Hormone dependent uterine epithelial-stromal communication for pregnancy support.

The uterus will only allow a blastocyst to attach and establish pregnancy during a finite period of time termed the window of receptivity [ 9 ]. In humans, this period lasts longer and occurs 7–10 days after ovulation during the mid-secretory phase of the menstrual cycle [ 7 ]. In domestic animals, the uterus becomes receptive on day 13 in pigs, day 16 in sheep and day 20 in cows [ 21 , 22 ]. In rodents, the window of receptivity is a short period on the fourth day of pregnancy or pseudopregnancy [ 20 ]. Prior to the window of receptivity, conceptuses cannot attach to the uterine epithelial cells, while the uterus is refractory to conceptus attachment after this period. The coordination of the timing of embryo development and uterine receptivity ensures that only healthy conceptuses can initiate pregnancy in a uterine environment that will support the developing fetus. As shown in Figure 1 , this process or uterine receptivity requires: 1) uterine epithelial cell proliferation to cease; 2) uterine epithelia cells to become permissive for conceptus attachment (domestic animals) and/or invasion (rodents and humans); and 3) stromal cells to undergo a differentiation termed decidualization which protects and supports the invading conceptus (rodents and humans) [ 9 , 10 , 12 ]. In humans, stromal decidualization occurs recurrently as part of the menstrual cycle (28–30 days) and is a blastocyst-independent process, whereby decidual reaction initiates near the spiral arteries and spreads throughout the endometrium [ 23 ]. In mice, decidual reaction is dependent upon mechanical stimulation, whereby the decidualization initiates at the site of blastocyst attachment and then spreads throughout the implantation chamber [ 24 ]. The coordination of conceptus development and the uterine window of receptivity is controlled by the ovarian steroids.

The timing of the window of receptivity is achieved by the ovarian hormones, E2 and P4 regulating both gene expression in the epithelial and stromal cells of the uterus, as well as the paracrine cross between these compartments [ 9 ]. The ovarian secretion of these hormones is coordinated with menstrual cycle (humans and non-human primates) or the estrus (rodents, sheep, cow and pigs). As the counterpart of menstrual cycle, the estrous cycle, particularly in mice last 4–5 days and can be divided into 4 phases: proestrus, estrus, metestrus and diestrus. This cycle lacks a prolonged luteal phase unless there is copulation with a male. In proestrus, prior to ovulation, the ovary accumulates a mature pool of follicles ready for ovulation that occurs during the subsequent estrus phase. Female mice are most receptive to mating during the estrus phase. A preovulatory surge of E2 acts upon the uterine epithelial cells to stimulate proliferation after mating, termed gestational day (GD) 0.5; E2 decreases by GD1.5. At GD2.5, the formation of a corpus luteum CL results in the production of P4 and maximum expression of progesterone receptor (NR3C3 or PGR) in the uterine glandular (GE) and luminal (LE) epithelia [ 9 ]. A nidatory E2 spike at GD3.5 is vital for inducing uterine receptivity [ 25 , 26 ], due to coordination of stromal cells to undergo decidualization. Similarly, the window of receptivity in human opens on day 19 or 20 of the menstrual cycle after a nidatory E2 spike and lasts for 4–5 days [ 27 ]. In rodents, domestic animals and humans, sustained progesterone signaling is critical for uterine receptivity and the maintenance of pregnancy. Ovarian steroid hormones govern uterine receptivity via their associated receptors, i.e. the estrogen receptors ( Esr1 and Esr2 , transcribed from two genes) and the progesterone receptor ( Pgr with two isoforms, PgrA and PgrB , transcribed from the same gene), that elicit transcription activation of downstream target genes that are necessary for a healthy pregnancy or normal menstrual cycle [ 7 , 28 ]. Although there are two estrogen receptor genes Esr1 and Esr2 , normal uterine physiology is governed by ESR1. The Esr1 -null female mouse exhibits 1) mating behavior disorder; 2) hypoplastic uteri that are unresponsive to estrogen stimulation; and 3) abnormal ovaries that display cystic and hemorrhagic follicles and the absence of a corpora luteum. On the other hand, ESR2 is only robustly expressed in the ovary [ 29 ], oviduct or mammary gland under normal conditions. Functionally, the Esr2 -null female mouse displays compromised ovulation, and therefore, subfertility. However, no uterine phenotype is observed. Taken together, these findings indicate that ESR1 is critical for the promotion of epithelial proliferation and conceptus attachment [ 30 ]. The mechanism by which ESR1 regulates conceptus attachment is through the regulation of the cytokine leukemia inhibitory factor (LIF, encoded by Lif ). LIF expression is regulated by E2 through the action of ESR1 [ 31 ]. LIF expression is induced in the uterine glands during the second nidatory peak of E2 [ 32 ]. LIF binds to its receptor, LIFR, in the uterine epithelial cells, which then interacts with its co-receptor, gp130, to phosphorylate STAT3 and initiate translocation to the nucleus to regulate target genes directly responsible for controlling the attachment of the conceptus [ 32 ]. Ablation of Lif results in infertility due to failure of conceptus attachment and a reduced decidual response. It was demonstrated that a single LIF injection was sufficient to replace the nidatory E2 surge at the time of implantation [ 31 ]. This confirms that the function of E2 on the day of implantation is to initiate the induction of LIF and the cascade of events ultimately results in the implantation of the conceptus. This important cytokine was quickly identified as functionally conserved in humans [ 33 ]. Epithelial specific ablation of Esr1 demonstrates that E2 regulation of LIF is due to ESR1 expression in the uterine epithelial cells [ 30 ]. On the other hand, unlike the direct regulation of Lif by ESR1 in the uterine epithelial cells, the ability of ESR1 to regulate uterine epithelial proliferation is not due to E2 acting directly through epithelial ESR1 but through E2 acting through its receptor in the endometrial stromal cells. This work was first demonstrated by reconstitution of combinations of Esr1 knockout and wild type epithelial and stromal cells in xenograph transplants to the kidney capsule of immunocompromised mice [ 34 ]. This was later confirmed by epithelial specific ablation of Esr1 in which the uterine epithelial cells showed E2 induction of proliferation in the absence of ESR1 in the epithelial cells [ 13 ]. In addition to ESR1, PGR is critical for the regulation of the window of receptivity and maintenance of pregnancy. The steroid hormone P4 acts via PGR to elicit transcription activation of downstream target genes that are necessary for a healthy pregnancy or normal menstrual/estrous cycle [ 7 , 28 ]. In humans, domestic animals and rodents, PGR consists of two isoforms, PGR-A and PGR-B, arising from alternative promoter usage in the same gene [ 35 ]. Since both PGR isoforms exist in the murine uterus, the total Pgr knockout (PRKO) resulted in the disruption of both PgrA and PgrB transcription [ 36 ]. Both female and male mice exhibit normal viability, yet the female PRKO mice are sterile with multiple defects, including 1) aberrant lordosis; 2) uterine failure to undergo decidualization; 3) compromised ability to inhibit E2 signaling; 4) inability to ovulate; and 5) deficiencies in ductal branching during mammary gland development [ 36 ]. To further dissect the function of the individual isoform of PGR, site directed mutagenesis using homologous recombination in ES cells to disrupt the translational start sites of PgrA and PgrB was utilized. This generated mice with ablation of PgrA (PRAKO) [ 37 ] and PgrB (PRBKO) [ 37 ]. The PRAKO mice exhibit infertility with similar uterine defects to PRKO mice [ 37 ]. Interestingly, the PRAKO mouse uterine epithelial cells are induced to proliferate in the presence of P4, highlighting a unique role for PgrB in the regulation of the mouse uterine epithelial cell cycle [ 37 ]. The PRBKO mice are fertile with no reported uterine defects; however, ductal failure to undergo proper branching during mammary gland development is present [ 38 ]. These results identify functional differences between these two isoforms in response to P4 in the regulation of uterine epithelial proliferation during early pregnancy, and indicates that the PGR-A isoform is the predominant functional isoform in mouse uterus. Not only are the isoforms and levels of PGR critical for uterine function, but also the timing of PGR expression in the various compartments is important for uterine receptivity. During pregnancy, PGR is first expressed in the uterine epithelial cells. As pregnancy proceeds, PGR expression is downregulated in the uterine epithelial compartment prior to conceptus implantation while being expressed in the uterine stromal cells [ 28 ]. The loss of epithelial expression of PGR during the window of receptivity is most likely due to the downregulation of PGR by its ligand, P4. The importance of this downregulation of PGR prior to conceptus implantation has been investigated by the generation of a mouse model which constitutively expresses PGR-A ( mPgrA LsL/+ ) in the uterine epithelial cells ( Wnt7a Cre/+ ) [ 14 ]. This mouse is infertile with defects in conceptus attachment and stromal decidualization. These mice show a maintained expression of the morphogen, Indian hedgehog, Ihh, as well as, reduction in the cytokine, leukemia inhibitory factor (LIF), a key regulator of conceptus receptivity, prior to the receptive period. Further transcriptomic and cistromic analyses of the mouse uterus at GD4.5 identified the PGR-binding events in regions upstream of the Lif gene, indicating PGR-A may inhibit transcription of the Lif gene. These results suggest that downregulation of the PGR-A isoform at the window of receptivity is necessary for the attenuation of hedgehog signaling and transcriptional activation of LIF signaling, thereby producing a receptive environment for the attaching conceptus. Although PGR isoforms are critical, they work with other transcription factors to regulate uterine epithelial function. One of these factors is GATA2. GATA binding protein 2 (GATA2) is a transcription factor that has been identified as a PGR target [ 39 ] and is a potential modifier of uterine P4 signaling. Gata2 is expressed in both uterine epithelial and stromal cells and its expression coincides with that of Pgr [ 40 ]. Mice with germline ablation of Gata2 display defects in hematopoietic cell development [ 41 ], vascular integrity [ 42 ], adipocyte differentiation [ 43 ], and pituitary function [ 44 ]. Mice with uterine ablation of Gata2 ( Pgr Cre/+ Gata2 flox/flox ) are infertile with defects in conceptus implantation and subsequent uterine stromal decidualization [ 45 ]. These mice display stratification in uterine epithelia with increased TRP63 expression, as well as, reduction in PGR target genes and Pgr itself. Further cistromic analysis with transcriptomic analysis identified a distinct uterine-specific GATA2 genomic occupancy profile that encompasses most PGR target genes, suggesting a cooperative relationship between the two factors in controlling P4-mediated transcription in the mouse uterus. Importantly, this GATA2-PGR regulatory axis is also observed in endometrial specimens of human subjects consisting of normal and recurrent pregnancy loss patients, which provides a clinical indication and evidence of evolutional conservation of this pathway [ 45 ].

During pregnancy, the increase in P4 signaling initiates a cascade of molecular events within the epithelial and stromal cells of the uterus. This compartmental crosstalk allows the uterine epithelia to stop proliferating, become permissive for proper attachment and implantation of the conceptus, and induces the decidual response in the stromal cells. We and others have mapped out these molecular pathways in this communication between the uterine epithelial and stromal cells as shown in Figure 2 . This crosstalk is initiated by the direct regulation of the morphogen Indian hedgehog (IHH) in the uterine epithelial cells by PGR [ 46 , 47 ]. IHH acts on the uterine stromal cells to regulate the expression of the orphan nuclear receptor Chicken Ovalbumin Upstream Promoter Transcription Factor 2 (COUP-TFII). Downstream of this signaling is the upregulation of the transcription factor neural crest derivative-expressed protein 2 (HAND2). COUP-TFII and HAND2 regulate uterine stromal decidualization, as well as, the inhibition of uterine epithelial cell proliferation. This cascade will be described in the following section. The Pgr -initiated epithelial to stromal crosstalk event begins with the induction of IHH in the epithelial cells. IHH is a member of the developmentally regulated morphogens, the Hedgehog ( Hh ) gene family [ 48 ]. It has been reported in the regulation of bone development [ 49 , 50 ], gastrointestinal tract development [ 51 ], and embryonic vasculogenesis [ 52 ]. It is also expressed in the uterine epithelial cells under the control of the steroid hormone P4 [ 46 , 47 ]. In humans, IHH decreases in the endometrium of women with endometriosis [ 53 ]. In mice, after ovulation, epithelial PGR regulates the transcription of the Ihh gene in the uterine epithelial cells with the maximum production at GD2.5. IHH signals through its receptors Patched 1 (PTCH1) and/or Patched 2 (PTCH2), which are located in uterine stromal cells [ 46 , 47 ]. Once IHH binds PTCHs, their repression of the membrane signaling protein smoothened (SMO) by PTCHs is removed. SMO then activates a signaling cascade that results in the activation of the glioma-associated oncogene (GLI) transcription factors and the orphan nuclear receptor COUP-TFII [ 46 , 54 ]. These elements function to prime the stromal cells for decidualization. Ihh mice exhibit perinatal lethality, presumably caused by respiratory failure due to malformation of the thorax [ 55 ]. Mice with uterine ablation of Ihh exhibit infertility due to defects in conceptus attachment and stromal cell decidualization [ 54 ]. A smoothened (SMO) agonist, administered to these mice results in partial rescue of the decidual phenotype, which demonstrates the importance of the IHH signaling in the induction of decidualization [ 54 ]. Furthermore, the generation of a murine model which constitutively activates SMO in the uterus by crossing Pgr cre/+ mice to SmoM2 + ( Gt(ROSA)26Sor tm2(Smo/EYFP)Amc ) mice has been generated to better understand the role of IHH signaling in the uterus [ 56 ]. Female Pgr cre/+ SmoM2 + mice are infertile due to an inability of the ova to be fertilized in vivo and a failure to undergo the artificially induced decidual response. It also exhibits uterine hypertrophy, and abnormal luminal epithelial cells and a reduction in the number of uterine glands. These findings together indicate a pivotal role of the hedgehog signaling in mediating the epithelial-stromal crosstalk. COUP-TFII (also known as Nr2f2 , nuclear receptor subfamily 2, group F, member 2) is expressed within early developing mesenchyme and is vital for epithelial to mesenchymal signaling to promote appropriate development of multiple organs [ 57 , 58 ]. Ablation of COUP-TFII results in conceptus lethality due to defects in vascularization [ 59 ]. Endometrial expression of COUP-TFII is observed primarily in the stromal and vascular cells. Uterine specific ablation of COUP-TFII results in infertility due to failure of attachment and the lack of a decidual response similar to the uterine ablation of Ihh mice [ 60 ]. Further analyses of the failure of decidualization in mice with uterine ablation of COUP-TFII shows the reduced expression of PGR levels in the endometrial stromal cells along with a decrease in stromal cell proliferation and angiogenesis. In addition to the defects in the uterine stromal cells, uterine epithelial estrogen receptor 1 (ESR1) is elevated and epithelial-specific E2 target genes, such as Mucin 1 ( Muc1 ) are up-regulated. The implantation and decidual defects observed in the COUP-TFII mice are rescued by the administration of a low dose of the antiestrogen ICI182780, demonstrating that stromal COUP-TFII is critical for regulating the activity of epithelial ESR1, as well as, uterine epithelial proliferation [ 61 ]. These results suggest that COUP-TFII is activated downstream of epithelial PGR and IHH, and promotes the expression of stromal the Pgr gene to maintain decidualization. The stromal cells signal to the epithelial cells to inhibit uterine epithelial proliferation and Esr1 gene targets, such as Muc1 to prepare uterine epithelial cells for conceptus attachment. Thus, COUP-TFII acts as an epithelial to stromal conduit to govern blastocyst attachment and decidualization by initiating the inhibition of epithelial E2 signaling to allow conceptus attachment and by promoting the activity of genes critical for preparing the stromal cells for decidualization. In addition to COUP-TFII, heart and neural crest derivatives expressed 2 (HAND2), epidermal growth factor receptor (EGFR), bone morphogenetic protein 2 (BMP2) and Wingless-Type MMTV Integration Site Family, member 4 (WNT4) have been shown to be part of the regulators in uterine stromal cells that regulate epithelial cell proliferation and stromal cell decidualization [ 62 , 63 ]. The mechanism by which uterine stromal cells regulate proliferation of the uterine epithelial cells and allows P4 to repress uterine proliferation has been uncovered by investigating the role of HAND2 in the mouse uterus. HAND2 was initially identified as necessary for heart development and tissue differentiation through the germline ablation of this gene in mice [ 64 ]. Uterine ablation of HAND2 demonstrates that this gene is a critical regulator of the uterine stromal-epithelial communication in response to P4. Loss of the Hand2 gene in the uterus results in the failure of conceptus attachment and the absence of a decidual response [ 62 ]. In addition, the uterine epithelial cells continue to proliferate during the period of receptivity. It was further identified that Hand2 suppresses the production of several fibroblast growth factors (FGF1, FGF2, FGF9, FGF18) in the stromal cells that act as paracrine mediators of epithelial cell proliferation [ 62 ]. Therefore, HAND2 functions to prepare the epithelial cells for conceptus attachment and to promote stromal decidualization during early pregnancy.

The differentiation of the stromal cells of the uterus from a fibroblastic phenotype to an epithelioid like phenotype is critical to nourish and protect the developing conceptus. In the regulation of mouse stromal cell decidualization during pregnancy, knockout mouse models have been instrumental in identifying the molecular mechanisms that regulate this process. With respect to the IHH-COUP-TFII pathway, epidermal growth factor receptor 1 (ERBB1), BMP2 and WNT4 have been shown to be critical for the regulation of this process. Members of the epidermal growth factor (EGF) family, including: amphiregulin (AREG), epidermal growth factor (EGF), transforming growth factor α (TGF-α), and heparin binding epidermal growth factor (HB-EGF) are expressed during the preimplantation period of the uterus, implying the role of EGF signaling in conceptus implantation [ 65 – 67 ]. The expression of HB-EGF, in the luminal epithelial cells surrounding blastocysts at the time of attachment and the effects it has on blastocysts have made it a focus of interest in driving conceptus implantation in the mouse uterus [ 68 , 69 ]. However, ablation of each of these ligands, either in the whole animal or in the uterus, results in either no effect on fertility or a reduction in fertility, implying functional redundancy in the ligands in regulating uterine function [ 70 , 71 ]. This may indicate that the receptors for the EGFs, i.e. the ERBBs, may be the limiting factors. There are four members of the ERBB family of receptor tyrosine kinases: the epidermal growth factor receptor (EGFR/ERBB1) and v-erb-b2 erythroblastic leukemia viral oncogene homolog 2–4 (HER2, ERBB3 and ERBB4, respectively). ERBB1–3 are expressed in the mouse endometrium while ERBB4 localizes predominantly to the muscular myometrium [ 72 – 74 ]. Microarray analysis identifies ERBB1 as downstream of the IHH-COUP-TFII signaling cascade that is required to drive decidualization [ 75 ]. Erbb1–3 genes have been ablated in the mouse uterus with the only member of the ERBB family to affect mouse fertility being ERBB1. These mice have defects in decidualization. Microarray analysis of the Egfr uterine knockout showed that ERBB1 is a master regulator of decidualization with BMP2 and WNT4 downstream of EGFR signaling [ 63 ]. Bone morphogenetic proteins (BMPs) are multifunctional growth factors that belong to the transforming growth factor β (TGF-β) superfamily. Numerous BMPs are expressed in the uterus during pregnancy in the mouse; yet, only the expression of BMP2, a soluble factor required for osteoblast and osteoclast differentiation [ 76 , 77 ] is spatiotemporally correlated with implantation [ 78 , 79 ]. Mice with a universal deletion of the Bmp2 gene are embryonic lethal due to a failure of proamniotic canal closure in the majority of mice or abnormal cardiac development in the surviving mice [ 80 ]. Uterine BMP2 levels are under the control of the major stromal regulator COUP-TFII, implicating a pivotal role of BMP2 in decidualization [ 60 ]. As expected, uterine ablation of Bmp2 results in female mice that are infertile and exhibit a defect in uterine decidualization at the beginning of pregnancy. Nagashima et al. generated a conditional deletion of Bmpr2 (encoded BMP receptor type 2) using Pgr -Cre mice, which demonstrates severe hemorrhage at the implantation sites and subsequent fetal growth retardation during midgestation, leading to placental abruption, fetal demise and sterility [ 81 ]. Microarray analysis of mice with the conditional ablation of BMP2 in the uterus demonstrate that Bmp2 regulates decidualization by regulating PGR expression through expression of FK-506-binding proteins (FKBPs), which stabilizes and promotes the activity of PGR [ 82 ]. Additionally, BMP2 regulates a multitude of targets in the stromal cells, including prostaglandin-endoperoxide synthase 1 (PTGS1) and the noncanonical WNT ligands, WNT4 and WNT6. Wnt genes encode secreted glycoproteins that are homologous to the Drosophila segment polarity gene wingless ( wg ) and regulate stem cell fate, cell differentiation, and tissue growth [ 83 ] via canonical and non-canonical signaling pathways [ 84 ]. The canonical WNT signaling pathway involves binding of frizzled (FZD) receptors and inhibition of β-catenin (encoded by Ctnnb1 gene) degradation, resulting in nuclear translocation and activation of target genes. The non-canonical WNT signaling encompasses a variety of signaling pathways that involve different WNT receptors (i.e. ROR2 and RYK) and mediators (i.e. calcium, MAPK8/9/10, and CAMK2) that regulate cell migration and movement [ 85 ]. A subset of WNTs (WNT4, WNT5A, and WNT7A) is involved in müllerian duct patterning and differentiation during development of the female reproductive tract in the embryo [ 86 ]. Thus, analysis of the in vivo role of these genes in adult uterine function has been limited [ 87 , 88 ]. Conditional ablation of Wnt4 in the uterus results in female sterility. These mice display alterations in the differentiation of the uterine luminal epithelial cells, a reduction in the number of uterine glands, as well as, failure in decidualization, thereby, exhibiting subfertility [ 89 ]. Like Bmp2, Wnt4 regulates the responsiveness of uterine stromal cells to P4, suggesting that WNT4 signaling is also required for optimal PGR signaling. Notably, WNT4 gene expression is under direct regulation of COUP-TFII, as indicated by transcriptome and COUP-TFII binding profiles in human endometrial stromal cells [ 90 ].

In all mammals, successful establishment and maintenance of pregnancy requires complex remodeling of the endometrium to orchestrate implantation, which includes alteration in gene expressions that coordinate with conceptus attachment to uterine epithelial cells (domestic animals), invasion to uterine stromal cells (rodents and primates) and then, cause phenotypic modification of stromal cells, silencing of receptors for P4 and E2, and suppression of genes for immune recognition, as well as, enhancing signals for pregnancy recognition. Advancements in technology and the use of genetically engineered mouse models have extensively enhanced our understanding of these timely orchestrated molecular events that are critical for pregnancy and P4-dependency. In particular, cistromic, transcriptomic coupled with proteomic tools helped identify multiple novel PGR targets, followed by the generation of uterine compartment-specific KO mouse models to discover their roles as either downstream effectors or coregulators involved in important uterine function during the window of receptivity. Recently, new gene editing technology, such as CRISPR/Cas9 system has opened a new era for the generation of transgenic animal models in a more efficient and economical way that can help further identify uterine-specific enhancer or promoter regions of these PGR-associated genes. For example, PGR and GATA2 both occupy at putative enhancers of the Sox17 and Ihh genes [ 45 ] that have pivotal uterine functions [ 54 , 91 , 92 ]. While results from in vitro assays suggest a cooperation of PGR and GATA2 in the regulation of enhancers for the Sox17 and Ihh genes, the in vivo function of such enhancers remains to be demonstrated, perhaps by CRISPR technology. Furthermore, advanced statistical and bioinformatics tools are available for integrated analysis of genome-wide data from both human subjects and mouse models, which can help to identify genetic networks and regulatory mechanisms that are conserved across species [ 45 , 93 – 95 ]. Importantly, mapping out evolutionary conservation and difference on regulatory pathways would help better interpretation on data generated from preclinical investigations. In summary, the PGR exerts pleiotropic effects by regulating multiple types of proteins including ligands, receptors, singling proteins, and transcription factors between uterine epithelial and stromal cells. Understanding these mechanisms is not only necessary for fertility improvement, but also the development of new targeted therapies for its associated disease like endometriosis and endometrial cancer.

Infertility affects 6.7 million women in the U.S. and 48 million women worldwide [ 1 ]. Although assisted reproductive technologies (ART), such as in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) can supply embryos, clinical evidence has shown that at least 30% of pregnancies still cease before the second trimester [ 2 ]. This failure is because a successful establishment and maintenance of pregnancy is not only dependent upon the appropriate development of a healthy conceptus, but also dependent upon the ability of the uterus to support the implanting conceptus [ 3 – 5 ]. The ability of the uterus to support pregnancy requires the uterus to be receptive for conceptus attachment and invasion and be able to support and protect the developing blastocyst during pregnancy [ 6 , 7 ]. The ovarian hormones estrogen (E2) and progesterone (P4), acting through their cognate nuclear receptors ESR1 and PGR are critical for this process [ 3 , 4 , 8 – 10 ]. The mechanisms by which ESR1 and PGR regulate the ability of the uterus to support pregnancy has been investigated in vivo by utilizing genetically engineered mouse models [ 11 – 14 ]. The genes associated with uterine function to establish pregnancy have been extensively reviewed [ 7 , 15 – 20 ]. Here, we will discuss the process of uterine receptivity and how genetically engineered mouse models have helped to identify the molecular mechanisms governing the ability of the uterus to support pregnancy, mainly based on our previous findings.