Embryo Implantation Process in Close Focus
Abstract: Embryo Implantation Process in Close Focus.
The article was designed as a retrospective overview of the scientific studies for distinguishing the peculiarities of Embryo Implantation process, with special emphasis given to the representation of the mechanism of preimplantation embryonic development and its further implantation in the maternal uterus. The main issues, interconnected with Embryo Implantation process were discussed. The article focuses on analyzing and representing the basic concepts of successful implantation paradigm, where under the term “successful implantation” must be considered the result of reciprocal interactions between the implantation–competent blastocyst and receptive uterus. It was established that the Embryo Implantation is the result of a synchronous sequence of events including cellular adhesion, invasion and immune regulatory mechanisms, some of which are controlled through genetic processes by the ovarian hormones. Taken together, these observations strongly imply that in contrast to the longstanding dogma of excessive maternal rejection, lack of embryo recognition and selection at implantation predisposes for subsequent clinical pregnancy loss.
Introduction
Infertility, early abortion, and inadequate placentation are increasing problems for couples in the modern world. To address these issues, it is imperative that we should understand the molecular events at the maternal–fetal interface during implantation. Furthermore, a better understanding of the mechanisms regulating Embryo Implantation may improve the ability of clinicians to treat infertility and to prevent early pregnancy loss. This knowledge might enable investigators to improve this critical step in modern reproductive therapies.
Embryo Implantation is a three–stage process (apposition, adhesion and invasion) involving synchronized crosstalk between a receptive endometrium and a functional blastocyst. This ovarian steroid–dependent phenomenon can only take place during the window of implantation, a self–limited period of endometrial receptivity spanning between days 20 and 24 of the menstrual cycle. Implantation involves a complex sequence of signalling events, consisting in the acquisition of adhesion ligands together with the loss of inhibitory components, which are crucial to the establishment of pregnancy [Achache and Revel, 2006].
Implantation is initiated by attachment of the trophectoderm to the endometrial epithelium and continues with its penetration of the epithelial layer and the underlying basal lamina and trophoblast invasion through the decidualized stroma until it engrafts the maternal vasculature. These events represent a continuum during which the phenotypes of the cells involved are constantly changing. Implantation of the trophoblast into the maternal endometrium cannot be studied in vivo and is difficult even ex vivo [Hannan et al., 2010].
Successful implantation is the result of reciprocal interactions between the implantation–competent blastocyst and receptive uterus. Although various cellular aspects and molecular pathways of this dialogue have been identified, a comprehensive understanding of the implantation process is still missing [Dey et al., 2004]. If properly annotated and explored, this information will expand the scientific knowledge regarding yet–to–be–identified unique, complementary, and/or redundant molecular pathways in implantation.
1. Fertilization: unique tandem of a spermatozoon and an oocyte, culminating into a new life
The assembly of a new life first depends on the union between a spermatozoon and an oocyte culminating in fertilization; failure to achieve such a union leads to their demise. The one–cell fertilized oocyte, termed embryo, undergoes several mitotic cell divisions, eventually forming a differentiated tissue called the blastocyst with two distinct cell populations, the Inner Cell Mass (ICM) and a layer of trophectoderm cells surrounding the Inner Cell Mass (ICM) [Dey et al., 2004]. If the fertilization has occurred, the embryo proper is derived exclusively from the Inner Cell Mass (ICM), whereas the hemochorial placenta and extraembryonic membranes are produced from cells contributed mainly by the trophectoderm. A two–way interaction between the blastocyst and maternal uterine luminal epithelium initiates the process of implantation, a process by which blood vessels of the embryo are brought into functional communication with the maternal circulation leading to the establishment of a functional placenta and pregnancy. Maternal resources filtered across the selective barrier of the placenta protect and nourish the conceptus [Dey et al., 2004].
If the fertilization was successful, and pre–implantation embryo development is considered to be perfect, there is no guarantee that the further process, Embryo Implantation, would result into pregnancy. A significant pregnancy loss resulting from preimplantation embryonic death is common and is considered to be a selection process leading to the survival of superior embryos for implantation. However, dysregulation of the events before, during, or immediately after implantation also may often be a cause for poor pregnancy rates. Understanding the mechanism of preimplantation embryonic development and implantation in the uterus has been a challenge to researchers and clinicians with the goal of alleviating the problems of infertility [Dey et al., 2004].
Implantation is a complex process involving spatiotemporally regulated endocrine, paracrine, autocrine, and juxtacrine modulators that span cell–cell and cell–matrix interactions. However, the precise sequence and details of the molecular interactions involved have not yet been defined. The successful implantation of an embryo is contingent upon cellular and molecular cross–talk between the uterus and the embryo. The coordination of the endocrine, cellular, and molecular events via paracrine, autocrine, and/or juxtacrine factors in a dynamic manner produces within the uterus a favorable environment, the receptive state, to support implantation. The embryo also functions as an active unit with its own molecular program of cell growth and differentiation. Thus, deficiencies in uterine receptivity, embryo development, or the embryo–uterine dialogue will compromise fertility [Dey et al., 2004]. Consequently, the scientific attention towards the implantation process should be focused, primarily, on the molecular basis of embryo homing and attachment, on elucidating the reciprocal signaling networks between the embryo and uterus, and on determining genetic causes of implantation failure [Dey et al., 2004].
2. Preimplantation embryo development and genomic activation
The preimplantation stage embryo normally develops from the one–cell zygote to the blastocyst stage within the zona pellucida, in other words, preimplantation embryo development normally occurs within the zona pellucida. However, zona removal by various experimental manipulations does not deter embryonic development in vitro, suggesting that this glycoprotein barrier is not essential for development to progress. The nonadhesive nature of the zona pellucida is thought to facilitate the journey of embryos through the oviduct and from the oviduct to the uterus [Dey et al., 2004]. Preimplantation embryo development and differentiation, which culminate in the formation of a blastocyst, require the activation of the embryonic genome, a process that is essential to implantation. Upon activation of the embryonic genome, the embryo grows rapidly to form a blastocyst. At the blastocyst stage, embryos mature and escape from their zona pellucidae to gain implantation competency. The differentiated and expanded blastocyst is composed of three cell types: the outer polarized epithelial trophectoderm, the primitive endoderm, and the pluripotent Inner Cell Mass (ICM) [Dey et al., 2004]. The Inner Cell Mass (ICM) provides the future cell lineages for the embryo proper, and the trophectoderm, the very first epithelial cell type in the developmental process, makes the initial physical and physiological connection with the uterine luminal epithelium. The formation of the trophectoderm and its subsequent development into trophoblast tissue are crucial steps for the initiation of implantation and the establishment of pregnancy. Trophoblast cells produce a variety of growth factors, cytokines, and hormones that influence the conceptus and maternal physiology in an autocrine, paracrine, and/or juxtacrine manner [Petraglia et al., 1998; Roberts et al., 1999].
Additionally, it should be noted that preimplantation embryos are capable of producing their own growth–promoting factors [Paria and Dey, 1990]. In fact, several growth factors, cytokines, and their receptors are expressed in the embryo, and the proliferative and differentiating effects of these factors on embryonic development and functions have been observed [Paria and Dey, 1990; Carson et al., 2000].
3. The window of implantation: a transient and unique moment
The rising estrogen level during the first part of the menstrual cycle enhances endometrial cell proliferation. The endometrium is a dynamic tissue that undergoes cyclic changes each month to prepare the uterus for Embryo Implantation. The window of implantation is accepted to be 5 days in length, spanning Cycle Days (CD) 20–24 in a 28–day cycle [Achache and Revel, 2006]. Although the relative importance of the state of the endometrium is appreciated, definitive biomarkers that predict endometrial receptivity have remained elusive [Kresowik et al., 2014]. Following ovulation, progesterone levels secreted by the luteinized follicles lead to the differentiation of these cells. At this point, the endometrium is mature and primed for Embryo Implantation. This process is rigorously controlled both temporally and spatially. The fine–tuning of the window of implantation timing is crucial and seems to be partially under the influence of PGs. When the blastocyst enters through one of the Fallopian ostia, 4 days after ovulation, it appears to move freely in the uterine cavity. Selectins were proposed to have an important role in this phase to ensure suitable rolling of the blastocyst. Because the embryo is required to attach to the endometrium in a polarized way and because the embryo is looking for the best area in the endometrium for implantation, this ‘rolling’ phenomenon is strictly regulated to ensure that the blastocyst will eventually settle in the proper spot and in the correct orientation. To prevent the blastocyst from adhering to an area with poor chances of implantation, an important role is played by the repellent activity of Mucin–1 (MUC–1) [Achache and Revel, 2006]. Mucins are high Molecular Weight (MW) glycoproteins, which contain at least 50% of carbohydrate O–linked to a threonine/serine rich peptide core [Gendler et al., 1990]. As detailed above, MUC–1 is widely expressed throughout the endometrium and, surprisingly, even increases before implantation. This phenomenon seems to be crucial in preventing the embryo from adhering to the wrong location [Achache and Revel, 2006].
In the case of implantation, a highly coordinated process is set into motion whereby specialized cells of the embryo, the trophectoderm and trophoblast, establish contact with a specialized tissue of the mother, the uterus. The exquisite coordination involves the regulated production of growth factors, cytokines, and hormones by embryonic as well as maternal tissues of both uterine and extrauterine origins. In concert, complementary receptors for these factors must be expressed by the appropriate tissues to propagate implantation signals. In addition, cell surface components must become functionally available to support attachment of trophectoderm/trophoblast and uterine cells [Carson et al., 2000]. To add to the challenge, it has been postulated that, there is only a restricted time during the uterine cycle during which implantation can occur [Psychoyos, 1986]. Failure to initiate the critical early events of implantation during this “window of receptivity” results in early pregnancy failure [Carson et al., 2000].
4. Embryo Implantation: a transition stage in the progress of pregnancy
Embryo Implantation represents the most critical step of the reproductive process. It [implantation] marks a transition stage in the progress of pregnancy during which the blastocyst assumes a fixed position and begins an altered physiological relationship with the uterus, but it is not a single event, nor can it be established as occurring at a single instant in time [Schlafke and Enders,1975].
Implantation is the process by which the blastocyst comes into intimate physical and physiological contact with the uterine endometrium. It consists of a unique biological phenomenon, by which the blastocyst becomes intimately connected to the maternal endometrial surface to form the placenta that will provide an interface between the growing fetus and the maternal circulation [Aplin, 2000]. Successful implantation requires a receptive endometrium, a normal and functional embryo at the blastocyst developmental stage and a synchronized dialogue between maternal and embryonic tissues [Simon et al., 2000].
Implantation involves a complex sequence of signalling events that are crucial to the establishment of pregnancy. A large number of identified molecular mediators, under the influence of ovarian hormones, have been postulated to be involved in this early feto–maternal interaction. These mediators embrace a large variety of inter–related molecules including adhesion molecules, cytokines, growth factors, lipids and others [Lessey et al., 1992; Simon et al., 2000]. Endometrial receptivity consists in the acquisition of adhesion ligands together with the loss of inhibitory components that may act as a barrier to an attaching embryo [Aplin, 2000].
The whole process of Embryo Implantation was classified into three stages: apposition, adhesion, and penetration. During blastocyst apposition, trophoblast cells adhere to the receptive endometrial epithelium. The blastocyst will subsequently anchor to the endometrial basal lamina and stromal Extracellular Matrix (ECM). At this point, the achieved embryo–endometrial linkage can no longer be dislocated by uterine flushing. Apposition is the stage when embryonic trophectoderm cells become closely opposed to the uterine luminal epithelium. This is followed by the adhesion stage in which the association of the trophectoderm and the luminal epithelium is sufficiently intimate as to resist dislocation of the blastocyst by flushing the uterine lumen. The stage of penetration involves the invasion of the luminal epithelium by the trophectoderm. Stromal cell differentiation into decidual cells (decidualization) is more extensive, and the loss of the luminal epithelium is evident at this stage. This is followed by the invasive blastocyst penetration through the luminal epithelium. These three stages of implantation form a continuum [Dey et al., 2004]. The attachment reaction coincides with a localized increase in stromal vascular permeability at the site of the blastocyst [Dey et al., 2004]. In other words, the process of Embryo Implantation is viewed as a stepwise process involving apposition and adherence of a blastocyst to the endometrium, followed by breaching of the luminal epithelium and, finally, invasion of maternal tissues.
Even though the blastocyst can implant in different tissues, surprisingly in the endometrium, this phenomenon can only occur during a self–limited period spanning between days 20 and 24 of a regular menstrual cycle (day LH+7 to LH+11). Throughout this period, namely the window of implantation [Psychoyos, 1973], the endometrium is primed for blastocyst attachment, given that it has acquired an accurate morphological and functional state initiated by ovarian steroid hormones [Finn and Martin, 1974; Yoshinaga, 1988; Paria et al., 2002].
This process seems analogous to an invading cancer, with the embryo driving the destruction of endometrial epithelial cells, the enzymatic digestion of stromal matrix, and finally, the invasion of the maternal decidua and inner myometrium. Within this context, the role of the maternal decidua is to both tolerate and control the invading semiallogeneic fetal trophoblast. Excessive trophoblast invasion of the uterus presents clinically as placenta accreta or choriocarcinoma, conditions associated with considerable maternal morbidity and, occasionally, death. Conversely, inadequate or deregulated invasion of trophoblast compromises the survival of the fetus and causes a spectrum of pregnancy complications, including miscarriage, intrauterine growth restriction, preeclampsia, and stillbirth. Hence, both excessive and inadequate implantation can have disastrous consequences [Quenby and Brosens, 2013].
Researchers Siobhan Quenby and Jan J. Brosens in their study “Human implantation: a tale of mutual maternal and fetal attraction”, presented two models to explain Embryo Implantation: the traditional view of implantation involves fetal trophoblast invasion into the maternal endometrium. The emerging view of implantation involves mutual attraction and migration of both maternal stromal cells and fetal trophoblast [Quenby and Brosens, 2013]. Through the inclusive discussion based on the scientific investigation’s results, both researchers completed the following implantation paradigm from the emerging view: notwithstanding the complexity at a cellular level, implantation in the conventional paradigm outlined above seems to require little more than a receptive endometrium and an invasive embryo capable of evading maternal immune detection. The implantation of an embryo is much more dynamically controlled by the endometrium than hitherto appreciated. This hypothesis is based on several intertwined observations. First, Human Endometrial Stromal Cells (HESCs) have been shown to mount a transient proinflammatory response upon differentiation into decidual cells. These proinflammatory signals trigger the expression of receptivity genes in the overlying surface epithelium and, thus, determine the duration for the window of implantation. Continuous progesterone signaling then drives an anti–inflammatory response essential for postimplantation embryo support. A restricted window of implantation is an extremely vital constituent to a successful pregnancy, because it ensures that the continuously changing endometrial environment is aligned to meet the requirements of an implanting blastocyst [Salker et al., 2012]. Second, decidualizing Human Endometrial Stromal Cells (HESCs) are highly secretory cells, suggesting that they create the microenvironment and provide the nutrients that enable the conceptus to thrive. Ultimately, experiments have also shown that decidualizing Human Endometrial Stromal Cells (HESCs) are programmed to sense signals from developmentally compromised embryos and then respond by shutting down the secretion of key cytokines and growth factors [Teklenburg et al., 2010]. Thus, once the luminal epithelium is breached, decidual cells engage in quality control and facilitate maternal rejection of undesirable embryos. Finally, rather than being passive bystanders, decidualizing Human Endometrial Stromal Cells (HESCs) have an intrinsic propensity to migrate toward trophoblast [Grewal et al., 2008; Gellersen et al., 2013; Schwenke et al., 2013]. In fact, the study of Helen Mardon and her colleagues vividly demonstrated that invasion of the embryonic trophoblast into stromal cell monolayers is blocked upon inhibiting the motility of decidualizing Human Endometrial Stromal Cells (HESCs) [Grewal et al., 2008]. In other words, active and directional migration of Human Endometrial Stromal Cells (HESCs) in response to local embryonic signals appears to be an integral component of the intense tissue remodeling at the implantation site. Taken together, these emerging concepts suggest that the embryos do not embed in the endometrium randomly but, rather, at receptive sites where stromal cells are poised to encapsulate the conceptus and create a microenvironment tailored to the individual embryo [Quenby and Brosens, 2013].
5. The vital role of the placenta in modulating the endocrinology of pregnancy with special emphasis on fetomaternal immunologic tolerance
There is an imperative in considering the placenta as an organ that is essential for the initiation, maintenance and successful conclusion of pregnancy. Consequently, the above–noted imperative presupposes the mainstream of further scientific investigations being focused almost exclusively around these three basic functions with special attention to the inclusive representation of the fundamental mechanisms. All three fundamental functions of the placenta are closely interconnected with the vital role of placenta in the fetus’s protection from infection and maternal rejection, which should be illustrated for integrative understanding of this process. It was established that placenta is a unique physical and immune barrier that protects the semiallogenic fetus in more than one way [Gurevich et al., 2007]. During pregnancy, maternal Immunoglobulin G (IgG) is transported from mother to fetus across the placenta to confer passive immunity to the fetus. On the one hand, it protects the fetus from infection, and on the other, it protects the fetus from immune rejection by the mother [Guleria and Sayegh, 2007]: acceptance of the fetus, which expresses paternally inherited alloantigens, by the mother during pregnancy is a unique example of how the immune system reshapes a destructive alloimmune response to a state of tolerance. Understanding the complex mechanisms of fetomaternal tolerance has important implications for developing novel strategies to induce immunologic tolerance in general and for prevention of spontaneous abortion in at–risk cases in particular.
The accurate studying of the placenta’s anatomy has led the researchers to the conclusion that the tissue most involved in immunoregulation at the maternal–fetal interface is the placenta. It is comprised of cells of maternal as well as fetal origin, both of which express molecules (HLA–G by trophoblast and Fas ligand (FasL)2 by maternal decidual cells) that play a vital role in fetomaternal tolerance [Guleria and Sayegh, 2007]. Generally, placenta is composed of layers of cells with distinct functions. The outer layer of placenta which mediates implantation and invasion into the uterus, is composed of invasive extravillous cytotrophoblast cells [Aplin, 1991; Hemberger and Cross, 2001]. The function of the middle layer of the placenta, the spongiotrophoblast, is largely unknown. However, some of the spongiotrophoblast cells can differentiate into trophoblast giant cells resembling the cytotrophoblast cell columns that anchor the villi of the placenta. The chorionic villi of the placenta is covered by syncytiotrophoblasts that lie in direct contact with maternal blood.
The fetus represents a foreign entity to the maternal immune system. Fifty years ago, Medawar P.B. (1953) proposed that immunological tolerance should be present during pregnancy to protect against an aggressive maternal alloimmune response directed at the paternal Ags expressed by the fetus [Medawar, 1953]. Recurrent pregnancy loss affects 1–3% of all couples, and about half of these cases have no identifiable cause [Hill, 1992]. Furthermore, a number of studies associate some pregnancy complications with abnormal maternal immune responses [Billington, 1992; Vince and Johnson, 1995]. Our understanding of tolerance mechanisms enabling a state of pregnancy is at this stage far from complete. Therefore, the known mechanisms of fetomaternal tolerance should be highlighted with particular emphasis on recent developments and future directions in the field.
Many mechanisms protect the fetus from the maternal immune system. These include the expression of nonclassical MHC molecules by trophoblast cells [King et al., 2000; Ishitani et al., 2003], tryptophan catabolism by the enzyme IDO [Munn et al., 1998], T cell apoptosis [Hunt et al., 1997], and the complement system [Holmes et al., 1990; Hsi et al., 1991]. Recent studies have also documented a role for regulatory T cells (Tregs) [Aluvihare et al., 2004; Sasaki et al., 2004; Somerset et al., 2004]. Finally, researchers Indira Guleria and Mohamed H. Sayegh and their colleagues have demonstrated a role for the inhibitory costimulatory molecule, Programmed Death Ligand (PDL)1, in maintenance of fetomaternal tolerance [Guleria et al., 2005]. Consequently, as it has been already mentioned, during pregnancy, maternal Immunoglobulin G (IgG) is transported from mother to fetus across the placenta to confer passive immunity to the fetus. On the one hand, it protects the fetus from infection, and on the other, it protects the fetus from immune rejection by the mother [Guleria and Sayegh, 2007]. The mechanism of this delicate antibody selection, transportation, and blockage phenomenon across the placenta has not been fully understood.
Normal development and function of the placenta requires invasion of the maternal decidua by trophoblasts, followed by abundant and organized vascular growth. There is much evidence indicating that the placenta plays a very important role in modulating the endocrinology of pregnancy. Furthermore, the placenta microenvironment has been implicated in playing a critical role in vascularization and angiogenesis. There are multiple cellular components present in the placenta microenvironment, which include trophoblast cells; immune cells like lymphocytes and macrophages; mesenchymal stromal cells; and vascular cells, such as endothelial cells, pericytes, and smooth muscle cells. Developmental expression and secretion of angiogenic factors by trophoblasts suggests their involvement in recruitment, maintenance of angiogenic cells, and placental vascularization. It should be emphasized that the vessel wall structure is different between placental arteries and veins. The artery has a smaller caliber than the vein, and the density of the smooth muscle cell population is significantly higher in artery. Similarly, large placental stem vessels contain smooth muscle actin positive cells [Cheng–Yi Chen et al., 2015]. Interactions between angiogenic factors expressed by trophoblasts and their receptors on endothelial cells may regulate placental vascularization in a paracrine and autocrine manner [Zhou et al., 2003; Demir et al., 2004]. However, placental vascularization is a complex interaction of different cells with various regulatory factors. The nature and mechanism of the interactions between stromal cellular components and endothelial cells remains to be clearly determined.
It is also important to explicate that the placental tissue produces cytokines, hormones and growth factors that are essential in the regulation of the feto–maternal unit. The production of these substances is regulated by a network of interactions within and between intra–uterine through paracrine and/or autocrine mechanisms. Communication between the gestational intra–uterine tissues is critical for successful pregnancy, from the earliest stage of implantation until the expulsive phase of delivery. This is confirmed by the demonstration that the placenta directly controls the release of substances such as Human Chorionic Gonadotropin (hCG), Human Placental Lactogen (hPL), steroid hormones and prostoglandins. Furthermore, there is evidence that intra–uterine tissues also can regulate Adrenocorticotropin Hormone (ACTH) release and have effects on the hypothalamus–pituitary–gonadal axis and the hypothalamus–pituitary–adrenal axis [Petragliaa et al., 1998]. The set of data reported in this article confirms that the placenta must be considered an organ that is essential for the initiation, maintenance and successful conclusion of pregnancy and that an imbalance between the complex network of placental regulating systems may cause serious gestational disorders [Petragliaa et al., 1998].
Conclusion
Embryo Implantation is the result of a synchronous sequence of events including cellular adhesion, invasion and immune regulatory mechanisms, some of which are controlled through genetic processes by the ovarian hormones. It is proposed that Embryo Implantation is a well–defined and precise process, in which various factors come into play one after the other, yet remaining in close collaboration. It is rather surprising that during most days of the menstrual cycle, the endometrium is essentially hostile towards the embryo. A major physiological endeavour is thus needed by the endometrium so as to reverse this paradoxical condition [Achache and Revel, 2006].
In the case of implantation, a highly coordinated process is set into motion whereby specialized cells of the embryo, the trophectoderm and trophoblast, establish contact with a specialized tissue of the mother, the uterus. The exquisite coordination involves the regulated production of growth factors, cytokines, and hormones by embryonic as well as maternal tissues of both uterine and extrauterine origins. In concert, complementary receptors for these factors must be expressed by the appropriate tissues to propagate implantation signals. In addition, cell surface components must become functionally available to support attachment of trophectoderm/trophoblast and uterine cells [Carson et al., 2000]. To add to the challenge, it has been postulated that, there is only a restricted time during the uterine cycle during which implantation can occur [Psychoyos, 1986]. Failure to initiate the critical early events of implantation during this “window of receptivity” results in early pregnancy failure [Carson et al., 2000].
In addition, development of functional in vitro systems to study embryo–uterine interactions will lead to better define the interactions existing between the mechanisms, involved in this process. Up to date, only a few modalities have been employed to treat failures of conception, despite the repeated transfer of apparently good–quality embryos. A better understanding of the signals that regulate uterine receptivity and implantation competency of the blastocyst is of clinical relevance because unraveling the nature of these signals may lead to strategies to correct implantation failure and improve pregnancy rates.
References
[1] Achache H., Revel A. Endometrial receptivity markers, the journey to successful embryo implantation. Hum. Reprod. Update, 2006; 12(6): 731–746.
[2] Aluvihare V.R., Kallikourdis M., Betz A.G. Regulatory T cells mediate maternal tolerance to the fetus. Nat. Immunol., 2004; 5: 266–271.
[3] Aplin J.D. Implantation, trophoblast differentiation and hemochorial placentation: mechanistic evidence in vivo and in vitro. J. Cell Sci. 1991; 99: (Pt. 4): 681–692.
[4] Aplin J.D. The cell biological basis of human implantation. Baillieres Best Pract. Res. Clin. Obstet. Gynaecol., 2000; 14: 757–764.
[5] Billington W.D. The normal fetomaternal immune relationship. Baillieres Clin. Obstet. Gynaecol., 1992; 6: 417–438.
[6] Carson D.D., Bagchi I., Dey S.K., Enders A.C., Fazleabas A.T., Lessey B.A., Yoshinaga K. Embryo implantation. Dev. Biol., 2000; 223: 217–237.
[7] Cheng–Yi Chen, Shu–Hsiang Liu, Chia–Yu Chen, Pei–Chun Chen, Chie–Pein Chen. Human Placenta–Derived Multipotent Mesenchymal Stromal Cells Involved in Placental Angiogenesis via the PDGF–BB and STAT3 Pathways. Biology of Reproduction, 2015; 93(4): 103, 1–10.
[8] Demir R., Kayisli U.A., Seval Y., Celik–Ozenci C., Korgun E.T., Demir–Weusten A.Y., Huppertz B. Sequential expression of VEGF and its receptors in human placental villi during very early pregnancy: differences between placental vasculogenesis and angiogenesis. Placenta, 2004; 25: 560–572.
[9] Finn C.A., Martin L. The control of implantation. J. Reprod. Fertil., 1974; 39: 195–206.
[10] Gellersen B, Wolf A, Kruse M, Schwenke M, Bamberger AM. Human endometrial stromal cell–trophoblast interactions: mutual stimulation of chemotactic migration and pro–migratory roles of cell surface molecules CD82 and CEACAM1. Biol. Reprod., 2013; 88: 80,1–13.
[11] Gendler S.J., Lancaster C.A., Taylor–Papadimitriou J., Duhig T., Peat N., Burchell J., Pemberton L., Lalani E.N., Wilson D. Molecular cloning and expression of human tumor–associated polymorphic epithelial mucin. J. Biol. Chem., 1990; 265: 15286–15293.
[12] Grewal S., Carver J.G., Ridley A.J., Mardon H.J. Implantation of the human embryo requires Rac1–dependent endometrial stromal cell migration. Proc. Natl. Acad. Sci. USA, 2008; 105: 16189–16194.
[13] Guleria I., Khosroshahi A., Ansari M.J., Habicht A., Azuma M., Yagita H., Noelle R.J., Coyle A., Mellor A.L., Khoury S.J., Sayegh M.H. A critical role for the programmed death ligand 1 in fetomaternal tolerance. J. Exp. Med. 2005; 202: 231–237.
[14] Guleria I., Sayegh M.H. Maternal acceptance of the fetus: true human tolerance. J. Immunol. 2007; 178: 3345–3351.
[15] Gurevich P., Elhayany A., Milovanov A.P., Halperin R., Kaganovsky E., Zusman I., Ben–Hur H. The placental barrier in allogenic immune conflict in spontaneous early abortions: immunohistochemical and morphological study. Am. J. Reprod. Immunol., 2007; 58: 460–467.
[16] Hannan J.N., Paiva P., Dimitriadis E., Salamonsen A.L. Models for Study of Human Embryo Implantation: Choice of Cell Lines? Biology of Reproduction, 2010; 82(2): 235–245.
[17] Hemberger M., Cross J.C. Genes governing placental development. Trends Endocrinol. Metab., 2001; 12: 162–168.
[18] Hill J.A. Immunological contributions to recurrent pregnancy loss. Baillieres Clin. Obstet. Gynaecol., 1992; 6: 489–505.
[19] Hsi B.L., Hunt J.S., Atkinson J.P. Differential expression of complement regulatory proteins on subpopulations of human trophoblast cells. J. Reprod. Immunol., 1991; 19: 209–223.
[20] Holmes C. H., Simpson K.L., Wainwright S.D., Tate C.G., Houlihan J.M., Sawyer I.H., Rogers I.P., Spring F.A., Anstee D.J., Tanner M.J. Preferential expression of the complement regulatory protein decay accelerating factor at the fetomaternal interface during human pregnancy. J. Immunol., 1990; 144: 3099–3105.
[21] Hunt J.S., Vassmer D., Ferguson T.A., Miller L. Fas ligand is positioned in mouse uterus and placenta to prevent trafficking of activated leukocytes between the mother and the conceptus. J. Immunol., 1997; 158: 4122–4128.
[22] Ishitani A., Sageshima N., Lee N., Dorofeeva N., Hatake K., Marquardt H., Geraghty D.E. Protein expression and peptide binding suggest unique and interacting functional roles for HLA–E, F, and G in maternal–placental immune recognition. J. Immunol., 2003; 171: 1376–1384.
[23] King A., Burrows T.D., Hiby S.E., Bowen J.M., Joseph S., Verma S., Lim P.B., Gardner L., Le Bouteiller P., Ziegler A., et al. Surface expression of HLA–C antigen by human extravillous trophoblast. Placenta, 2000; 21: 376–387.
[24] Kresowik D.K.J., Devor J.E., Van Voorhis J.B., Kimberly K.L. MicroRNA–31 is Significantly Elevated in Both Human Endometrium and Serum During the Window of Implantation: A Potential Biomarker for Optimum Receptivity. Biology of Reproduction, 2014; 91(1): 1–6.
[25] Lessey B.A., Damjanovich L., Coutifaris C., Castelbaum A., Albelda S.M., Buck C.A. Integrin adhesion molecules in the human endometrium. Correlation with the normal and abnormal menstrual cycle. J. Clin. Invest., 1992; 90: 188–195.
[26] Medawar P.B. Some immunological and endocrinological problems raised by the evolution of viviparity in vertebrates: prevention of allogeneic fetal rejection by tryptophan catabolism. Symp. Soc. Exp. Biol., 1953; 7: 320–338.
[27] Munn D.H., Zhou M., Attwood J.T., Bondarev I., Conway S.J., Marshall B., Brown C., Mellor A.L. Prevention of allogeneic fetal rejection by tryptophan catabolism. Science, 1998; 281: 1191–1193.
[28] Paria B.C., Dey S.K. Preimplantation embryo development in vitro: cooperative interactions among embryos and role of growth factors. Proc. Natl. Acad. Sci. USA, 1990; 87: 4756–4760.
[29] Paria B.C., Reese J., Das S.K., Dey S.K. Deciphering the cross–talk of implantation: advances and challenges. Science, 2002; 296: 2185–2188.
[30] Petraglia F., Santuz M., Florio P., Simoncini T., Luisi S., Plaino L., Genazzani A.R., Genazzani A.D., Volpe A. Paracrine regulation of human placenta: control of hormonogenesis. J. Reprod. Immunol., 1998; 39(1–2): 221–233.
[31] Psychoyos A. Hormonal control of ovoimplantation. Vitam. Horm., 1973; 31: 205–225.
[32] Quenby S., Brosens J.J. Human Implantation: A Tale of Mutual Maternal and Fetal Attraction. Biology of Reproduction, 2013; 88(3): 81,1–2.
[33] Roberts R.M., Ealy A.D., Alexenko A.P., Han C.S., Ezashi T. Trophoblast interferons. Placenta, 1999; 20: 259–264.
[34] Sasaki Y., Sakai M., Miyazaki S., Higuma S., Shiozaki A., Saito S. Decidual and peripheral blood CD4+CD25+ regulatory T cells in early pregnancy subjects and spontaneous abortion cases. Mol. Hum. Reprod., 2004; 10: 347–353.
[35] Schlafke S., Enders A.C. Cellular basis of interaction between trophoblast and uterus at implantation. Biol. Reprod.,1975; 12: 41–65.
[36] Schwenke M., Knofler M., Velicky P., Weimar C.H., Kruse M., Samalecos A., Wolf A., Macklon N.S., Bamberger A.M., Gellersen B. Control of human endometrial stromal cell motility by PDGF–BB, HB–EGF and trophoblast–secreted factors. PLoS ONE, 2013; 8: e54336.
[37] Simon C., Martin J.C., Pellicer A. Paracrine regulators of implantation. Baillieres Best Pract. Res. Clin. Obstet. Gynaecol., 2000; 14,815–826.
[38] Somerset D.A., Zheng Y., Kilby M.D., Sansom D.M., Drayson M.T. Normal human pregnancy is associated with an elevation in the immune suppressive CD25+CD4+ regulatory T cell subset. Immunology, 2004; 112: 38–43.
[39] Teklenburg G, Salker M, Molokhia M, Lavery S, Trew G, Aojanepong T, Mardon HJ, Lokugamage AU, Rai R, Landles C, Roelen BA, Quenby Set al Natural selection of human embryos: decidualizing endometrial stromal cells serve as sensors of embryo quality upon implantation. PLoS ONE, 2010; 5: e10258.
[40] Vince G.S., Johnson P.M. Materno–fetal immunobiology in normal pregnancy and its possible failure in recurrent spontaneous abortion? Hum. Reprod., 1995; 10: (Suppl. 2): 107–113.
[41] Yoshinaga K. Uterine receptivity for blastocyst implantation. Ann. N.Y. Acad. Sci., 1988; 541: 424–431.
[42] Zhou Y., Bellingard V., Feng K.T., McMaster M., Fisher S.J. Human cytotrophoblasts promote endothelial survival and vascular remodeling through secretion of Ang2, PlGF, and VEGF–C. Dev. Biol., 2003; 263: 114–125.