Trophoblasts, the specialized cells of the placenta, play a major role in implantation and formation of the maternal-fetal interface. Through an unusual differentiation process examined in this review, these fetal cells acquire properties of leukocytes and endothelial cells that enable many of their specialized functions. In recent years a great deal has been learned about the regulatory mechanisms, from transcriptional networks to oxygen tension, which control trophoblast differentiation. The challenge is to turn this information into clinically useful tests for monitoring placental function and, hence, pregnancy outcome.
: The success of embryonic implantation relies on an ideal cross‐talk between the embryo and the receptive endometrium. This article focuses on the role of leukemia inhibitory factor (LIF) and its receptors in human embryo implantation. LIF is a secreted glycoprotein first described as a factor that induced the differentiation of mouse myeloid leukemic M1 cells into macrophages and later proposed as a marker of the embryo implantation process. An important role for LIF in implantation was shown on LIF knockout mice, when embryo implantation did not occur. In endometrium of healthy women, LIF and LIF mRNA are expressed throughout the menstrual cycle with a striking increase in the midsecretory phase, coinciding with a supposed window of implantation. Correlation in the expression of LIF and some other markers of implantation has been reported. LIF acts on cells by binding to the LIF receptor (LIFR) and gp130. Human blastocysts express mRNAs for LIFR and gp130, participating actively in establishing contact with the endometrium. In the endometrium, LIFR and gp130 are expressed in the endometrial epithelium throughout the cycle with strong increase in the midsecretory phase. Endometrium of infertile women produces significantly less LIF during the period of receptivity. The role of LIF gene mutations in unexplained infertility and implantation failures in IVF patients is not clear yet. Infertile patients showed reduced secretion of LIFR and gp130 compared with fertile controls during the implantation window. Recombinant human LIF might help to improve the implantation rate in women with unexplained infertility.
A pregnancy rate of approximately 15% per cycle renders the process of human reproduction inefficient. The cycle-dependent expression of molecules involved in the embryo–endometrial dialogue has lead to the identification of a ‘window of implantation’. This is the unique temporal and spatial expression of factors that allows the embryo to implant (via signalling, appositioning, attachment and invasion) in a specific time frame of 48 h, 7–10 days after ovulation. Integrin molecules, L-selectin ligands, mucin-1, heparin-binding epidermal growth factor and pinopodes are involved in appositioning and attachment. The embryo produces cytokines and growth factors [interleukins, prostaglandins, vascular endothelial growth factor (VEGF)] and receptors for endometrial signals (leukaemia inhibitory factor receptor, colony stimulating factor receptor, insulin-like growth factors and heparin binding epidermal growth factor receptor). The immune system plays an important role. Immunomodulatory factors such as glycodelin, inhibin and interleukin prevent a graft-versus-host reaction. Angiogenesis controlled by VEGF and prostaglandins is needed for formation of a receptive endometrium and a placenta. Identification of these factors has led to their use as markers of implantation that may identify defects causing subfertility. An ideal marker of implantation is sensitive and specific, and easy to obtain without disturbing implantation. Glycodelin and leukaemia inhibitory factor (serum) and integrins and pinopodes (biopsies) are promising candidates.
The endometrium is receptive to embryo implantation only for a short period in each reproductive cycle: development of receptivity requires alterations in endometrial gene expression. Calbindin (CaBP)-d9k and CaBP-d28k are related proteins containing EF hand motifs that have a high affinity for Ca . We previously demonstrated that endometrial expression of CaBP-d9k mRNA is highly regulated during implantation in the mouse. This project aimed to determine the temporal and spatial expression of both CaBP proteins during early pregnancy and to establish whether they are necessary for blastocyst implantation. CaBP-d28k protein, like CaBP-d9k, was up-regulated in the endometrial epithelium just before implantation but disappeared at implantation sites after attachment. By the judicious intrauterine injection of morpholino oligonucleotides (MO) against CaBP-d9k into WT and CaBP-d28k null mice just before implantation, we selectively eliminated one or both CaBPs from the uterine epithelium. Implantation was blocked only when both CaBP-d9k and CaBP-d28k were absent: treated WT mice and untreated CaBP-d28k null mice were fertile. Furthermore, the effect on implantation was highly dependent on the timing of injection of MO. This report examining the function of implantation-related genes in the uterus using MO demonstrates that this technique is a highly effective means to specifically target uterine proteins in vivo. This study provides evidence for an absolute requirement for CaBPs during the early phase of embryo implantation, and thus that regulation of Ca availability in the uterine environment of the implanting embryo is critical for successful implantation.
Transformation of the endometrium into the receptive phase is under the control of ovarian steroid hormones and is modulated by embryonic signals during implantation. We have previously shown that this differentiation process is accompanied by a suppression of gap junction connexins (Cx) 26 and 43 before implantation followed by a local induction of both connexins in the implantation chamber. In the present study, we demonstrate that connexin gene expression in the rodent endometrium is regulated via two distinct signaling pathways during these different stages of early pregnancy. During preimplantation, transcription of connexins can be induced by estrogen via an estrogen receptor (ER)-dependent pathway. Additionally, Cx26 and Cx43 are induced by embryonic signals during implantation and delayed implantation as well as during artificially induced decidualization. In contrast to the estrogen-induced expression, this embryonic/decidual-associated induction of Cx26 and Cx43 could not be blocked by antiestrogen, thus pointing to another regulatory pathway independent of the ER. Studies in ERÎ± and ERÎ² knockout mice confirmed these different pathways, demonstrating that in the endometrium, estrogen-mediated Cx26 gene induction, but not induction during decidualization, is dependent on functional ERÎ±. To evaluate potential embryonic signals regulating Cx26 expression, uteri of pseudopregnant animals were incubated with different mediators in an organ-culture model, showing that catechol estrogen and mediators of the inflammatory cascade such as prostaglandin F 2Î± and interleukin-1Î² are able to induce Cx26 expression through the ER-independent pathway. Thus, the present study demonstrates that endometrial expression of Cx26 and Cx43 is induced via estrogen and ERÎ± during preimplantation but then utilizes an ER-independent signaling pathway during embryo implantation and decidualization.
The protection of the embryo from the maternal adverse environment during early pregnancy is considered to be achieved by the establishment of a transitory permeability barrier created by decidual cells immediately surrounding the implanting embryo. Normally, the polarized epithelium acts as a barrier by regulating paracellular passage of substances through tight junctions. The expression of tight junction proteins in the uterine luminal epithelium prior to implantation is consistent with this idea. However, limited information is available regarding, the nature and regulation of the permeability barrier that is created by decidualizing stromal cells during implantation. We show here that the tight junction proteins, occludin, claudin-1, zonula occludens-1 and zonula occludens-2, are all expressed and physically associated in decidualizing stromal cells of the primary decidual zone forming a barrier surrounding the embryo with the loss of adjacent luminal epithelium. The blastocyst trophectoderm appears to be the stimulus for the creation of this barrier, since isolated inner cell mass or artificial stimuli failed to induce such a barrier. Furthermore, the primary decidual zone induced by the normal blastocyst is impermeable to immunoglobulin molecules. These findings suggest that trophoblast-induced expression of tight junctions forms a temporary barrier in cells of the primary decidual zone that restricts the passage of injurious stimuli such as maternal immunoglobulins to the embryo.
Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. The window of implantation is the favorable time period when the uterus develops a receptive approach to the invading embryo. Various signaling cascades are likely to become active at the window of implantation both in the uterus and the embryo. This helps create maternal embryo dialogue leading to successful embryo implantation. In this study we report for the first time the presence and nuclear translocation of the adaptor molecule CrkL both in the uterine and embryonic partners at the window of implantation. We also report that estrogen, which initiates and guides crucial changes in the uterus and the embryo at the window of receptivity, causes a massive surge in the expression and subsequent nuclear translocation of CrkL. We have also identified the existence of one LXXLL motif in the CrkL amino acid sequence and a single LXD is sufficient for activation by the estrogen receptor. This is suggestive that CrkL can bind to estrogen receptors and act as a coactivator. (C) 2004 Elsevier Inc. All rights reserved.
Human embryonic germ (hEG) cell is a very important alternative pluripotent stem cell resource. We describe the derivation of hEG cells from human embryonic fetal gonads over 6–8 weeks postconception. A large number of EG-like cell clumps were obtained at passage 1 and thus facilitated the following routine culture when the donor tissues were trypsinized with gentle pipetting and plated on feeder layer cells in the initial culture. Eight diploid hEG cell lines have been cultivated in vitro for extended periods while maintaining expression of markers characteristic of pluripotent stem cells. Human EG cells expressed transcription factor Oct4, a marker of pluripotency in mouse EG cells, at a high and steady level. Expression of markers indicative of differentiation along the three germ lineages was also observed in EBs. High level of alkaline phosphatase activity was shown in EG cells. These encouraging findings provide a starting point for potential applicability of hEG cells.