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 Table of Contents  
REVIEW ARTICLE
Year : 2021  |  Volume : 5  |  Issue : 2  |  Page : 119-127

Research progress on human endometrium decidualization In vitro cell models


1 Department of Reproductive Endocrinology, Obstetrics and Gynecology Hospital, Fudan University; Shanghai Key Laboratory of Female Reproductive Endocrine Related Diseases, Shanghai 200011, China
2 Department of Reproductive Endocrinology, Obstetrics and Gynecology Hospital, Fudan University, Shanghai 200011, China

Date of Submission06-Oct-2020
Date of Decision23-Nov-2020
Date of Acceptance22-Mar-2021
Date of Web Publication08-Jul-2021

Correspondence Address:
Wei Zhang
Obstetrics and Gynecology Hospital, Fudan University, 419 Fangxie Road, Shanghai 200011
China
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2096-2924.320882

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  Abstract 


Decidualization is a special type of differentiation of endometrial stromal cells into secretory decidualized cells, which is closely related to the occurrence of menstruation and establishment of pregnancy. Decidualization abnormalities can cause female infertility and abortion, and the decidualization model in vitro is an important tool for studying relevant mechanisms. This article summarizes several in vitro decidualization models in recent research from three aspects, including the selection of model cells and culture systems, evaluation of decidualization markers, and induction schemes. These models can be appropriately selected and applied in specific endometrium-related disease models, such as endometriosis, recurrent pregnancy loss, and preeclampsia.

Keywords: Cyclic Adenosine Monophosphate; Decidualization; Endometrium; Estrogen; In vitro Cell Model; Progestogen


How to cite this article:
Tang ZJ, Guan HY, Wang L, Zhang W. Research progress on human endometrium decidualization In vitro cell models. Reprod Dev Med 2021;5:119-27

How to cite this URL:
Tang ZJ, Guan HY, Wang L, Zhang W. Research progress on human endometrium decidualization In vitro cell models. Reprod Dev Med [serial online] 2021 [cited 2021 Jul 31];5:119-27. Available from: https://www.repdevmed.org/text.asp?2021/5/2/119/320882




  Introduction Top


The successful establishment of pregnancy depends on embryo quality, implantation capability, and the synchronous decidualization of the endometrium, which becomes receptive to the embryo, accommodating the embryo for implantation. Unlike rodent decidualization, which relies on the presence of a conceptus, human endometrium decidualization occurs from the mid-secretory phase after ovulation, regulated by periodic changes in ovarian steroid hormone levels and other factors.[1] The morphology and function of endometrial stromal cells (ESCs) change to prepare for implantation. If there is a lack of implanted embryos, the endometrium disintegrates and menstruation occurs due to a remarkable decrease in ovarian steroid hormone levels. Impaired decidualization is a vital reason for early pregnancy loss and pregnancy-related complications, such as preeclampsia (PE).[2],[3],[4] Therefore, it is important to understand the underlying mechanism for diagnosing and treating related disorders.

Several methodologies have been utilized to explore the mechanisms of decidualization and related disorders. The mouse is one of the most commonly used experimental animals for in vivo experiments. Decidualization in the mouse uterus can be artificially induced through mechanical stimulation or oil injection in a hormonally primed uterus, which provides the appearance of blastocyst presence.[4],[5] There are translational complications associated with animal models and ethical dilemmas regarding in vivo experiments in humans. Therefore, in vitro decidualization models are practical and have been most successfully studied. This review summarizes several in vitro decidualization models from various aspects, including tool cells, culture systems, evaluation, and induction schemes.


  Sources of Tool Cells Top


Primary human endometrial stromal cells

Most decidualization in vitro models has been established in primary ESCs, which can be isolated from human endometrium by scraping uterine tissues resected due to benign diseases, such as uterine fibroids, or obtained by hysteroscopy or vaginal curettage.[6] However, histopathological identification is required to exclude the influence of the diseased endometrium on the experimental results. Most of these studies histologically identified the phase of the menstrual cycle when endometrium samples were collected. For various research purposes, different stages of the menstrual cycle, in which the cells are originally isolated, should be distinguished. Decidualization models are usually used as foundational models of dysregulated decidualization-related implantation failure and subsequent early pregnancy loss. In the study conducted by Szwarc et al. on the mechanism by which promyelocytic leukemia zinc finger regulates decidualization, endometrial biopsies were obtained during menstrual cycle day 7 to 12 from healthy women of reproductive age.[7] Furthermore, to reveal the role of autophagy gene ATG16L1 in decidualization, Oestreich et al. collected endometrial biopsies during menstrual cycle day 8 to 12.[8] To study the mechanism by which FK506 binding protein 51 (FKBP51) regulates decidualization, endometrium tissues were collected by Wei et al. during the mid-follicular phase and induced decidualization for 1–4 days with 1 μmol/mL medroxyprogesterone acetate (MPA) and 0.5 mM cyclic adenosine monophosphate (cAMP). The mRNA levels of decidualization markers, insulin-like growth factor-binding protein-1 (IGFBP-1) gene and prolactin (PRL) gene, elevated nearly 10,000- and 1,000-fold, respectively, on day 4.[9] In contrast, to explore the mechanism of the clearance of senescent decidual cells for endometrial rejuvenation and remodeling during embryo implantation, Ochiai et al. collected tissues during secretory phase because endometrium remodeling was the most intense during the mid-luteal phase.[10] Brighton et al. obtained biopsies between 5 and 13 days post-luteinizing hormone surge.[11] Decidualization is essential for embryo implantation and placental development. Therefore, the sampling time in the study conducted by Katoh et al. on differentially expressed genes in ESCs and in vitro decidualized cells is described as a secretory phase. However, they admitted their limitations in elucidating the factors that possibly affect the results, such as phases of menstrual cycle and cell culture conditions (passage numbers, types, and concentrations of reagents to induce decidualization and duration of their administration).[12] Some studies did not discriminate or clearly state the phase;[5],[13] however, no significant difference was found in their induction schemes or observation indices. ESCs from the mid-proliferative phase to secretory phase have been applied in some studies and have achieved specific decidualization effects. However, the methods established above are based on the fact that PRL and IGFBP-1 are typical biomarkers of decidualization and can respond to ovarian hormones or some other induction factors in ESCs of both proliferative and secretory phases.[14] It is still unclear whether ESCs in the secretory phase can respond to artificial induction to the same degree as ESCs in the proliferative phase can do. Thus, endometrium sample collection should comply with research purposes.

The selection and quality control of the samples are critical factors for primary cells. In addition to the absence of endometrial lesions, the participants are required to be of childbearing age, use no hormones or non-steroidal anti-inflammatory drugs for nearly 3 months, and have no systemic diseases. Strict screening criteria limit the expansion of sample size. With the increased number of passages, the purity of primary cells would be improved; however, the cell status, function, and some molecular markers might also be distorted. Consequently, experiments should be conducted with cells that are limited to five passages.[8],[15]

Immortalized human endometrial stromal cells

Primary cells do not expand in long-term culture and endometrial biopsies obtained are limited in size.[16] Therefore, ESCs immortalized with human telomerase reverse transcriptase are also frequently applied in decidualization models,[17],[18],[19],[20],[21],[22] improving the situation in which the processes of isolation and purification of primary cells change some molecular markers. However, the immortalized cells are derived from only a single sample; therefore, conclusions drawn from such experiments have relatively poor popularization. However, the use of immortalized cell line is a considerably efficient method in verification research.

Primary human decidual stromal cells

James-Allan et al. found that decidual stromal cells (DSCs) from the first trimester can re-decidualize exposure to cAMP and progesterone.[23] Moreover, Saleh et al. showed that in DSCs isolated from the first-trimester placenta, morphological changes and growth arrest were induced using cAMP.[24] Similar to decidual fibroblast cells, progesterone with estradiol had no effect; however, it enhanced the induction effect of cAMP. In addition, the viability of DSCs after 6 days of induction decreased, indicating that this substance should be preferentially used in short-term experiments.[24]

Since stromal cells from the first trimester are mostly decidualized cells, they can be used to explore the relationship between decidualization abnormalities and reproductive diseases or the influence of exogenous factors, such as cytokines and hormones, on decidualization. For example, James-Allan et al. developed a pre-eclampsia model with DSCs and extravillous trophoblast cells (EVTs). They also revealed that DSCs played an important role in controlling EVT invasion during the first trimester of pregnancy.[23]

Primary human decidual fibroblast cells

A study by Richards et al. demonstrated that the decidualized uterine endometrium maintains a proliferating population of cells, namely decidual fibroblast cells, which can be recruited for differentiation to the decidual cell phenotype throughout pregnancy.[25] They found that decidual fibroblast cells possess morphological features and cellular antigens similar to those of ESCs and respond to treatment with cAMP, prostaglandin E2 (PGE2), and steroids in a manner almost identical to that of ESCs.[25] However, the decidual fibroblast cells cannot respond to steroid treatment without cAMP or PGE2 as sensitively as ESCs, which might be explained by a significantly lower basal level of progesterone receptor (PR) in the decidual fibroblast cells.[24],[25]

Brar et al. also extracted decidual fibroblast cells from true decidua of placenta.[26] Treatment of primary cultures of decidual fibroblasts with cAMP can induce PRL expression and decidualization; however, treatment with progesterone, alone or in combination with estradiol, has no effect. The treatment of cells with progesterone can potentiate the induction of decidualization in response to cAMP.[26]

Other researchers isolated human uterine fibroblasts from the decidua parietalis dissected from the placental membrane after normal vaginal delivery.[27],[28],[29],[30],[31] These cells are nondifferentiated stromal fibroblasts that respond to ovarian hormones and proinflammatory cytokines as well. Such cells can also be used as a cell model for endometrial remodeling disorders, such as endometriosis[31] and PE.[32]


  In vitro Culture Systems Top


Two-dimensional culture system

Two-dimensional (2D) culture system is still the most prevalent and economical method for decidualization-related studies. According to a published protocol, monolayer ESCs were incubated in conditional media with induction factors and collected after a few days to validate decidualization.[4] Without complex bioengineering platforms, such systems are convenient and practical for operation. However, 2D culture system is actually a simplified model and cannot completely simulate the microenvironment in vivo. The influencing factors that the model can explore are relatively single. For example, it cannot reveal the influence of the spatial structure or some temporal factors, and it is inadequate to study the integrated influences of multiple factors simultaneously.

Microfluidic organ-on-chip model

Gnecco et al. utilized an “Organ-on-a-Chip” microfluidic model of the human endometrial perivascular stroma to examine the crosstalk between stromal and endothelial cells.[33] Primary human ESCs were co-cultured for 14 days with primary uterine microvascular endothelial cells within a dual-chamber microfluidic device. Cultures were maintained with estradiol and progestin, with or without continuous laminar perfusion, to mimic hemodynamic forces derived from blood flow.[33] This study demonstrated that hemodynamic forces induced the secretion of specific endothelial cell-derived prostanoids that enhanced endometrial perivascular decidualization through a paracrine mechanism.[33] The microfluidic design provides both spatial and temporal characterization of the cellular communication between endometrial cell types and has been reported to be sustainable for up to 4 weeks, remains sensitive to steroids, and is suitable for quantitative biochemical analysis.[34]

Three-dimensional culture system

Compared with the 2D cell culture system, the cells in the three-dimensional (3D) organoid culture system more closely resemble architectural and functional characteristics of in vivo tissues.[16] Earlier studies established monocellular endometrial organoids by culturing endometrial epithelial cells within Matrigel in tissue culture inserts and stromal cells on plastic below the epithelial compartment.[35] Wang et al. established a human endometrium-like 3D culture system.[36] It was constructed with fibrin-agarose as a matrix scaffold, and using epithelial and stromal cells from both human primary cultures and established cell lines. Epithelial cells concentrated on top of the matrix form a monolayer, and stromal cells reside within the matrix, resembling a normal endometrial structure.[36] Abbas et al. also built upon endometrial organoids and developed a similar model.[37] They used porous collagen scaffolds produced with controlled lyophilization to direct cellular organization, integrating organoids with primary isolates of stromal cells.[37] In this model, both cell types are hormone-responsive, with hormone stimulation resulting in epithelial differentiation and stromal decidualization.[37] Olalekan et al. described a novel model of decellularized human endometrial tissue repopulated with primary endometrial cells.[38] Primary endometrial cells within recellularized scaffolds proliferated and remained viable for an extended period in vitro.[38] The cells within the endometrial scaffold expressed estrogen and PRs, and decidualization markers were secreted upon the addition of a cAMP analog, indicative of a decidualization response.[38]

Such bioengineered systems provide another unique model for studying interactions between the different endometrial cell compartments via soluble-paracrine signals and cell-to-cell interactions. They are a helpful tool to study certain aspects of decidualization and implantation.[36],[39]


  Features of Decidualization Top


Morphological changes

In the proliferative phase, ESCs have long and serrated nuclei and less cytoplasm. A few days after ovulation, cytoplasm increases and nucleus becomes large, round, with an increased number of nucleoli.[14] These changes gradually extend from the vicinity of the spiral artery to the entire endometrial stroma.[14] The most typical morphological change observed under an optical microscope is the transformation of ESCs from long fusiform fibroblast-like cells to large, round, polygonal epithelioid decidual cells.[14]

Abundant glycogen particles, lipid droplets, mitochondria, and endoplasmic reticulum can be observed intracellularly.[40] The remodeling of cytoskeletal actin filaments is usually detected by fluorescently labeled phalloidin combined with F-actin.[4]

Acquisition of secretory function

Another feature of decidualization is enhanced secretion function. DSCs secrete PRL, IGFBP-1, tissue factor, plasminogen activator inhibitor, and extracellular matrix components such as laminin, fibronectin, and collagen IV.[14] Among them, PRL and IGFBP-1 are the most convincing markers and are widely used to assess the degree of decidualization.

PRL secreted by stromal cells rises on the 22nd day of the menstrual cycle with the beginning of decidualization. Its expression level is positively correlated with the size of decidua cells.[14] At the maternal-fetal interface, PRL secreted by DSCs stimulates the growth and invasion of trophoblast cells, promotes angiogenesis, modulates uterine NK cells, and regulates amniotic fluid transport.[14] PRL of rat decidua inhibits the expression of progesterone-catabolizing enzyme 20α-hydroxysteroid dehydrogenase (20α-HSD) and proinflammatory cytokine Interleukin (IL)-6, which are potentially detrimental to pregnancy.[14],[40]

IGFBP-1 secreted by decidualized stromal cells is consistent with morphological changes in ESCs.[40] IGFBP-1 regulates endometrial cell division by regulating the bioavailability of IGF-1 and promotes trophoblast invasion through integrin-αβI and the mitogen-activated protein kinase pathway.[40]

Molecular biomarkers

With the widespread dissemination of omics and bioinformatics techniques, it has become a common method to identify genes differentially expressed before and after decidualization via high-throughput screening and low-throughput verification, yielding new researches and systematic reviews, which will not be enumerated in this study.[1],[41],[42] These differentially expressed genes of stromal cells are mostly involved in biological processes such as cell proliferation and differentiation, structural protein transformation, binding nuclear receptor protein, and secretion function.[43] Transcription regulatory factors such as PR, homeobox protein A10 (HOXA10), CCAAT enhancer binding protein β (C/EBPβ), signal transducer and activator of transcription 3 (STAT3), forkhead box O1 (FOXO1), transforming growth factors such as left-right determination factor 2, bone morphogenetic protein 2, and a series of molecules of the Wnt/β-catenin signaling pathway, including Wnt-5a and Dickkopf-related protein 1,[1],[6],[14],[40] are typical representatives. Remarkably, some biomarkers of DSCs in the late secretory phase are often expressed in endometrial epithelial cells of the proliferative phase.[5] Considering stromal cell transformation into epithelioid decidual cells, the process of mesenchymal-epithelial transition is suggested to be involved in decidualization,[14] and the mechanism of Snail in this process is one of the most widely studied.[5]


  Decidualization Induction Schemes Top


Master regulators of decidualization

Progesterone-PR signaling pathway plays a key role in the initiation and maintenance of decidualization. In ESCs, PR PR-A is the main PR isoform and has stronger transcriptional activity on the IGFBP-1 and PRL promoter regions of decidua cells compared with PR-B.[14] Estradiol (E2) plays an important role in the expression of PR, which promotes the expression of PR in the secretory phase, creating conditions for progesterone-mediated decidualization.[44]

cAMP is a widely distributed second messenger produced upon binding of extracellular ligands to G protein-coupled receptors. Subsequent activation of membrane-bound adenylyl cyclase leads to the generation of cAMP from adenosine triphosphate.[14] The level of cAMP in the endometrium of mid-secretory phase is significantly higher than that in proliferative phase, and increases with the degree of decidualization.[14] It can strengthen the in vitro decidualization reaction induced by progesterone;[45] however, it can also induce PRL expression to induce in vitro decidualization.[13] cAMP regulates decidualization-related transcriptional factors such as STAT5, C/EBP, FOXO1, and homeobox protein A11(HOXA11) through a series of phosphorylation reactions by activating the protein kinase A (PKA) signaling pathway, thus regulating the process of decidualization.[1],[14]

Decidualization induction schemes

In early years, E2 combined with progesterone (P4), which stimulates the hormone environment in vivo, is often used to induce decidualization in primary cultured endometrial cells, although it usually takes 12 to 20 days. However, additional cAMP can shorten the process to 3–4 days to observe a good effect.[15],[44]

As summarized in [Table 1], the most commonly used induction schemes are progesterone combined with cAMP with (EPC) or without (PC) estrogen, and evident decidualization characteristics can be observed as early as the first day of induction.[20] However, by progesterone with estrogen, morphology, and secretory function changes still cannot be observed until the 4th day after induction.[46]
Table 1: Induction schemes and evaluation index of in vitro decidualization

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The effective induction concentration of P4 or MPA ranges from 1 μmol/mL to 1 mmol/mL. However, 1 μmol/mL is most commonly used in different combinations. A recent study demonstrated that the addition of progesterone at an excess concentration of 0.16–20 μmol/mL based on the regimen of 1 μmol/mL MPA combined with 0.5 mmol/mL cAMP would result in decreased decidualization markers IGFBP-1, FOXO1 and impaired endometrial receptivity.[21] Another study also found that the expression of IGFBP-1 decreased with an increase in MPA concentration when 0.5 mmol/mL cAMP was combined with 10-8–10-4 mol/mL MPA.[9] In EPC schemes, the commonly used estrogen concentration is 10 nmol/mL, while in the study conducted by Oestreich et al., 100 nmol/mL estrogen could still induce a good decidualization effect on the 6th day.[8] However, studies have shown that estrogen can resist progesterone-promoting differentiation, and the induction scheme with E2 is less effective than that without E2.[46] The concentration of cAMP in these induction schemes usually took 0.5 mmol/mL, which significantly shortened decidualization time. Some recent studies have reported that some genes expressed in decidualization can only be regulated by progesterone but not by cAMP.[44] Therefore, there should be a certain difference between progesterone-mediated and cAMP-mediated pathways, suggesting that using progesterone or cAMP alone to induce in vitro decidualization has some shortcomings.

As mentioned above, for DSCs in the first trimester of pregnancy and decidual fibroblast cells of term placenta, decidualization effect relies on the appearance of cAMP; otherwise, treatment with progesterone, alone or in combination with estradiol, has no effect. Treatment of the cells with progesterone can potentiate the induction of decidualization in response to cAMP.[26] Therefore, cAMP is an indispensable factor in the decidualization of these cell types.

In addition, incorporating human chorionic gonadotropin, relaxin, IL-11, IL-1β, PGE2, or activin A into induction schemes containing progesterone can also increase decidualization markers.[14],[40] Studies have shown that relaxin, IL-1β, and PGE2 promote the expression of decidualization-related molecular markers through the PKA pathway and increases intracellular cAMP levels.[1],[14]


  Application of Decidualization In vitro Models Top


Endometriosis

Endometriosis is characterized by the presence of endometrial-like tissue outside of the uterine cavity. It affects 10%–15% of all women of reproductive age, 70% of women with chronic pelvic pain, and 20%–50% of women with infertility.[16] Incomplete endometrial decidualization and failure to suppress cell cycle genes are key factors in impaired implantation, a factor increasingly recognized in endometriosis-associated infertility.[53] Impaired decidualization in cells from patients with endometriosis is further showed by a 2-fold decrease in the expression of IGFBP-1 and PRL.[54] One potential mechanism is progesterone resistance in endometriosis. The expression of the PR-B isoform and P target genes, such as glycodelin and 17β-HSD-2, decreases in endometriotic tissue. Other gene products that are important for decidualization, such as HOXA10 and HOXA11, are also downregulated.[55] Endometrium decidualization in an in vitro model could be of great use for dissecting the relationship between dysregulated decidualization and endometriosis.

To study the mechanism of impaired decidualization in infertile women with endometriosis, Kim et al. established a decidualization in vitro model with samples of human endometrium from endometriosis patients with infertility and fertile disease-free control women undergoing tubal ligation.[56] The cells were cultured and treated with EPC (10 nmol/mL E2, 1 mmol/mL MPA, and 50 μmol/mL cAMP) for 6 days. After HDAC3 knockdown, the decreased level of PRL mRNA was found on the first day of EPC treatment; however, significantly decreased IGFBP-1 mRNA level was not found until day 6.[56]

Impaired decidualization can be a consequence of endometriosis and a potential pathogenic mechanism for endometriosis.[57] In a study of mechanisms for inadequate decidual response in stromal cells from ovarian endometriomas (OsisSC), stromal cells of the endometrium from women without endometriosis (HSC) or from OsisSC were treated with 100 nmol/mL MPA and 0.5 mmol/mL dibutyryl cAMP for 48 h.[57] The mRNA levels of IGFBP-1 and PRL significantly increased over 60- and 10-fold, respectively, in HSCs, but OsisSC expressed dramatically lower levels of IGFBP-1 and PRL mRNA with decreased expression of FOXO1 in response to PC treatment. In their subsequent explanation of the role of FOXO1 in OsisSC decidualization, they found that the PI3K/AKT pathway was the predominant pathway that attenuated short-term MPA + cAMP-induced IGFBP-1 expression based on a decidualization model using OsisSC.[57] The PI3K/AKT pathway is known to promote survival and proliferation; however, the pathway compromises OsisSC decidualization. Accordingly, researchers speculated that resistance to decidualization of ectopic lesions might promote their proliferation and/or survival, and sensitization of ectopic lesions to decidualization would promote the clearance or eradication of ectopic tissues.[57]

Comparing these two studies, we found that decidualization models could be applied to reveal the mechanism of endometriosis leading to pathological phenotypes such as infertility and explore the pathogenic mechanisms of endometriosis. Treatment with both PC and EPC schemes successfully induced a decidualization effect marked by increased mRNA levels of IGFBP-1 and PRL in 1–2 days. However, with different target genes manipulated, significant differential decidualization levels could be detected after different induction days. In the study conducted by Kim, the decreased level of IGFBP-1 could only be detected on day 6 of decidualization after HDAC3 knockdown,[56] while a significant decrease was observed on day 1 after FOXO1 knockdown in a previous study.[57] This suggested that for different target genes, the duration of induction might affect our estimate of their roles in decidualization. Meanwhile, the significant changes in IGFBP-1 and PRL occurring on different days of induction suggest that it is necessary to seek more comprehensive biomarkers of decidualization. Although both PC and EPC schemes induced decidualization effect successfully in these studies, the use of induction schemes with or without estrogen should be based on the effect of estrogen on the target gene, which was not evident in these two studies.

Recurrent pregnancy loss

Recurrent pregnancy loss (RPL) affects approximately 1%–2% of reproductive women.[58] Aberrant responsiveness of human ESCs to deciduogenic cues is strongly associated with RPL.[59] Differentiating ESCs from RPL patients is characterized by lower induction of decidual marker genes,[18],[60] and failure to discriminate between high-and low-quality human embryos in cell migration assays.[61] In addition, RPL is associated with a prolonged and highly disordered proinflammatory response.[14] During decidualization of ESCs, IL-33 is secreted, triggering autocrine and/or paracrine activation of its cell surface receptor, ST2L, which in turn drives an acute-phase inflammatory response.[62] Downregulation of ST2L and simultaneous induction of soluble decoy receptor sST2 ensures that this initial proinflammatory decidual response is intrinsically self-limiting.[62] However, in some RPL patients, the IL-33/ST2L/sST2 axis is activated in disorders.[62] This study utilized a traditional 2D culture system that cannot mimic the complex environment of maternal-fetal interface. Immune cells at the interface play a significant role in modulating the function of both maternal decidua and embryo trophoblast.[63]

Preeclampsia

PE complicates 5%–7% of all pregnancies.[64] It is a pregnancy-specific disorder characterized by new-onset hypertension and maternal organ damage after 20 weeks of gestation.[64] The typical pathologic basis of PE is impaired trophoblast invasion and failure of spiral artery remodeling.[64] Garrido-Gomez identified a transcriptomic fingerprint characterizing a decidualization defect in the endometrium of women with a history of severe PE (sPE), which is linked to impaired trophoblast invasion.[65]

Annexin A2 (ANXA2) is involved in mediating decidualization, and deficiency of maternal ANXA2 contributes to low decidual invasion by placental cytotrophoblast cells.[47] In this study, ESCs isolated from biopsies were collected in early secretory phase from women with previous sPE or normal obstetric outcomes. Treatment with both EP and PC for 5 days can evoke increased ANXA2 secretion; however, the latter evoked more strongly, and the increase was sharply reversed in sPE. When ANXA2 was silenced, PRL and IGFBP-1 secretion was not detected during decidualization. To evaluate the functional effect of ANXA2 inhibition on ESCs modulating trophoblast invasion, they performed in vitro models of embryo spreading on stromal cells, wound-healing assay to evaluate cell motility, and invasion assays.[47] This study used both in vitro and in vivo models to address the proof-of-concept that maternal deficiency of ANXA2 could be a potential biomarker for shallow decidual invasion by placental cytotrophoblasts that leads to PE, which is considerably persuasive. It is also an excellent example of integrated usage of diverse models such as embryo in vitro implantation model and ESC-trophoblast co-culture system, which closely associated decidualization resistance with sPE. However, the pathogenesis of PE is associated with crosstalk between decidual cells and trophoblast cells.[66],[67] In the rat uterus, intracellular basic fibroblast growth factor (bFGF) content in luminal epithelial and decidual cells increased with embryonic implantation; however, similar cells outside of the implantation site and in the artificially decidualized uterus did not express analogous bFGF levels, indicating that a unique signal from the embryo triggers bFGF expression.[68] DSCs or decidual fibroblast cells could be more suitable as tool cells for PE-related studies, similar to many other studies.[32],[69] Since DSCs and decidual fibroblast cells are not as sensitive to ovarian hormones as ESCs,[24],[25],[26] an induction scheme with cAMP is recommended.


  Conclusion and Prospective Top


The successful decidualization of the endometrium is essential for the establishment and maintenance of pregnancy.[1] Abnormal decidualization may cause infertility, abortion, and other reproductive problems, and in vitro decidualization models have become important tools for studying the underlying mechanisms and identifying potential solutions. For different disease models and research purposes, researchers should select appropriate tool cells and adjust custom schemes based on the established cocktail schemes above to find the most compatible combination with current experimental conditions. With the development of novel bioengineering technologies, researchers should aim to establish more integrated and efficient models in the future that can simulate the structural and functional changes of decidualization in vivo more realistically and can also be used for multiple-factor research. In addition, it is necessary to explore more representative biomarkers of decidualization using the developed omics and sequencing technologies. Finally, the decidualization of in vitro models in a clinical context should be studied.

Financial support and sponsorship

The work was supported by grant from the National Natural Science Foundation of China (81771587).

Conflicts of interest

There are no conflicts of interest.



 
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