After the initial injury to the endometrium, immune cells, particularly macrophages, migrate to the damaged area.15,17 They primarily adopt a predominantly profibrotic phenotype within the damaged endometrium. Continuous secretion of inflammatory mediators, including interleukins, tumor necrosis factor-α, and chemokines, promotes fibroblast activation and differentiation into α-smooth muscle actin–expressing myofibroblasts.15,17,18 These cells are the principal effectors of fibrosis, producing excessive collagen types I and III, as well as other extracellular matrix components, which form avascular fibrous bands characteristic of intrauterine adhesions.3,18

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From a wound-healing perspective, TGF-β1 plays a central role.14,19,20 At the molecular level, the activation of TGF-ß1 and Smad2/3 signaling was found in the mechanism of intrauterine adhesions.19,21,22 TGF-β1 promotes myofibroblast differentiation and inhibits matrix metalloproteinases, leading to fibrosis persistence.16,19,22

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In addition, one of the main pathophysiological mechanisms of Asherman's syndrome is the depletion of endometrial resident stem cells, which are necessary for the cyclic regeneration of the basal layer.12,13 This raises doubts about the potential positive outcomes of stem cell therapy.

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Mesenchymal stem cells exert potent immunomodulatory and antifibrotic effects primarily through paracrine mechanisms. Notably, preclinical studies in Asherman's syndrome demonstrated reduced fibrosis and partial reconstruction of endometrial architecture following MSC-based treatments.6,7,21

 

Stem cell dysfunction and impaired endometrial regeneration

Physiological recovery of the endometrium depends on stem cells located in the basal layer.12,13 In Asherman's syndrome, not only is the structural integrity of the endometrium damaged, but also the endogenous regenerative system. Recent studies clearly show that the central determining factor of Asherman's syndrome is precisely the reduction in the number of stem cells in the basal layer.1-3 Disruption of the stem cell niche by chronic inflammation and hypoxia leads to the replacement of tissue regeneration mechanisms with scar formation.13,15,17

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Experimental and clinical observations demonstrate that Wnt–β-catenin signaling is an essential pathway for endometrial stem cell activation and epithelial proliferation.23,24 In Asherman's syndrome, suppression of the syndrome has been observed to reduce tissue regeneration. Eliminated Wnt activity limits β-catenin nuclear translocation and transcription of regeneration-associated genes, thereby locking the tissue in a non-proliferative state.23 In parallel, TGF-ß1 signaling antagonizes Wnt signaling and reinforces fibrotic differentiation by promoting the phosphorylation and degradation of β-catenin, which is essential for repairing the endometrial lining.19,23,24

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These changes are accompanied by a decrease in the expression of HOXA10 and leukemia inhibitory factor (LIF), which are critical for endometrial receptivity.25-27 HOXA10 is required for glandular maturation. At the same time, LIF is responsible for epithelial differentiation. Downregulation of these factors causes regenerative failure and reproductive impairment.25,26

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Together, these changes create a pathological feedback loop in which stem cell loss, niche disruption, and signaling imbalance prolong fibrosis and prevent the restoration of a functional endometrium.3,7,10 This framework highlights that endometrial restoration in regenerative therapies should not only consider cellular replacement but also reprogramming of the molecular microenvironment.

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Epigenetic alterations, epithelial-mesenchymal transition, and hypoxia-driven fibrosis

The main cause of Asherman's syndrome is not only inflammation or stem cell damage.1-3 One factor to consider here is epigenetic changes.28,29 After severe endometrial damage, when the intensity of inflammatory and profibrotic factor release increases, epigenetic modifications also occur.15,19,28 The DNA methylation and histone remodeling that develop at this time fix the stromal and epithelial cells in a fibrogenic phenotype.28-30 Accordingly, fibrosis occurs, which is the leading problem in this disease.28-30 The tissue's regenerative function is suppressed, and recovery of the damaged tissue is limited.13,15,28

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The result of this reprogramming is the pathological activation of the epithelial-mesenchymal transition (EMT) in the endometrium.14,31,32 EMT results in loss of epithelial polarity and adhesion, leading to downregulation of E-cadherin and increased expression of N-cadherin and vimentin.31-33 Experimental models of intrauterine adhesions demonstrate that TGF-β1 signaling is a major inducer of EMT.14,19,31 This is achieved through Smad-dependent and non-canonical pathways to inhibit epithelial gene expression and promote mesenchymal differentiation.14,19,31 Clinically, reduced E-cadherin expression can correlate with certain hysteroscopic findings, such as thinning or complete absence of the endometrial lining.9 On ultrasound, such alterations might present as irregular endometrial surfaces or the formation of scars, giving practitioners a visual cue to the molecular changes occurring during EMT.14,31,32

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Expanding on diagnostic implications, these molecular changes can also serve as biomarkers to improve the diagnosis and prognosis of Asherman's syndrome.9,33 For instance, levels of E-cadherin and expression of N-cadherin and vimentin in tissue samples could serve as indicators of disease progression or treatment efficacy.9,34 Monitoring these changes could help tailor individualized treatment plans, enhancing the connection between molecular mechanisms and clinical practice.33

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It is noteworthy that hypoxia amplifies these processes. Endometrial ischemia resulting from vascular disruption stabilizes hypoxia-inducible factor-1α (HIF-1α), which synergizes with TGF-β signaling to promote EMT and fibrogenesis.35,36 HIF-1α release induces the transcription of profibrotic mediators. This, in turn, improves the expression of mesenchymal genes and changes endometrial cellular architecture.35-37 It is important to highlight those epigenetic changes induced by hypoxia may persist even after revascularization.28,29

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However, there is growing evidence suggesting that both epigenetic and hypoxia-induced changes can be partially reversible. These therapeutic approaches are still being explored and have the potential to find new ways to reduce the progression of Asherman's syndrome.28,29,35 Collaborative efforts highlight why regenerative therapies should be established so that they not only restore cells but also prevent future pathological epigenetic programming.

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Myofibroblast activation and extracellular matrix remodeling failure in Asherman's syndrome

From a pathomorphological perspective, one of the central mechanisms of Asherman's syndrome is the pathological transition of stromal fibroblasts into matrix-producing myofibroblasts.15,17,18 TGF-ß1 induces myofibroblast differentiation, which can occur through the Smad2/3-dependent pathway or non-canonical signaling cascades, such as MAPK, Rho/ROCK, and PI3K/AKT.19,30 It should be highlighted that TGF-ß1 strongly upregulates collagen and fibronectin synthesis while suppressing matrix degradation.15,16,19 When the receptors are activated, phosphorylated Smad complexes enter the nucleus and lead to transcription of fibrogenic genes: encoding α-smooth muscle actin, α-SMA, COL1A1, and COL3A1.18,19

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The reciprocal regulation of matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) is critical in suppressing matrix degradation.15,16 Myofibroblasts form strong integrin-mediated attachments to the extracellular matrix, stimulating the transmission of cytoskeletal tension to the extracellular matrix.14 This induces the contraction of the tissues, as it is the hallmark of dense intrauterine adhesions.3 A progressively dense extracellular matrix enhances myofibroblast activation via integrins, focal adhesion kinase (FAK), and YAP/TAZ signaling in Asherman's syndrome.15,19 Matrix stiffness functions not only as a consequence of fibrosis but also as an active driver of sustained myofibroblast persistence.

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In the endometrium, this remodeling failure has additional consequences beyond structural scarring. Extracellular matrix deposition damages glandular architecture and impairs oxygen diffusion.5,35 As mentioned, hypoxia activates profibrotic signaling, which can also result from compressed microvasculature.35,36 This creates a self-reinforcing circuit in which TGF-β1 signaling, myofibroblast contractility, and extracellular matrix stiffening mutually support one another.15,19,35

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Importantly, a key feature of antifibrotic interventions in Asherman's syndrome is targeting the fibroblast-myofibroblast transition and extracellular matrix remodeling.15,16,19

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Paracrine anti-inflammatory and antifibrotic mechanisms of stem cells in Asherman's syndrome

The therapeutic effects of mesenchymal stem cells in Asherman's syndrome are mostly guided by paracrine signaling rather than direct tissue replacement.8,9,20 As experimental evidence increases, it becomes more obvious that transplanted stem cells show long-term persistence within the endometrium. Stem cell therapies provide better antifibrotic and pro-regenerative effects.7,20 Growth factors, chemokines, and lipid mediators collectively modulate inflammation and lead to epithelial regeneration.7,38

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A central antifibrotic mechanism involves suppression of TGF-β1–dominant signaling. This is the main cause of myofibroblast activation in Asherman's syndrome.6,21,22 MSC-derived factors downregulate TGF-β/Smad signaling and reduce collagen deposition in intrauterine adhesions.38,39 This effect is achieved by the secretion of hepatocyte growth factor, prostaglandin E2, and antifibrotic microRNAs. Altogether, they inhibit profibrotic transcriptional programs.39,40

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Mesenchymal stem cells also have strong immunomodulatory effects that recover endometrial injury.7,20 Through secretion of interleukin-10 and prostaglandin E2, MSCs suppress pro-inflammatory cytokine production. In addition, MSC paracrine signaling supports angiogenic and epithelial repair pathways.6,8,9 Experimental models of Asherman's syndrome demonstrate increased vascular density and glandular restoration following stem cell therapy.6,8,9

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Extracellular vesicles and exosomes represent a concentrated and biologically active component of the mesenchymal stem cell secretome.39,40 These vesicles suppress myofibroblast differentiation with microRNAs and proteins.21,39,40 It also activates collagen synthesis and increases epithelial proliferation. Because exosomes demonstrate antifibrotic and anti-inflammatory effects, they are promising therapeutic methods for endometrial fibrosis.39

Regenerative functional profiles of stem cell–based therapies in endometrial repair

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Stem cell therapy is being investigated for the treatment of Asherman's syndrome. One of the most studied methods remains the bone marrow-derived mesenchymal stem cells.6,7,11 It demonstrates immunomodulatory and antifibrotic effects by suppressing TGF-ß1 signaling.10,11,20,38 Studies have shown that in laboratory rats with intrauterine adhesions, bone marrow stem cells improved endometrial thickness and vascular density.6,21,22 A main regenerative feature of stem cell-based therapies is microenvironmental immunoregulation.7,34 Active stem cells secrete soluble mediators that suppress pro-inflammatory cytokines.11,20,38 In parallel, its antifibrotic signaling is mediated by inhibition of α-SMA expression and downregulation of collagen synthesis.18,19

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Secondly, stem cells express pro-angiogenic and perfusion-restorative activity. Effective regenerative cell therapies produce vascular endothelial growth factor and angiopoietins, thereby stabilizing microvessels in the endometrium.8,9 Menstrual blood-derived stem cells demonstrated strong proliferative and angiogenic capacity as they originate from endometrial tissue.12,41 Preclinical studies show that menstrual blood-derived stem cells reduce fibrosis in injury models.12,41 (Fig.2).

 

FIGURE 2. Mesenchymal stem cell therapy for Asherman's syndrome