensive bioinformatic analyses of large datasets and multiple experimental models, including humans, monkeys, rats, and mice, they identified S100A8 and S100A9 as the most consistent candidate age‐related proteins across species and tissues [77]. Indeed, S100A8 and S100A9 expression levels were markedly higher in aged tissues, such as the kidney, lung, liver, and brain, compared with younger tissues in rodents and monkeys. Moreover, they demonstrated elevated serum levels of S100A8/S100A9 in healthy elderly individuals compared with younger individuals, suggesting their potential utility as biomarkers for assessing human aging at both tissue and organismal levels [77]. Conversely, in sheep oviduct epithelial cells, S100A8 is regulated by estradiol 17‐β, and knockdown of S100A8 resulted in an excessive immune response, indicating that S100A8 contributes to maintaining immune homeostasis in the oviductal mucosa [78]. Therefore, the biological roles of S100A8 and S100A9 may differ depending on tissue context and physiological conditions.

Additionally, recent research has reported the presence of microbiota in the human fallopian tube and its association with ovarian cancer [79]. In the future, further investigations of fallopian tube microbiota in both humans and animal models are expected to clarify their relationship with reproductive function.

In the oviduct/fallopian tube, multiple “hallmarks of oviductal aging” may be proposed, analogous to the established framework in ovarian and uterine aging research [4, 80, 81]. Specifically (Figure 3), these include cellular senescence, chronic inflammation, mitochondrial dysfunction, and alterations in intercellular communication. Furthermore, fibrosis of the oviduct/fallopian tube and abnormalities in ciliated oviductal/fallopian epithelial cells may be considered age‐related changes specific to these tissues.

9. Uterine Aging Negatively Impacts Pregnancy

Studies using rodent models have suggested that uterine aging contributes to the decline in reproductive function. For example, when female mice are categorized into young (3 months old), middle‐aged (8–9 months old), and old (11–13 months old) groups, abnormal pregnancy outcomes, such as miscarriage and an increased incidence of low‐birth‐weight fetuses, become more frequent with advancing age [82]. Woods et al. [83] also reported that, although overall fertility was preserved in aged mice (12 months old) compared with young mice (2–3 months old), abnormalities in placental development and fetal growth were observed more frequently in the aged maternal group. When embryos derived from aged mice were transferred into young recipients, abnormalities in implantation, placental development, and embryonic growth were largely resolved [83]. These findings suggest that many pregnancy complications are not attributable to intrinsic oocyte abnormalities but rather to an aged maternal environment that is unable to adequately support normal pregnancy.

In humans, research has been conducted to clarify whether age‐related changes in uterine function contribute to decreased fertility or abnormal pregnancy outcomes by analyzing IVF embryos generated through oocyte donation. Sebastian‐Leon et al. [84] analyzed data from 33,141 embryo transfers using high‐quality embryos derived from donor oocytes obtained from women under 35 years of age, stratified according to the recipient's age. Their results revealed that implantation failure increased beginning at approximately 39 years of age, live birth rates declined from approximately 40 years, and miscarriage rates increased from approximately 43 years [84]. Furthermore, they demonstrated that after 40 years of age, the risk of implantation failure increased by 4.2% per year and the risk of miscarriage increased by 3.2% per year. In addition, analyses using euploid embryos have reported that pregnancy and live birth rates decrease in older women [85].

These findings suggest that, in addition to embryonic factors contributing to age‐related reproductive decline, uterine aging and senescence also play an important role in the deterioration of reproductive function (Figure 4).

FIGURE 4.

The possible hallmarks of uterine aging. Based on the hallmarks of uterine aging proposed by other reviews [81, 86, 87], this review presents the eight hallmarks of uterine aging discussed herein.

10. In Aging Individuals, the Uterus Undergoes Cellular Senescence and Chronic Inflammation

The endometrium is a complex tissue composed of various cell types, including endometrial epithelial cells, stromal cells, immune cells, and vascular cells. A distinctive characteristic of the endometrium, not observed in most other organs, is its cyclical proliferation and shedding in accordance with the menstrual cycle [80].

In the aging human endometrium, the expression of cellular senescence–related genes (p16 and p21) and SASP factors (IL‐6 and IL‐8) increases, suggesting the presence of chronic inflammation [81, 88, 89, 90]. In rodents, the expression of senescence‐associated genes (p16 and p21), as well as inflammation‐related molecules such as C‐X‐C motif chemokine ligand 12, IL‐17 receptor B, and CCL2, increases in the endometrium with age [91, 92]. These findings suggest that cellular senescence and chronic inflammation progressively develop in the uterus during aging [81, 86]. Of particular importance is the paracrine effect of SASP factors, which is thought to influence surrounding cells, thereby propagating senescence signals and ultimately contributing to organ dysfunction (Figure 4).

Because humans exhibit diverse genetic backgrounds, it is necessary to account for genetic variability when interpreting research findings. To address this issue, Denis et al. [93] generated somatic cell–cloned cows derived from the same donor and compared young (4–7 years old, first calving) and older (13–15 years old, multiparous) cows under conditions of matched genetic backgrounds. They found that inflammation‐ and immune response–related genes were activated in the endometrium of aged cloned cows, whereas the expression of ECM‐ and collagen‐related genes was reduced [93]. Similarly, we previously reported that inflammation‐related genes (IL1A, NFKB, CCL5) and interferon signaling pathways were significantly upregulated in aged bovine endometrial cells compared with younger cells, based on comprehensive gene expression analysis [94]. Another study reported that inflammatory mediators, including IL1β, CCL21, and CXCL1 (GRO1), were elevated in the endometrium of aged cows compared with younger cows [95]. On the other hand, inflammatory mediators, including IL1β, TNFα, and CCL2, were elevated in the endometrium of subfertile cows [96], suggesting the possible association between age‐related inflammaging and infertility in the uterus.

These findings suggest that age‐associated increases in SASP factors and inflammatory responses within the uterus may contribute to reduced fertility and the occurrence of abnormal pregnancy outcomes [86, 87, 97] (Figure 4).

11. In the Aging Uterus, Multiple Hallmarks of Aging Are Observed

In endometrial tissue and cells, not only does hormonal responsiveness change with age, but several of the so‐called hallmarks of aging are also implicated, including cell cycle dysregulation resulting from the accumulation of DNA damage, epigenetic alterations, and increased oxidative stress [86, 87].

In the human uterus after 35 years of age, the expression of genes associated with cell cycle arrest, telomere maintenance, and inhibition of DNA repair changes significantly, leading to reduced proliferative capacity of endometrial cells [98]. Kobayashi et al. [88] suggested that mitochondrial dysfunction serves as a central hub underlying these changes. When mitochondrial function declines, ROS production increases, resulting in DNA damage and cellular senescence. Concurrently, the proliferation and differentiation of endometrial cells are impaired, thereby hindering the establishment of a uterine environment suitable for implantation and disrupting uterine tissue remodeling [88]. Furthermore, mitochondrial dysfunction promotes chronic inflammation, which in turn exacerbates mitochondrial abnormalities, creating a vicious cycle that accelerates cellular senescence [88]. These alterations occur not independently but as interconnected processes. The cumulative effect of these abnormalities reduces endometrial receptivity, leading to implantation failure and age‐related fertility decline.

12. Changes in the Structure and Function of Uterine Cells With Aging

With aging, the proliferative capacity of endometrial cells declines, leading not only to thinning of the endometrium but also to reduced development of uterine glandular structures and decreased uterine blood flow. These alterations are associated with decreased pregnancy rates and increased incidence of abnormal pregnancy outcomes [97].

Ciliated endometrial epithelial cells, which are rarely observed in younger women, increase in number with age, suggesting disruption of the differentiation balance of endometrial epithelial cells [89]. Another study reported activation of multiple pathways and genes involved in ciliary formation in the endometrium of aging women [98]. Therefore, abnormal ciliation of endometrial epithelial cells represents an early cellular alteration associated with maternal aging and is considered an important factor contributing to reduced embryo receptivity in the aging endometrium (Figure 4).

Xie et al. [99] analyzed uterine tissue from early pregnancy in young (22–30 years old) and older (35–49 years old) women. In the endometrium of older women, the number of uterine glands decreases, and abnormal fibrosis is frequently observed in the surrounding stroma. Detailed scRNA‐seq analysis indicated activation of pathways involved in ECM organization and collagen fibril organization in older women [100] similar to mice experimental data [100]. These alterations are thought to be related to enhanced fibroblast activity and reduced function of immune cells, such as macrophages and natural killer cells, along with excessive activation of TGF‐β signaling, which promotes fibrosis and collagen production [99]. Thus, aging may disrupt the interaction between fibroblasts and immune cells in the endometrium, creating a vicious cycle that further promotes uterine fibrosis [99, 101].

Detailed scRNA‐seq analysis using an aging mouse model indicated that diverse cell types are present in the uterus and that macrophages, in particular, which are involved in anti‐inflammatory and tissue repair processes, play a crucial role [65]. Furthermore, fibroblasts constitute a substantial proportion of uterine cells and regulate cell proliferation across the estrous cycle through cytokine‐mediated signaling. In young mice, inflammatory signaling and ECM reorganization are appropriately regulated, enabling restoration of the estrous cycle. However, with aging, this regulatory process becomes impaired, and chronic inflammation and fibrosis progressively develop in uterine fibroblasts [65].

Therefore, “hallmarks of uterine aging” may be proposed, analogous to the established framework in aging research [80, 81, 86, 102]. Specifically (Figure 4), these include cellular senescence, chronic inflammation, genomic instability (DNA damage and reduced repair capacity), and mitochondrial dysfunction. Furthermore, decreased hormonal responsiveness may represent altered intercellular communication as an additional age‐related feature (Figure 4). In addition, recognition of a uterine microbiota and age‐related shifts in its composition suggests that disruption of the uterine microbial environment may also be an important contributing factor [103] (Figure 4). Moreover, endometrial fibrosis [99] and differentiation abnormalities of ciliated endometrial epithelial cells [89] may be considered uterus‐specific age‐related alterations.

13. Conclusion

The risks associated with pregnancy and childbirth at advanced maternal age were traditionally attributed primarily to oocyte‐related abnormalities. However, accumulating evidence indicates that, in addition to age‐related changes in follicles and ovaries responsible for oocyte production and maturation, the functional integrity of the ovary, oviduct/fallopian tubes, and uterus also plays a critical role in reproductive outcomes. As a consequence of the inherent physiological process of aging, reproductive tissues and cellular functions progressively decline over time.

The aging and senescence of reproductive tissues and cells are not driven by a single mechanism but rather progress through the interaction of multiple hallmarks of aging [8, 9]. Further research into female reproductive aging is expected to facilitate the identification of biomarkers for objectively assessing age‐related alterations in reproductive tissues and to promote the development of novel therapeutic strategies targeting fibrosis and other aging‐associated changes in the female reproductive system.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.