The cell biological basis of human implantation

Early stages of implantation as revealed by an in vitro model

Our limited understanding of the processes underlying steroid hormonal control of human endometrial receptivity is largely due to the lack of a relevant model system. To overcome scarcity of material, we have developed a model in which mouse embryos attach to human Ishikawa cells, which express functional steroid hormone receptors. Blastocysts flushed from day 4 pregnant superovulated mice were transferred to confluent Ishikawa cell monolayers. After 48 h of co-culture, 85% of the blastocysts had attached loosely, but only 40% attached stably to the epithelial cell surface. In contrast, 95% of the embryos attached stably to tissue culture plastic. Thus, weak attachment of a majority of the embryos was followed by stronger adhesion of a smaller proportion. Seventeen percent of the transferred blastocysts modified the epithelial cell surface with loss of MUC1 at the attachment site, extending variably to adjacent epithelial cells. Initially, stable attachment occurred without disruption to the integrity of the epithelial monolayer, but at later stages after the embryo had spread laterally, displacement of subjacent cells was observed. A modest increase in stable attachment, but no changes to MUC1 clearance, was observed after assisted hatching. After 24 h priming of Ishikawa cells by 17β-oestradiol (OE2) followed by 72-h incubation with medroxyprogesterone acetate and OE2, stable attachment increased from 40 to 70%. Initial attachment is efficient either in the presence or in the absence of hormone; steroid treatment increased the incidence of stable attachment. Implantation failure is predicted to occur in this model when embryos fail to progress from initial to stable attachment.

3-Phosphoglycerate dehydrogenase expression is regulated by HOXA10 in murine endometrium and human endometrial cells

3-Phosphoglycerate dehydrogenase (PHGDH, 3-PGDH) is an enzyme necessary for de novo l-serine biosynthesis. HOXA10 expression is required for endometrial receptivity; however, few target genes of HOXA10 regulation are known. Using a microarray we identified Phgdh as a target of HOXA10 regulation in murine endometrium and confirmed this regulatory relationship in human endometrial cells. PHGDH was downregulated 2.0-fold by HOXA10 and upregulated 4.4-fold by HOXA10 antisense in vivo. In human endometrial cells, real-time PCR results show that pcDNA3.1/HOXA10 transfection decreased PHGDH mRNA expression to 40% of pretreatment level (P<0.05), while PHGDH mRNA expression was increased 2.1-fold (P<0.05) by HOXA10 siRNA. Western blot results confirmed the regulatory relationship in both primary human endometrial stromal and epithelial cells, as well as in human endometrial stromal cells and Ishikawa cells. In human cycling endometrial tissue, immunohistochemical results showed that PHGDH expression is relatively high in the proliferative phase in glandular cells and lower in the secretory phase. Here we report novel expression and regulation of PHGDH in murine and human endometrium. PHGDH is expressed in both endometrial epithelial and stromal cells. HOXA10 represses endometrial PHGDH expression. PHGDH is necessary for serine biosynthesis, which serves as a substrate for protein synthesis. One mechanism by which HOXA10 regulates cellular differentiation may involve limiting protein synthesis in the secretary phase.

Adhesion molecules in endometrial epithelium: tissue integrity and embryo implantation

Cell adhesion in endometrial epithelium is regulated to maintain the continuity and protectiveness of the luminal covering cell layer while permitting interstitial implantation of the embryo during a restricted period of about 4 days. Many apparently normal embryos fail to implant, and epithelial-embryo adhesion remains a poorly understood phenomenon. After menstruation, epithelial regeneration occurs by epiboly from the basal residues of glands, an activity that requires migration on extracellular matrix as well as cell-cell cohesion. Here we review current knowledge of adhesion molecules in the epithelium.

Bioinformatics and transcriptomics studies of early implantation

Little is known of the genes that mediate cell interactions at implantation. A combined transcriptomics/bioinformatics protocol is described that identifies cell surface glycoproteins in endometrium with potential involvement in implantation. This is linked to an in vitro model in which functional assays of candidate gene products can be carried out.

Gene expression profiling reveals putative HOXA10 downstream targets in the periimplantation mouse uterus

HOXA10 encodes a transcription factor required for endometrial receptivity and embryo implantation. The objective of this study was to identify and to characterize those molecular markers regulated by HOXA10 expression. The authors have identified putative HOXA10 target genes identified by microarray analysis employing a murine model of transient HOXA10 expression during the anticipated implantation window. Microarray analysis identified 40 statistically significant genes regulated by HOXA10 overexpression of which 31 genes were downregulated greater than 2-fold over control and 9 genes were upregulated. Cellular ontogenies of differentially expressed genes include cell adhesion molecules, signal transduction factors, and metabolic regulators. Semiquantitative real-time reverse transcriptase polymerase chain reaction confirmed regulation of selected candidate genes. Examples included clusterin (Clu), phoshoglycerate 3-dehydrogenase (3-Pgdh), and tumor-associated calcium signal transducer 2 (Tacstd2). Elucidation of these pathways will allow further characterization of the molecular mechanisms governing endometrial development, which also may function to enhance uterine receptivity.

Spiral artery blood volume in normal pregnancies and those compromised by pre-eclampsia

The aim of this work was to use intravoxel incoherent motion (IVIM) to provide a non-invasive in vivo assessment of the function of the maternal spiral arteries that feed the placenta in normal pregnancy and in pre-eclampsia. Eleven normal pregnant women were scanned at 16, 22, 29 and 35 weeks gestation in a longitudinal study. Nine normal pregnant women and six women with pre-eclampsia were scanned in a cross-sectional study, within 10 days of delivery. The MRI IVIM technique was used to measure the moving blood fraction (f%) at the basal plate. There was no evidence that f% changed with gestational age (P = 0.84), but considering the cross-sectional groups, f% in women with pre-eclampsia was reduced compared with normal pregnancy (mean +/- SD: 36 +/- 5% and 27 +/- 5%; P < 0.005). In conclusion, pregnancies complicated by pre-eclampsia exhibit a reduced fraction of moving blood within the region of the spiral arteries. IVIM performed in the mid-trimester may provide an early means of predicting those pregnancies with an increased likelihood of being complicated by pre-eclampsia.

An integrated view of L-selectin and trophinin function in human embryo implantation

Determining molecular mechanisms of human embryo implantation is an extremely challenging task due to the limitation of materials and significant differences underlying this process among mammalian species. Recently, L-selectin and its ligand carbohydrate have been proposed as a system that mediates initial adhesion of human blastocysts to the uterine epithelia. We have also identified trophinin as a unique apical cell adhesion molecule potentially involved in the initial adhesion of trophectoderm of the human blastocyst to endometrial surface epithelia. In the mouse, the binding between ErbB4 on the blastocyst and heparin-binding epidermal growth factor-like growth factor on the endometrial surface enables the initial step of the blastocyst implantation. The evidence suggests that L-selectin and trophinin are included in human embryo implantation. This review summarizes findings relevant to the functions of L-selectin and trophinin in human embryo implantation, and proposes a model that reconciles these cell adhesion mechanisms.

Extracellular matrix-dependent regulation of angiogenin expression in human placenta

Knowledge of the rapidly developing hierarchy of controls affecting vascular development in placenta is required to understand how the growth factors and their receptor-mediated signals actually produce vessels. At the cell biological level, these events clearly require stable interactions between the cells, and cells with the surrounding ECM. The objective of the study was to understand the role of integrins and ECM on the expression and secretion of angiogenin in placentas and from trophoblasts in culture. Functionally active term placental explant culture and trophoblast cultures were used to demonstrate the differential secretion profile of angiogenin and real-time quantitative RT-PCR to demonstrate the mRNA expression in the presence or absence of ECM proteins. In this study, a significant increase in expression and secretion of angiogenin occurred in the presence of vitronectin (VN) and fibronectin (FN). Using antibody-blocking experiments it was also demonstrated that the angiogenin secretion is mediated by placental integrins, alpha(V)beta3 and alpha5beta1. In addition, exposure to hypoxic conditions resulted in diminished angiogenin secretion in the presence of both ECMs suggesting that angiogenin expression in the presence of ECM is modulated by local O2 concentration. In conclusion, this study provides evidence for the regulatory role of ECM and integrins on the mRNA expression and secretion of angiogenin in human placenta. ECMs may have a pivotal role in enhancing secretion of this peptide necessary for placental angiogenesis and provides the impetus as additional targets for the control of angiogenesis in pathological pregnancy.

Implantation and early embryonic development: implications for pregnancy

Implantation represents the limiting step in governing reproductive outcomes. The process is driven by both embryo and endometrium alike. Minute perturbations in this orchestration lay foundation for pregnancy-associated complications that may manifest throughout the gestational course. Furthermore, placental function dictates many aspects of fetal development inclusive of preeclampsia and intrauterine growth restriction (IUGR). Novel techniques such as Multigate Spectral Doppler Analysis (MSDA) may allow for early detection and diagnosis of potentially deleterious fetal outcomes.

Human uterine epithelial RL95-2 and HEC-1A cell-line adhesiveness: the role of plexin B1

OBJECTIVE

To study the expression of plexin-B1 in high- and low-receptive epithelial-endometrial cell lines, and its possible role in endometrial adhesiveness.

DESIGN

Controlled, laboratory study.

SETTING

Laboratory for Research in Reproductive Sciences, Department of Obstetrics and Gynecology, Ha'Emek Medical Center, Afula, Israel.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

This study was designed to explore and compare the expression and role of plexin-B1 in endometrial cell lines RL95-2 and HEC-1A, used as models of receptive and nonreceptive cells, respectively. The expression of plexin-B1 was analyzed by Western blotting and reverse-transcriptase polymerase chain reaction. The possible role of plexin-B1 in endometrial-trophoblast adhesiveness was studied with attachment and invasion assays. For further validation, we transfected HEC-1A cells with an expressing vector encoded for plexin-B1.

RESULT(S)

Significant differences in spheroid attachment was observed between RL95-2 and HEC-1A cells. Western blotting and reverse-transcriptase polymerase chain reaction revealed that in RL95-2 cells, the expression of plexin-B1 was significantly higher. An attachment assay that used RL95-2 cells in the presence of inhibiting antibodies against plexin-B1 significantly decreased the attachment rates of spheroids. A comparison between HEC-1A and transfected HEC-1A (HEC-1A-2) cells showed significant differences in spheroid attachment. No significant difference was found between HEC-1A-2 and RL95-2. An attachment assay using inhibitory antibodies against plexin-B1 significantly decreased the spheroid-attachment rate.

CONCLUSION(S)

Based on our results, we think that plexin-B1 contributes to trophoblast-endometrium interactions, most likely by enhancing adhesion properties.

Embryo implantation: the molecular mechanism remains elusive

Low rates of implantation are an impediment to more efficient assisted reproduction techniques. Improved endometrial receptivity and embryo preparation should lead to higher pregnancy rates, lower rates of early pregnancy failure and fewer multiple pregnancies. As the first site of contact between embryo and endometrium, the luminal epithelium (LE) is responsible for the non-receptive status of proliferative and early secretory tissue, and transformation to receptivity in the mid-secretory phase presumably requires alterations in expression, organization or activation of adhesion systems. Luminal cells are less abundant than their glandular counterparts, and are under-represented in global tissue datasets. Furthermore, alterations in cell surface composition can be readily accomplished by mechanisms that do not rely on altered transcription or translation. Current data from in-vitro models are consistent with initial attachment to mucin in the apical glycocalyx, perhaps via a carbohydrate-mediated interaction, after which the epithelial phenotype is modified by a medium- or short-range embryonic signal. A cascade of interactions follows, mediating embryo migration across the epithelium. Strikingly, numerous potential mediators of adhesion at implantation are located in the lateral rather than the apical surface of LE cells. Attached embryos appear to gain rapid access to this highly adhesive lateral membrane domain.

Endometrial receptivity markers, the journey to successful embryo implantation

Human 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. Histological evaluation, now considered to add little clinically significant information, should be replaced by functional assessment of endometrial receptivity. A large number of molecular mediators have been identified to date, including adhesion molecules, cytokines, growth factors, lipids and others. Thus, endometrial biopsy samples can be used to identify molecules associated with uterine receptivity to obtain a better insight into human implantation. In addition, development of functional in vitro systems to study embryo-uterine interactions will lead to better definition of the interactions existing between the molecules involved in this process. The purpose of this review was not only to describe the different players of the implantation process but also to try to portray the relationship between these factors and their timing in the process of uterine receptivity.

Functional characteristics of NaS2, a placenta-specific Na+-coupled transporter for sulfate and oxyanions of the micronutrients selenium and chromium

NaS2 is a Na+-coupled transporter for sulfate that belongs to the SLC13 gene family. This transporter was originally cloned from high endothelial venule endothelial cells, but nothing is known about the functional characteristics of this transporter except that it transports sulfate in a Na+-coupled manner. Northern blot analysis indicates that NaS2 is expressed most robustly in placenta. In the present study, we cloned NaS2 from rat placenta and characterized its transport function in detail using the Xenopus laevis oocyte expression system. Rat NaS2 consists of 629 amino acids and is highly similar to human NaS2. In situ hybridization studies with mouse placental sections show that NaS2 transcripts are expressed primarily in trophoblasts of the labyrinth zone. The expression of the transporter is confirmed in primary cultures of trophoblasts isolated from human placenta. When expressed in X. laevis oocytes, rat NaS2 mediates Na+-coupled transport of sulfate. The transport of sulfate is inhibited by oxyanions of selenium, chromium, arsenic, molybdenum, and phosphorous, suggesting that the transporter may mediate the transport of these oxyanions in addition to sulfate. The Kt for sulfate is 153+/-30 microM and the Na+:sulfate stoichiometry is 3:1. The transport process is electrogenic as evidenced from the inhibition of the uptake process by K+-induced depolarization. We conclude that NaS2 is a placenta-specific Na+-coupled, electrogenic, transporter for sulfate expressed in trophoblasts and that it is also responsible for the transport of oxyanions of the micronutrients selenium and chromium.

Growth and function of the normal human placenta

The placenta is the highly specialised organ of pregnancy that supports the normal growth and development of the fetus. Growth and function of the placenta are precisely regulated and coordinated to ensure the exchange of nutrients and waste products between the maternal and fetal circulatory systems operates at maximal efficiency. The main functional units of the placenta are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. After implantation, trophoblast cells proliferate and differentiate along two pathways described as villous and extravillous. Non-migratory, villous cytotrophoblast cells fuse to form the multinucleated syncytiotrophoblast, which forms the outer epithelial layer of the chorionic villi. It is at the terminal branches of the chorionic villi that the majority of fetal/maternal exchange occurs. Extravillous trophoblast cells migrate into the decidua and remodel uterine arteries. This facilitates blood flow to the placenta via dilated, compliant vessels, unresponsive to maternal vasomotor control. The placenta acts to provide oxygen and nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. The placenta can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. In addition, it releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many placental functional changes occur that accommodate the increasing metabolic demands of the developing fetus throughout gestation.

Luteal phase defect: myth or reality

Although the diagnosis of luteal phase defect (LPD) has been described convincingly in the research setting, it remains a controversial clinical entity. Apart from many uncertainties that surround the diagnosis of LPD, there is no convincing evidence that LPD is associated with infertility and recurrent abortion. Once diagnosed, the treatment options are empiric and include those that are recommended for unexplained infertility. The efforts to diagnose LPD in patients who have infertility or recurrent abortion are not justified.

Implantation-dependent expression of trophinin by maternal fallopian tube epithelia during tubal pregnancies: possible role of human chorionic gonadotrophin on ectopic pregnancy

Trophinin, tastin, and bystin have been identified as molecules potentially involved in human embryo implantation. Both trophoblasts and endometrial epithelial cells express trophinin, which mediates apical cell adhesion through homophilic trophinin-trophinin binding. We hypothesized that trophinin's function in embryo implantation is unique to humans and investigated the expression of trophinin, tastin, and bystin in ectopic pregnancy, a condition unique to humans. In tubal pregnancies, high levels of all three were found in both trophoblasts and fallopian tubal epithelia. Trophinin expression in maternal cells was particularly high in the area adjacent to the trophoblasts, whereas trophinin was barely detectable in intact fallopian tubes from women with in utero pregnancies or without pregnancies. When explants of intact fallopian tube were incubated with the human chorionic gonadotrophin (hCG), trophinin expression was enhanced in epithelial cells. Since the trophectoderm of the human blastocyst secretes hCG before and after implantation, these results suggest that hCG from the human embryo induces trophinin expression by maternal cells. As both beta-subunit of hCG and trophinin genes have diverged in mammals, the present study suggests a unique role of hCG and trophinin in human embryo implantation, including the pathogenesis of ectopic pregnancy.

Ultrasonography as a predictor of embryo implantation after in vitro fertilization: a controlled study

OBJECTIVE

To investigate the predictive value of ultrasonographic parameters as prognostic indicators of implantation after IVF when measured on the day of embryo transfer.

DESIGN

Comparative, observational study.

SETTING

University teaching hospital.

PATIENT(S)

Two hundred eighty patients undergoing IVF.

INTERVENTION(S)

Ovarian stimulation, IVF.

MAIN OUTCOME MEASURE(S)

Variables related to patients' clinical characteristics, treatment characteristics, ovarian response, ovum retrieval, outcome of IVF and ICSI, embryo transfer, ultrasonographic and Doppler endometrial measurements, and uterine blood flow that have been proposed as potential predictive factors of implantation. All transvaginal ultrasonographic assessments were performed on the day of embryo transfer.

RESULT(S)

Among 240 patients finally evaluable, 67 (group 1) became pregnant after IVF, and 173 (group 2) failed to conceive. The 111 nonpregnant patients who had the same embryo score per replacement (group 3) as did patients in group 1 were selected for comparison purposes. The only significant differences between groups 1 and 3 were the type A endometrium and the absence of a protodiastolic notch in the uterine arteries, both of which were more frequently found in group 1. However, a considerable overlap existed between conception and nonconception cycles regarding both variables.

CONCLUSION(S)

Ultrasonographic parameters as predictors of implantation in assisted reproduction have a limited value in the clinical setting.