Lipidomics as an emerging tool to predict endometrial receptivity

From the first histologic dating methods to the new “-omics” technologies, there has been a lot of effort put into understanding and characterizing receptive endometrium. The development of new diagnostic approaches to using biologic fluids has opened up a new field of investigation in noninvasive endometrial diagnosis techniques. Moreover, improvements in the field of mass spectrometry and nuclear magnetic resonance have made the precise detection of lipids possible; these organic molecules are involved in important functions such as modulating energy reserves, forming structural features, and promoting regulatory functions. Developments in endometrial receptivity diagnosis using lipidomics are discussed in this review paper. In summary, the results currently available indicate that prostaglandins E2 and F2α are particularly abundant during the window of implantation and that they might serve to nurse the blastocyst at the time of embryo implantation; they may also serve as important biomarkers to define the receptive phase of the endometrium. The importance of understanding the mechanisms that influence the production of these individual prostaglandins in the endometrium is clinically relevant because it may shed light on the sequence of events that leads to successful embryo implantation. From the first histologic dating methods to the new “-omics” technologies, there has been a lot of effort put into understanding and characterizing receptive endometrium. The development of new diagnostic approaches to using biologic fluids has opened up a new field of investigation in noninvasive endometrial diagnosis techniques. Moreover, improvements in the field of mass spectrometry and nuclear magnetic resonance have made the precise detection of lipids possible; these organic molecules are involved in important functions such as modulating energy reserves, forming structural features, and promoting regulatory functions. Developments in endometrial receptivity diagnosis using lipidomics are discussed in this review paper. In summary, the results currently available indicate that prostaglandins E2 and F2α are particularly abundant during the window of implantation and that they might serve to nurse the blastocyst at the time of embryo implantation; they may also serve as important biomarkers to define the receptive phase of the endometrium. The importance of understanding the mechanisms that influence the production of these individual prostaglandins in the endometrium is clinically relevant because it may shed light on the sequence of events that leads to successful embryo implantation. Discuss: You can discuss this article with its authors and with other ASRM members at http://fertstertforum.com/vilellaf-lipidomics-endometrial-receptivity/ Discuss: You can discuss this article with its authors and with other ASRM members at http://fertstertforum.com/vilellaf-lipidomics-endometrial-receptivity/ The endometrium is a dynamic tissue that undergoes growth, differentiation, and regression periods throughout the menstrual cycle in response to hormonal regulation to prepare the uterus for embryo implantation. The human endometrium is receptive to the embryo at the blastocyst stage for a short period of time during the menstrual cycle, referred to as the “window of implantation” (WOI). This period occurs in the middle of the secretory phase, coinciding with days 19–21 in a normal menstrual cycle (1Bergh P.A. Navot D. The impact of embryonic development and endometrial maturity on the timing of implantation.Fertil Steril. 1992; 58: 537-542Crossref PubMed Google Scholar), although it varies according personal factors. Three critical events must occur simultaneously during the WOI: proper embryo development, the acquisition of a receptive endometrium, and the proper dialogue between them, the latter being a crucial step in the implantation process and therefore in the initiation of pregnancy (2Domínguez F. Pellicer A. Simón C. The chemokine connection: hormonal and embryonic regulation at the human maternal-embryonic interface—a review.Placenta. 2003; : S48-S55Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar). The endometrial dating system was established with the use of specific morphologic criteria known as the “Noyes criteria” (3Noyes R.W. Hertig A.T. Rock J. Dating the endometrial biopsy.Am J Obstet Gynecol. 1975; 122: 262-263PubMed Scopus (684) Google Scholar). Although histologic assessment of the endometrium is still in use for research purposes, it adds little information of clinical significance (4Coutifaris C. Myers E.R. Guzick D.S. Diamond M.P. Carson S.A. Legro R.S. et al.Histological dating of timed endometrial biopsy tissue is not related to fertility status.Fertil Steril. 2004; 82: 1264-1272Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar, 5Murray M.J. Meyer W.R. Zaino R.J. Lessey B.A. Novotny D.B. Ireland K. et al.A critical analysis of the accuracy, reproducibility, and clinical utility of histologic endometrial dating in fertile women.Fertil Steril. 2004; 81: 1333-1343Abstract Full Text Full Text PDF PubMed Scopus (239) Google Scholar), and therefore the endometrial factor is still not properly assessed in the infertility work-up. The search for biomarkers in biologic fluids (including the endometrial fluid) has emerged as an alternative to the use of invasive methods. This is supported by similar work in other fields: Wong's group have recently identified microRNAs in human saliva that distinguish patients with oral cancer (6Yoshizawa J.M. Wong D.T.W. Salivary microRNAs and oral cancer detection.Methods Mol Biol. 2013; 936: 313-324Crossref PubMed Google Scholar), proteins that differentiate patients with Alzheimer disease have been identified in blood and plasma (7Lista S, Faltraco F, Hampel H. Blood and plasma-based proteomic biomarker research in Alzheimer's disease. Prog Neurobiol. Published online June 26, 2012. Available at: http://www.ncbi.nlm.nih.gov/pubmed?term=Blood%20and%20plasma-based%20proteomic%20biomarker%20research%20in%20Alzheimer's%20disease. Last accessed January 14, 2013.Google Scholar), and specific lipids have been identified in blood serum that profile patients with pancreatitis (8Stevens T. Berk M.P. Lopez R. Chung Y.-M. Zhang R. Parsi M.A. et al.Lipidomic profiling of serum and pancreatic fluid in chronic pancreatitis.Pancreas. 2012; 41: 518-522Crossref PubMed Scopus (21) Google Scholar). The investigation of new molecules that are differentially expressed throughout the WOI, will help us not only to identify putative markers, but also to guide the clinical acquisition of a receptive endometrium (9Berlanga O. Bradshaw H.B. Vilella-Mitjana F. Garrido-Gómez T. Simón C. How endometrial secretomics can help in predicting implantation.Placenta. 2011; : S271-S275Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). It has already been demonstrated that aspiration of endometrial fluid does not affect pregnancy rates in the same cycle (10van der Gaast M.H. Beier-Hellwig K. Fauser B.C.J.M. Beier H.M. Macklon N.S. Endometrial secretion aspiration prior to embryo transfer does not reduce implantation rates.Reprod Biomed Online. 2003; 7: 105-109Abstract Full Text PDF PubMed Scopus (96) Google Scholar). Different protein profiles have been defined in the endometrial fluid (11Casado-Vela J. Rodriguez-Suarez E. Iloro I. Ametzazurra A. Alkorta N. García-Velasco J.A. et al.Comprehensive proteomic analysis of human endometrial fluid aspirate.J Proteome Res. 2009; 8: 4622-4632Crossref PubMed Scopus (106) Google Scholar), as well as the profile of cytokines that are released into the lumen of the uterine cavity (12Domínguez F. Martinez S. Quiñonero A. Loro F. Horcajadas J.A. Pellicer A. et al.CXCL10 and IL-6 induce chemotaxis in human trophoblast cell lines.Mol Hum Reprod. 2008; 14: 423-430Crossref PubMed Scopus (60) Google Scholar, 13Boomsma C.M. Kavelaars A. Eijkemans M.J.C. Amarouchi K. Teklenburg G. 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Localization of interleukin-1 type I receptor and interleukin-1 beta in human endometrium throughout the menstrual cycle.J Clin Endocrinol Metab. 1993; 77: 549-555Crossref PubMed Google Scholar), therefore opening up a new field of study for the analysis of the functional changes experienced by the endometrium during the menstrual cycle with the use of noninvasive approaches (16Boomsma C.M. Kavelaars A. Eijkemans M.J. Lentjes E.G. Fauser B.C. Heijnen C.J. et al.Endometrial secretion analysis identifies a cytokine profile predictive of pregnancy in IVF.Hum Reprod. 2009; 24: 1427-1435Crossref PubMed Scopus (174) Google Scholar). However, the technique's biggest handicap is its low sensitivity, which limits the number of proteins that can be evaluated (13Boomsma C.M. Kavelaars A. Eijkemans M.J.C. Amarouchi K. Teklenburg G. Gutknecht D. et al.Cytokine profiling in endometrial secretions: a noninvasive window on endometrial receptivity.Reprod Biomed Online. 2009; 18: 85-94Abstract Full Text PDF PubMed Scopus (98) Google Scholar). Lipidomics can be defined as the large-scale study of lipid species and their related networks and metabolic pathways that exist in cells or any other biologic system. The aim of this “-omics” is the full characterization, identification, and quantification of molecular lipid species and their biologic roles regarding the expression of proteins involved in lipid metabolism and function, including gene regulation (17Lagarde M. Géloën A. Record M. Vance D. Spener F. Lipidomics is emerging.Biochim Biophys Acta. 2003; 1634: 61Crossref PubMed Scopus (126) Google Scholar). As an emerging “-omics” field, lipidomics provides a powerful approach to understanding lipid biology (18Quehenberger O. Armando A.M. Brown A.H. Milne S.B. Myers D.S. Merrill A.H. et al.Lipidomics reveals a remarkable diversity of lipids in human plasma.J Lipid Res. 2010; 51: 3299-3305Crossref PubMed Scopus (869) Google Scholar). Lipids are organic biomolecules essentially formed by carbon, hydrogen, and oxygen, although the latter forms a much lower proportion of these molecules. In addition, they may also contain phosphorus, nitrogen, and sulfur. Lipids do not have a characteristic functional group that allows us to classify them according to specific functions, and different lipid species are present in diverse substances, such as fats, oils, certain vitamins, and hormones. It is also important to note that most of the nonprotein components of membranes are lipids, which are generated and metabolized by enzymes, which are in turn influenced by the environment of a given biologic system. Lipids are a very heterogeneous group of substances that share a common feature: They are water insoluble but are soluble in organic solvents such as chloroform or ether. These molecules play an important role in diverse biologic functions, constituting the main energy reserve of living beings (triglycerides), forming cell membranes and giving them structure (i.e., the phospholipids of lipid bilayers), and regulating the activity of cells and tissues (as steroids and other hormones). Lipidomics has emerged as a new field to integrate the investigation of the role of genomics, transcriptomics, proteomics, and metabolomics in cell function (18Quehenberger O. Armando A.M. Brown A.H. Milne S.B. Myers D.S. Merrill A.H. et al.Lipidomics reveals a remarkable diversity of lipids in human plasma.J Lipid Res. 2010; 51: 3299-3305Crossref PubMed Scopus (869) Google Scholar). The lipidomic approach is possible today owing to the newly developed instrumentation, protocols, and bioinformatic tools that are now available (19Brown H.A. Lipidomics and bioactive lipids: specialized analytical methods and lipids in disease. Preface.Meth Enzymol. 2007; 433: xv-xviCrossref PubMed Scopus (11) Google Scholar). The lipidome, characterized by global changes in lipid metabolites, is part of the metabolome; however, the widely differing physicochemical properties of many lipids compared to most water soluble metabolites means that they deserve separate analysis (20Griffiths W.J. Ogundare M. Williams C.M. Wang Y. On the future of “omics”: lipidomics.J Inherit Metab Dis. 2011; 34: 583-592Crossref PubMed Scopus (30) Google Scholar). The lipidome is defined by the qualitative and quantitative molecular determination of lipids isolated from tissues, cells, or biologic fluids. To isolate these molecules, the samples must be stored frozen at −80°C to avoid degradation by oxidation of the polyunsaturated fatty acids. Samples coming from tissues must have been previously homogenized. The usual technique for lipid extraction from biologic samples consists of precipitating the sample with organic solvents and concomitantly washing the precipitate with a saline solution. In classic studies, radioactivity assays were used (e.g., using 3H- or 32P-labeled carbon sources) to identify lipids (21Marinetti G.V. Morris T.W. Leaky P. Effects of Ca2+, Mg2+, and depolarizing agents, on the 32Pi-labeling and degradation of phosphatidylinositols in rat brain synaptosomes.Neurochem Res. 1993; 18: 345-351Crossref PubMed Scopus (3) Google Scholar), or fluorescent precursors were used to metabolically label lipids in tissues and cells (such as NBD or rhodamine labels) (22Arvinte T. Hildenbrand K. N-NBD-L-alpha-dilauroylphosphatidylethanolamine. A new fluorescent probe to study spontaneous lipid transfer.Biochim Biophys Acta. 1984; 775: 86-94Crossref PubMed Scopus (33) Google Scholar). However, in the case of low lipid abundance, improvements to these analytic methods are required. The primary technology used for lipid research is mass spectrometry (MS), a technique that can provide the structure, molecular mass, and concentration of the molecules analyzed (20Griffiths W.J. Ogundare M. Williams C.M. Wang Y. On the future of “omics”: lipidomics.J Inherit Metab Dis. 2011; 34: 583-592Crossref PubMed Scopus (30) Google Scholar, 23Ho C.S. Lam C.W.K. Chan M.H.M. Cheung R.C.K. Law L.K. Lit L.C.W. et al.Electrospray ionisation mass spectrometry: principles and clinical applications.Clin Biochem Rev. 2003; 24: 3-12PubMed Google Scholar). However, to study complex lipid mixtures, new technologies have been developed to improve the measurement of diverse components while using only small sample volumes. Methods using MS, such as electrospray MS, have been used to gently ionize biologic molecules and are known as electrospray ionization (ESI) techniques. These allow the structure of molecular species, such as polar lipids and some nonpolar lipids, to be determined (24Fortier M.A. Krishnaswamy K. Danyod G. Boucher-Kovalik S. Chapdalaine P. A postgenomic integrated view of prostaglandins in reproduction: implications for other body systems.J Physiol Pharmacol. 2008; : 65-89PubMed Google Scholar). ESI has proven to be a useful technique for large-molecular-mass biologic molecules, because they are not fragmented into particles; instead, they are ionized into small droplets with the use of electrical energy to transfer ions from a solution into a gaseous phase. ESI is a sensitive and reliable technique that is able to determine sample compositions from microliter volumes, and it can identify nonvolatile and thermally labile biomolecules that can not be studied by other techniques (23Ho C.S. Lam C.W.K. Chan M.H.M. Cheung R.C.K. Law L.K. Lit L.C.W. et al.Electrospray ionisation mass spectrometry: principles and clinical applications.Clin Biochem Rev. 2003; 24: 3-12PubMed Google Scholar). Another ionization method is matrix-assisted laser desorption/ionization (MALDI), which has the advantage of having a higher tolerance for salts and other contaminants. This method can analyze not only proteins, but also lipids in small sample volumes with high sensitivity, a wide mass range, and little fragmentation. The MALDI technique works by dissolving the sample in a matrix that is able to absorb the laser radiation, thus causing the volatilized ions to flow down a tube allowing their separation according to mass (25Jackson S.N. Woods A.S. Direct profiling of tissue lipids by MALDI-TOFMS.J Chromatogr B Analyt Technol Biomed Life Sci. 2009; 877: 2822-2829Crossref PubMed Scopus (60) Google Scholar). Particularly for analysis of complex lipid mixtures, tandem mass spectrometry (MS/MS) has been designed not only for the identification of molecules based on their molecular weights, but also to provide structural information of the molecule, which is achieved by combining two mass spectrometers. The first mass analyzer is used to select a precursor ion that is specifically fragmented by collision with an inert gas, producing ion fragments that are then passed through a second mass analyzer (26Griffiths W.J. Tandem mass spectrometry in the study of fatty acids, bile acids, and steroids.Mass Spectrom Rev. 2003; 22: 81-152Crossref PubMed Scopus (218) Google Scholar). These methods have been coupled with techniques such as time of flight to improve the resolution, as well as with complementary mass analyzers such as ion traps and Fourier transform ion cyclotron resonance to determine the mass of ions. Those techniques are mainly focused on directly profiling lipids, increasing the sensitivity and the mass measurement accuracy (24Fortier M.A. Krishnaswamy K. Danyod G. Boucher-Kovalik S. Chapdalaine P. A postgenomic integrated view of prostaglandins in reproduction: implications for other body systems.J Physiol Pharmacol. 2008; : 65-89PubMed Google Scholar, 27Wenk M.R. The emerging field of lipidomics.Nat Rev Drug Discov. 2005; 4: 594-610Crossref PubMed Scopus (952) Google Scholar). However, MS is also limited by its poor ability to ionize, and therefore quantify, low-abundance lipids within a complex mixture. Independently from the specific method used, MS gives information about the molecular weight of the analytes by measuring the mass to charge ratio, m/z (20Griffiths W.J. Ogundare M. Williams C.M. Wang Y. On the future of “omics”: lipidomics.J Inherit Metab Dis. 2011; 34: 583-592Crossref PubMed Scopus (30) Google Scholar). The methods usually used for lipid analysis are gas chromatography (GC), thin layer chromatography (TLC), and high-performance liquid chromatography (HPLC) (28Willmann J. Thiele H. Leibfritz D. Combined reversed phase HPLC, mass spectrometry, and NMR spectroscopy for a fast separation and efficient identification of phosphatidylcholines.J Biomed Biotechnol. 2011; 2011: 1-8Crossref Scopus (14) Google Scholar). GC is an important technique generally used for the analysis of fatty acids. However, its restriction to volatile lipids requires the derivatization of the nonvolatile lipid components before the method can be applied, as well as the use of internal standards. HPLC is a simpler method in which derivatization is not required. It can be applied across diverse chromatographic conditions in which reverse-phase and normal-phase chromatography would normally be used. HPLC is highly selective and efficient in detecting lipid classes, allowing it to determine and quantify analytes with high resolution. For lipid class separations, TLC is usually used for lipid mixtures and has the advantage of being cheaper and faster than other methods, including HPLC and GC; however, low resolution and sensitivity restricts the use of TLC (29Clogston J.D. Patri A.K. Lipid component quantitation by thin layer chromatography.Methods Mol Biol. 2011; 697: 109-117Crossref PubMed Scopus (8) Google Scholar). The nuclear magnetic resonance (NMR) technique, designed especially for lipid membranes, uses a principle based on the presence of active nuclei atoms that form lipids (30Grélard A. Loudet C. Diller A. Dufourc E.J. NMR spectroscopy of lipid bilayers.Methods Mol Biol. 2010; 654: 341-359Crossref PubMed Scopus (21) Google Scholar). Although this method studies the physical properties of membrane compounds, nowadays the focus of this approach has been changed toward studying the properties of complex lipid mixtures to determine their function and structure. NMR has even contributed to the development of other “-omics,” such as metabolomics and proteomics, although its sensitivity is a significant limiting factor. Because NMR is a powerful technique for structural analysis many new technologies have focused on improving it (31Gawrisch K. Eldho N.V. Polozov I.V. Novel NMR tools to study structure and dynamics of biomembranes.Chem Phys Lipids. 2002; 116: 135-151Crossref PubMed Scopus (69) Google Scholar). The availability of instrumentation, protocols, and bioinformatic tools, such as LIPID MAPS, have made the characterization of changes in lipid metabolites and the comprehensive analysis of the mammalian lipidome possible (18Quehenberger O. Armando A.M. Brown A.H. Milne S.B. Myers D.S. Merrill A.H. et al.Lipidomics reveals a remarkable diversity of lipids in human plasma.J Lipid Res. 2010; 51: 3299-3305Crossref PubMed Scopus (869) Google Scholar). A schematic summarizing this section is depicted in Figure 1. In recent years, several studies in animal models have shown the importance of lipids at the time of embryo implantation. Lipidomics studies have enabled the identification and characterization of these lipids at the time of endometrial receptivity. Furthermore, functional studies in mice have confirmed the important role played by these lipids in endometrial receptivity and implantation. Lipid molecules such as endocannabinoids, lysophosphatidic acid (LPA), and prostaglandins (PGs) have been reported to be some of the most widely studied mediators of embryo implantation (32Sordelli M.S. Beltrame J.S. Cella M. Gervasi M.G. Perez Martinez S. Burdet J. et al.Interaction between lysophosphatidic acid, prostaglandins and the endocannabinoid system during the window of implantation in the rat uterus.PLoS One. 2012; 7: e46059Crossref PubMed Scopus (40) Google Scholar). Consistency between the results observed in different animal models suggests that this trend might also be found in humans, meaning that the receptive state of the endometrium may be characterized by a specific pattern of lipids. Endocannabinoids are lipids that have been detected in different human tissues and reproductive fluids. The anandamide N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol have been described as the two principal endocannabinoids involved in mouse implantation events; moreover, it has been described that aberrant levels of these lipids lead to deferred implantation and compromised pregnancy outcome (33Maccarrone M. DeFelici M. Klinger F.G. Battista N. Fezza F. Dainese E. et al.Mouse blastocysts release a lipid which activates anandamide hydrolase in intact uterus.Mol Hum Reprod. 2004; 10: 215-221Crossref PubMed Scopus (48) Google Scholar). These studies are supported by genetic evidence showing that fatty acid amide hydrolase (FAAH), the major degrading enzyme for endocannabinoids, is linked with successful continued pregnancy after in vitro fertilization (IVF) and subsequent embryo transfer (32Sordelli M.S. Beltrame J.S. Cella M. Gervasi M.G. Perez Martinez S. Burdet J. et al.Interaction between lysophosphatidic acid, prostaglandins and the endocannabinoid system during the window of implantation in the rat uterus.PLoS One. 2012; 7: e46059Crossref PubMed Scopus (40) Google Scholar). Although low FAAH levels are associated with the up-regulation of AEA and are therefore related to nonreceptivity to embryo implantation, down-regulation of AEA correlates with uterine receptivity (34Schmid P.C. Paria B.C. Krebsbach R.J. Schmid H.H. Dey S.K. Changes in anandamide levels in mouse uterus are associated with uterine receptivity for embryo implantation.Proc Natl Acad Sci U S A. 1997; 94: 4188-4192Crossref PubMed Scopus (221) Google Scholar). These data support work that shows that cannabinoid AEA signaling is important in both the mouse embryo and uterus during the implantation period (35Paria B.C. Deutsch D.D. Dey S.K. The uterus is a potential site for anandamide synthesis and hydrolysis: differential profiles of anandamide synthase and hydrolase activities in the mouse uterus during the periimplantation period.Mol Reprod Dev. 1996; 45: 183-192Crossref PubMed Scopus (127) Google Scholar). Lysophosphatidic acid is a water-soluble phospholipid that acts as a potent signaling molecule with wide-ranging effects on many different target tissues. LPA, acting through its receptor LPA3, is essential for normal embryo size and spacing in mice which is linked with a positive effect on implantation (36Ye X. Hama K. Contos J.J. Anliker B. Inoue A. Skinner M.K. et al.LPA3-mediated lysophosphatidic acid signalling in embryo implantation and spacing.Nature. 2005; 435: 104-108Crossref PubMed Scopus (435) Google Scholar). It has also been shown that deficiency of LPA3 in uteri during preimplantation leads to down-regulation of cyclooxygenase-2 (COX-2) and therefore reduces the levels of PGs, thus directly affecting the process of implantation and decidualization. Studies with ovine trophectoderm and porcine and bovine endometrium also demonstrated that LPA is an important mediator in the process of implantation. A recent study has suggested that LPA controls the levels of endocannabinoid and PG mediators that act via LPA3 to rearrange the endometrium for implantation during the receptive stage (32Sordelli M.S. Beltrame J.S. Cella M. Gervasi M.G. Perez Martinez S. Burdet J. et al.Interaction between lysophosphatidic acid, prostaglandins and the endocannabinoid system during the window of implantation in the rat uterus.PLoS One. 2012; 7: e46059Crossref PubMed Scopus (40) Google Scholar, 37Mizugishi K. Li C. Olivera A. Bielawski J. Bielawska A. Deng C.X. et al.Maternal disturbance in activated sphingolipid metabolism causes pregnancy loss in mice.J Clin Invest. 2007; 117: 2993-3006Crossref PubMed Scopus (112) Google Scholar). Prostaglandins are lipids derived from membrane phospholipids by the action of phospholipase A2 (PLA2) and COX-2. These enzymes are responsible for the production of PGs that become elevated in both the lumen and the stroma throughout the receptive period, although only at the site of blastocyst implantion in murine models. The lack of either PLA2 or COX-2 leads to an absence of PG synthesis, which in mice results in several implantation defects (38Achache H. Revel A. Endometrial receptivity markers, the journey to successful embryo implantation.Hum Reprod Update. 2006; 12: 731-746Crossref PubMed Scopus (587) Google Scholar), thus confirming the importance of PGs in murine reproductive efficiency (24Fortier M.A. Krishnaswamy K. Danyod G. Boucher-Kovalik S. Chapdalaine P. A postgenomic integrated view of prostaglandins in reproduction: implications for other body systems.J Physiol Pharmacol. 2008; : 65-89PubMed Google Scholar). It has also been demonstrated that any alteration in the PG production pathway has a dramatic effect on the process of implantation, decreasing the probability of achieving pregnancy at the end of the process. For that reason it has been established that PGs significantly contribute to establishing the microenviroment required during implantation and decidualization, and are therefore strong molecular candidates for implantation regulation in humans (39Tranguch S. Daikoku T. Guo Y. Wang H. Dey S.K. Molecular complexity in establishing uterine receptivity and implantation.Cell Mol Life Sci. 2005; 62: 1964-1973Crossref PubMed Scopus (76) Google Scholar). In mice, PGE2 and PGF2α have been revealed as the most important PGs during the implantation stage (40Pakrasi P.L. Prostaglandins and ovum implantation in mice.J Exp Zool. 1997; 278: 53-57Crossref PubMed Scopus (8) Google Scholar). That work reported that PGE2 was increased at implantation sites, whereas PGF2α was higher at interimplantation areas (40Pakrasi P.L. Prostaglandins and ovum implantation in mice.J Exp Zool. 1997; 278: 53-57Crossref PubMed Scopus (8) Google Scholar). However, a more recent investigation differed from earlier studies, showing that PGI2 levels were the highest, followed by PGE2 and no differences in the PGF2α levels during implantation were found compared with other phases of the menstrual cycle (41Cong J. Diao H.-L. Zhao Y.-C. Ni H. Yan Y.-Q. Yang Z.-M. Differential expression and regulation of cylooxygenases, prostaglandin E synthases and prostacyclin synthase in rat uterus during the peri-implantation period.Reproduction. 2006; 131: 139-151Crossref PubMed Scopus (31) Google Scholar). Due to the properties of PGI2 as a vasoactive agent that participates in vascular permeability changes, it has been associated with localized vascular permeability in the uterus during implantation, indicating that PGE2 and PGI2 are important for ovulation but also for the initiation of implantation in mouse uteri (42Lim H. Paria B.C. Das S.K. Dinchuk J.E. Langenbach R. Trzaskos J.M. et al.Multiple female reproductive failures in cyclooxygenase 2–deficient mice.Cell. 1997; 91: 197-208Abstract Full Text Full Text PDF PubMed Scopus (1235) Google Scholar). In contrast, studies performed with rats as a model showed that PGE2, b