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• W2000547968 abstract "The physiologic importance of the endometrium in assisted reproductive technology is now receiving much research attention. The historical importance of ultrasound for determination of endometrial pattern, thickness and contractility is reviewed and speculation provided for a possible mechanism of the adverse effect of thin endometrium on implantation. The physiologic importance of the endometrium in assisted reproductive technology is now receiving much research attention. The historical importance of ultrasound for determination of endometrial pattern, thickness and contractility is reviewed and speculation provided for a possible mechanism of the adverse effect of thin endometrium on implantation. In this issue of Fertility and Sterility two review articles, by Dr. Bruce Lessey (1Lessey B.A. Assessment of endometrial receptivity.Fertil Steril. 2011; 213: 522-529Abstract Full Text Full Text PDF Scopus (159) Google Scholar) and Dr. Rick Paulson (2Paulson R.J. Hormonal induction of endometrial receptivity.Fertil Steril. 2011; 213: 530-535Abstract Full Text Full Text PDF Scopus (137) Google Scholar), focus on the endometrium and embryo implantation. Dr. Lessey’s article reviews the physiology of endometrial receptivity, biomarkers for the window of implantation, and future areas for research into endometrial receptivity. Dr. Paulson’s review concerns the role of estrogen (E) and P in endometrial preparation for implantation, with specific emphasis on creating a receptive endometrium in assisted reproduction together with practical clinical suggestions. In general, the endometrium has received much less attention in reproductive research compared with the ovary, fertilization, or embryo development. Most of the investigations up until 1990 were based on endometrial biopsy and were aimed at endometrial morphology and dating. Other research focused on determining whether a luteal-phase defect existed by hormonal monitoring. Endometrial histologic dating was used to delineate the mid-luteal phase, and the presence of pinopodes on electron microscopy of endometrial samples had been suggested as marking the window of implantation. This latter suggestion was initially based on the cyclic presence of pinopodes in rat endometrium, in which a definite and a well-demarcated period of onset and disappearance for these structures was observed around the time of implantation (3Psychoyos A. Mandon P. Study of the surface of the uterine epithelium by scanning electron microscope. Observations in the rat at the 4th and 5th day of pregnancy.C R Acad Sci Hebd Seances Acad Sci D. 1971; 272: 2723-2725PubMed Google Scholar, 4Singh M.M. Chauhan S.C. Trivedi R.N. Maitra S.C. Kamboj V.P. Correlation of pinopod development on uterine luminal epithelial surface with hormonal events and endometrial sensitivity in rat.Eur J Endocrinol. 1996; 135: 107-117Crossref PubMed Scopus (52) Google Scholar). Unfortunately for their use as an implantation marker, more recent evidence in both mice and humans (5Quinn C.E. Casper R.F. Pinopodes: a questionable role in endometrial receptivity.Hum Reprod Update. 2009; 15: 229-236Crossref PubMed Scopus (93) Google Scholar) suggests that this narrow window of pinopode appearance is a unique phenomenon in rats and likely does not occur in other species. In the human, specifically, we showed that pinopodes developed shortly after ovulation and persisted right into the first trimester of pregnancy (6Quinn C.E. Ryan E. Claessens E.A. Greenblatt E. Hawrylyshyn P. Cruickshank B. et al.The presence of pinopodes in the human endometrium does not delineate the implantation window.Fertil Steril. 2007; 87: 1015-1021Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). The search for biomarkers for the window of implantation has been challenging. Recently, microarray studies of human endometrium have been used to compare genomic, microRNA, and proteomic patterns in both the nonreceptive and putative receptive phases of the menstrual cycle. As described by Dr. Lessey (1Lessey B.A. Assessment of endometrial receptivity.Fertil Steril. 2011; 213: 522-529Abstract Full Text Full Text PDF Scopus (159) Google Scholar), several groups have used a microarray platform to determine genes that are either up- or down-regulated during the estimated window of implantation. These studies have identified hundreds of differentially expressed genes that are potential markers, and the difficulty now is in determining accurate and reproducible biomarkers that could help clinically with the identification of the window of implantation. Almost all of these studies of differential expression involve two endometrial biopsies often around day 2 after ovulation and again at day 7 after ovulation to compare the nonreceptive and receptive phases of the cycle. It is possible that the first endometrial biopsy may itself cause an inflammatory or injury reaction that could alter the gene expression at the time of the second biopsy. This issue makes it difficult to study gene expression at two different times in the same patient during the same cycle and needs to be considered in interpretation of results. Regarding noninvasive investigation of endometrial receptivity, the advent of vaginal ultrasound for monitoring ovarian stimulation in assisted reproduction made it possible to accurately follow endometrial development throughout the menstrual cycle. Even so, interest in an ultrasound focus on the endometrium was slow to develop. Although it is now the standard of care to measure endometrial thickness and pattern during ultrasound monitoring for controlled ovarian stimulation, the initial manuscript describing the relationship of sonographic endometrial thickness and endometrial pattern to pregnancy was rejected by Fertility and Sterility and not published until almost a year later in another journal (7Gonen Y. Casper R.F. Prediction of implantation by the sonographic appearance of the endometrium, during controlled ovarian stimulation for IVF.J In Vitro Fert Embryo Transfer. 1990; 7: 146-152Crossref PubMed Scopus (294) Google Scholar). The adverse effect of clomiphene citrate on endometrial development was described more than 20 years ago (8Gonen Y. Casper R.F. Sonographic determination of an adverse effect of clomiphene citrate on endometrial growth.Human Reprod. 1990; 5: 670-674PubMed Google Scholar), but many physicians today still prescribe clomiphene citrate without ultrasound monitoring to determine whether there is any negative effect on the endometrium. More recently, ultrasound measurement of uterine contractions during ET and the impact of contraction frequency on pregnancy rates have been described (see review by Fanchin and Ayoubi [9Fanchin R. Ayoubi J.M. Uterine dynamics: impact on the human reproduction process.Reprod Biomed Online. 2009; 18: 57-62Abstract Full Text PDF PubMed Scopus (62) Google Scholar]). Physiologically, uterine contraction frequency slows from a maximum on the day of the LH surge to a nadir around the mid-luteal phase. The same pattern of uterine contractions occurred in IVF cycles, and vaginal P administration for luteal support decreased uterine contraction frequency and increased implantation rates. One suggestion for the increased pregnancy rates seen with blastocyst transfer is that uterine contraction frequency is reduced after 2 extra days of P exposure compared with day-3 ET (10Fanchin R. Ayoubi J.M. Righini C. Olivennes F. Schönauer L.M. Frydman R. Uterine contractility decreases at the time of blastocyst transfers.Hum Reprod. 2001; 16: 1115-1119Crossref PubMed Scopus (189) Google Scholar). More research is needed now to determine whether women who have repeated implantation failure with consistently high-quality embryos have increased uterine contractility and could be helped by administration of agents that reduce contractions. Ultrasound monitoring of endometrial development plays a key role in steroid replacement for donor egg programs or frozen embryo transfer. As Dr. Paulson outlines in his review (2Paulson R.J. Hormonal induction of endometrial receptivity.Fertil Steril. 2011; 213: 530-535Abstract Full Text Full Text PDF Scopus (137) Google Scholar), E and P are the only hormones required to obtain a receptive endometrium for embryo implantation. It is likely that there is no specific period of E support that is absolutely necessary for priming the endometrium to respond to P. Whether E is increased in a step-wise manner to simulate a natural follicular phase, or given continuously at the same dose (11Shapiro H. Cowell C.A. Casper R.F. The use of vaginal ultrasound for monitoring endometrial preparation in a donor oocyte program.Fertil Steril. 1993; 59: 1055-1058Crossref PubMed Google Scholar), is likely not important. The use of a higher dose of E continuously has the added advantage of suppressing endogenous gonadotropin secretion and follicle development, allowing better timing of ET. As an illustration of the resilience of the endometrium to follicular E exposure, we recently published a case report of a patient undergoing controlled ovarian stimulation for IVF in which GnRH antagonist injections resulted in follicular demise and a drop in E concentrations. Continuing gonadotropin administration was followed by growth of a second wave of follicles and increasing E. Consequently, endometrial thickness advanced, regressed, and then advanced again without any menstrual bleeding, and the patient subsequently conceived despite a follicular phase lasting 30 days with tremendous variation in E levels (12Bentov Y. Esfandiari N. Goktur A. Burstein E. Fainaru O. Casper R.F. An ongoing pregnancy from two waves of follicles developing during long follicular phase of the same cycle.Fertil Steril. 2010; 94 (350.e8–11)Abstract Full Text Full Text PDF Scopus (23) Google Scholar). Dr. Paulson suggests (2Paulson R.J. Hormonal induction of endometrial receptivity.Fertil Steril. 2011; 213: 530-535Abstract Full Text Full Text PDF Scopus (137) Google Scholar) that the simplest E replacement regimen that is also the easiest for the patient to implement should be encouraged for endometrial preparation. This suggestion makes sense and will likely give pregnancy rates similar to more complicated regimens. The key issue is the target endometrial thickness achieved by E preparation, which by consensus seems to be >7 mm in cross-section, with a triple-line endometrial pattern. Our experience is that endometrial thickness <6 mm is associated with a lower rate of full-term pregnancy, whereas many other publications suggest that thickness >9 mm is important (13Dickey R.P. Olar T.T. Curole D.N. Taylor S.N. Rye P.H. Endometrial pattern and thickness associated with pregnancy outcome after assisted reproduction technologies.Hum Reprod. 1992; 7: 418-421PubMed Google Scholar, 14Bergh C. Hillensjö T. Nilsson L. Sonographic evaluation of the endometrium in in vitro fertilization IVF cycles. A way to predict pregnancy?.Acta Obstet Gynecol Scand. 1992; 71: 624-628Crossref PubMed Scopus (32) Google Scholar). More recently, advances in three-dimensional ultrasound, together with automated software for endometrial measurements, have allowed accurate and reproducible measures of endometrial volume that are less operator dependent than standard two-dimensional measurements. These changes may improve reliability and consistency of endometrial measurements when the lining is uneven or difficult to visualize throughout its length in a single two-dimensional plane, but more research is needed to determine clinical utility. Thin endometrium observed on ultrasound may be iatrogenic, related to Asherman’s syndrome or to the use of clomiphene citrate with endometrial receptor depletion but is sometimes seen in the absence of any apparent endometrial trauma or hormonal etiology. We have recently determined that a persisting thin endometrium, which does not seem to respond normally to E, is associated significantly more frequently with long-term use of birth control pills (i.e., >10 years) (unpublished data). This apparent adverse effect on the endometrium of long-term use of combined oral contraceptive pills is infrequent but when present seems to persist for years after cessation of the contraceptives. Why does a thin endometrium result in lower implantation? The obvious hypothesis is an E receptor abnormality like that seen with clomiphene citrate. However, receptor dysfunction or abnormality may only be part of the answer. As pointed out by Dr. Lessey (1Lessey B.A. Assessment of endometrial receptivity.Fertil Steril. 2011; 213: 522-529Abstract Full Text Full Text PDF Scopus (159) Google Scholar), loss of epithelial steroid receptors during the window of implantation is consistent across almost all mammals studied to date. In addition, ectopic pregnancies can implant on the tubal epithelium, or on the peritoneum or the surface of intraperitoneal organs in the absence of endometrium. In neither of these cases is the occurrence of implantation likely to be explained easily by a receptor hypothesis or by adequate endometrial thickness. One speculation regarding why a thin endometrium may be associated with difficulty for implantation could relate to oxygen tension. As seen in a classic figure of endometrial vascularity (Fig. 1) published by Bartelmez in 1957 (15Bartelmez G.W. The form and the functions of the uterine blood vessels in the rhesus monkey.Contrib Embryol. 1957; 36: 154-183Google Scholar), the functional layer of the endometrium has a vascular capillary structure, in contrast to the larger spiral arteries in the basal layer. Other early studies demonstrated constriction of the spiral arteries after ovulation, with diminished blood flow to the surface endometrium (16Rossman I. Bartelmez G.W. The injection of the blood vascular system of the uterus.Anat Rec. 1957; 128: 223-231Crossref PubMed Scopus (10) Google Scholar). One would presume that blood flow and, therefore, oxygen tension would be reduced in the functional or surface epithelium after ovulation and around the time of implantation. It is well known that embryos do not develop well in high oxygen tensions as a result of the production of reactive oxygen species (17Yang H.W. Hwang K.J. Kwon H.C. Kim H.S. Choi K.W. Oh K.S. Detection of reactive oxygen species (ROS) and apoptosis in human fragmented embryos.Hum Reprod. 1998; 13: 998-1002Crossref PubMed Scopus (383) Google Scholar, 18Catt J.W. Henman M. Toxic effects of oxygen on human embryo development.Hum Reprod. 2000; 15: 199-206Crossref PubMed Scopus (143) Google Scholar). This observation is utilized clinically with triple-gas incubators in IVF laboratories to keep oxygen concentration in the range of 5%. In the endometrium, when the thickness measured by ultrasound is <7 mm, it is the functional layer that is thin or absent, and the implanting embryo would be much closer to the spiral arteries and the higher vascularity and oxygen concentrations of the basal endometrium. The high oxygen concentrations near the basal layer could be detrimental compared with the usual low oxygen tension of the surface endometrium. This same speculation could also explain the possibility of implantation occurring in the fallopian tube or on the peritoneal surface, where oxygen tension is likely quite low compared with elsewhere in the body (19Bornside G.H. Donovan W.E. Myers M.B. Intracolonic tensions of oxygen and carbon dioxide in germfree, conventional, and gnotobiotic rats.Proc Soc Exp Biol Med. 1976; 151: 437-441Crossref PubMed Scopus (16) Google Scholar). Whatever the cause of impaired pregnancy rates with thin endometrium, to date there have been very few options for increasing a persistently thin endometrium. A fascinating recent case series by Gleicher et al. (20Gleicher N. Vidali A. Barad D.H. Successful treatment of unresponsive thin endometrium.Fertil Steril. 2011; 95 (2123.e13–17)Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar) demonstrated an endometrial growth-inducing role of a cytokine (granulocyte colony-stimulating factor) instilled into the endometrial cavity of four women with thin endometrium, leading to pregnancy. This very interesting new finding needs to be explored further. In summary, it seems that the endometrium, and its physiologic importance in reproduction, is now receiving the research attention it deserves. Novel biomarkers to delineate the window of implantation will likely be discovered in the next few years, and new ultrasound techniques to monitor endometrial development or contractility are under investigation. New therapies to enhance endometrial thickness and implantation should follow soon after. Improving endometrial receptivity will add to the ongoing developments in embryo culture and selection and lead to increased live birth rates in assisted reproduction. It's time to pay attention to the endometrium, including the nucleolar channel systemFertility and SterilityVol. 96Issue 6PreviewIn the context of the excellent Views and Reviews devoted to the endometrium (1), Dr. Bruce A. Lessey offers a thorough analysis of 186 publications that are of significance to the window of implantation (WOI) (2). We feel this already exhaustive effort needs to be expanded further. Although one histological hallmark of midluteal endometrium—pinopodes (whose significance as markers of endometrial receptivity has been questioned)—is reviewed in detail, another, the nucleolar channel system (NCS), went unmentioned. Full-Text PDF" @default.
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