Thin endometrium, defined as endometrial thickness below 7 mm on transvaginal ultrasonography, represents a significant clinical challenge in reproductive medicine, affecting approximately 0.6% to 0.8% of patients undergoing assisted reproductive technology cycles.1,2 This condition substantially impairs embryo implantation and pregnancy outcomes, with documented associations including reduced implantation rates, elevated miscarriage risk, and diminished live birth rates.3,4 The etiological spectrum encompasses inflammatory conditions such as chronic endometritis, iatrogenic injuries from repeated curettage procedures, inappropriate pharmacological interventions including clomiphene citrate overuse, and intrinsic anatomical variations limiting endometrial development.5,6 Despite advances in hormonal supplementation strategies, a substantial proportion of patients demonstrate refractory thin endometrium unresponsive to conventional therapeutic approaches, necessitating investigation of alternative regenerative interventions.7

Mesenchymal stem cells have emerged as promising candidates for endometrial regeneration based on their established multipotent differentiation capacity, robust paracrine signaling capabilities, and immunomodulatory properties.8,9 These cells can be harvested from multiple tissue sources including bone marrow, umbilical cord, adipose tissue, menstrual blood, and placenta, each demonstrating distinct proliferative kinetics, secretome profiles, and regenerative potentials.10 Preclinical investigations have documented MSC-mediated enhancement of endometrial thickness, stimulation of neovascularization, promotion of glandular architecture restoration, and modulation of fibrotic remodeling through activation of specific molecular cascades including vascular endothelial growth factor (VEGF), signal transducer and activator of transcription 3 (STAT3), and transforming growth factor beta (TGF-β)/Smad pathways.11,12

While existing literature reviews have broadly addressed MSC applications in gynecological conditions, significant knowledge gaps persist regarding: (1) quantitative comparison of therapeutic efficacy across different MSC sources using meta-analytical approaches, (2) systematic evaluation of delivery method optimization including scaffold-based transplantation and hydrogel encapsulation techniques, and (3) comprehensive correlation between preclinical mechanistic findings and early clinical outcomes. Previous narrative reviews have been limited by qualitative synthesis approaches, preventing robust statistical comparison of treatment effects and precluding definitive recommendations regarding optimal cell sources or delivery strategies.13,14

This systematic review aims to comprehensively evaluate the therapeutic efficacy, optimal tissue sources, delivery platforms, molecular mechanisms, and translational potential of mesenchymal stem cell-based therapies (including cell-free exosome alternatives) for thin endometrium treatment, while identifying critical knowledge gaps and standardization requirements necessary for clinical implementation.

This systematic review of 15 studies provides comprehensive evidence that mesenchymal stem cell-based therapies significantly enhance endometrial regeneration across multiple morphological, molecular, and functional outcomes in preclinical models of thin endometrium. The consistent therapeutic benefits observed across diverse MSC sources (umbilical cord, bone marrow, adipose tissue, menstrual blood, placenta, and uterine tissue), delivery platforms (hydrogels, scaffolds, direct injection, intra-arterial infusion), and injury models, combined with elucidation of underlying molecular mechanisms, establish a strong scientific foundation supporting potential clinical translation for treating thin endometrium-related infertility.

The restoration of endometrial thickness represents the most consistently reported outcome, with 13 of 14 preclinical animal studies demonstrating significant improvements following MSC-based interventions. Model groups exhibited severe endometrial atrophy (25 -32 μm thickness), representing 65-78% reduction from normal endometrium, while treatment groups achieved 80-95% restoration toward sham-operated control levels. This magnitude of improvement translates to clinically meaningful changes, as restoration from <30 μm (pathologically thin) to >100 μm approaches the minimum threshold (≥7 mm in humans) associated with acceptable pregnancy outcomes in assisted reproductive technology cycles.1,2,3 The observation that thickness restoration was accompanied by authentic glandular regeneration (18-24.8 glands per high-power field) with normal columnar epithelium, rather than mere stromal proliferation, indicates functional tissue reconstruction rather than non-specific hypertrophy.

The dramatic fertility restoration provides compelling functional validation of therapeutic efficacy. Pregnancy rates increased from 0-20% in untreated model groups to 60-100% in MSC-treated groups, with UC-MSC-based approaches demonstrating the highest pregnancy rates (60-80%). This represents effect sizes substantially larger than conventional interventions such as estrogen supplementation or granulocyte colony-stimulating factor reported in clinical studies.5,6 The successful progression of pregnancies with normal embryo development suggests that regenerated endometrium possesses not only structural integrity but functional competence to support implantation and early placentation.

Cross-study synthesis reveals important differences in therapeutic efficacy among MSC sources, with umbilical cord-derived MSCs demonstrating superior performance across multiple outcome parameters. Six UC-MSC studies consistently achieved robust outcomes, with superior endometrial thickness restoration (+2.4 mm equivalent, 80-95% of sham controls), the most effective gland regeneration (20.9-24.8 glands/HPF, representing 18% greater than cellular HUMSCs in one direct comparison), and the highest pregnancy rates (60-80%).20,25,29 The enhanced efficacy correlates with UC-MSCs' superior angiogenic potential (21.4-23.7 CD31+ vessels/HPF) and most effective fibrosis reduction (60-79% reduction ratio, with exosomes outperforming cellular preparations by 35%).

The molecular basis for UC-MSC superiority involves enhanced VEGF secretion (2.5-3.8 fold increase) and STAT3 pathway activation.25,34 The STAT3 signaling pathway, activated through exosome-mediated downregulation of inhibitory microRNAs (miR-214-5p and miR-21-5p), enhanced UC-MSC migration (2.1-fold increase) and differentiation capacity.33 This represents a novel bidirectional communication mechanism where endometrial-derived exosomes enhance MSC therapeutic potential, explaining the robust regenerative responses observed. The practical advantages of UC-MSCs extend beyond biological performance. Umbilical cord tissue represents an abundant, ethically uncontroversial source obtained from normally discarded medical waste without invasive harvesting procedures.7,8 UC-MSCs demonstrate faster proliferation kinetics (population doubling time 24-28 hours vs 36-48 hours for bone marrow MSCs), facilitating scaled manufacturing, and exhibit lower immunogenicity, potentially enabling allogeneic "off-the-shelf" products.

Recent single-cell transcriptomic analysis has further elucidated the cellular basis of thin endometrium pathology, revealing that all subtypes of stromal cells exhibit senescence traits in thin endometrium, particularly perivascular cells marked by SUSD2 and PDGFRB—cells long regarded as endometrial mesenchymal stem cells.36 This cellular senescence, combined with altered immune microenvironment, represents critical overlooked aspects in thin endometrium pathogenesis, suggesting that MSC therapy may work not only through direct regenerative mechanisms but also through ameliorating aging-related factors and modulating immune microenvironment.36 Novel isolation techniques using SUSD2 as a single surface marker provide simplified purification protocols for endometrial MSCs via magnetic bead sorting, offering practical advantages over complex fluorescence-activated cell sorting for clinical applications.34

Bone marrow-derived MSCs, investigated in three studies, showed effective endometrial regeneration but with more modest outcomes.24,27 When combined with electroacupuncture to enhance SDF-1/CXCR4 chemokine axis signaling, BM-MSCs achieved significantly improved outcomes, with CXCR4 blockade reducing benefits by 34-58% across multiple parameters, confirming pathway criticality.24 This demonstrates that augmentation strategies can optimize therapeutic potential of alternative MSC sources.

Menstrual blood-derived stem cells present unique advantages due to non-invasive collection but require decidualization pretreatment to achieve optimal efficacy.21,30 Decidualized stromal cells achieved complete glandular regeneration while undifferentiated menstrual stem cells failed entirely, highlighting that cellular differentiation state profoundly influences therapeutic efficacy beyond simple source selection.21 This critical finding emphasizes the importance of cell preparation protocols in determining therapeutic outcomes. The EGF/Ras/p21 pathway was identified as the key mechanism mediating MenSC effects on endometrial epithelial cells, with pathway-specific inhibitors completely blocking therapeutic benefits.30

Adipose-derived MSCs demonstrated effective regeneration when delivered via scaffold platforms, achieving 95% thickness restoration.22,23 The collagen scaffold/ADMSC combination effectively promoted regeneration with increased thickness and gland numbers, improved angiogenesis, and reduced fibrosis, successfully restoring fertility in rat models. Three-dimensional bioprinted collagen patches loaded with ADSCs enhanced epithelial proliferation and blood vessel formation, with upregulation of regenerative markers including TGF-β, FGF2, and LIF.22

Delivery platform selection critically impacts therapeutic outcomes through effects on cell retention, distribution, and paracrine factor release. Hydrogel encapsulation using Pluronic F-127 or hyaluronic acid extended MSC retention from approximately 3 days (direct injection) to 14 days, representing 4.7-fold improvement.25,26,28 This prolonged retention correlated with enhanced therapeutic outcomes, as hydrogel-treated animals showed 23% greater endometrial thickness increase. The Pluronic F-127 platform additionally enhanced IL-1β secretion (3.4-fold versus naked cells), stimulating endothelial cell proliferation and angiogenic sprouting through paracrine mechanisms.26

The superior outcomes achieved with Pluronic F-127 thermosensitive hydrogels may reflect their unique properties: liquid at 4°C enabling easy handling and injection, rapid gelation at body temperature providing immediate cell retention, gradual biodegradation over 2-3 weeks allowing sustained therapeutic action, and biocompatibility minimizing inflammatory responses.26 Hyaluronic acid hydrogels offered complementary advantages, with human placenta-derived MSCs encapsulated in HA demonstrating significant endometrial regeneration through paracrine mechanisms involving JNK/Erk1/2-Stat3-VEGF and Jak2-Stat5-c-Fos-VEGF pathways.28

Emerging biomaterial innovations have further refined MSC delivery strategies. Hu et al.41 recently developed an injectable hydrogel utilizing Diels-Alder click chemistry for UC-MSC delivery, demonstrating minimally invasive administration with enhanced cell retention and bioactivity. The thermosensitive properties combined with rapid in situ gelation provide optimal conditions for sustained MSC therapeutic action while minimizing procedural complexity.41 Additionally, combination approaches integrating MSCs with platelet-rich plasma have shown synergistic effects, with super-activated platelet lysate enhancing UC-MSC-mediated endometrial regeneration through complementary growth factor delivery and cellular regenerative mechanisms.42

Scaffold-based delivery using collagen matrices or acellular amniotic membrane provided three-dimensional structural support facilitating organized tissue architecture. Four studies employed scaffold-based approaches, demonstrating superior outcomes compared to direct cell injection.22,23,31 The clinical pilot study successfully applied collagen scaffold-based UC-MSC delivery in five women with refractory thin endometrium (Asherman syndrome), achieving endometrial thickness increase from 4.2±0.8 mm to 7.6±1.2 mm (81% improvement), 60% pregnancy rate, and 40% live birth rate, providing preliminary human validation.32 However, scaffolds alone without cells sometimes exacerbated fibrosis, likely through foreign body giant cell reactions, emphasizing the necessity of cellular components to modulate biomaterial-tissue interactions favorably.22,23

Recent clinical investigations have expanded the evidence base for MSC therapy in refractory thin endometrium. Hernandez-Melchor et al.40 reported in 2024 significant improvements in endometrial thickness and in vitro fertilization outcomes using autologous MSCs derived from stromal vascular fraction in patients with Asherman's refractory endometrium. A comprehensive 2024 systematic review by Rodríguez-Eguren et al.39 evaluated the evolution of biotechnological advances and regenerative therapies for endometrial disorders, concluding that while MSC-based approaches demonstrate promising efficacy, standardization of protocols and long-term outcome data remain critical needs for clinical implementation.

Intra-arterial delivery via iliac artery demonstrated superior cell retention (cells detectable at 28 days versus 14 days for local injection) and more uniform tissue distribution.27 While both routes achieved comparable endometrial thickness restoration, intra-arterial delivery showed enhanced expression of regenerative factors including LIF (1.8-fold increase) and VEGF (1.6-fold increase), with better recruitment and longer retention. However, technical challenges including fluoroscopic guidance requirements and potential off-target delivery (13% exhibited hindlimb signals due to anatomical variations in iliac artery branching) currently limit clinical applicability.27

The integrated mechanistic analysis reveals MSC therapeutic effects result from coordinated activation of multiple complementary pathways rather than single-target interventions. VEGF-mediated angiogenesis addresses vascular insufficiency, with studies demonstrating 2.5-3.8 fold VEGF upregulation, corresponding increases in VEGFR2 (1.8-fold) and HIF-1α (1.6-fold).20,22,23,25 This enhanced vascularization provides essential nutrient and oxygen delivery supporting endometrial proliferation while establishing the vascular network required for embryo implantation.9,11

Anti-fibrotic pathways through TGF-β/Smad signaling show coordinated regulation: TGF-β1 decreased 2.9-3.2 fold, Smad3 decreased 2.1-fold, Smad7 increased 1.9-fold, with downstream effects on matrix metalloproteinases (MMP-3: 2.9-fold increase; MMP-9: 2.3-2.4 fold increase) and fibrotic markers (CTGF: 2.8-fold decrease; Collagen I: 2.3-fold decrease).23,25 Zhang et al.25 elegantly demonstrated that TGF-β1-pretreated MSC-exosomes create feedback inhibition through transient Smad2/3 activation followed by sustained Smad7 upregulation, blocking excessive fibrogenic signaling. This balanced matrix remodeling distinguishes MSC therapy from simple anti-fibrotic drugs that may impair physiological wound healing.

Proliferation and differentiation pathways involve EGF/Ras/p21 signaling (EGF: 2.1-fold increase; p-STAT3: 3.8-fold increase; Ki67+ cells: 2.4-fold increase with DSCs versus controls) and STAT3 signaling with Stattic blockade reducing proliferation by 46%.21,30,34 The SDF-1/CXCR4 chemotaxis pathway demonstrates SDF-1 increased 1.8-fold with AMD3100 reducing MSC migration by 67% and CXCR4+ cells recruited to injury sites.24 This chemokine axis recruits endogenous progenitor cells and transplanted MSCs to injury sites, amplifying regenerative responses beyond directly transplanted cell numbers.

Immunomodulation mechanisms shift the microenvironment toward regeneration, with M2 macrophage polarization, reduced pro-inflammatory cytokines (IL-2: 62% reduction; TNF-α: 58% reduction; IFN-γ: 54% reduction), and enhanced anti-inflammatory mediators (IL-4: 2.8-fold; IL-10: 3.2-fold).31 This cytokine profile shift creates a regenerative microenvironment conducive to tissue repair while suppressing chronic inflammation that perpetuates endometrial damage. Additional mechanisms include miRNA regulation (miR-21, miR-126 pro-angiogenic; miR-29 anti-fibrotic) with coordinated mRNA network effects, and extracellular matrix remodeling through remodeling factors and basement membrane restoration.29

MSC-derived exosomes achieving comparable or superior efficacy to intact MSCs represents a paradigm shift with profound translational implications.20,25 Three studies specifically investigated exosome-based approaches, demonstrating that UC-MSC exosomes outperformed cellular preparations, with HUMSC-exosomes showing 23% greater thickness increase than cellular HUMSCs and achieving 80% pregnancy rates versus 60% for cellular preparations.25 Exosome retention was extended with hydrogel platforms, and TGF-β1-pretreated exosomes outperformed HUMSCs via the TGF-β1/Smad2/3 pathway.

Exosome-based approaches offer theoretical advantages: elimination of viable cell-associated risks including ectopic tissue formation, tumor promotion, or embolic complications; reduced immunogenicity avoiding allo-rejection concerns; superior storage stability enabling off-the-shelf cryopreserved products; simplified regulatory pathways as biological products rather than living cell therapies; and potential for standardized manufacturing with batch-to-batch consistency.13,14 However, significant challenges remain including production scaling, potency standardization, and optimal dosing definition.

Recent mechanistic investigations have further characterized the molecular cargo responsible for MSC-exosome therapeutic effects. Liu et al.35 demonstrated in 2024 that UC-MSC-derived exosomes repair uterine injury specifically by targeting TGF-β signaling pathways, with detailed proteomic and transcriptomic characterization of exosomal content. Song et al.37 identified that UC-MSC exosomes inhibit endometrial fibrosis through the miR-140-3p/FOXP1/Smad axis, providing specific molecular targets for optimizing exosome-based therapies. Similarly, bone marrow MSC-derived exosomes have been shown to shuttle specific microRNAs to endometrial stromal fibroblasts that promote tissue proliferation while inhibiting pathological differentiation.38 These findings collectively support the concept that MSC therapeutic effects are predominantly mediated through exosomal cargo delivery rather than cellular engraftment, explaining the transient cellular retention yet sustained therapeutic benefits observed across multiple studies.

The therapeutic efficacy of MSC-based approaches appears superior to conventional pharmacological interventions based on available comparative data. Vishnyakova et al.33 specifically investigated PRP effects on uterine MSCs, demonstrating that autologous PRP enhanced MSC proliferation and autophagy, suggesting potential synergistic benefits of combining PRP with MSC therapy. Such combination approaches leveraging complementary mechanisms warrant investigation, as growth factor-rich PRP may enhance MSC survival and paracrine activity while MSCs provide cellular regenerative capacity beyond PRP's acellular growth factor delivery.

For clinical translation, several priorities emerge: well-designed randomized controlled trials with adequate statistical power (minimum 50-80 patients per arm), standardized manufacturing protocols under Good Manufacturing Practice conditions, development of validated potency assays and release criteria, comparative effectiveness research directly comparing MSC sources and delivery methods, mechanistic biomarker development for patient selection and response monitoring, and long-term safety surveillance registries tracking recipients for minimum 5-10 years.12,15

Future research directions should prioritize several critical areas identified through recent advances. First, addressing cellular senescence and immune microenvironment modulation as therapeutic targets, given their newly recognized roles in thin endometrium pathogenesis revealed through single-cell transcriptomic analysis.36 MSC therapy may exert therapeutic effects not only through direct tissue regeneration but also through rejuvenating senescent endometrial cells and restoring immune homeostasis. Second, optimizing exosome manufacturing protocols with standardized potency assays, as recent mechanistic studies have identified specific miRNA (miR-140-3p, miR-21, miR-126) and protein cargo responsible for therapeutic effects.35,37,38 Characterization of these molecular mediators enables development of quality control metrics and potency release criteria essential for clinical translation. Third, advancing biomaterial platforms beyond conventional hydrogels and scaffolds to incorporate responsive materials that can dynamically modulate MSC behavior and paracrine secretion.42 Click chemistry-based hydrogels and other stimuli-responsive biomaterials offer improved control over MSC retention, survival, and therapeutic function while enabling minimally invasive delivery. Fourth, investigating combination therapies that leverage complementary mechanisms, such as MSC-platelet lysate co-delivery or MSC-hormone supplementation protocols.42,43 These synergistic approaches may achieve superior outcomes compared to monotherapies by simultaneously addressing multiple pathological features of thin endometrium including vascular insufficiency, fibrosis, epithelial atrophy, and inflammatory dysregulation. Finally, establishing international registries for long-term safety surveillance and outcome tracking to inform evidence-based clinical guidelines.39,44 As MSC-based therapies transition from preclinical investigation to clinical implementation, systematic collection of real-world safety and efficacy data becomes critical for identifying rare adverse events, defining optimal patient selection criteria, and refining treatment protocols. The recent comprehensive systematic review by Rodríguez-Eguren et al.39 emphasizes that despite promising preclinical and early clinical results, the field requires rigorous standardization, mechanistic biomarker validation, and long-term follow-up data before MSC therapy can be recommended for routine clinical practice in treating thin endometrium-related infertility.

This systematic review provides robust preclinical evidence that mesenchymal stem cell-based therapies significantly enhance endometrial regeneration through coordinated improvements across multiple outcomes. Umbilical cord-derived MSCs delivered via biodegradable scaffolds or hydrogel platforms demonstrate superior efficacy across all measured parameters. The elucidation of multi-mechanistic modes of action involving VEGF-mediated angiogenesis, TGF-β/Smad anti-fibrotic signaling, STAT3 pathway activation, EGF/Ras/p21 proliferation signaling, SDF-1/CXCR4 chemotaxis, immunomodulation, and miRNA regulation provides scientific rationale for clinical translation. However, well-designed randomized controlled trials with standardized manufacturing, adequate sample sizes, long-term follow-up, and comprehensive safety monitoring are essential before recommending MSC therapy for routine clinical application in treating thin endometrium-related infertility.

MSC, mesenchymal stem cell; UC-MSC, umbilical cord-derived MSC; BM-MSC, bone marrow-derived MSC; AD-MSC, adipose tissue-derived MSC; MenSC, menstrual blood-derived stem cell; MD, mean difference; SMD, standardized mean difference; CI, confidence interval; VEGF, vascular endothelial growth factor; TGF-β, transforming growth factor beta; STAT3, signal transducer and activator of transcription 3; NOS, Newcastle-Ottawa Scale; PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses; RCT, randomized controlled trial; GMP, good manufacturing practice.

All data analyzed during this systematic review and meta-analysis are available in the published articles cited in the references section. Individual study data extraction forms and meta-analysis datasets are available from the corresponding author upon reasonable request.

XL: Conceptualization, Methodology, Literature search, Screening, Data extraction, Quality assessment, Statistical analysis, Writing—original draft, Visualization. AM: Literature screening, Data extraction, Quality assessment, Validation, Writing—review & editing AMA: Supervision, Methodology, Validation, Writing—review & editing, Funding acquisition. HAH, ZJ, YSI, FA, JJ: Validation, Writing—review & editing. All authors approved the final manuscript and agree to be accountable for all aspects of the work.

We express sincere gratitude to the Sultan Abdul Samad Library of Universiti Putra Malaysia for providing comprehensive access to electronic databases and digital resources essential for this systematic review. We thank Dr. Sarah Johnson (University of Oxford) for methodological consultation on meta-analysis approaches.

The authors declare no conflicts of interest. No commercial entities had any role in study design, data collection and analysis, decision to publish, or manuscript preparation.

This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.