Uterine factor infertility-current understanding and limitations

Despite limited advances in the field of female reproduction, a growing number of studies reflect increasing interest in diagnostic and therapeutic strategies targeting the immune-cytokine-decidual interface. One such approach is endometrial cytokine profiling, which quantifies immune-related signaling molecules in endometrial biopsy tissue or uterine fluid using Enzyme-Linked Immunosorbent Assay (ELISA), Reverse Transcription-quantitative Polymerase Chain Reaction (RT-qPCR), or protein arrays. These evaluations will extend beyond structural assessment of the endometrium and hormonal timing to include evaluation of the inflammatory state and immune competence during the implantation window [ 146 ]. For example, elevated IL-10 and reduced MUC1 expression have been associated with successful implantation, whereas dysregulation of IL-1, IL-15, LIF, and IL-12 have been associated with implantation failure [ 39 , 64 , 119 , 135 , 136 ]. The cytokine network in the endometrium during the peri-implantation period functions as a “finely tuned signaling platform” through a, usually transient, pro-inflammatory shift (involving IL-1, IL-6, TNFα, and LIF), followed by a transition to an anti-inflammatory (Th2-type) environment (involving IL-10, IL-4, and TGFß) that support implantation, embryo–maternal communication, and regulates immune activation and vascular adaptation [ 59 , 108 ]. Thus, changes in cytokine levels during the menstrual cycle make it difficult to define reliable baseline values for implantation, but the use of clinical cytokine arrays could be a first step toward providing individualized baselines for women throughout menstruation to better understand inflammation-based relationships in the uterus. These challenges are further compounded by substantial interindividual variability, as even small differences in endometrial biopsy location or tissue volume can significantly affect cytokine evaluation and standardization across patients. This has made studying these processes challenging, as timing is critical to implantation. These challenges have yet to be fully resolved, and the area of investigation remains highly promising, as overcoming these limitations could substantially advance our understanding of implantation biology and the earliest stages of human life. Thus, cytokine spatial and temporal expression patterns and signaling thresholds are critical in the endometrium and can shift the endometrium from a receptive to a non-receptive state, contributing to unexplained infertility, RIF, or early pregnancy loss. In recent years, several studies have comprehensively cataloged human endometrial cell populations during the mid-luteal phase, allowing for detailed analysis of gene expression patterns within the implantation window. These investigations have revealed extensive crosstalk between decidual stromal cells and immune subsets. One of them is CXCL12-driven recruitment of uNK and myeloid cells, a mechanism that is likely essential for coordinating immune support and tissue remodeling during implantation [ 63 , 147 ]. Also, a spatial transcriptomic study of RIF endometrial samples has revealed distinct regional gene expression changes in women with implantation failure compared to fertile controls. Notably, these studies identified upregulation of pro-inflammatory genes within the superficial luminal epithelium in RIF patients [ 148 ]. In contrast, immune cell clusters were predominantly localized to the endometrial glands and surrounding blood vessels. In addition, spatial analyses have shown that uNK cells preferentially cluster around the endometrial glands, where they are thought to contribute to glandular remodeling and functional maturation of the endometrium [ 149 ]. Spatial transcriptomics has emerged as a tool to contextualize single cells in multicellular neighborhoods and to identify spatially recurrent phenotypes, or ecotypes. These advancements in molecular science have significantly contributed to reproductive research through single-cell RNA sequencing (scRNA-seq). Single-cell transcriptomic analysis of vast datasets, with targeted-transcriptome and whole-transcriptome profiles of hundreds to millions of cells, has broadened our understanding of cellular diversity and dynamic gene expression, cellular plasticity, and diverse tissue microenvironments in healthy and diseased tissues. Profiling gene expression at the level of individual endometrial cells enables the identification of specific cell types that produce specific cytokines, along with their corresponding receptors [ 112 , 150 ]. These studies will also allow for a better understanding of the crosstalk experienced between the blastocyst and the endometrium. Although these technologies are primarily used as research tools, they may provide a roadmap for future diagnostics and support hormone-based timing and structural assessment toward a more precise functional evaluation of the endometrium, as well as clinical avenues that warrant study for their potential to improve fertility outcomes in women with unexplained UFI.

Although inflammation is typically associated with infection or pathology, embryo implantation represents a tightly regulated, hormonally orchestrated, and transient physiological inflammatory response that supports endometrial receptivity (Figs.  3 and 4 ). This inflammatory process peaks during the mid-secretory phase, coinciding with the window of implantation [ 71 , 102 ]. Clinical studies demonstrating improved implantation rates following endometrial injury further supported the concept that a brief transient pro-inflammatory state facilitates successful embryo attachment [ 103 ]. During the window of implantation, the embryo-uterine interaction initiates a pro-inflammatory response that is both temporary and necessary for priming leukocyte recruitment, promoting vascular and ECM remodeling, and supporting endometrial attachment and trophoblast invasion [ 71 ]. Although immune cells such as uNKs and uterine dendritic cells predominate the decidual environment, their functional phenotype shifted away from cytotoxicity. Instead, these cells primarily contribute to angiogenesis, ECM remodeling, and the establishment of maternal–fetal tolerance, reflecting the uniquely permissive inflammatory milieu of the peri-implantation endometrium. This transient pro-inflammatory state is sustained by cytokines including TNF-α, IL-1β, and IL-6 [ 100 , 104 ]. Cytokines are secreted by immune cells, epithelial cells, and decidualizing stromal fibroblasts that play pivotal roles in shaping the peri-implantation endometrial environment. Cytokine expression is regulated by steroid hormones, including E2 and P4, embryo-derived signals such as hCG, and local paracrine factors. Once secreted, cytokines coordinate key processes necessary for successful implantation, including embryo development, epithelial receptivity, stromal remodeling, angiogenesis, and immune cell recruitment [ 25 ]. Dysregulation of cytokine signaling pathways has been strongly associated with implantation failure and abnormal placental development [ 104 , 105 ]. These findings reposition inflammation not as an intrinsic barrier to pregnancy, but as a tightly regulated, beneficial physiological process. Whereas dysregulation of inflammatory signaling in timing, magnitude, or localization becomes a pathological condition, thereby impairing implantation success. We highlight a few cytokines and their roles in endometrial crosstalk, inflammatory tuning during implantation, and UFI (Table  1 ). Table 1 Role of cytokines of the endometrium Cytokine Location Target cell(s) Receptors Functions Dysregulation outcome LIF Glandular and luminal epithelium, decidual leukocytes [ 59 ] Preimplantation embryo, epithelial cells [ 87 ] LIFR/gp130 complex [ 51 , 52 ] Promotes pinopod expression, epithelial receptivity, and trophoblast adhesion [ 59 , 80 ] ↓ LIF → impaired STAT3 signaling, reduced implantation potential [ 52 , 112 ] IL-1 Epithelium, macrophages, blastocyst [ 135 ] Endometrial epithelial cells, stromal cells [ 135 ] IL-1R [ 135 ] Promotes prostaglandin E2, LIF, and integrin expression; primes implantation site [ 136 ] ↓ IL-1 → reduced adhesion and impaired implantation [ 136 ] IL-6 Glandular epithelium, stroma [ 66 ] Cells expressing gp130 [ 111 , 119 ] IL-6R (membrane-bound or soluble) + gp130 [ 111 , 119 ] Promotes decidualization, regulates immune balance, STAT3 signaling [ 91 – 93 ] ↓ IL-6 → linked to recurrent miscarriage, poor receptivity [ 120 , 121 ] IL-8 Endometrial epithelial and glandular cells [ 123 ] Neutrophils, endothelial cells [ 123 ] CXCR1, CXCR2 [ 66 ] Stimulates angiogenesis, endothelial proliferation, and immune cell recruitment [ 66 , 95 , 151 ] ↓ IL-8 → impaired vascularization, ↓ decidualization [ 122 ] IL-11 Decidualized stromal cells [ 59 , 128 ] Immune cells (uNK), endometrial cells [ 128 ] IL-11R + gp130 [ 38 ] Promotes stromal differentiation, immune cell maturation [ 128 ] ↓ IL-11 → poor uNK maturation, stromal dysfunction [ 59 ] IL-15 Decidualizing stromal cells; endometrial endothelial cells; dendritic cells [ 64 ] uNK cells; peripheral NK cells recruited to the endometrium [ 74 ] IL-15Rα/IL-2Rβ/γc (shared IL-2/IL-15 receptor complex) [ 91 ] uNK cell recruitment, proliferation, survival, and functional differentiation [ 74 ] absence of uNK cells, impaired spiral artery remodeling, and compromised vascular adaptation [ 74 ] TNF-α Macrophages, uNK cells, stromal and epithelial cells, possibly embryo [ 141 , 152 ] Stromal cells, epithelial cells [ 140 ] TNFR1 (apoptosis), TNFR2 (repair/tolerance) [ 153 , 154 ] ECM remodeling, apoptosis, immune recruitment [ 25 , 140 ] ↑ TNF-α → impaired decidualization, apoptosis, infertility [ 155 , 156 ] Role of cytokines of the endometrium LIF is a multifunctional cytokine belonging to the IL-6 cytokine family that regulates the stimulation of proliferation, differentiation, and survival processes, which are essential for pinopod formation, trophoblast invasion and implantation, and blastocyst development [ 49 , 59 ]. LIF is primarily localized to the glandular and luminal epithelium and stromal compartment of the endometrium, with particularly strong expression in decidual leukocytes at the implantation site. This spatial distribution suggests an important role for LIF in mediating interactions between the maternal decidual cells and the invading blastocyst [ 52 ]. Studies in fertile women demonstrated that both LIF protein and mRNA are detectable throughout the menstrual cycle, with expression increasing during the window of implantation in the mid- to late-secretory phase and being retained during early pregnancy [ 106 – 108 ]. Upon release, LIF exerts pleiotropic effects by binding to its heterodimeric receptor complex, which consists of the LIF receptor (LIFR) and glycoprotein 130 (gp130). gp130 serves as a shared transmembrane signal-transducer for the IL-6 cytokine family [ 109 ] and is predominantly expressed in the glandular and luminal epithelium of the human endometrium [ 110 ]. Ligand binding activates multiple intracellular signaling cascades, including the Janus kinase (JAK)/signal transducer and activator of transcription (STAT), mitogen-activated protein kinase (MAPK), and phosphatidylinositol-3-kinase (PI3K) pathways [ 51 , 80 , 111 ]. Evidence from human studies suggests that LIF signaling is stimulated indirectly by P4 release from the CL, along with local paracrine factors such as heparin-binding epidermal growth factor–like growth factor (HB-EGF) and transforming growth factor-β1 (TGF-β1), which together promote LIF secretion in cultured endometrial stromal cells [ 25 , 112 ]. Activation of downstream STAT3 signaling facilitates trophoblast adhesion, epithelial receptivity, and supports coordinated implantation processes that are critical for successful decidualization and establishment of endometrial receptivity [ 113 , 114 ]. LIF enhances embryo viability and implantation competence by binding to receptors on both the preimplantation embryo and the uterine epithelium. These findings further support the association between altered LIF expression and unexplained infertility, allowing for it to be most recently used as a biomarker in IVF patients for predictive clinical pregnancy outcomes [ 115 , 116 ]. IL-6 is a pleiotropic cytokine classically associated with host defense, immune response, acute-phase reactions, and hematopoiesis, and is also recognized as a critical determinant of several physiological processes essential for successful pregnancy [ 117 , 118 ]. Within the female reproductive tract, IL-6 is predominantly localized to the glandular epithelium and stromal layer throughout the menstrual cycle. IL-6 expression is relatively modest during the proliferative stage but demonstrates strong epithelial immunoreactivity during the mid-secretory phase, coinciding with the window of implantation [ 90 ]. Notably, endometrial IL-6 levels are significantly lower in women with unexplained recurrent miscarriage than in fertile women, supporting the concept that IL-6 plays a pivotal role in establishing endometrial receptivity during the secretory phase [ 119 ]. Interestingly, IL-6 exerts both pro-inflammatory and anti-inflammatory effects, with regulation that is tightly controlled throughout the menstrual cycle. During the proliferative phase, IL-6 is weakly expressed and acts in a pro-inflammatory manner via IL-6 trans-signaling involving the soluble IL-6R (sIL-6R). In humans, sIL-6R is generated through two primary mechanisms: first, by alternative splicing of the IL-6R gene; and second, by proteolytic cleavage of the classical transmembrane IL-6R by the disintegrin and Metalloproteinase 17 (ADAM17), a process largely confined to epithelial and stromal compartments. Following cleavage, the IL-6/sIL-6R complex can circulate systemically and bind gp130, a signal-transducing receptor ubiquitously expressed on most cell types, thereby broadening the inflammatory response to IL-6. In contrast, during the mid- to late-secretory phase following ovulation, IL-6 predominantly mediates anti-inflammatory effects through classical signaling involving membrane-bound IL-6R and gp130 heterodimerization [ 111 , 119 ]. In this context, IL-6 binds to transmembrane IL-6R and increases the receptor complex’s affinity for gp130, leading to gp130 dimerization and downstream signaling activation. This signaling cascade engages the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathways, which initiate a negative feedback loop that limits IL-6 activity. Activated STAT3 induces expression of suppressor of cytokine signaling 3 (SOCS3), a key regulator that restrains excessive inflammation and supports successful implantation during the secretory phase [ 120 , 121 ]. Notably, IL-6 shares downstream signaling pathways with LIF, suggesting potential synergistic interactions between these cytokines in regulating endometrial receptivity and implantation [ 87 ]. However, the functional interplay between IL-6 and LIF remains incompletely understood and represents an important area for future investigation. IL-8, a member of the CXC chemokine family, is a major pro-inflammatory cytokine that is primarily localized to the epithelial and glandular cells of the endometrium. IL-8 expression varies in a cyclical and time-dependent fashion throughout the menstrual cycle. IL-8 has two distinct expression peaks throughout the menstrual cycle, each corresponding to different physiological roles. The biphasic expression pattern of IL-8 supports distinct functions in tissue priming and decidual maintenance, underscoring its diagnostic potential and therapeutic relevance [ 13 , 122 ]. The first peak of IL-8 occurs during the early to mid-proliferative stage [ 9 ], primarily associated with immune cell recruitment, monocyte maturation and differentiation, endothelial cell survival, angiogenesis, and proliferation. To mediate these functions, IL-8 binds to its G-coupled protein receptors, CXCR1 or CXCR2, which are expressed on neutrophils, endothelial cells, and potentially uNK cells. Within the endometrium, CXCR2 is more widely expressed in the epithelial and stromal layers during the proliferative and early secretory phases. Activation of CXCR1 or CXCR2 initiates downstream signaling through the Akt/Protein Kinase B (PKB) pathway, the mitogen-activated protein kinase (MAPK) pathway, and the protein kinase C (PKC) pathway, thereby driving the biological effects described in the previous section. The second IL-8 peak occurs during the early secretory phase and is associated with increased vascular permeability, epithelial remodeling, and potential immune cell–cell–mediated guidance of the implanting embryo [ 66 , 122 , 123 ]. Decidualizing stromal fibroblasts also contribute to IL-8 production in response to P4 and cAMP signaling, generating chemotactic gradients that recruit neutrophils and may influence uNK cell positioning and activation [ 100 , 101 ]. Consistent with this, IL-8 levels increase at the onset of decidualization, a pattern that parallels elevated IL-8 expression observed in the presence of an embryo within the endometrium. Broadly, this first peak in IL-8 is associated with neovascularization and early decidualization, whereas the second peak is more closely linked to continued decidualization and immune cell recruitment [ 95 , 123 – 125 ]. Neutrophils recruited in response to IL-8 act as a transient but essential source of vascular VEGF-A during the proliferative phase, promoting endothelial proliferation and subepithelial capillary plexus formation [ 94 ]. Elevated IL-8 expression has also been correlated with improved IVF outcomes, likely reflecting its role in vascular priming and immune cell recruitment [ 126 , 127 ]. Taken together, these findings position IL-8 as a critical mediator linking hormonal cues to endometrial structural adaptation by orchestrating immune cell recruitment and vascular expansion in a tightly controlled temporal manner. However, IL-8’s role as a biomarker remains investigational. IL-11, a member of the IL-6 cytokine family, is a pleiotropic factor implicated in hemopoiesis, macrophage differentiation, and mucosal protection in the intestines. Within the endometrium, IL-11 plays a critical role in establishing an immunotolerant decidual environment by attenuating cytotoxic inflammatory responses and promoting tissue remodeling [ 59 , 128 ]. Mouse model studies demonstrate that IL-11 deficiency results in impaired decidualization, stromal disorganization, and early pregnancy loss due to defective implantation [ 129 ]. Consistent with other IL-6 family members, IL-11 signals through binding to its specific receptor, followed by recruitment of the shared gp130 coreceptor. Although both IL-11 and LIF activate gp130-dependent STAT3 signaling, IL-11 appears to act preferentially on the stromal compartment, particularly during the late secretory phase, whereas LIF primarily regulates epithelial receptivity. This spatial differential action suggests functional specialization despite shared downstream pathways. Activation of gp130 subsequently engages the JAK/STAT pathway, which suppresses pro-inflammatory signaling pathways during the secretory phase, thereby supporting a receptive and stable decidual environment [ 51 , 130 , 131 ]. In the endometrium, IL-11 is cyclically expressed by all cells, with peak mRNA expression in decidualized stromal cells during the late secretory phase. In contrast, IL-11 receptor expression remains relatively constant across the menstrual cycle and is broadly distributed among endometrial cell populations, indicating that endometrial IL-11 function is primarily regulated through ligand availability rather than changes in receptor expression [59]. This pattern is supported by gene array studies showing robust IL-11 mRNA upregulation during P4- or cAMP-induced in vitro decidualization of endometrial stromal fibroblasts [ 132 ]. These findings further suggest that IL-11 plays a role in decidual-specific immune maturation, including the regulation of uNK cells, thereby contributing to immune tolerance, which is essential for successful pregnancy [ 45 , 57 ]. Clinically, reduced IL-11 expression has been reported in endometrial biopsies from women with unexplained infertility and RIF, implicating insufficient IL-11 signaling, impaired stromal receptivity, or immune tolerance. Despite growing evidence for IL-11’s importance, the downstream targets and mechanism of IL-11 signaling in human decidua remain poorly defined [ 59 , 118 ]. Further studies are needed to clarify how IL-11 integrates with P4 signaling, stromal–immune crosstalk, and trophoblast invasion to sustain early pregnancy. The endometrial epithelium and macrophages secrete IL-1β, which promotes blastocyst formation and contributes to implantation competence [ 133 ]. IL-1 is a pro-inflammatory cytokine that plays an important role in establishing a receptive feto-maternal interface during implantation. Although both IL-1α and IL-1β signal through the IL-1 receptor (IL-1R), IL-1β is the predominant secreted isoform in peri-implantation and is among the earliest embryo-derived cytokines influencing endometrial function [ 119 , 133 ]. IL-1β acts upstream of key implantation mediators, including LIF and cyclooxygenase-2 (COX-2), thereby promoting epithelial remodeling and priming the implantation site for inflammation. The binding of IL-1β to IL-1R type I (IL-1R1) recruits the adaptor molecule MyD88 and activates downstream signaling molecules, including NF-κB and MAPK, leading to COX-2 induction in epithelial and stromal cells and increased prostaglandin E2 (PGE2) synthesis, which supports vascular adaptation and stromal remodeling [ 72 , 84 ]. Concurrently, IL-1β enhances LIF transcription via NF-κB activation, creating an inflammatory–receptive environment essential for blastocyst adhesion. In vitro mouse model studies have demonstrated that IL-1 increases endometrial PGE2, LIF, and integrin expression [ 134 , 135 ]. In contrast, administration of an IL-1R antagonist before implantation significantly reduces implantation rates, suggesting that IL-1 may play a significant role in implantation [ 136 ]. However, excessive expression or prolonged IL-1 activity contributes to pathological inflammation. Elevated IL-1β levels have been linked to endometriosis and RPL [ 137 , 138 ], underscoring its context-dependent role in peri-implantation and pregnancy maintenance. TNFα plays a complex and tissue-specific role at the maternal–fetal interface. TNFα has well-established pathological roles in endometriosis and endometritis, but is also physiologically upregulated during implantation to support ECM turnover and remodeling, regulate apoptosis, and recruit immune cells, including leukocyte priming and trophoblast guidance [ 103 ]. Studies have demonstrated that TNFα expression is highly localized in individuals who received successful implantation, suggesting its context‑dependent function during early pregnancy [ 139 , 140 ]. In the endometrium, TNFα is produced by multiple sources, including decidual macrophages, uNK cells, and endometrial epithelial and stromal cells, and may also be embryo-derived during early decidualization, highlighting its function in the normal implantation program [ 140 , 141 ]. Functionally, TNFα induces MMP expression and coordinates with uNK-derived cytokines to facilitate spiral artery remodeling during early decidualization [ 74 , 139 ]. The downstream signaling of TNFα through two receptors. TNFα receptor 1 (TNFR1), which activates NF-κB, caspase cascades, and apoptosis, and TNFR2, which mediates tissue repair and immune modulation [ 139 , 142 , 143 ]. The balance of receptor activation likely determines whether TNF-α supports endometrial receptivity or drives pathology. It is well established that higher TNFα concentrations or dysregulated TNFα disrupt stromal cell function, impair decidualization, and promote apoptosis or senescence within the endometrial compartment [ 141 ]. Similarly, elevated TNFα levels have been reported in endometrial biopsies from patients with endometriosis and RIF, and correlate with impaired decidualization, increased epithelial apoptosis, and disrupted vascular remodeling [ 144 ]. P4 mitigates TNFα signaling by downregulating TNFR1 and inducing anti-apoptotic factors, such as BCL2, in decidualizing stromal cells, thereby limiting inflammatory stress [ 142 ]. Distinguishing physiological TNF-α signaling from pathological inflammatory stress is therefore critical for UFI. Thus, TNF-α expression, concentration, timing, and context-dependent signaling distinguish its physiological and pathophysiological responses as a key mediator of implantation and a potential contributor to implantation failure, prompting interest in selective immunomodulatory strategies that preserve its physiological functions. A growing number of patients are experiencing RIF or unexplained infertility challenges despite normal hormonal levels (E2, P4, gonadotropins), adequate endometrial thickness, and receptive ERA. Increasing evidence suggests that cytokine dysregulation, undetectable by standard diagnostic tools, may be a critical and underrecognized contributor to UFI [ 3 , 28 ]. Interestingly, implantation success depends not only on optimal cytokine concentrations but also on their precise expression of timing, spatial localization, and concentrations. Excessive cytokine expression can lead to pathological inflammation, whereas insufficient cytokine expression may impair decidualization, immune cell recruitment, and feto–maternal interface formation. Spatial and temporal imbalances of cytokine signaling can disrupt trophoblast guidance or shift key signals outside the implantation window [ 12 ]. Studies have demonstrated that reduced levels of IL-11 impair decidual stromal cell maturation [ 128 , 129 , 132 ]; reduced levels of IL-15 compromise uNK cell differentiation and angiogenesis [ 91 ]; low levels of LIF disrupt epithelial receptivity via impaired STAT3 signaling [ 108 , 115 ]; and elevated levels of TNF-α increase stromal apoptosis and vascular dysfunction [ 91 , 139 , 140 ]. Although not all infertility etiologies are cytokine-driven, cytokine signaling defects are common despite normal ERA results, underscoring the limitations of current UFI diagnostic methods, which overlook the inflammatory microenvironment [ 145 ]. Thus, understanding cytokine imbalance as a central mechanism of uterine dysfunction not only helps explain idiopathic infertility but also enables the development of molecular diagnostics and immunomodulatory therapies that may restore endometrial receptivity without altering hormonal regulation.

Despite significant progress in reproductive science, the molecular mechanisms underlying UFI remain poorly understood. It is well documented that during the menstrual cycle, the endometrium undergoes hormone-regulated structural transformation that supports embryonic implantation and development. In this process, the underlying molecular signaling pathways that govern uterine glands, epithelial, and stromal layer survival, remodeling, immune regulation, and cellular stress responses remain incompletely characterized during the implantation window. As a result, current clinical tools capture only a fraction of the signaling processes and the various molecules involved in implantation and in determining implantation success. Addressing these gaps requires a conceptual framework that extends beyond anatomical assessment to incorporate the cellular, molecular, and immunological determinants of endometrial receptivity. A more comprehensive understanding of how hormonal signaling, cytokine networks, immune modulation, and local cellular stress responses shape the peri-implantation environment is essential for identifying biomarkers of uterine dysfunction and developing targeted therapeutic strategies. Therefore, future research should consider high-resolution molecular profiling using functional ex vivo and in vivo models, as well as longitudinal studies of the menstrual cycle to identify specific biomarkers and therapeutic targets for use in clinical studies. Moreover, identifying the immune cytokine decidual interface may hold promise for advancing diagnostics and personalized treatments in idiopathic infertility. Furthermore, in patients with idiopathic infertility, a comprehensive assessment of endometrial abnormalities may incorporate endometrial cytokine profiling using a clinical cytokine panel, TVUS, and direct endometrial tissue sampling for histopathological evaluation, with histology serving as the gold-standard diagnostic reference. Integrating these complementary approaches would enable a more detailed characterization of the endometrial microenvironment and its relationship to reproductive outcomes. Such multimodal endometrial analyses could help identify immune mediators and inflammatory pathways associated with successful versus unsuccessful pregnancy outcomes. This approach would not only strengthen the evidence supporting a role for immune dysregulation in RPL and UFI, but also improve diagnostic precision and facilitate the development of targeted, personalized therapeutic interventions aimed at optimizing endometrial receptivity and pregnancy success. Ultimately, these studies may bridge the gap between cellular and molecular signaling and clinical practice, which could be essential to improving reproductive outcomes for the growing population affected by UFI.

Infertility is a global health challenge that affects approximately 10–15% of couples of reproductive age, with recent estimates indicating that 1 in 6 people (~ 17.5% of adults) experience infertility in their lifetime [ 1 , 2 ]. Infertility can originate from either partner, whereas uterine factors may account for up to 15% of infertility cases in women [ 3 ]. Despite the substantial contribution of uterine factors, clinical interventions and current research have been focused on oocyte and embryo quality and ovarian functions. The optimal endometrial development is critical for successful implantation and maintenance of pregnancy, even in the presence of a competent oocyte. The endometrium, the inner lining of the uterus, is a highly dynamic tissue that undergoes cyclical remodeling throughout the menstrual cycle under the coordinated influence of estrogen (E2) and progesterone (P4) [ 4 , 5 ]. Structurally, the endometrium has two distinct layers, namely the stratum basalis, which contains stromal and glandular cells that remain relatively stable throughout the menstrual cycle, and the stratum functionalis, which is hormonally responsive and sheds during menstruation. Each menstrual cycle, the endometrium proliferates under the influence of E2, differentiates and becomes receptive to implantation during the mid-luteal phase under P4 dominance, and is subsequently shed in the absence of implantation (Fig.  1 ) [ 6 – 8 ]. Fig. 1 Hormonal regulation of the endometrium during the menstrual cycle in women. The schematic diagram depicts the changes in the endometrium during the menstrual phase, which is divided into the menstrual, proliferative, and secretory phases. During the proliferative phase, follicle-stimulating hormone (FSH) promotes follicular growth, resulting in rising levels of estrogen (E2, red color in the diagram) produced by the follicles, which then drives the proliferation of the functional layer of the endometrium. In mid-cycle, a peak in E2 levels triggers a surge in luteinizing hormone (LH) and the release of the oocyte (ovulation). This marks the start of the secretory phase, dominated by progesterone (P4, blue color in the diagram), which drives the differentiation (decidualization) of the endometrium to prepare for implantation. In the absence of implantation, P4 levels drop, triggering menstruation, the shedding of the functional layer, to restart the cycle. The basal layer is not shed. The lower panel depicts corresponding structural changes in the luminal epithelium, uterine glands, and uterine stroma. Key surface events at the window of implantation (Days 19–21) include Mucin-1 (MUC1) downregulation, αvβ3 integrin activation, pinopode formation, and blastocyst arrival at the receptive luminal surface. Abbreviations: MUC1, mucin-1; uNK, uterine natural killer cells; WOI, window of implantation Hormonal regulation of the endometrium during the menstrual cycle in women. The schematic diagram depicts the changes in the endometrium during the menstrual phase, which is divided into the menstrual, proliferative, and secretory phases. During the proliferative phase, follicle-stimulating hormone (FSH) promotes follicular growth, resulting in rising levels of estrogen (E2, red color in the diagram) produced by the follicles, which then drives the proliferation of the functional layer of the endometrium. In mid-cycle, a peak in E2 levels triggers a surge in luteinizing hormone (LH) and the release of the oocyte (ovulation). This marks the start of the secretory phase, dominated by progesterone (P4, blue color in the diagram), which drives the differentiation (decidualization) of the endometrium to prepare for implantation. In the absence of implantation, P4 levels drop, triggering menstruation, the shedding of the functional layer, to restart the cycle. The basal layer is not shed. The lower panel depicts corresponding structural changes in the luminal epithelium, uterine glands, and uterine stroma. Key surface events at the window of implantation (Days 19–21) include Mucin-1 (MUC1) downregulation, αvβ3 integrin activation, pinopode formation, and blastocyst arrival at the receptive luminal surface. Abbreviations: MUC1, mucin-1; uNK, uterine natural killer cells; WOI, window of implantation A successful conception depends on tightly synchronized signaling between the developing embryo and the maternal endometrium, particularly during the ”window of implantation,” a brief period when the endometrium becomes transiently receptive to embryo invasion. Endometrial receptivity during this window is recognized as a major limiting factor in human fertility, as implantation can only occur when the uterine environment is appropriately primed, even in the presence of a developmentally competent embryo [ 9 , 10 ]. Despite this recognition, diagnosing and treating subtle endometrial dysfunction in clinical practice remains challenging. Conventional diagnostic approaches, such as transvaginal ultrasound imaging and hysteroscopy, are effective in identifying overt structural abnormalities, including fibroids, polyps, uterine septum, and intrauterine adhesions. However, many individuals with normal uterine morphology continue to experience implantation failure or recurrent pregnancy loss (RPL) that remains poorly understood [ 11 ]. These challenges are further underscored in assisted reproductive technologies (ART), including in vitro fertilization (IVF). The impaired uterine receptivity in IVF is thought to be a primary contributor to low implantation rates despite the transfer of high-quality embryos [ 12 , 13 ]. These observations emphasize the central role of endometrial biology in successful reproduction and highlight the urgent need for improved technologies to assess and characterize endometrial receptivity. In this review, we highlighted current knowledge of uterine factor infertility (UFI) regarding endometrial remodeling, immune–endocrine crosstalk, and cytokine-mediated regulation of endometrial physiology, including the roles of the luminal epithelium, the stromal compartment, uterine glands, endometrial vasculature, and immune cells. Parallelly this review emphasizes unmet mechanistic questions and emerging research directions in UFI, the cellular and molecular determinants of implantation that cause infertility. UFI encompasses structural, functional, hormonal, inflammatory, or molecular abnormalities of the uterus that impair implantation and lead to early pregnancy loss. Clinically apparent uterine abnormalities are caused by submucosal fibroids, intrauterine adhesions, endometrial polyps, or congenital uterine anomalies [ 1 , 11 ]. Although uterine dysfunction may also be unexplained, it may present with normal imaging and standard diagnostic results despite recurrent implantation failure (RIF) [ 9 ]. These unexplained uterine dysfunctions can arise from impaired decidualization, dysregulated cytokine signaling, aberrant immune activation, or subtle molecular defects affecting endometrial receptivity [ 11 ]. Interestingly, endometrial receptivity is further compromised by abnormal endometrial thickness, a parameter not adequately captured by standard hormone assays or gene expression profiling alone. Both thin ( 20 mm) endometria have been associated with reduced implantation rates, altered hormonal responsiveness, and unopposed E2 exposure [ 14 , 15 ]. These structural abnormalities of the endometrium contribute to reproductive failure by disrupting embryo adhesion, invasion, or early embryonic development. Successful implantation requires tightly coordinated remodeling across distinct endometrial compartments, namely the luminal epithelium, uterine glands, stromal compartment, immune cell populations, and vasculature. These compartments of the endometrium coordinate activity through hormonally dependent, tightly regulated molecular signals to establish a transiently receptive microenvironment for embryo invasion and attachment. Due to endometrial dysfunction, impaired coordination of endometrial activity can lead to defective implantation and subsequent pregnancy loss. There are several well-established uterine abnormalities known to impair implantation and early pregnancy loss, which constitute the traditional clinical definition of UFI. Structural lesions, including submucosal fibroids, endometrial polyps, and intrauterine adhesions, can distort the uterine cavity and disrupt endometrial morphology, which can interfere with embryo apposition or trophoblast invasion [ 11 ]. Congenital uterine abnormalities, such as septate, bicornuate, or arcuate uteri, may compromise implantation by altering uterine shape, vascular distribution, and the mechanical properties of the endometrial lining [ 16 , 17 ]. Beyond structural or inflammatory pathology, endometrial dysfunction arising from systemic disorders has also been implicated in infertility. Inflammatory conditions, due to chronic endometritis and pelvic inflammatory disease (PID), including sexually transmitted diseases (STDs), can further impair endometrial receptivity by affecting the local immune environment, leading to cytokine dysregulation and defective tissue remodeling [ 18 ]. In women with endometriosis, endometrial stromal cells frequently exhibit P4 resistance, altered inflammatory signaling, and defective decidualization even in the absence of detectable lesions within the uterine cavity [ 19 – 22 ]. Comparable abnormalities, including impaired stromal differentiation and dysregulated cytokine production, have also been observed in patients with autoimmune diseases, metabolic disorders, and chronic systemic inflammation [ 23 – 25 ]. Similarly, inadequate endometrial proliferation or delayed endometrial maturation, despite normal circulating ovarian hormone levels, can compromise endometrial receptivity and lead to implantation failure [ 26 , 27 ]. Although these contributors are well recognized clinically, they account for only a subset of UFI cases. Standard imaging techniques, including transvaginal ultrasound (TVUS), saline infusion sonohysterography, and hysteroscopy, can identify structural uterine abnormalities, such as fibroids, polyps, and adhesions. Still, they provide limited insight into endometrial function and cannot assess cellular and molecular abnormalities, as well as immune and endocrine signaling, that are critical for implantation [ 11 ]. Even advanced diagnostic approaches, such as the Endometrial Receptivity Array (ERA), which evaluates gene-expression profiles to estimate the timing of the “window of implantation”, offer only a short snapshot of endometrial status and fail to capture the full complexity of endometrial remodeling, inflammatory signaling, and cellular stress responses [ 10 ]. A recent retrospective study indicated there could be some promise in ERA in personalized embryo transfer conditions, but many patients present with RIF or unexplained infertility despite normal imaging, appropriate endometrial thickness, and a nominally “receptive” ERA result [ 28 ]. Analyses of endometrial biopsies from women with RIF or early pregnancy loss have revealed impaired decidualization responses despite normal ultrasound findings and appropriate endometrial thickness [ 29 ]. In these patients, altered expression of key transcription factors, cytokines, and endometrial receptivity markers, including evidence of dysregulated P4 responsiveness, has been observed even when uterine imaging appears normal [ 25 ]. Similarly, aberrant immune activation or inadequate regulation of the endometrial inflammatory milieu has been reported in women with unexplained infertility, despite histologically “normal” endometrium [ 30 – 32 ]. These findings indicate that endometrial dysfunction may occur at the level of gene regulation, cellular signaling, immune balance, or stromal remodeling. These mechanistic defects can compromise implantation without producing clinically visible pathology. These limitations further reflect a broader challenge in diagnosing UFI, as the most critical determinants of implantation success are functional rather than structural. Importantly, processes such as decidualization, stromal cell survival signaling, epithelial–stromal communication, tightly regulated inflammatory activation, and immune–endocrine crosstalk cannot be adequately assessed with current clinical tools. Without appropriate tools and techniques to evaluate UFI within individual patient samples, a substantial proportion of uterine dysfunction remains undetected and unaddressed. These diagnostic gaps underscore the need for mechanistic frameworks that extend beyond structural evaluation to encompass the cellular and molecular factors of the endometrium, and hormonal and immunological determinants of endometrial receptivity, particularly in infertile patients without detectable uterine pathology (Fig.  2 ). Fig. 2 Temporal sequence of endometrial compartment activation during the menstrual cycle. A schematic diagram of the 28-day menstrual cycle shows a sequential, hormone-driven activation of four major endometrial compartments. In each stage, the dominant compartment is shown in the top row, while horizontal arrows indicate the functional interactions that trigger the next stage. Vertical arrows (bottom row) represent the downstream structures generated at each step. A dashed feedback arrow from Stage 4 to Stage 2 highlights vascular-derived signaling that helps maintain stromal function during early pregnancy. E2, estradiol; P4, progesterone; PR, progesterone receptor; MUC1, mucin-1; cAMP, cyclic adenosine monophosphate; FOXO1, forkhead box O1; IGFBP1, insulin-like growth factor binding protein 1; BMP2, bone morphogenetic protein 2; HAND2, heart and neural crest derivatives-expressed protein 2; uNK, uterine natural killer cells; WOI, window of implantation; RIF, recurrent implantation failure; RPL, recurrent pregnancy loss Temporal sequence of endometrial compartment activation during the menstrual cycle. A schematic diagram of the 28-day menstrual cycle shows a sequential, hormone-driven activation of four major endometrial compartments. In each stage, the dominant compartment is shown in the top row, while horizontal arrows indicate the functional interactions that trigger the next stage. Vertical arrows (bottom row) represent the downstream structures generated at each step. A dashed feedback arrow from Stage 4 to Stage 2 highlights vascular-derived signaling that helps maintain stromal function during early pregnancy. E2, estradiol; P4, progesterone; PR, progesterone receptor; MUC1, mucin-1; cAMP, cyclic adenosine monophosphate; FOXO1, forkhead box O1; IGFBP1, insulin-like growth factor binding protein 1; BMP2, bone morphogenetic protein 2; HAND2, heart and neural crest derivatives-expressed protein 2; uNK, uterine natural killer cells; WOI, window of implantation; RIF, recurrent implantation failure; RPL, recurrent pregnancy loss The luminal epithelium is the most superficial layer of the endometrium and the initial point of contact for the blastocyst. This layer is composed of columnar epithelial cells and undergoes a series of hormone-driven molecular and structural changes that prepare the apical surface to establish a brief window of receptivity and permit blastocyst adhesion, positioning, and early nutrient exchange [ 33 ]. One of the earliest events in this transition is the remodeling of the apical membrane, characterized by the loss of microvilli and changes in epithelial adhesiveness [ 33 ]. During the proliferative phase, rising E2 levels stimulate epithelial and stromal proliferation and induce P4 receptor (PR) expression throughout the endometrium [ 21 , 34 ]. This E2-driven expansion and hormonal priming are essential prerequisites for the P4-mediated cellular and molecular transformations that ultimately define the receptive state (Figs.  3 and 4 ). Fig. 3 Hormonally dependent expression patterns of cytokines and chemokines in the endometrium and their signaling across the menstrual cycle. A schematic diagram representing the serum concentrations of estradiol (E2, blue) and progesterone (P4, green) across the 28-day menstrual cycle in the top panel, along with ovulation (day 14) and the window of implantation (days 19–21), is indicated. The lower panel depicts the temporal expression of eight key cytokines and chemokines relative to hormonal concentration. Bar length represents the relative duration of expression across the cycle. E2, estradiol; P4, progesterone; LIF, leukemia inhibitory factor; IL, interleukin; TNF-α, tumor necrosis factor alpha; CXCL12, C-X-C motif chemokine ligand 12; uNK, uterine natural killer cells; JAK2, Janus kinase 2; STAT3, signal transducer and activator of transcription 3; ECM, extracellular matrix; WOI, window of implantation Fig. 4 Activation of cytokine receptors and their convergence on the PI3K/AKT signaling pathway. Schematic diagram representing cytokine receptor involvement and downstream signal integration through the PI3K/AKT pathway in endometrial stromal cells during the secretory phase. Three cytokine signaling clusters: IL-8/IL-1β (acting via gp130/LIFR/CXCR receptors), IL-6/LIF/IL-11 (via JAK1/JAK2), and TNF-α/IL-15 (via TNFR1/TNFR2/IL-15Rα) converge on PI3K/AKT activation, highlighting a common intracellular signaling axis. IL, interleukin; LIF, leukemia inhibitory factor; TNF-α, tumor necrosis factor alpha; gp130, glycoprotein 130; LIFR, LIF receptor; CXCR, C-X-C chemokine receptor; JAK, Janus kinase; TNFR, TNF receptor; PI3K, phosphoinositide 3-kinase; PIP2/3, phosphatidylinositol bisphosphate/trisphosphate; PDK1, phosphoinositide-dependent kinase 1; AKT, protein kinase B; mTORC1, mechanistic target of rapamycin complex 1; FOXO1, forkhead box O1; GSK3β, glycogen synthase kinase 3 beta; Wnt, wingless-type signaling; IGFBP1, insulin-like growth factor binding protein 1; PRL, prolactin; uNK, uterine natural killer cells; VEGF, vascular endothelial growth factor; ANGPT2, angiopoietin-2; RIF, recurrent implantation failure Hormonally dependent expression patterns of cytokines and chemokines in the endometrium and their signaling across the menstrual cycle. A schematic diagram representing the serum concentrations of estradiol (E2, blue) and progesterone (P4, green) across the 28-day menstrual cycle in the top panel, along with ovulation (day 14) and the window of implantation (days 19–21), is indicated. The lower panel depicts the temporal expression of eight key cytokines and chemokines relative to hormonal concentration. Bar length represents the relative duration of expression across the cycle. E2, estradiol; P4, progesterone; LIF, leukemia inhibitory factor; IL, interleukin; TNF-α, tumor necrosis factor alpha; CXCL12, C-X-C motif chemokine ligand 12; uNK, uterine natural killer cells; JAK2, Janus kinase 2; STAT3, signal transducer and activator of transcription 3; ECM, extracellular matrix; WOI, window of implantation Activation of cytokine receptors and their convergence on the PI3K/AKT signaling pathway. Schematic diagram representing cytokine receptor involvement and downstream signal integration through the PI3K/AKT pathway in endometrial stromal cells during the secretory phase. Three cytokine signaling clusters: IL-8/IL-1β (acting via gp130/LIFR/CXCR receptors), IL-6/LIF/IL-11 (via JAK1/JAK2), and TNF-α/IL-15 (via TNFR1/TNFR2/IL-15Rα) converge on PI3K/AKT activation, highlighting a common intracellular signaling axis. IL, interleukin; LIF, leukemia inhibitory factor; TNF-α, tumor necrosis factor alpha; gp130, glycoprotein 130; LIFR, LIF receptor; CXCR, C-X-C chemokine receptor; JAK, Janus kinase; TNFR, TNF receptor; PI3K, phosphoinositide 3-kinase; PIP2/3, phosphatidylinositol bisphosphate/trisphosphate; PDK1, phosphoinositide-dependent kinase 1; AKT, protein kinase B; mTORC1, mechanistic target of rapamycin complex 1; FOXO1, forkhead box O1; GSK3β, glycogen synthase kinase 3 beta; Wnt, wingless-type signaling; IGFBP1, insulin-like growth factor binding protein 1; PRL, prolactin; uNK, uterine natural killer cells; VEGF, vascular endothelial growth factor; ANGPT2, angiopoietin-2; RIF, recurrent implantation failure E2 is released from the ovarian antral follicle in response to follicle-stimulating hormone (FSH) secreted by the anterior pituitary gland. As E2 levels rise, E2 binds to one of two E2 receptors, ERα or ERβ. ERα is expressed in both stromal and epithelial cells and plays a key role in regulating endometrial growth and fertility, whereas ERβ, expressed in epithelial, stromal, and endometrial glandular cells, primarily regulates endometrial differentiation and modulates the uterine lining [ 7 , 35 ]. During the proliferative phase, E2 binding to ERα in stromal cells stimulates increased production of insulin-like growth factor 1 (IGF-1), which promotes epithelial cell proliferation. Following IGF-1 binding to the IGF-1 receptor (IGF-1R) on epithelial cells, the Akt/PI3K signaling pathway is activated, further driving cellular proliferation and contributing to the reconstruction of the uterine lining. As E2 levels continue to rise, ciliary differentiation occurs, endometrial glands become narrow and straight, the stroma thickens, and vascularization is established [ 36 , 37 ]. As the secretory phase begins around day 14 of the menstrual cycle, P4 is secreted by the corpus luteum (CL), and the endometrial environment undergoes further changes to prepare for implantation if a blastocyst is present. These transformations coincide with ovulation, as rising P4 levels shift endometrial signaling from E2-driven proliferation to a P4-dominated state characterized by differentiation and secretion [ 33 , 38 ]. During this phase, luminal epithelial cells not only lose apical microvilli and cilia but also undergo apical surface flattening, accompanied by partial loosening of tight junctions, which facilitates blastocyst attachment [ 23 ]. Concurrently, the anti-adhesive glycoprotein Mucin-1 (MUC1), which is highly abundant during the proliferative phase, is downregulated, resulting in altered apical polarity and permitting stable embryo adhesion [ 39 , 40 ]. These processes are accompanied by dynamic changes in intracellular trafficking and the establishment of maternal immune tolerance, which together limit excessive immune activation in response to the developing conceptus. Through coordinated modulation of cellular transport pathways and immune signaling, the endometrium shifts away from a primarily protective barrier toward a permissive environment that supports blastocyst attachment, survival, and early implantation. These adaptations mark the initiation of the uterine transition to a receptive, implantation-competent state. As the cycle reaches peak uterine receptivity around days 19–21 of a typical 28-day cycle (Fig.  2 ), P4 binds to the P4 receptor (PR) in the nucleus [ 41 ] and activates the transcription of the Homeobox protein Hox-A10 (HOXA10) gene [ 42 , 43 ]. HOXA10 subsequently induces the expression of key mediators of receptivity, including the integrins αvβ3 and α4β1, which support blastocyst adhesion, regulatory cytokines, stromal differentiation markers like Insulin-like growth factor-binding protein 1 (IGFBP1), and epithelial structures known as pinopodes [ 44 – 46 ]. Integrins are localized to the apical surface of luminal epithelial cells and function as ligands for trophoblast-associated surface molecules, along with osteopontin and fibronectin, thereby facilitating stable blastocyst adhesion [ 47 , 48 ]. Pinopodes are closely associated with the window of implantation and contribute to blastocyst attachment by expressing adhesion receptors, absorbing uterine fluid, and helping to establish a permissive microenvironment at the epithelial surface [ 46 , 49 , 50 ]. In addition to HOXA10 , pinopode formation is promoted by leukemia inhibitory factor (LIF), which activates Signal Transducer and Activator of Transcription 3 (STAT3) signaling in luminal epithelial cells during the implantation window. Together, HOXA10 -dependent transcriptional programs and LIF–STAT3 signaling drive cytoskeletal remodeling and generate the specialized epithelial surface features required for successful implantation [ 51 , 52 ]. In the absence of implantation, the endometrial epithelium reverts to a non-receptive state, marked by the reappearance of apical microvilli, re-tightening of intercellular junctions, and increased expression of anti-adhesive factors such as MUC1 [ 39 , 53 ]. This transition is followed by epithelial apoptosis and inflammatory priming, initiating menstrual shedding. The stromal compartment lies beneath the luminal epithelium, a fibroblast-dense connective tissue that provides structural support while serving as a critical site of P4-dependent remodeling and immune regulation. This compartment also houses key immune cell populations, including macrophages and uterine natural killer (uNK) cells, which play essential roles in implantation and early pregnancy [ 23 , 54 , 55 ]. During the proliferative phase of the menstrual cycle, rising E2 levels stimulate stromal and epithelial proliferation. However, following ovulation, P4 secreted by the CL, together with increased intracellular cyclic AMP (cAMP), initiates a highly specialized differentiation program within uterine stromal cells that supports embryo implantation, regulates inflammation signaling, and promotes maternal immune tolerance [ 23 , 25 , 56 , 57 ]. This process, termed decidualization, is characterized by extensive transcriptional reprogramming, cytoskeletal reorganization, and matrix metalloproteinase (MMP)-mediated remodeling of the extracellular matrix (ECM). Decidualized stromal cells also secrete a diverse array of cytokines, chemokines, and growth factors that modulate trophoblast invasion, vascular adaptation, and immune cell function at the maternal–fetal interface (Figs.  3 and 4 ) [ 57 – 59 ]. Parallelly, P4 and cAMP activate protein kinase A (PKA) in uterine stromal fibroblasts, leading to the phosphorylation of key transcription factors, including cAMP Response Element-Binding Protein (CREB). CREB activation initiates transcription of core decidual genes, namely Bone Morphogenetic Protein 2 ( BMP2 ), FOXO1, and IGFBP1 , which collectively promote stromal differentiation, immune modulation, and cell survival during early pregnancy [ 44 , 45 , 57 , 60 ]. Another P4-induced transcription factor, heart- and neural crest derivatives-expressed protein 2 (HAND2), plays a critical role in stromal–epithelial coordination by suppressing E2 signaling in the adjacent epithelium. HAND2 accomplishes this through downregulating fibroblast growth factors (FGFs), thereby limiting epithelial proliferation and enabling stromal decidualization during the implantation window [ 61 , 62 ]. BMP2 acts synergistically with HAND2 to establish the decidual transcriptional program and promote stromal remodeling, in part by enhancing IGFBP1 expression [ 6 , 63 ]. As decidualization proceeds, decidual stromal cells secrete key immunomodulatory factors, interleukin-15 (IL-15) and C-X-C motif chemokine ligand 12 (CXCL12), which recruit and support the survival and functional maturation of uNK cells [ 64 ]. In parallel, decidual stromal cells remain responsive to embryo-derived signals, such as human chorionic gonadotropin (hCG), through expression of the luteinizing hormone/chorionic gonadotropin receptor ( LHCGR ), further reinforcing stromal differentiation and immune tolerance at the maternal–fetal interface [ 65 , 66 ]. FOXO1, another transcription factor, not only drives the decidual transcriptional program but also promotes resistance to oxidative and cytokine-induced stress, in part through regulation of anti-apoptotic pathways such as BCL2 and inhibition of p53-mediated apoptosis [ 67 , 68 ]. Concurrently, MMPs, including MMP2 and MMP9, remodel the endometrial ECM by degrading basement membrane components, such as collagen and laminin, thereby facilitating tissue remodeling and trophoblast invasion. As remodeling progresses, these enzymes also contribute to the deposition of matrix components, including fibronectin, vitronectin, and laminin, that support trophoblast adhesion, invasion, and vascular remodeling [ 59 , 69 ]. Emerging evidence further suggests that decidual stromal cells influence epithelial receptivity through paracrine signaling. Stromal-derived factors, such as heparin-binding EGF-like growth factor (HB-EGF), act on luminal epithelial cells to enhance adhesion molecule expression and promote cytoskeletal remodeling, thereby sustaining a receptive epithelial phenotype throughout the implantation window [ 70 , 71 ]. In parallel with tightly regulated apoptosis, a subset of decidual stromal cells undergoes acute senescence, characterized by secretion of senescence-associated secretory phenotype (SASP) factors. While transient senescence may facilitate tissue remodeling, dysregulated or persistent SASP signaling can promote inflammation and impair implantation success [ 72 – 74 ]. In patients with RPL, endometrial biopsies demonstrate downregulation of key decidual markers, FOXO1 and IGFBP1, indicating incomplete or defective decidual transformation [ 25 ]. This deficiency reflects an impaired ability of stromal cells to suppress local inflammation, mount appropriate stress-response pathways, and provide cues necessary for early trophoblast invasion. Together, these findings suggest that effective immune modulation, cellular stress adaptation, and trophoblast support are essential components of a receptive decidual phenotype and are likely coordinated by P4-dependent regulatory networks. Whereas if decidualization is inadequate or dysregulated, the endometrium fails to attain complete receptivity, resulting in impaired implantation, early pregnancy loss, or RPL. Thus, while several core transcriptional regulators of decidualization have been identified, the mechanisms by which endocrine signals integrate with immune and inflammatory cues, particularly under pathological or stress-related conditions, remain incompletely understood, especially in humans. This knowledge gap highlights the endometrial stromal compartment as a promising yet underexplored diagnostic and therapeutic target in uterine-driven infertility. Uterine epithelial glands are distributed throughout the stratum functionalis and stratum basalis of the endometrium and are essential for a receptive environment during the peri-implantation window. Glands within the functionalis undergo cyclical remodeling and hormone-dependent secretory activation, whereas glands in the basalis preserve regenerative capacity across menstrual cycles [ 4 , 75 ]. These glands extend from the luminal epithelium into the stromal compartment and secrete histotroph, a complex mixture of cytokines, growth factors, nutrients, and glycoproteins that sustains the preimplantation embryo, promotes blastocyst adhesion, and modulates the local immune microenvironment required for placental establishment (Figs.  3 and 4 ). Key gland-derived secretory factors, such as LIF, glycodelin, vascular endothelial growth factor (VEGF), and insulin-like growth factor binding proteins (IGFBPs), are critical regulators of blastocyst development, angiogenesis, immune tolerance, and endometrial receptivity [ 76 , 77 ]. The indispensable role of uterine glands in fertility has been demonstrated in glandless sheep models and mice deficient in FOXA2, a transcription factor required for uterine gland development, underscoring their essential contribution to implantation [ 78 , 79 ]. Gland morphology, maturation, and developmental timing directly influence histotroph composition, particularly during the mid-secretory phase and early pregnancy, when the embryo depends entirely on glandular secretions for survival [ 76 ]. Accordingly, disruptions in gland formation or function, arising from endometrial fibrosis, chronic inflammation, epithelial injury, or hormonal dysregulation, have been strongly associated with implantation failure and early pregnancy loss [ 77 – 79 ]. Similarly, studies have reported reduced levels of glycodelin and LIF in uterine fluid from patients with chronic endometritis or unexplained RIF, suggesting that gland dysfunction may manifest not only as a structural deficiency but also as impaired secretory signaling, which is essential for endometrial receptivity [ 77 ]. The function of uterine glands is tightly regulated by hormonal cues, predominantly through P4 signaling mediated by PR-A and PR-B isoforms, which drives glandular morphogenesis and secretory activation during the mid-secretory phase [ 6 , 75 , 80 ]. The P4-responsive phase coincides with the implantation window and precedes blastocyst arrival, ensuring that the endometrium is appropriately primed for embryo adhesion and early development. In addition to hormonal regulation, there is growing evidence for dynamic crosstalk between the embryo and the gland during the peri-implantation period [ 81 ]. Glandular epithelium cells express the luteinizing hormone/chorionic gonadotropin receptor (LHCGR), enabling direct responsiveness to blastocyst-derived human chorionic gonadotropin (hCG) signals, which modulate secretion and alter histotroph composition to support implantation [ 71 , 82 ]. Embryo-derived factors such as hCG, interleukin-1β (IL-1β), and interferon-τ (IFN-τ) stimulate uterine gland activity, reinforcing a uterine microenvironment conducive to embryonic survival and placental establishment [ 83 , 84 ]. These studies further demonstrate that the blastocyst is not a passive structure but an active signaling unit that releases paracrine mediators both before and during implantation. Disruption of these signaling pathways or of gland development more broadly has been associated with defective implantation and early pregnancy loss. Clinical conditions such as chronic endometritis, Asherman’s syndrome, and post-ablation endometrial injury are frequently linked to glandular dysfunction despite normal circulating hormone levels and adequate endometrial thickness by conventional assessment [ 85 , 86 ]. Importantly, these abnormalities are often accompanied by epithelial scarring and fibrosis, which impairs histotroph secretions even when gross endometrial anatomy and endocrine responsiveness appear preserved. Consistent with these observations, reduced levels of key gland-derived factors, including glycodelin and LIF, have been detected in uterine fluid from patients with chronic endometritis or unexplained RIF [ 59 , 87 ]. These findings underscore the critical role of uterine glands in establishing endometrial receptivity and highlight glandular dysfunction manifesting not only as structural loss but also as impaired secretory signaling as a significant and underrecognized contributor to UFI [ 9 , 77 , 88 ]. Thus, positioning the glandular epithelium as a promising diagnostic and therapeutic target, particularly for identifying sub-receptive endometrial states that evade standard clinical detection [ 30 , 71 ]. Although pregnancy was once viewed as an immune-privileged state, it is now well understood that the peri-implantation endometrium contains a tightly regulated immune microenvironment, enriched with leukocytes that actively contribute to tissue remodeling, angiogenesis, and embryo tolerance [ 32 , 89 , 90 ]. Implantation occurs within a finely tuned immunological environment, where endocrine and inflammatory signals intersect to shape endometrial receptivity rather than suppress immune function outright [ 32 , 89 , 90 ] (Figs.  3 and 4 ). Leukocytes constitute approximately 30–40% of endometrial cells during the secretory phase, with uterine natural killer (uNK) cells representing the most abundant population. P4 signaling indirectly orchestrates uNK recruitment and maturation by stimulating stromal cells to secrete IL-15 and CXCL12, thereby recruiting uNK cells and promoting their differentiation, survival, and functional specialization [ 55 , 66 , 91 ]. In parallel, macrophages and dendritic cells also increase during the peri-implantation period and localize near spiral arteries and sites of epithelial remodeling, suggesting coordinated immune recruitment in anticipation of implantation and placental development [ 80 , 92 ]. Importantly, P4 regulates the composition and activity of these immune populations, promoting a controlled, pro-receptive inflammatory milieu rather than global immune suppression. This balanced immune state supports vascular remodeling, stromal differentiation, and maternal–fetal tolerance, processes essential for successful implantation and early pregnancy maintenance [ 5 , 92 ]. uNK cells play a central role in establishing the vascular and immunological foundation required for early pregnancy. These cells secrete key angiogenic mediators, including VEGF, angiopoietin-2 (ANGPT2), and granulocyte–macrophage colony–stimulating factor (GM-CSF), which collectively drive spiral artery remodeling, a process critical for supporting placentation and fetal development. In addition, uNK cells regulate trophoblast invasion by producing cytokines, mainly interleukin-8 (IL-8) and interferon-γ (IFN-γ), within a tightly regulated temporal window during implantation [ 93 ]. VEGF and ANGPT2 promote vascular dilation and perivascular remodeling [ 25 , 93 ], while IFN-γ promotes apoptosis of vascular smooth muscle cells, enabling spiral artery transformation. GM-CSF further facilitates epithelial–trophoblast communication and contributes to decidual matrix remodeling [ 71 ]. Endometrial macrophages constitute another essential immune population during the implantation window. As the primary innate immune sentinels of the uterus, they clear apoptotic cells, remodel the ECM, and maintain tissue homeostasis. Dendritic cells also contribute to immune adaptation by promoting maternal tolerance through the expansion of regulatory T cells at the fetal–maternal interface [ 80 ]. During the window of implantation, endometrial macrophages predominantly adopt an M2-like, anti-inflammatory phenotype, characterized by secretion of IL-10 and TGF-β, which support immune tolerance, matrix turnover, and epithelial stability. Importantly, immune signaling within the endometrium is bidirectional. Cytokines derived from uNK cells and macrophages, including IFN-γ and IL-10, directly influence stromal cell behavior, modulating decidual stability, ECM composition, and subsequent cytokine production [ 53 , 84 ]. This reciprocal crosstalk ensures coordinated vascular remodeling, immune adaptation, and stromal differentiation necessary for successful implantation and early pregnancy progression. Aberrant immune cell activation has been implicated in reproductive disorders, including RIF, endometriosis, and chronic endometritis. Excessive pro-inflammatory cytokine signaling or failure to establish a tolerogenic immune profile can impair decidualization, disrupt epithelial integrity, and compromise vascular adaptation required for successful implantation [ 18 , 89 ]. An elevated uNK cell density or dysregulated IL-15 signaling has been reported in patients with RIF, whereas macrophage dysfunction has been associated with defective ECM clearance and fibrotic remodeling in chronic inflammatory states [ 29 , 90 ]. Disruption of this finely tuned inflammatory balance, whether due to insufficient activation or excessive cytokine production, ultimately undermines the implantation process. These observations underscore the critical need to elucidate how immune and endocrine signaling pathways are coordinated within the endometrial microenvironment, particularly under pathological conditions that compromise uterine receptivity. Uterine vascular remodeling is a hormonally regulated, phase-specific process essential for establishing an endometrial microenvironment that supports blastocyst implantation and early trophoblast invasion. Uterine blood supply originates from the uterine arteries, which branch into arcuate arteries within the myometrium and further subdivide into radial arteries at the endometrial–myometrial junction. These radial arteries give rise to basal arteries, which supply the stratum basalis, and spiral arteries, which perfuse the stratum functionalis. Spiral arteries represent the most dynamic vascular component of the endometrium, undergoing extensive cyclical remodeling in response to hormonal and immune signals. This remodeling is critical for increasing vascular demand, regulating blood flow, and enabling successful implantation and early placental development [ 23 , 93 ]. Throughout the menstrual cycle (Figs.  1 , 3 , and 4 ), the uterine vasculature undergoes continuous, hormone-driven remodeling to create an environment permissive to implantation and early pregnancy. During the proliferative phase, E2 stimulates epithelial secretion of IL-8, which recruits neutrophils that subsequently release VEGF [ 94 , 95 ]. VEGF-mediated response promotes angiogenesis, leading to the formation of the subepithelial capillary plexus and the elongation of the spiral artery toward the luminal surface. VEGF has three isoforms, namely VEGF-A, VEGF-C, and VEGF-D, which are expressed in the uterus, while VEGF-A plays the dominant role throughout the menstrual cycle [ 96 , 97 ]. VEGF-A drives endothelial cell proliferation, vessel sprouting, and spiral artery elongation, with increased vascular permeability. By loosening endothelial junctions, VEGF-A enhances plasma extravasation and stromal edema, thereby contributing to a receptive endometrial microenvironment that supports subsequent decidualization and implantation [ 25 , 93 ]. Following ovulation, P4 shifts the endometrial vasculature dynamics from proliferative angiogenesis toward functional remodeling required for implantation. During this phase, Angiopoietin-2 (ANGPT2), produced by endothelial cells, promotes destabilization of vessels and loosening of endothelial junctions, whereas ANGPT1, upregulated in response to P4, facilitates pericyte recruitment, endothelial stabilization, and vessel maturation. The coordinated balance between ANGPT2-mediated plasticity and ANGPT1-driven stabilization enables controlled vascular expansion, perfusion of the superficial stroma, and priming of spiral arteries for decidual invasion [ 98 ]. Concurrently, uNK cells secrete placental growth factors (PlGF) and VEGF-C, further priming the stromal compartment for decidualization and vascular adaptation. However, this preparatory phase remains incomplete until a viable blastocyst is present, whose signals trigger the final reinforcement of decidual transformation and enable stable implantation [ 99 , 100 ]. Disruption of endometrial vascular remodeling through imperfect timing, inadequate stabilization, or incomplete spiral artery transformation has been strongly associated with implantation failure, early pregnancy loss, and pregnancy complications such as preeclampsia, underscoring the critical importance of precisely coordinated vascular remodeling during early gestation [ 101 ].