Molecular mechanisms of ovarian fibrosis.

Introduction
The ovaries are central to the female reproductive system and play a critical role in oocyte development, hormone secretion, and the regulation of reproductive health (Sisodia and Del Carmen, 2022). The ovary comprises two main types of tissue: parenchymal and stromal. The parenchymal tissue includes the functional part, which is further composed of the cortex and medulla. The ovarian stromal tissue around the ovarian follicles contains immune cells, blood vessels, nerves, lymph vessels, and fibroblast-like interstitial cells. While the ovarian stroma provides structural and biochemical support to the ovary, tissue-specific fibrils, together with network-forming proteins, proteoglycans, and glycosaminoglycans (GAGs) in the extracellular matrix (ECM), play a critical role in maintaining and regulating ovarian physiology (Kinnear et al., 2020). One of the main functions of ovarian tissue is the production of several hormones that are essential for female reproductive life and the maintenance of systemic homeostasis (Dunlop and Anderson, 2014; Han et al., 2023). After puberty, GnRH is secreted from the hypothalamus. Under the stimulation of GnRH, FSH and LH, secreted from the pituitary gland, regulate folliculogenesis. In addition, theca cells produce androgens in response to LH, and granulosa cells respond to FSH by converting the androgen produced in the theca cells to oestradiol through aromatization (Oduwole et al., 2021; Recchia et al., 2021). Beyond its role in folliculogenesis and oocyte maturation during each menstrual cycle, the dynamic remodelling of the ECM is essential for follicle wall breakdown during ovulation, the transformation of the follicle into the corpus luteum (CL) after ovulation, the structural degradation of the CL, as well as the regression of atretic follicles. After ovulation, the remnants of the ovulated follicle is converted into the CL, while atretic follicles undergo degeneration. These processes are necessary for continuous restructuring of the components of the ECM. The dynamic modification of the ECM is crucial for the proper progression of processes such as follicular development, ovulation, and the structural degradation of the CL (Curry and Osteen, 2003; Asadzadeh et al., 2016) (Table 1).
Tissue fibrosis is a pathological and complex process characterized by excessive and persistent deposition of ECM components, particularly collagen and fibronectin (FN)—resulting from chronic or dysregulated wound-healing responses. This leads to tissue stiffening, architectural distortion, and pathological remodelling, ultimately causing functional impairment of the affected organ (Wynn, 2008; Wynn and Ramalingam, 2012) (Table 1). Repeated ovulation throughout the reproductive life leads to progressive and permanent collagen accumulation (Lind et al., 2006) with an increase in inflammatory proteins involved in tissue healing. The accumulation of collagen in the tissue may lead to ovarian fibrosis and an increased risk of developing ovarian cancer (Zhou et al., 2017; Harper et al., 2018; Yeung et al., 2019; Sarwar et al., 2022; Gu et al., 2024; Abbaspour et al., 2025). After ovulation, the microenvironment of the follicle initiates a process of tissue repair through remodelling of the ECM (Curry and Smith, 2006; Zhang et al., 2024; Wang et al., 2025). Repeated and sustained tissue injury incurred during the ovulatory process has been shown to result in a significant decline in both the quantity and quality of oocytes. This damage contributes to follicular atresia and depletion, disrupts the delicate processes of folliculogenesis, and precipitates alterations in endocrine function (Bendarska-Czerwińska et al., 2022). Collectively, these pathological changes adversely affect ovarian reserve and hormonal homeostasis, which may underlie various reproductive disorders associated with ovarian ageing. Ovarian fibrosis has been linked to disorders such as polycystic ovary syndrome (PCOS), premature ovarian insufficiency (POI), ovarian endometriosis, and even ovarian cancer (Harper et al., 2018; Yeung et al., 2019; Landry et al., 2020; Bendarska-Czerwińska et al., 2022; Du et al., 2023; Wang et al., 2023; Gu et al., 2024; Vissers et al., 2024). These abnormalities are characterized by a reduction in follicle number, impaired folliculogenesis, and diminished oocyte quality (Gu et al., 2024) (Fig. 1 and Table 2).

Ovarian fibrosis
Fibrosis is characterized by fibroblast proliferation, myofibroblast differentiation, and excessive collagen deposition in the extracellular space during the process of repetitive repair of damaged tissue. Ovarian fibrosis, a condition that significantly affects female reproductive health, represents a major concern from both scientific and clinical perspectives. Because it leads to infertility and various endocrine disorders by impairing reproductive function, its pathological impact is particularly severe (Briley et al., 2016; Sisodia and Del Carmen, 2022; Wang et al., 2024) (Table 2).
Ovarian fibrosis is characterized by fibroblast hyperproliferation and ECM deposition. Ovulation during the menstrual cycle can cause inflammation, disruption of tissue homeostasis, signalling pathways, and ECM accumulation (Curry and Smith, 2006; Duffy et al., 2019; Gu et al., 2024) (Table 2). The characteristic features of ovarian fibrosis include thickening of the tunica albuginea surrounding the ovarian tissue, mesenchymal connective tissue proliferation, accumulation of atretic follicles, and a decrease in the number of follicles. ECM deposition not only alters the stromal microenvironment but also disrupts cell–cell communication, which is essential for proper follicular development (Fig. 1). In particular, the disruption of signalling pathways between follicles may lead to compromised oocyte quality and developmental competence (Briley et al., 2016; Zhou et al., 2017; Wang and Khalil, 2018; Gu et al., 2024) (Table 2). Structural changes with ovarian fibrosis have the potential to significantly impact the reproductive and endocrine functions of the ovary, which can result in pathological conditions such as PCOS, POI, and ovarian cancer (Zhou et al., 2017; Landry et al., 2020; Gu et al., 2024). Ultimately, aberrant ECM dynamics contribute to ovarian ageing and follicular dysfunction by increasing fibrosis and stiffness, disrupting growth factor signalling, impairing granulosa–oocyte interactions, and inducing chronic inflammation. These changes compromise both oocyte quality and follicle development, ultimately reducing reproductive lifespan.

Myofibroblast activation and role in fibrosis process
Fibroblasts are connective tissue cells that synthesize and abundantly secrete components of the ECM, thereby providing crucial support for tissue integrity. In addition to maintaining the tissue structure and organization by being in connective tissue, it has significant functions in embryonic development, tissue injury, repair, regeneration, and various pathological conditions (Plikus et al., 2021). Fibroblasts are usually quiescent when there is no tissue repair or regeneration. The process of fibroblast cell activation is initiated by disruption of tissue homeostasis, followed by increased inflammation (Wynn and Ramalingam, 2012; Roman, 2023). To promote the process of repair, regeneration, and re-establishment of homeostasis following injury, it is essential for the activation and differentiation of fibroblasts in the tissue into myofibroblasts (Wynn and Ramalingam, 2012). However, if this process is not properly controlled, it may cause pathological conditions in the later stages of the healing (Wynn and Ramalingam, 2012). During wound repair or the fibrosis, quiescent fibroblasts begin to differentiate into myofibroblasts with various signals, physical factors, and an increase in the expression of contractile proteins such as α-smooth muscle actin (α-SMA) (Darby et al., 1990, 2014). Myofibroblasts are cells that play pivotal roles in pathological processes originating from epithelial and endothelial cells or bone marrow stem cells transformed by epithelial/endothelial–mesenchymal transition (EMT). The presence of filamentous actin and myosin in myofibroblasts confers contractile properties (Wynn and Ramalingam, 2012; Garcia Garcia et al., 2023) (Fig. 2).
Myofibroblast activation, which plays a critical role in tissue repair, is typically a transient process. Although its vital functions in acute injury repair, its abnormal and sustained activation in conditions of chronic injury and persistent inflammation can lead to excessive ECM accumulation and fibrosis-like changes (Tai et al., 2021) (Table 1). Moreover, it also has the potential to cause tumours (Plikus et al., 2021). The activation, proliferation, and persistence of myofibroblast cells are regulated by inflammatory cytokines, including interleukins IL-1, IL-6, IL-13, and tumour necrosis factor-α (TNF-α) (Plikus et al., 2021; Garcia Garcia et al., 2023). This process also involves the production of TGF-β, connective tissue growth factor (CTGF), and PDGF, in addition to growth factors; ECM-derived factors, such as mechanical stress and matrix stiffness, can trigger inflammation, driven by hyaluronan (HA) fragments. These factors influence quiescent fibroblasts, thereby increasing the population of myofibroblast cells (Plikus et al., 2021) (Fig. 1).
Myofibroblasts, found in nearly every tissue, are spindle-shaped fusiform cells with an extensive cell matrix and an irregular network structure (Tai et al., 2021; Xu et al., 2022) (Table 3). The differentiation of fibroblasts into myofibroblasts is crucial in fibrosis (Adapala et al., 2021; Sun et al., 2022). Myofibroblasts can be identified by the accumulation of type I and type III fibrillar collagens, HA, FN and fibronectin extra domain A (FN-EDA) in the ECM (Kohan et al., 2010; Tai et al., 2021; Garcia Garcia et al., 2023). These accumulations have a detrimental effect on the process of fibrosis in myofibroblasts, causing alterations in the tissue structure (Gabbiani, 1981; Klingberg et al., 2013; Tai et al., 2021; Xu et al., 2022) (Fig. 2 and Table 3). The transformation of fibroblast cells into myofibroblast cells is crucial in the repair of acute tissue damage (Gabbiani, 1981; Klingberg et al., 2013; Tai et al., 2021). However, myofibroblasts may acquire resistance to apoptosis and are chronically activated due to abnormal persistence of these cells within fibrotic tissues, leading to persistent ECM overproduction and pathological tissue stiffening (Tai et al., 2021; He et al., 2025) (Tables 2 and 3). The elevated expressions of FN-EDA and HA contribute notably to fibrosis progression (Tai et al., 2021) (Fig. 2). However, the persistence of this transformation and the excess accumulation of myofibroblast cells can hinder the normal function of tissues (Younesi et al., 2024). When the process of damage repair is complete, the cells are eliminated by apoptosis (Hinz and Lagares, 2020). However, in cases of chronic inflammation, the continued activation of fibroblasts may lead to the development of fibrosis in the tissue. In the presence of persistent inflammation, myofibroblasts may over-produce ECM, which can contribute to pathological tissue stiffness and dysfunction (Wynn and Ramalingam, 2012; He et al., 2025) (Table 3). Myofibroblasts can exhibit both active and inactive phenotypes. In the inactive form, myofibroblasts secrete ECM components and matrix metalloproteinases (MMPs), thus contributing to the maintenance of tissue homeostasis. Conversely, in the active form, myofibroblasts exhibit migratory and proliferative properties, resulting in ECM accumulation (Tai et al., 2021). In ovarian fibrosis, chronic inflammation, ovarian dysfunction, radiation exposure, environmental factors, and surgical interventions trigger fibroblast proliferation and differentiation of fibroblasts into myofibroblasts. This differentiation becomes more pronounced under conditions of elevated extracellular stress, accelerated ECM deposition, and increased TGF-β expression (Doolin et al., 2021; Baik et al., 2022). Moreover, an increased synthesis of FN-EDA and HA synthesis is conducive to this process, resulting in fibrosis (Tai et al., 2021).

Ovarian fibrosis and changes in ECM
The ECM has been defined as a non-cellular, three-dimensional macromolecular network composed of collagen, proteoglycan, GAG, elastin, FN and laminin. In addition to serving as a structural support, the ECM is a key regulatory component that provides an optimal environment for cell migration, division, differentiation, and adhesion (Birkedal-Hansen et al., 1993; Theocharis et al., 2016). The maintenance and remodelling of this dynamic structure are regulated by MMPs and tissue inhibitors of metalloproteinases (TIMPs) (McIntush and Smith, 1998; Bałkowiec et al., 2018) (Table 3). MMPs are pivotal mediators of connective tissue remodelling and the maintenance of tissue homeostasis. Their role in collagen degradation is of crucial for the dynamic restructuring of the ECM (Gu et al., 2024). In addition, they play a pivotal role in a multitude of biological processes, including embryonic development, organ morphogenesis, angiogenesis, cartilage remodelling, bone growth, and wound healing (Wang and Khalil, 2018).
MMPs and their inhibitors, TIMPs, have been shown to play key roles in the remodelling of the ECM and wound repair in the ovarian tissue (Curry and Osteen, 2003; Asadzadeh et al., 2016; An et al., 2019; Butler et al., 2025) (Table 3). For instance, collagen degradation and MMP activation are required during ovulation. Consistent with these findings, Umehara et al. (2022) demonstrated that treatment with antifibrotic agents, specifically BGP-15 and pirfenidone, upregulated MMP13 expression in the ovaries of aged and obese animals exhibiting fibrotic tissue, thereby contributing to the regulation of normal ovulatory processes.
ECM remodelling is fundamental to ovarian physiology, coordinating processes such as folliculogenesis, ovulation, and the preservation of ovarian architecture across the reproductive lifespan. During follicle development, ECM components, including collagen, laminin, and FN form a dynamic scaffold that facilitates granulosa cell adhesion, proliferation, and oocyte maturation. MMPs (MMP-2 and MMP-9) play a central role in ECM degradation, enabling follicle expansion and antral cavity formation (Irving-Rodgers and Rodgers, 2005). In the periovulatory phase, the LH surge induces the expression of proteolytic enzymes such as MMPs (Goldman and Shalev, 2004) (Table 3), ADAMTS-1, and plasminogen activators, which mediate follicle wall breakdown and cumulus–oocyte complex (COC) expansion (Robker et al., 2000; Russell et al., 2003) (Fig. 2).
Ovulation itself represents a controlled, injury-like event in which granulosa, theca, and stromal cells respond to ECM degradation and inflammatory cues through tissue remodelling and repair pathways. LH-induced signalling promotes granulosa cell-mediated ECM remodelling and upregulation of angiogenic factors necessary for post-ovulatory tissue repair (Curry and Osteen, 2003; Robker et al., 2018). Concurrently, theca and stromal fibroblasts detect ECM disruption via integrin-FAK and Toll-like receptor pathways, releasing cytokines such as IL-6 and TNF-α that orchestrate immune cell recruitment and neovascularization (Orisaka et al., 2023) (Fig. 2, Tables 2 and 3).
However, ageing profoundly alters ovarian ECM dynamics (Pennarossa et al., 2022) (Table 2). Pathological ECM remodelling arises, characterized by progressive collagen accumulation, increased matrix stiffness, and the onset of fibrosis. These alterations disrupt key mechanotransduction pathways, including integrin–FAK and Hippo–YAP/TAZ signalling, leading to impaired granulosa cell function, reduced protease expression, and diminished follicle activation (Umehara et al., 2018; Amargant et al., 2020) (Tables 1 and 2). Theca cells increasingly activate TGF-β/SMAD-driven fibrotic signalling, while stromal fibroblasts acquire a senescence-associated secretory phenotype, exacerbating chronic inflammation and ECM deposition (Briley et al., 2016) (Table 2). Together, these maladaptive responses transform the ovarian microenvironment from a regenerative to a fibrotic state, accelerating reproductive decline and compromising oocyte quality. These age-associated ECM alterations are recapitulated in models of chemotherapy-induced ovarian damage, underscoring the conserved role of ECM dysregulation in follicular dysfunction and reproductive ageing.
Ovarian tissue remodelling is governed by the intricate interplay between MMPs and TIMPs. An imbalance between their activities leads to excessive ECM deposition, driving ovarian fibrosis subsequent ovarian dysfunction (Curry and Osteen, 2003; Gu et al., 2024; Nikanfar and Amorim, 2025) (Table 2). The ovarian stroma contains HA, another essential component of the ECM, alongside collagen. HA is one of the main ovarian GAGs and plays a crucial role in the formation of a mucoelastic matrix in the COC that develops during the follicular phase of the ovarian cycle. Studies have demonstrated that the increased collagen levels and reduced HA content in the ageing human ovarian matrix are associated with the development of ovarian fibrosis (Amargant et al., 2020) (Table 1). Given that the ECM is central to follicular growth, ovulation and CL remodelling, maintaining its dynamic balance is essential for female ovarian functions.
EMT significantly contributes to fibrotic remodelling by enabling ovarian epithelial or granulosa cells to acquire mesenchymal characteristics, such as increased expression of vimentin and α-SMA, reduced adhesion, and enhanced ECM secretion. In the ovary, EMT is physiologically regulated; for example, granulosa cells exhibit a mesenchymal phenotype during folliculogenesis, and ovulatory rupture involves EMT-like transitions of the ovarian surface epithelium to facilitate tissue remodelling (Bilyk et al., 2017). Pathologically, TGF-β acts via canonical SMAD activation and crosstalk with PI3K/Akt and Wnt/β-catenin-pathways to induce EMT and promote fibrotic ECM accumulation in multiple tissues, likely including the ovary (Gu et al., 2024). In animal models of ovarian hyperfibrosis, pharmacological inhibition of TGF-β signalling (using a TGF-β receptor I inhibitor) has been shown to attenuate fibrotic progression by modulating EMT mediators, enhancing MMP activity and reducing collagen deposition (Wang et al., 2018) (Tables 2 and 3).

Important signalling pathways in ovarian fibrosis

Transforming growth factor-beta
TGF-β is recognized as an essential regulator of both physiological and pathological tissue repair mechanisms. Various cell types, including platelets, monocytes, macrophages, epithelial cells, and fibroblasts, demonstrate increased TGF-β expression (Györfi et al., 2018) (Table 4). The pro-TGF-β monomer is synthesized in the ribosome and folded in the endoplasmic reticulum. The latent complex associated with the latency-associated peptide (LAP) plays a crucial role in the storage, localization, and subsequent activation of TGF-β in the ECM (Khalil et al., 2017). Upon release from the latent complex and subsequent activation, TGF-β binds to its receptors (Khalil et al., 2017). Binding of TGF-β to TGF-β receptor II and subsequent phosphorylation of TGF-β receptor I initiates both Smad-dependent and Smad-independent signalling cascades that lead to fibroblast proliferation and collagen synthesis (Moustakas and Heldin, 2009) (Tables 1 and 4). Genes of the TGF-β superfamily, which are expressed in a manner specific to mammalian ovarian somatic cells and oocytes, regulate folliculogenesis in the ovary (Knight and Glister, 2006; Santibañez et al., 2011). Additionally, these genes function as essential cytokines in maintaining the CL (Haas et al., 2019; Guo et al., 2022) (Table 2). The TGF-β superfamily comprises genes encoding bone morphogenetic proteins (BMP2, BMP4, BMP5, BMP6, BMP7, and BMP15) as well as growth differentiation factor 9, which is expressed during folliculogenesis and plays a crucial role in regulating follicle growth and development (Santibañez et al., 2011; Xu et al., 2018).
The regulation of fibrotic tissue is controlled by the TGF-β superfamily, interleukins, oxidative stress, and inflammatory cytokines. TGF-β1 induces the overexpression of pro-fibrotic genes, directly activating the Smad signalling pathway and causing fibrosis (Biernacka et al., 2011; Hu et al., 2018; Ghafouri-Fard et al., 2024) (Table 1). TGF-β, comprising three isoforms (TGF-β1, TGF-β2, and TGF-β3), is a cytokine that has important roles in cell differentiation, migration, proliferation, tissue remodelling, morphological changes, and immune regulation. TGF-β1 is primarily expressed in endothelial, haematopoietic, and connective tissue cells, while TGF-β2 is expressed in epithelial and neuronal cells, and TGF-β3 is expressed in mesenchymal cells (Santibañez et al., 2011; Hu et al., 2018; Xu et al., 2018).
All three TGF-β isoforms are known to bind to TGF-β receptor-2 (TGFBR2). Subsequently, TGFBR2 activates TGFBR1, thereby initiating receptor signalling (Meng et al., 2016). TGF-β1 directly activates the SMAD2 and SMAD3 signalling pathways, resulting in the overexpression of profibrotic genes through TGF-β1/SMAD signalling. As evidenced by numerous studies, the dysregulation of the TGF-β1/Smad pathway is a significant pathological mechanism contributing to tissue fibrosis (Xu et al., 2016; Hu et al., 2018; Yu et al., 2022; Li et al., 2023; Wang and Yang, 2025) (Table 4). Smad-2 and Smad-3 have been known to be the two primary downstream regulators that promote TGF-β1-induced tissue fibrosis (Hu et al., 2018). Conversely, Smad7 functions as a negative feedback regulator of the TGF-β1/Smad pathway, thereby protecting against TGF-β1-mediated fibrosis (Hu et al., 2018) (Table 4). Wang et al. administered sitagliptin (a DPP4 inhibitor) in a PCOS rat model and evaluated the effects of the drug on ovarian fibrosis via TGF-β signalling. The experimental results showed that sitagliptin reduced the mRNA levels of CTGF and TGF-β1. The study shows that the TGF-β pathway plays a critical role in the development of ovarian fibrosis (Wang et al., 2019) (Tables 1 and 4).
TGF-β has been observed to activate several other pathways, including the extracellular signal-regulated kinase (ERK), mitogen-activated protein kinase pathways mediated by p38 and JUN N-terminal kinase (JNK), RHO-related kinase (ROCK), and RAC-α serine/threonine protein kinase (AKT) pathways in fibrotic tissues (Distler et al., 2019) (Table 4). The process of activation of TGFβR results in the phosphorylation of Smad2 and Smad3 proteins (Leask and Abraham, 2004; Wynn and Ramalingam, 2012; Hu et al., 2018). Following this process, Smad2/3 interacts with Smad4 to form a trimer (Zhou et al., 2021). Consequently, this heteromeric complex is transported to the nucleus, where it regulates the expression of fibrosis-related genes (Nakao et al., 1997; Xu et al., 2016; Khalil et al., 2017). It is well known that this signalling is also negatively regulated by the Smad7 expression (Hu et al., 2018; Humeres et al., 2022).
In ovarian fibrosis, TGF-β acts as the primary initiator of ECM deposition, but its effects are amplified through crosstalk with Wnt/β-catenin and PI3K/Akt signalling; this interconnected network stabilizes β-catenin, enhances SMAD transcriptional output, and promotes fibroblast survival, creating a self-reinforcing loop of fibrosis and stromal stiffening (Guo et al., 2012; Yokoyama et al., 2012; Zhang et al., 2021) (Table 4).
Numerous studies have reported increased expression and activation of profibrotic TGF-β1 in ovarian fibrosis (Zhou et al., 2021, 2022, 2023; Liu et al., 2022; Morsi et al., 2022; Yang et al., 2024) (Tables 2 and 3). These findings highlight TGF-β1 as a central regulator of fibrogenesis. Overactivation of TGF-β1 increases ECM deposition, leading to tissue stiffening and stromal remodelling. Furthermore, TGF-β1-mediated signalling can profoundly alter ovarian function by modulating key processes such as inflammation, cellular differentiation, and myofibroblast activation (Zhang et al., 2013; Takahashi et al., 2017; Wang et al., 2019; Zhou et al., 2023; Lan et al., 2024) (Table 3).

Connective tissue growth factor
CTGF, a member of the CCN family, is a multifunctional protein that was first identified in human vascular endothelial cells in 1991 (Zhou et al., 2017) (Tables 1 and 4). It is involved in cell proliferation, differentiation, EMT, apoptosis, angiogenesis, and ECM remodelling. Recent studies have shown that CTGF is expressed in various cell types, including chondrocytes, fibroblasts, muscle cells, tumour cells, epithelial cells, central nervous system cells, and some cancer cell lines (Ihn, 2002; Nagashima et al., 2011; Chung and Han, 2022) (Tables 2 and 4).
CTFG has also been demonstrated to play crucial role in follicular development. Nagashima et al. investigated the processes of ovulation and folliculogenesis in a CTFG knockout mouse model, and their results showed that CTGF-deficient mice exhibited deregulated steroidogenesis, characterized by elevated serum progesterone levels. This hormonal imbalance was associated with subfertility, impaired follicular development, reduced ovulation rates, and an increased number of CL (Nagashima et al., 2011).
CTGF, a downstream effector of TGF-β, is highly expressed and shares many biological functions with TGF-β1 (Zhou et al., 2017; Wang et al., 2019) (Tables 1 and 4). The functional relationship between CTGF and fibrosis-related growth factor TGF-β has been highlighted in several studies (Yang et al., 2012; Kennedy et al., 2018). In rat ovarian tissue, CTGF expression has been shown to be linked to the TGF-β1/Smad2/3 signalling pathway, which is associated with tissue fibrosis (Wang et al., 2019) (Table 3). The expression of CTGF is regulated by multiple factors, including growth factors, hormones, and cytokines. Wang et al. (2019) investigated ovarian fibrosis in a DHEA-induced PCOS rat model and found that it was mediated by the TGF-β signalling pathway. In this model, fibrotic biomarkers were analysed using the TGF-βRI inhibitor (SB431542). The results demonstrated increased expression of fibrin and collagen in DHEA-induced ovaries, as well as upregulation of fibrosis markers including TGF-β, CTGF, FN and α-SMA. The inhibitor SB431542 significantly decreased the expression of pro-fibrotic molecules (TGF-β, Smad3, Smad2, α-SMA) and increased the anti-fibrotic factor MMP2 (Wang and Khalil, 2018). This study supports the concept that tissue fibrosis results from interactions between TGF-β1 and CTGF.
TGF-β1 has been shown to modulate the cellular responses of monocytes, neutrophils, and lymphocytes, while simultaneously inducing CTGF expression in these cells. During fibrosis, elevated TGF-β1 expression further enhanced CTGF levels, leading to increased ECM protein production. These interconnected signalling pathways collectively promote collagen accumulation within the ECM and accelerate the progression of fibrotic remodelling (Zhou et al., 2017) (Table 1).

Vascular endothelial growth factor
Angiogenesis is a vital process in the female reproductive system that promotes tissue growth and development. It is essential for the transport of oxygen, nutrients, and hormones to the ovary (Kaczmarek et al., 2005). EGF promotes ovarian angiogenesis by increasing vascular permeability and stimulating endothelial cell proliferation and migration (Geva and Jaffe, 2000; Zhao et al., 2024) (Table 2). It also protects cells against apoptosis by inducing antioxidant and anti-apoptotic proteins. Moreover, vascular endothelial growth factor (VEGF) plays a crucial role in follicular growth, CL formation, and its maintenance throughout reproductive life (Yeh et al., 2008; Qiu et al., 2012; Morsi et al., 2022; Guzmán et al., 2023) (Tables 1, 2, and 4).

Pala et al. (2015) evaluated the dose-dependent effects of tamoxifen on ovarian histopathology, serum VEGF, and endothelin-1 levels in a rat model of ovarian hyperstimulation syndrome (OHSS). Tamoxifen administration significantly increased serum VEGF levels, CL angiogenesis, and ovarian fibrosis, while markedly reducing in ovarian follicular reserve. These findings suggest that tamoxifen may exacerbate OHSS-related hypoxia and follicular depletion in a dose-dependent manner through increased VEGF expression (Pala et al., 2015). Ali et al.(2020) investigated the effects of insulin and metformin treatments on ovarian VEGF and TGF-β expression in a diabetic rat model. Their results revealed pronounced structural alterations in the ovarian cortex and medulla of diabetic rats. Collagen fibre density and organization were significantly increased, whereas follicle count and mean follicular diameter were decreased. Moreover, a notable rise in the number of atretic follicles and CLs was observed. Immunohistochemical analyses showed decreased VEGF expression and elevated of TGF-β expression in diabetic rats' ovaries. When comparing treatment outcomes, both insulin and metformin significantly reduced TGF-β expression and collagen deposition, but insulin exerted more pronounced beneficial effects on ovarian morphology, modulating both VEGF and TGF-β expression (Ali et al., 2020) (Table 4). Morsi et al. (2022) examined the effects of stevia leaf extract (SLE) on VEGF, TGF-β, and ovarian fibrosis in a letrozole-induced PCOS rat model (Table 1). They compared the modulatory effects of SLE with metformin, revealing that PCOS induction caused cystic follicular degeneration characterized by attenuated granulosa cell layers and hypertrophied theca layers, along with increased ovarian fibrosis and altered expression of angiogenic and fibrotic markers, including VEGF and TGF-β. Treatment with SLE, metformin, or their combination alleviated these histological abnormalities, restoring granulosa cell architecture and reducing theca cell thickening. These findings indicate that SLE may regulate ovarian remodelling and serve as a potential adjunct or alternative to metformin in ameliorating PCOS-related ovarian dysfunction (Morsi et al., 2022).
Collectively, the studies by Morsi et al. and Ali et al. demonstrated that of VEGF and TGF-β expression levels are increased in PCOS and diabetic mouse models (Ali et al., 2020; Morsi et al., 2022) (Table 1). Therapeutic interventions such as metformin, SLE, and insulin have been shown to reverse ovarian fibrotic histomorphology. Experimental findings suggest that these treatments attenuate fibrosis progression, primarily through the downregulation of VEGF and TGF-β expression. However, further investigations are warranted to clarify the dose-dependent efficacy of these interventions and to evaluate their translational potential in clinical applications.

Ageing and inflammation in ovarian fibrosis
The ovarian stroma, a critical somatic compartment, supports folliculogenesis, undergoes progressive inflammation and fibrotic remodelling with advancing age (Rowley et al., 2020). A crucial role of the female reproductive system is its ability to respond rapidly to inflammatory stimuli, thereby enabling the ovary to preserve its functional integrity (Wynn and Ramalingam, 2012). Ageing and environmental factors can lead to in heightened ovarian inflammation which may progress to chronic inflammation or fibrosis. Persistent inflammation reduces the ovarian reserve and contributes to the development of ovarian fibrosis by damaging tissues and organs (Winkler et al., 2024; Zeng et al., 2024). Inflammatory cytokines, such as IL-1 and IL-6, are overexpressed in aged mice compared with younger counterparts, whereas anti-inflammatory mediators, including cortisol and IL-10, are downregulated. This dysregulation disrupts fibroblast homeostasis, leading to excessive ECM production (Winkler et al., 2024; Zeng et al., 2024) (Table 1).

Ageing and ovarian fibrosis
Accumulating evidence indicates that organs of the female reproductive system, particularly the ovaries, are among the earliest to exhibit an age-related decline. Ovarian ageing has been mechanistically linked to elevated levels of oxygen species (ROS), mitochondrial dysfunction, ATP depletion, DNA damage accumulation, and elevated cellular stress responses (Broekmans et al., 2009; Liang et al., 2023; Zeng et al., 2024) (Table 3).
Ovarian ageing is characterized by a progressive decline in ovarian reserve and oocyte quality which in turn leads to reduce ovarian function and fertility (Briley et al., 2016; Zeng et al., 2024). Lliberos et al. (2021) demonstrated that the numbers of primordial, primary, secondary, and antral follicles decrease with age, accompanied by an increase in pro-inflammatory gene expression. Inflammasome-related genes and proteins were found to be upregulated, indicating an age-associated inflammatory response in mice (Lliberos et al., 2021) (Tables 2 and 4). Briley et al. (2016) also examined the accumulation of collagen types I and III in ovarian tissues in reproductive-aged mice, and reported collagen deposition around of follicles, the ovarian surface epithelium and blood vessels. Their findings indicate that the amount of collagen accumulation and fibrotic tissue formation increase with age (Briley et al., 2016; Amargant et al., 2020; Mara et al., 2020; Umehara et al., 2022) (Table 2).
An important feature of ovarian ageing is mitochondrial dysfunction within the ovarian stroma, which is closely associated with various intracellular stress pathways. Specifically, oxidative stress activates both pro-inflammatory (M1) and anti-inflammatory (M2) immune responses, leading to a disruption of the immune homeostasis. As a result, fibrotic collagen proteins accumulate, and the stromal architecture undergoes remodelling (Zeng et al., 2024). The literature further indicates that IL-6 expression increases with age, promoting collagen deposition and fibrosis (Lliberos et al., 2021) (Table 2). Supporting these findings, Amargant et al. (2020) investigated the impact of fibrosis on ovarian biomechanics and its underlying mechanisms (Table 2). The researchers demonstrated that the age-related collagen accumulation results in ovarian stiffness, as assessed by the instrumental indentation method. Nanoindentation and microindentation techniques are increasingly employed to evaluate the mechanical properties of ovarian tissue, such as stiffness (elastic modulus) and viscoelastic behaviour (Amargant et al., 2020). These methods provide valuable insight into ovarian biomechanical alterations associated with physiological or pathological conditions, including ageing, fibrosis, PCOS, or OHSS (Li et al., 2007; Méndez et al., 2022; Sumbul et al., 2022; Stewart et al., 2023) (Table 2). Their analytical findings revealed that the ovaries from reproductive-age mice required ∼2.5 times more force for deformation than those from younger mice, implying that ovarian stiffness increases with age. In addition to collagen, which is a major contributor to ovarian stiffness, the researchers also quantified HA content in young and aged ovaries. A significant decline in HA levels was observed with advancing age, corresponding to increased hyaluronidase (Hyal1) expression and reduced HA synthase (Has3) expression (Amargant et al., 2020) (Table 2).

Inflammation and ovarian fibrosis
Ovarian tissue contains numerous immune cells, such as adaptive lymphocytes (T and B cells), monocytes, macrophages, eosinophils, and natural killer cells (Shen et al., 2023; Zhu et al., 2025). Immune cells provide essential physiological support to the ovary through phagocytosis, antigen recognition, secretion of inflammatory mediators, and ECM remodelling (Shen et al., 2023; Zhu et al., 2025). Inflammation plays a crucial role in many physiological reproductive processes including menstruation, embryo implantation, and onset of labour (Lliberos et al., 2021; Shen et al., 2023). Defects in immune cell regulation can lead to chronic inflammation and ovarian dysfunction (Shen et al., 2023). Fibrosis, characterized by excessive deposition of ECM in tissue, is closely associated with inflammation. Recurrent cycles of inflammation and the repair during ovulation may act as triggers for the development of ovarian fibrosis. Reproductive ageing is also linked to a pro-inflammatory ovarian microenvironment (Rowley et al., 2020; Lliberos et al., 2021; Shen et al., 2023; Zeng et al., 2024) (Tables 1 and 2). The expression of pro-inflammatory cytokines, such as IL-1β, IL-6, and TNF-α, which possess pro-fibrotic effects, is elevated in the ovaries of ageing mice. In addition, the presence of multinucleated giant macrophages has been observed in ovarian stroma. Consequently, researchers proposed that increased immune cell proliferation in the ageing ovary contributes to the development of ovarian fibrosis (Briley et al., 2016; Zeng et al., 2024) (Table 2).
The recent findings indicate a strong correlation between ovarian fibrosis and ovarian ageing. In support of this hypothesis, a study analysing inflammatory markers and immune cell populations in the ovaries of C57BL/6 mice demonstrated that the decline in follicle number and quality during reproductive life was accompanied by an increase in ovarian CD4+ T cells, B cells, and macrophages. Furthermore, significant increases in serum levels and mRNA expression of pro-inflammatory cytokines and inflammasome-related genes, including IL-1α/β, TNF-α, IL-6, ASC and NOD-like receptor family, pyrin domain containing 3 (NLRP3), were observed with advancing age (Lliberos et al., 2021). Consistent with these observations, McCloskey et al. (2020) examined the cell populations associated with chronic inflammation in ovarian fibrosis using immunohistochemical staining for CD8, CD4, and FOXP3 markers. Their analysis revealed the presence of fibrotic tissue in senescent ovaries and a marked increase in T cell population expressing CD8, CD4, and FOXP3 (McCloskey et al., 2020; Lliberos et al., 2021). To further investigate the effects of age-related inflammation on ovarian fibrosis, Zhang et al. (2020) analysed macrophage populations in young and old mouse ovaries. Macrophages are the most abundant immune cells in ovarian tissue, and the activity of M2 macrophages is known to be associated with chronic inflammation and tissue fibrosis (Briley et al., 2016). The study revealed that although the total macrophages population decreased in aged ovaries, the proportion of M2 macrophages increased, reflecting a shift towards a pro-fibrotic phenotype (Zhang et al., 2020).
Additionally, analysis of CSF1 and CSF2 cytokines, which play an important role in the macrophages' proliferation and maturation, showed significantly higher gene expressions in the ovaries of 18-month-old mice compared to those of 2- and 6-month-old mice (Lliberos et al., 2021). These data are consistent with the findings reported by McCloskey et al. (2020). In another study, Umeraha et al. (2022) demonstrated that fibrotic remodelling in the ovarian stroma causes oxidative damage, inflammation, and collagen deposition due to mitochondrial dysfunction. Treatment of isolated stromal cells with pirfenidone and the antifibrotic drug O-[3-piperidino-2-hydroxy-1-propyl]-nicotinic amidoxime (BGP-15) alleviated ovarian fibrosis. This therapeutic approach improved mitochondrial bioenergetics, reduced mitochondrial ROS production, and suppressed inflammatory mediators such as IL-6, TGF-β, and IL-4 (Umehara et al., 2022) (Table 3).
Existing literature also demonstrates that the accumulation of low-molecular-weight HA fragments in ageing tissues may act as a trigger for inflammation. Rowley et al. (2020) showed that these fragments induce an inflammatory response in the ovarian stroma, contributing to female reproductive senescence by impairing oocyte quality.

NOD-like receptor family, pyrin domain containing 3
The nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing 3 (NLRP3) inflammasome is a member of the Nod-like receptor family and a key component of the innate immune system. It is activated by caspase-1, which subsequently promotes the production pro-inflammatory cytokines, including interleukin-18 (IL-18) and interleukin-1β (IL-1β) (Lliberos et al., 2021; Moustakli et al., 2025) (Table 1). Several factors have been identified as potential triggers of NLRP3 activation, including excess ATP, glucose, oxidative stress, elevated androgen levels, and obesity. Furthermore, NLRP3 activation induces caspase-1-dependent pyroptosis in microglial cells, cardiomyocytes, and mesangial cells (Navarro-Pando et al., 2021) (Table 3). Recent studies have demonstrated that the inhibition of caspase-1, apoptosis-associated speck-like protein containing a CARD (ASC) (Zheng et al., 2023) (Table 3) and NLRP3 significantly reduces the expression of TGF-β, collagen I, tissue inhibitor of metalloproteinase 1 (TIMP1) and hyaluronic acid. These findings suggest that such inhibition may prevent or attenuate the progression of fibrosis (Wang et al., 2020, 2024; Lliberos et al., 2021) (Tables 1 and 4). Although NLRP3 activation is typically associated with tissue injury and inflammation, it has also been implicated in persistent inflammatory responses within ovarian tissue.

Wang et al. (2020) investigated the role of NLRP3 inflammasome signalling in the pathogenesis of ovarian fibrosis. In a testosterone-induced PCOS mouse model, ovarian expression of α-SMA was significantly elevated compared to controls (Wang et al., 2020) (Table 1). Treatment with INF39, a selective NLRP3 inhibitor, effectively attenuated this increase, suggesting a functional link between NLRP3 activation and fibrotic remodelling. Moreover, theca cells exposed to elevated NLRP3 signalling exhibited upregulated expression of key pro-fibrotic mediators, including α-SMA, CTGF, TGF-β, β-catenin, and collagen I, thereby implicating these cells as active participants in fibrosis development. These findings highlight theca cells as central mediators of ovarian stromal fibrosis and suggest their potential as therapeutic targets for antifibrotic interventions (Wang et al., 2020). Additionally, NLRP3 expression has been shown to increase with age in mammalian ovaries, further supporting its role in age-associated inflammatory and fibrotic processes (Navarro-Pando et al., 2021) (Table 4). Liberos et al. (2021) evaluated NLRP3 activation at the mRNA level in ovarian tissue from young and aged mice. To further clarify its functional relevance, IL18 and CASP1 expression levels were also assessed. The results revealed that NLRP3 expression was significantly higher in aged ovaries compared with young one. Moreover, IL-18 and CASP-1 expressions were markedly increased in older mouse ovaries, reinforcing the link between NLRP-3 activation and ovarian inflammation (Lliberos et al., 2021). Similarly, age-dependent analyses of murine ovaries revealed a significant increase in inflammatory markers and immune cell infiltration with advancing age. Specifically, pro-inflammatory cytokines such as interleukin-1α/β (IL-1α/β), TNF-α and interleukin-6 (IL-6), along with inflammasome-associated genes including apoptosis-associated speck-like protein containing a CARD (ASC) and NLRP3, were markedly upregulated. These molecular changes coincided with a decline in follicle number, indicating a mechanistic link between chronic inflammation, immune activation, and follicular depletion during ovarian ageing (Moustakli et al., 2025) (Table 3). Further analysis conducted on the ovaries of 2-, 6-, 12, and 18-month-old C57BL/6 female mice demonstrated that expression levels and mRNA contents of IL-1α/β, TNF-α, IL-6, and inflammasome-related Asc and Nlrp3 genes were significantly elevated in correlation with the age-related decline in follicle numbers (Zeng et al., 2024). Umehara et al. (2018) demonstrated that fibrotic remodelling in the ovarian stromal leads to oxidative damage, inflammation, and collagen deposition as a result of mitochondrial dysfunction. Similarly, Sun et al. (2023) showed that Aristolochic acid I (AAI) enhances inflammation through the NLRP3 signalling pathway and causes ovarian dysfunction due to mitochondrial damage. Collectively, these inflammatory responses impair follicular development by promoting ovarian fibrosis (Gu et al., 2024).

Peroxisome proliferator-activated receptor-γ
Peroxisome proliferation-activated receptors-γ (PPAR-γs) are nuclear hormone receptors that belong to the steroid receptor superfamily. PPAR-γs play a crucial role in regulating several processes, including steroidogenesis, angiogenesis, glucose and lipid metabolism, cell cycle, cell differentiation, apoptosis, and inflammatory responses (Komar, 2005) (Tables 1 and 4). In addition, these receptors are involved in processes such as inflammation, wound healing, and embryo implantation. While PPAR-γs are widely expressed in the liver, kidneys, and adipose tissue, members of the three PPAR isoforms, including alpha (PPAR-α), delta (PPAR-δ), and gamma (PPAR-γ), are also expressed in the ovary. The importance of PPAR function in ovarian physiology is highlighted by its regulation by LH (Zaree et al., 2015). PPAR-γ expression is induced during the early stages of folliculogenesis and remains detectable in granulosa cells through follicle development (Froment et al., 2003; Faut et al., 2011). Members of the PPAR family have been shown to play a role in tissue remodelling during the follicular and luteal phases of ovarian cycle (Komar, 2005) (Table 1), in addition to regulating the expression of proteases associated with angiogenesis (Faut et al., 2011; Vitti et al., 2016). The critical role of PPAR was further demonstrated using Cre/LoxP technology to specifically delete PPAR expression, which resulted infertility in mice (Cui et al., 2002) (Table 4).
PPARs also regulate the expression of proteinases involved in tissue regeneration and angiogenesis. In a different study, enhanced activation of PPAR-α and PPARγ was found to reduce MMP-9 expression, indicating a potential anti-fibrotic mechanism (Komar, 2005; Bogacka et al., 2015). Prabhu and Valsala Gopalakrishnan (2020) confirmed that linolenic acid attenuates fibrotic tissue formation in PCOS rat ovaries via the PPAR-γ signalling pathway, demonstrating the anti-fibrotic potential PPAR-γ. Specifically, increased PPAR-γ expression promotes ECM regeneration, regulates myofibroblast differentiation, and prevents fibrotic tissue deposition by suppressing the TGF-β/SMAD signalling pathway (Prabhu and Valsala Gopalakrishnan, 2020).
In the literature, therapeutic approaches for ovarian fibrosis have primarily focused on reducing collagen accumulation, suppressing inflammation, and inhibiting fibrosis-related signalling pathways. The anti-fibrotic, anti-inflammatory, and ECM regenerating properties of Stevia rebaudiana leaf extract, Irpex lacteus mushroom polysaccharides, Sinapic acid, γ-linolenic acid, and AAI have been studied for their potential effects on ovarian fibrosis (Prabhu and Valsala Gopalakrishnan, 2020; Morsi et al., 2022; Zhou et al., 2022, 2023; Sun et al., 2023; Lan et al., 2024) (Tables 3 and 4).
Rhamnocitrin (Rha, kaempferol-7-O-methylether) is a flavonoid molecule with antioxidant, anti-inflammatory, and anti-tumour properties, found in a variety of plant sources. Given its documented beneficial effects on the reproductive system, antifibrotic effects of Rha were investigated on PCOS rat models. The primary objective of this study was to examine the antifibrotic effects of Rha on ovarian tissue and to purpose it as a possible alternative to metformin, a drug commonly used to regulate insulin levels in PCOS patients. Masson trichrome staining revealed that collagen accumulation was significantly lower in the Rha-treated group compared to the metformin-treated group Zhou et al. (2022). Similar results were reported in additional studies showing that Rha treatment inhibited activation of the TGF-β1/Smad pathway while increasing PPAR-γ expression, thereby providing new therapeutic perspectives for ovarian fibrosis (Zhou et al. 2023) (Table 3). Synaptic acid (SA) has been reported to possess antioxidant, anticancer, intestinal barrier-protective, anti-inflammatory, immunomodulatory, antifibrotic, antimicrobial, and lipid metabolism-improving properties. In a study by Lan et al. (2024), the effects of SA, a polyphenolic compound, on ovarian fibrosis were investigated based on its multifaceted bioactivities. The results revealed that, consistent with previous studies, SA reduces TGF-β1/Smad expression and downregulates fibrotic markers including collagen I, α-SMA, and CTGF in PCOS rat ovaries. The study concluded that SA may serve as a promising alternative treatment for ovarian fibrosis by regulating metabolic activity and significantly reducing oxidative stress in letrozole-induced PCOS rats (Lan et al., 2024) (Table 3).

Environmental factors increasing inflammation
Several female reproductive disorders have been identified as risk factors for the development of ovarian fibrosis, including PCOS, POI, ovarian ageing, endometriosis, and chocolate cyst ovarian cancer (Gu et al., 2024). In addition to these conditions, elevated levels of inflammatory cytokines such as ROS, IL-6, IL-8, along with accumulation of ECM proteins and excessive activation of TGF-β and NF-κB signalling pathways, have been shown to trigger inflammation and promote ovarian fibrosis. Furthermore, endometriosis, radiotherapy, and surgical interventions have been identified as additional contributing factors to ovarian fibrosis (Plikus et al., 2021; Umehara et al., 2022; Garcia Garcia et al., 2023; Gu et al., 2024; Guo et al., 2024) (Table 2). Importantly, beyond these medical and physiological causes, environmental factors also play a significant role in enhancing inflammatory responses. Specifically, exposure to environmental chemicals, pollutants, food contamination, and chronic stress can exacerbate of inflammation and accelerate the progression of ovarian fibrosis (Zhou et al., 2021; He et al., 2023; Ma et al., 2024; Yang et al., 2024) (Tables 1, 2, and 4). Zhao et al. (2021) examined the impact of heavy metal exposure on ovarian homeostasis and oocyte quality. Heavy metals, a common environmental contaminant, are also a pervasive component of modern life. This experiment involved the intraperitoneal injection of 6-week-old mice with nickel sulphate, the dose of which was administered in a dose-dependent manner, to ascertain the level of exposure in the environment (Zhao et al., 2021) (Table 4). The subsequent analysis focused on the localization of inflammation-related proteins within the ovaries. The results revealed that the expression of TNF-α, a pro-inflammatory cytokine, and IL-10, an anti-inflammatory cytokine, exhibited dose-dependent expression pattern in the ovaries of mice exposed to nickel sulphate. Specifically, TNF-α expression increased in the group exposed to 20 mg/kg nickel sulphate, while IL-10 expression decreased in comparison to the other groups (Zhao et al., 2021). Furthermore, while the number of preovulatory follicles diminished in comparison to the control group, the number of atretic follicles increased significantly in the groups exposed to nickel sulphate. The experimental results demonstrated that nickel sulphate induced apoptosis in granulosa cells and ovarian fibrosis by triggering inflammation through the TGF-β and NF-κB signalling pathways (Zhao et al., 2021) (Tables 1 and 4). To support these findings, ovarian tissues were analysed by Sirius red staining, which revealed that collagen accumulation was increased in a dose-dependent manner. In addition to this finding, it was demonstrated that nickel sulphate doses resulted in higher levels of MMP2, α-SMA, and TGF-β expression compared to the other groups. Moreover, nickel sulphate exposure increases ROS levels and induces DNA damage. Consequently, it has been demonstrated that nickel sulphate adversely affects ovarian homeostasis by disrupting the structure and function of mitochondria in oocytes (Zhao et al., 2021).
A subsequent study examined the effects of exposure to atrazine (ATR) on ovarian fibrosis and embryo development. ATR is a widely used herbicide that contaminates groundwater, streams, and rivers, posing a significant environmental hazard. The experimental model was designed to evaluate the effects of ATR on the oestrous cycle, follicular development, inflammatory mediator expression, and fibrosis-associated proteins, as well as oocyte maturation and early embryonic development. Experimental results revealed that ATR exposure hinders follicle development and adversely affected reproductive capacity in mice. Western-blot analysis showed increase protein levels of IL-1β, TNF-α, p-NF-κB, NF-κB, and IL-6 in the ovaries of ATR-exposed mice. These findings indicate that ATR increases the inflammatory response in the ovary, causes α-SMA accumulation, and may lead to ovarian fibrosis by activating TGF-β signalling (Yang et al., 2024) (Tables 1 and 3).
In addition to pollutant and chemical exposure, prolonged chronic stress has also been implicated ovarian dysfunction. To investigate this, female C57BL/6 mice aged 6, 9, and 12 months were subjected to 8 months of chronic mild stress at sexual maturity and continuing through decreased fertility. The animals were exposed to various stressors, including water deprivation, starvation, tail pinching, continuous lighting during the night, immersion in cold water immersion, and hot-air exposure. The experimental outcomes demonstrated that chronic unpredictable mild stress (CUMS) accelerated follicular atresia and increased α-SMA and collagen I expression in ovarian tissue, both of which are dysfunctional and contribute to fibrotic remodelling within the ovary (Ma et al., 2024).

Clinical perspective of ovarian fibrosis
Ovarian fibrosis, from a clinical standpoint, refers to the excessive accumulation of fibrous connective tissue in the ovaries, often as a result of chronic inflammation, injury, or underlying pathological conditions. While not a widely recognized standalone diagnosis, it is commonly associated with conditions such as PCOS, endometriosis, or repeated ovarian surgeries, which can lead to scarring and fibrotic changes. Conditions like endometriosis or pelvic inflammatory disease trigger prolonged inflammatory responses that activate fibroblasts and promote collagen deposition (Oală et al., 2024) (Tables 2 and 3).
In PCOS, hyperandrogenism and insulin resistance may contribute to stromal hyperplasia and fibrosis in the ovarian cortex. Procedures such as ovarian cystectomy or repeated IVF oocyte retrieval can cause mechanical injury, resulting in scar tissue formation. Although fibrosis is commonly linked to pathological conditions, the observed increase in ovarian fibrosis with age in healthy control animals likely reflects normal physiological ageing, characterized by stromal remodelling and reduced follicular activity, rather than a pathological process (Goldzieher and Axelrod, 1963; Leung and Yuen, 2006; Nezhat et al., 2024; Zeng et al., 2024).
Ovarian fibrosis in PCOS is closely intertwined with chronic inflammation, advanced glycation end products (AGEs), and hyperandrogenism, forming a complex pathogenic network that exacerbates ovarian dysfunction. Chronic inflammation in PCOS promotes the activation of fibroblasts and their differentiation into myofibroblasts, resulting in excessive ECM deposition and fibrotic remodelling of the ovarian stroma (Plikus et al., 2021; Gu et al., 2024). AGEs, which accumulate in metabolic disorders such as PCOS, contribute to oxidative stress and inflammatory responses by binding to their receptor (RAGE), thereby further stimulating fibroblast activation and ECM synthesis (Gu et al., 2024). Hyperandrogenism, a hallmark of PCOS, not only disrupts normal folliculogenesis but also enhances inflammatory and fibrotic pathways by upregulating profibrotic markers such as TGF-β and CTGF, as well as activating inflammasomes like NLRP3, which promote fibrotic signalling pathways (Wang and Khalil, 2018; Wang et al., 2020; Gu et al., 2024). Collectively, these factors create a self-perpetuating cycle of inflammation, metabolic disturbance, and fibrotic tissue remodelling, contributing to impaired ovarian function, follicular depletion, and infertility associated with PCOS (Zhou et al., 2017; Gu et al., 2024).
The clinical manifestations of ovarian fibrosis are nonspecific and often overlap with those of the underlying condition. Patients may experience irregular menstrual cycles or amenorrhea, pelvic pain or discomfort, especially if associated with adhesions or endometriosis, infertility (due to impaired follicular development or ovulatory dysfunction), and hormonal disturbances, such as elevated androgens or altered oestrogen levels (Leung and Yuen, 2006; Zeng et al., 2024).
Ovarian fibrosis is challenging to diagnose directly, as it is typically identified incidentally or inferred through imaging and clinical correlation. Ultrasound may reveal thickened ovarian stroma, as seen in PCOS, or irregular ovarian architecture suggestive of scarring. MRI offers better visualization of fibrotic tissue and adhesions, particularly in endometriosis-related cases. Laparoscopy is the gold standard for confirming fibrosis, allowing direct visualization of scar tissue or adhesions. Biopsy (rarely performed) for histopathology investigation can confirm excessive collagen deposition, though this is invasive and not routine (Leung and Yuen, 2006; Zeng et al., 2024).
Fibrosis can reduce ovarian reserve and impair follicular maturation, which can lead to infertility, complicating both natural conception and assisted reproductive technologies. Altered ovarian structure may disrupt steroidogenesis, worsening conditions like PCOS. Fibrotic changes, especially with adhesions, can also contribute to persistent pelvic pain. Together, these features represent key clinical consequences of ovarian fibrosis that profoundly affect women’s reproductive health and quality of life (Gu et al., 2024; Zeng et al., 2024).
Ovarian fibrosis has been implicated as a contributing factor in the pathophysiology of POI, a disorder characterized by diminished ovarian reserve and early loss of fertility. Fibrotic remodelling of the ovarian stroma disrupts the microenvironment necessary for folliculogenesis, leading to follicle depletion and impaired oocyte quality, which are hallmark features of POI (Gu et al., 2024). Chronic inflammation and oxidative stress within the ovary further exacerbate fibrotic changes by promoting myofibroblast activation and excessive ECM deposition, further impairing ovarian function (Wang et al., 2021; Bendarska-Czerwińska et al., 2022; Umehara et al., 2022) (Tables 2 and 3). Although direct evidence linking fibrosis as a primary cause of POI remains limited, the presence of stromal fibrosis correlates with deteriorating ovarian function and may represent a key pathological mechanism underlying POI progression (Du et al., 2023; Gu et al., 2024). Therapeutic strategies targeting fibrosis, such as modulation of TGF-β signalling or employing antifibrotic agents, hold promise in preserving ovarian function and delaying the onset of POI (Wang et al., 2019; Cui et al., 2020) (Tables 1 and 3). Therefore, ovarian fibrosis represents a critical target for understanding and potentially mitigating the development POI.

Treatment of ovarian fibrosis
Treatment focuses on addressing the underlying aetiology rather than fibrosis itself, as reversing established fibrosis is limited even with surgical intervention (Roman, 2018; Martire et al., 2025), medical therapy including hormonal treatments, antifibrotic drugs, stem cell-based therapies, exosomes, immunomodulators, and approaches targeting biological signalling pathways which have been explored for managing ovarian fibrosis (Amargant et al., 2020; Cui et al., 2020; Gu et al., 2024). Surgical procedures such as adhesiolysis or removal of endometriotic lesions may alleviate symptoms, though surgery risks further scarring (Roman, 2018; Martire et al., 2025). Hormonal treatments (e.g., oral contraceptives for PCOS or GnRH agonists for endometriosis) aim to suppress inflammation and disease progression. In PCOS-associated fibrosis, lifestyle modification and insulin-sensitizing agents (e.g., metformin) may mitigate stromal changes. The investigation of the prevention and reversibility of fibrotic tissue formation demonstrates the potential of pharmacological treatment to yield beneficial outcomes. Notably, Umehera et al. (2022) have been at the frontline of this research, showing that acute ovarian fibrosis can be reversed through pharmacological targeting of multiple mechanisms. Their findings revealed that pharmacological intervention can restored healthy follicular development and ovulation, highlighting a promising avenue for ovarian fibrosis management (Umehara et al., 2022) (Table 3).
As previously stated, mitochondrial dysfunction, a major driver of chronic inflammation, triggers cellular stress and promotes fibrotic processes. BGP-15 has been identified as a potential therapeutic agent that enhances mitochondrial function in several disease models (Sumegi et al., 2017; Horvath et al., 2021). A study investigating the effects of BGP-15 on the ovaries of both young and aged mice found that, while BGP-15 had no significant effect on the number of ovulated oocytes in young mice, it reversed ovarian fibrosis in aged mice and promoted ovulation. They concluded that the inhibition of fibrotic signalling pathways holds substantial potential for treating ovarian fibrosis (Umehara et al., 2022). The therapeutic potential of targeting biological signalling pathways in the treatment of ovarian fibrosis has yielded encouraging outcomes. For instance, Yu et al. (2024) demonstrated that mesencephalic astrocyte-derived neurotrophic factor (MANF) reduced inflammatory response in PCOS by suppressing the TLR4–NF-κB–NLRP3 pathway, emphasizing the importance of modulating inflammatory signalling pathways in the treatment of ovarian fibrosis.
Pirfenidone, a drug approved by the FDA for pulmonary fibrosis, exhibits antifibrotic activity primarily by reducing TGF-β1 expression (Meyer and Decker, 2017; Di Martino et al., 2021). In addition, pirfenidone has been shown to inhibit the progression of liver and cardiac fibrosis in various animal models (Sartiani et al., 2022; Silva et al., 2024). Based on these findings, Amargant et al. (2020) examined the effect of pirfenidone on ovarian fibrosis. The study, which involved the gavage of low-dose pirfenidone to 7-month-old mice over a period of 6 weeks, demonstrated the potential of pirfenidone to reverse age-related ovarian fibrosis. Furthermore, it was observed that pirfenidone-treated mice exhibited higher serum anti-Müllerian hormone levels in comparison to the control group (Amargant et al., 2020) (Table 2). In another study, ovarian tissues from transmasculine donors of reproductive age were examined, and drugs with anti-inflammatory and antioxidant properties, such as pirfenidone, metformin, and mitoquinone, were shown to reduce collagen accumulation in the ovarian cortex and improve early folliculogenesis (Del Valle et al., 2025). Similarly, administration of pirfenidone and nintedanib, both with anti-inflammatory and antioxidant activity, effectively prevented pulmonary fibrosis development (Sato et al., 2017; Finnerty et al., 2021). The accumulation of collagen I/III, a hallmark of fibrosis, was evaluated using Picrosirius red staining which revealed that pirfenidone treatment significantly suppressed collagen deposition in the ovarian tissue of aged mice. These findings indicate that pirfenidone may enhance the ovarian microenvironment and promote favourable follicle development by reducing fibrotic tissue in ageing ovaries.
Emerging evidence suggests that therapeutic strategies targeting ovarian fibrosis can preserve ovarian function by mitigating fibrotic tissue accumulation. In this context, transplantation of mesenchymal progenitor cells (MPCs) derived from human embryonic stem cells has been shown to protect fertility and maintain ovarian function in perimenopausal mouse models, likely through antifibrotic and regenerative mechanisms (Shin et al., 2024). In addition to this study, antifibrotic agents such as metformin, rhamnocitrin, and rosiglitazone, as well as p66Shc protein inhibition, have been shown to alleviate ovarian fibrosis (Miao et al., 2012; McCloskey et al., 2020; Wang et al., 2020; Zhou et al., 2022). However, no current therapy has demonstrated consistent efficacy in reversing established fibrotic tissue within the ovary.
Ovarian fibrosis is a secondary clinically significant phenomenon with major implications for reproductive and gynaecological health. Its management hinges on early intervention in the causative condition to prevent progression, as established fibrosis is largely irreversible with current therapies. In these patients, IVF or ovulation induction may be employed, depending on the extent of ovarian damage. A multidisciplinary clinical approach involving gynaecologist and reproductive endocrinologist is essential, tailored to each patient’s individual clinical context.

Conclusions
Ovarian fibrosis is a pathological process characterized by excessive deposition of ECM components, stromal remodelling, and activation of myofibroblasts. Key profibrotic signalling pathways, including TGF-β, CTGF, and VEGF, play central roles in the initiation and progression of fibrotic changes. These molecular events are further exacerbated by ageing, chronic inflammation, oxidative stress, and environmental insults, which collectively contribute to stromal stiffening and impaired ovarian function. The morphological and molecular differences between healthy and fibrotic ovarian tissues are illustrated in Fig. 2.
At the cellular level, fibrosis arises the persistent activation of fibroblasts and myofibroblasts, driven by persistent inflammatory signalling. This process leads to the accumulation of collagens (types I and III), FN and HA, disrupting follicular architecture, and interfering with steroidogenesis and oocyte quality. The fibrotic ovarian microenvironment has been implicated in reproductive pathologies such as PCOS, POI, and possibly ovarian cancer, although causal relationships remain to be fully elucidated.
At the molecular level, ovarian fibrosis is orchestrated by interconnected signalling cascades including TGF-β/Smad, Wnt/β-catenin, and PI3K/Akt pathways, that collectively regulate fibroblast activation, ECM synthesis, and tissue remodelling. In parallel, immune and inflammatory cells, particularly polarized macrophages and inflammasome components (e.g., NLRP3), play critical roles in modulating the fibrotic milieu. These pathways interact with metabolic and environmental stressors, underscoring the multifactorial nature of ovarian fibrosis.
Despite its clinical relevance, ovarian fibrosis currently lacks approved treatments. Existing strategies are largely experimental and focus on targeting inflammation and fibrogenesis to preserve ovarian function. Antifibrotic agents, such as pirfenidone and TGF-β inhibitors, have shown potential in preclinical models by reducing collagen synthesis and fibroblast activation; however, they have yet to undergo rigorous clinical evaluation in reproductive medicine.
In addition to small-molecule inhibitors, several novel therapeutic modalities are being explored: antioxidant therapies (to mitigate ROS-induced fibrotic signalling), stem cell and exosome-based treatments (for tissue regeneration and immune modulation), gene editing technologies (targeting pro-fibrotic genes and pathways), biomaterial scaffolds (to support ovarian tissue repair and revascularization), nanotechnology-based theranostic systems (enabling targeted, cell-specific delivery of antifibrotic agents to minimize systemic toxicity), and microRNA-based therapeutics (designed to regulate fibrosis-associated gene networks and enhance chemoresistance) (Fig. 3).
While cell therapies and ovarian tissue transplantation have shown promise in managing POI, their application to fibrotic ovarian restoration remains under active investigation. Future therapeutic development will require a deeper understanding of fibroblast biology, inflammatory signalling, and ECM dynamics. Importantly, well-designed clinical trials are crucial for translating these experimental approaches into effective interventions.
In summary, a comprehensive understanding of the molecular and cellular mechanisms driving ovarian fibrosis is imperative. Future investigations should focus on delineating precise fibrotic triggers, identifying biomarkers for early detection, and conducting rigorous clinical trials to evaluate antifibrotic therapeutics. Addressing these challenges will be paramount to preserving ovarian function, improving fertility outcomes, and enhancing women’s reproductive health.

Supplementary Material
gaaf058_Supplementary_Data