Immunochemical detection and characterization of pregnancy-associated endometrial alpha 1- and alpha 2-globulins secreted by human endometrium and decidua

Antisera raised against the soluble antigens of the endometrium of early pregnancy detected two antigenic proteins of alpha 1 and alpha 2 mobility in extracts of this tissue and were termed antigens A and B. Neither antigen was detected in pregnancy sera or extracts of proliferative endometrium, but antigen B was detected in extracts of secretory endometrium and both were present in amniotic fluid and medium from in-vitro incubations of pregnancy endometrium. Fractionation of radiolabelled medium on ion-exchange chromatography demonstrated that antigens A and B co-eluted with the proteins from which EP14 and EP15 were derived and which were the major secretory polypeptides of pregnancy endometrium in vitro. Further biochemical purification revealed that EP14 (Mr 32 000) was derived from a protein of native molecular weight 36 000 which existed in two forms, whereas EP15 (Mr 28 000) was derived from a dimeric glycoprotein of native molecular weight 56 000. Immunochemical studies demonstrated that antigens A and B are identical to these two secretory proteins and have been termed pregnancy-associated endometrial alpha 1- and alpha 2-globulins (alpha 1- and alpha 2-PEG).     

Immunochemical detection, physicochemical characterization and levels of pregnancy-associated endometrial alpha 2-globulin (alpha 2-PEG) in seminal plasma of men

Pregnancy-associated endometrial alpha 2-globulin (alpha 2-PEG), the major secretory protein of the human uterine endometrium during the luteal phase of the menstrual cycle and early first trimester of pregnancy, has been detected by immunochemical techniques in seminal plasma. Biochemical analysis and immunoblotting has verified that immunoreactive alpha 2-PEG in seminal plasma exhibits properties identical to those of endometrial alpha 2-PEG, i.e. Concanavalin A-binding dimeric glycoprotein of native Mr 56,000, subunit Mr 28,000, average pI 4.7 and of alpha 2-mobility. Concentration of alpha 2-PEG in seminal plasma was 22.13 +/- 2.82 micrograms/ml (mean +/- s.e.m., n = 110) which compared to 12.02 +/- 1.65 micrograms/ml (mean +/- s.e.m., n = 48) found in amniotic fluid at 11-20 weeks of pregnancy, to 4.29 +/- 1.66 micrograms/ml (mean +/- s.e.m., n = 15) in uterine luminal fluid in women during the luteal phase and to 0.245 +/- 0.025 micrograms/ml (mean +/- s.e.m., n = 10) in sera at 10 weeks of pregnancy. This distribution is very different from that observed for pregnancy-associated placentally-derived serum proteins detected in seminal plasma. The source of alpha 2-PEG in seminal plasma is unknown but is unlikely to be the testis because of the normal levels observed in vasectomized men. In the endometrium alpha 2-PEG synthesis and secretion appears to be related to progesterone-dependent differentiation of the glandular epithelium. Therefore these observations suggest that a different mechanism of regulation of the gene for alpha 2-PEG operates in the male reproductive tract.     

Progesterone-associated proteins PP12 and PP14 in the human endometrium

Two proteins, designated as PP12 and PP14 were originally isolated from soluble extracts of the human placenta and its adjacent membranes. We have shown that they are synthesized by decidualized/secretory endometrium and not by placenta. Both proteins occur at high concentrations in human amniotic fluid, which is therefore an excellent source for purification. PP12 is a 34-kDa glycoprotein, which has an N-terminal amino acid sequence of Ala-Pro-Trp-Gln-Cys-Ala-Pro-Cys-Ser-Ala. This is identical with that of somatomedin-binding protein purified from the amniotic fluid. PP12 too binds somatomedin-C, or IGF-I (insulin-like growth factor-I). Human secretory endometrium synthesizes and secretes PP12, and progesterone stimulates its secretion. PP14 is a 28-kDa glycoprotein. Its N-terminal sequence shows homology to that of beta-lactoglobulins from various species. We have found PP14 in the human endometrium, serum and milk. Immunologically, PP14 is related to progestagen-associated endometrial protein (PEP), alpha-2 pregnancy-associated endometrial protein (alpha-2, PEG), endometrial protein 15 (EP15), alpha-uterine protein (AUP) and chorionic alpha-2 microglobulin (CAG-2). In ovulatory menstrual cycles, the concentration of PP14 increases in endometrial tissue as the secretory changes advance. In serum, the PP14 concentration begins to rise later than the progesterone levels, and high serum PP14 levels are maintained for the first days of the next cycle. By contrast, no elevation of serum PP14 level is seen in anovulatory cycles. Our results show that progesterone-associated proteins are synthesized by the human endometrium and appear in the peripheral circulation, where they can be quantitatively measured using immunochemical techniques.     

Synthesis and release of polypeptides by the baboon (Papio anubis) uterine endometrium in culture

This study was designed to identify proteins released in culture by the baboon uterine endometrium. Endometrial tissues from cyclic baboons were minced and cultured in the presence of L-[3H]leucine or L-[35S]methionine for 24 h. The culture media and solubilized tissues were analyzed by one- and two-dimensional gel electrophoresis for secretory products that were uterine-specific. The fluorographs of the one- and two-dimensional gels demonstrated that the proteins released into culture media could be divided into two groups. Group I proteins were present throughout the menstrual cycle and showed minor cyclic variations in intensity, and Group II proteins were those that appeared to be hormonally modulated. Group I was comprised of several high molecular weight proteins (Mr greater than 200,000) and at least five additional proteins ranging in molecular weight from 80,000 to 37,000, with isoelectric points (pIs) of 5.1 to 6.0. Group II consisted of a protein (Mr 33,000; pI 7.6) that was observed only during the follicular stages of the cycle and two other groups of proteins (Mr 130,000 and 88,000) that were present during the luteal stage. Western blots of tissue culture media incubated with antibodies against human placental proteins (PP) and prolactin demonstrated that PP4 and PP7 were secreted throughout the cycle while PP12, PP16, and prolactin were only present during the luteal stage of the cycle. Thus, it appears that the baboon uterine endometrium, like that of the human, secretes a wide array of proteins in culture. Our results also suggest that a few of these proteins are immunologically similar. Endometrial differentiation during the menstrual cycle altered the secretion of some proteins, whereas the synthesis of others appeared to be dependent on either estrogen or progesterone and were stage-specific.     

The effect of progesterone and human interferon alpha-2 on the release of PGF2 alpha and PGE from epithelial cells of human proliferative endometrium.

Progesterone and interferon-like trophoblastic proteins modulate prostaglandin (PG) synthesis from endometrium in early ovine and bovine pregnancy. Enriched epithelial cells were prepared from human endometrium removed in the proliferative phase of menstrual cycle (n = 8). Progesterone at a concentration of 1 microM suppressed PGE release from the cells during the first 24 hours in culture. After 48 hours in culture progesterone at a dose of 100 nM and 1 microM suppressed both the release of PGF2 alpha and PGE from the cells and this suppression was maintained for a further two days. Addition of exogenous 30 microM arachidonic acid (AA) abolished this effect of progesterone on both PGF2 alpha and PGE release. Interferon alpha-2 did not suppress the basal release of PGF2 alpha nor PGE. In the presence of progesterone, interferon alpha-2 attenuated the progesterone mediated suppression of PGF2 alpha but not PGE release from endometrial cells. These findings suggest that progesterone suppresses the basal release of PGs from human endometrium, but unlike the sheep, interferon alpha-2 does not exert this action on human endometrium.     

Regulation of bcl-2 gene family members in human endometrium by antiprogestin administration in vivo.

It is likely that the changes which occur in the endometrium throughout the menstrual cycle involve apoptosis, and that expression of associated genes, such as the bcl-2 family, are regulated by sex steroids. The aim of this study was to investigate the presence of bcl-2, Bax and oestrogen receptor proteins in secretory endometrium collected from ten patients with normal ovulatory cycles 4 or 6 days after the LH surge, and on the same days in a subsequent cycle in which the formation of secretory changes was inhibited by the administration of the antiprogestin mifepristone (RU486) 2 days after the onset of the LH surge. Since some stromal cells display positive immunoreactivity, leucocyte subpopulations of macrophages (CD68-positive) and large granular lymphocytes (CD56-positive) were identified in serial sections. After administration of mifepristone on day 2 after the LH surge, a significant increase in bcl-2 immunoreactivity was observed in glandular and surface epithelium. A positive correlation (0.874) with nuclear oestrogen receptor immunoreactivity in endometrial glands was demonstrated. Subsets of stromal cells, identified as CD68-positive macrophages and CD56-positive large granular lymphocytes displayed positive immunoreactivity for the bcl-2 epitope, which was unaffected by mifepristone administration. Bax immunostaining was similar in control and antiprogestin-treated endometrium. These data indicate that antiprogestin administration inhibits progesterone downregulation of steroid receptors in endometrial glands, resulting in persistence of a proliferative endometrium and accompanying bcl-2 secretion.     

Localisation of the Notch family in the human endometrium of fertile and infertile women

To investigate the spatial and temporal immunolocalisation and staining intensity of the Notch signalling family in endometrium of fertile and infertile women, endometrial biopsies were collected by curettage from 25 fertile women across the menstrual cycle and 10 infertile women in the mid secretory phase of menstrual cycle. Immunohisotchemistry was completed for NOTCH1, -2, -3, -4, cleaved Notch, DLL1, -3, -4, JAGGED1, -2, HES and NUMB and immunostaining intensity measured in both the endometrial glandular and luminal epithelium. NOTCH1 and the ligands DLL1 and JAGGED1 were key proteins displaying increased staining intensity during the receptive phase of the menstrual cycle and dysregulated in infertile endometrium. Conversely, NUMB a negative regulator of Notch signalling was decreased in the mid secretory phase of the menstrual cycle in fertile women and increased with infertility.     

Effects of ovine conceptus secretory proteins and progesterone on oxytocin-stimulated endometrial production of prostaglandin and turnover of inositol phosphate in ovariectomized ewes.

This study examined the effects of progesterone and intrauterine injection of ovine conceptus secretory proteins (oCSP) on endometrial responsiveness to oxytocin. Twelve ewes were ovariectomized on day 4 of the cycle (oestrus = day 0) and assigned in a 2 x 2 factorial arrangement, to receive either 1.5 mg ovine serum proteins (SP) or oCSP containing 25 micrograms ovine trophoblast protein 1 (oTP-1) (by radioimmunoassay) in 1.5 mg total protein into each uterine horn, via catheters, twice a day on days 11, 12, 13 and 14. Ewes received 200 mg progesterone per day (i.m.) from day 4 to day 10 or 15. Oxytocin-induced prostaglandin F2 alpha was measured as 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) on days 11, 12, 13 and 14 in plasma from three integrated, 10 min (10 ml) blood samples (0-10, 10-20, 20-30 min) obtained after intravenous injection of 20 iu oxytocin, and in a pre-oxytocin (-10 to 0 min) sample collected via an indwelling jugular catheter. The pre-oxytocin samples were also assayed for progesterone. Oxytocin-induced turnover of inositol phosphate was determined in endometrium on day 15 after hysterectomy. In ewes receiving progesterone to day 10, plasma progesterone decreased from about 12 to 2 ng ml-1 (SEM +/- 2.6) during the treatment period (days 11-14), but remained high (12-20 +/- 2.6 ng ml-1) in ewes that received progesterone to day 15. Intrauterine injection of oCSP resulted in high basal concentrations of PGFM on days 12 and 13 compared with SP-treated ewes (P less than 0.01). Treatments with progesterone did not affect basal PGFM concentrations. Treatment with oCSP abolished oxytocin-induced endometrial secretion of prostaglandin only if progesterone was maintained to day 15 (P less than 0.01); in ewes receiving such treatment, oCSP inhibited (P less than 0.01), but SP did not inhibit, oxytocin-induced endometrial turnover of inositol phosphate (P less than 0.06), which was greater in ewes treated with progesterone to day 10 than in those treated to day 15 (P less than 0.05). Ewes that responded to oxytocin with increased PGFM exhibited increased oxytocin-stimulated turnover of inositol phosphate on day 15. These results indicate that the antiluteolytic action oTP-1 exerts on the endometrium requires progesterone and that this mechanism involves inhibition of oxytocin-stimulated turnover of inositol phosphate.     

The effects of danazol, mefenamic acid, norethisterone and a progesterone-impregnated coil on endometrial prostaglandin concentrations in women with menorrhagia

The effects of four medical treatments have been assessed on menstrual blood loss (MBL) and endometrial prostaglandin (PG) concentrations in 30 women with objectively confirmed menorrhagia. Patients were randomly treated with danazol, 200 mg daily (n = 6), mefenamic acid, 500 mg three times daily during menses (n = 8), norethisterone, 5 mg twice daily from day 15-25 of the cycle (n = 8) or a progesterone-impregnated coil releasing 65 micrograms progesterone daily (n = 8). Endometrial biopsies were obtained in the mid-luteal phase before and after treatment in 23 cases, and assayed for PG content using radioimmunoassay. Treatment with norethisterone had no effect on either MBL or the concentration of PGs in the endometrium. MBL was significantly reduced after treatment with mefenamic acid (P = 0.05, n = 6) and the progesterone coil (P less than 0.05, n = 6), and was reduced in each of 4 cases treated with danazol in whom endometrial biopsies were available. Although there was no consistent change in endometrial PG concentrations in either the mefenamic acid or danazol groups, the lower MBL after insertion of the progesterone coil was associated with a reduced endometrial content of PGE, PGF2 alpha and "total" PG (6oxo PGF1 alpha + PGE + PGF2 alpha)-P = 0.05. Whereas the cyclooxygenase inhibitor mefenamic acid is likely to exert its effect on endometrial PGs at the time of menstruation itself, the continuous administration of progesterone throughout the menstrual cycle could result in both an impairment in estrogen receptor generation leading to reduced estrogen-mediated cyclooxygenase activity, and an increase in endometrial PG metabolism.     

Spatial and temporal characterization of endometrial mesenchymal stem-like cells activity during the menstrual cycle

The human endometrium is a highly dynamic tissue with the ability to cyclically regenerate during the reproductive life. Endometrial mesenchymal stem-like cells (eMSCs) located throughout the endometrium have shown to functionally contribute to endometrial regeneration. In this study we examine whether the menstrual cycle stage and the location in the endometrial bilayer (superficial and deep portions of the endometrium) has an effect on stem cell activities of eMSCs (CD140b⁺CD146⁺ cells). Here we show the percentage and clonogenic ability of eMSCs were constant in the various stages of the menstrual cycle (menstrual, proliferative and secretory). However, eMSCs from the menstrual endometrium underwent significantly more rounds of self-renewal and enabled a greater total cell output than those from the secretory phase. Significantly more eMSCs were detected in the deeper portion of the endometrium compared to the superficial layer but their clonogenic and self-renewal activities remained similar. Our findings suggest that eMSCs are activated in the menstrual phase for the cyclical regeneration of the endometrium.     

Concentration of plasminogen activators and inhibitor in the human endometrium at different phases of the menstrual cycle.

The concentrations of tissue plasminogen activator (t-PA), urokinase plasminogen activator (u-PA) and plasminogen activator inhibitor (PAI-1) have been determined in endometrial curettings obtained from 46 subfertile women during proliferative, early or late secretory phases of the menstrual cycle. t-PA activity and antigen concentrations was significantly higher (P < 0.001) in late secretory endometrium than in proliferative or early secretory endometrium. Higher concentrations of PAI-1 antigen (P < 0.05) were also noted in late secretory phase than in proliferative and early secretory endometrium. However, u-PA concentration was not significantly different and no PAI activity could be demonstrated in the menstrual phases studied. Zymography studies confirmed the presence of both t-PA and u-PA in the endometrium. Ovarian hormonal patterns may therefore influence the activity of plasminogen activators especially of t-PA in the endometrium during various phases of the menstrual cycle.     

Inhibin/activin subunits alpha, beta-A and beta-B are differentially expressed in normal human endometrium throughout the menstrual cycle

Inhibins are dimeric glycoproteins composed of an alpha (alpha) subunit and one of two possible beta (beta-) subunits (betaA or betaB). The aims of this study were to assess the frequency and tissue distribution patterns of the inhibin subunits in normal human endometrium. Samples from human endometrium from proliferative phase (PP; n=32), early secretory phase (ES; n=10) and late secretory phase (LS; n=12) were obtained. Immunohistochemistry, immunofluorescence and a statistical analysis were performed. All three inhibin subunits were expressed by normal endometrium by immunohistochemistry and immunofluorescence. Inhibin-alpha was primarily detected in glandular epithelial cells, while inhibin-beta subunits were additionally localised in stromal tissue. Inhibin-alpha staining reaction increased significantly between PP and ES (P<0.05), PP and LS (P<0.01), and ES and LS (P<0.02). Inhibin-betaA and -betaB were significant higher in LS than PP (P<0.05) and LS than ES (P<0.05). All three inhibin subunits were expressed by human endometrium varying across the menstrual cycle. This suggests substantial functions in human implantation of inhibin-alpha subunit, while stromal expression of the beta subunits could be important in the paracrine signalling for adequate endometrial maturation. The distinct expression in human endometrial tissue suggests a synthesis of inhibins into the lumen and a predominant secretion of activins into the stroma.     

Expression of the chemokine eotaxin and its receptor, CCR3, in human endometrium

Eosinophils are present in human endometrium only immediately before and during menstruation, suggesting a role in that process. The expression of the eosinophil chemoattractant, eotaxin, and its receptor, CCR3, within the human endometrium were investigated by immunohistochemical analysis of tissue sections spanning the entire menstrual cycle. Eotaxin was localized to perivascular cells in the late secretory phase, and it was also identified in eosinophils. However, the highest levels of this chemokine were present in both luminal and glandular epithelial cells during the proliferative and secretory phases of the cycle. Treatment of endometrial tissue with monensin, which blocks protein secretion, increased epithelial immunoreactive eotaxin, substantiating synthesis in these cells. Although the CCR3 receptor was expressed by eosinophils, it was also strongly expressed by endometrial epithelial cells. The CCR3 receptor on purified, cultured endometrial epithelial cells was functional, as assessed by a transient Ca(2+) flux in response to eotaxin. These analyses demonstrate that eotaxin is expressed by endometrial cells and may therefore be involved in the recruitment of eosinophils into this tissue premenstrually. However, the observation that this chemokine and the CCR3 molecule are strongly expressed by epithelial cells throughout the cycle suggests that these proteins may have additional important functions within the endometrium.     

Differential effects of progesterone treatment on the oxytocin-induced prostaglandin F2 alpha response and the levels of endometrial oxytocin receptors in ovariectomized ewes.

The oxytocin-induced uterine prostaglandin (PG) F2 alpha response and the levels of endometrial oxytocin receptors were measured in ovariectomized ewes after they had been given steroid pretreatment (SP) with progesterone and estrogen to induce estrus (day of expected estrus = Day 0) and had subsequently been treated with progesterone over Days 1-12 and/or PGF2 alpha over Days 10-12 postestrus. The uterine PGF2 alpha response was measured after an i.v. injection of 10 IU oxytocin on Days 13 and 14, using the PGF2 alpha metabolite, 13,14-dihydro-15-keto-PGF2 alpha (PGFM), as an indicator for PGF2 alpha release. The levels of oxytocin receptors in the endometrium were measured on Day 14. During the treatment with progesterone, the peripheral progesterone concentrations were elevated and remained above 1.8 ng/ml until the morning of Day 14. The PGFM responses to oxytocin in untreated controls and SP controls were low on both Days 13 and 14 whereas the levels of endometrial oxytocin receptors in the same ewes were high. Treatment with progesterone either alone or in combination with PGF2 alpha significantly (p less than 0.04) increased the PGFM response on Day 14 and reduced the levels of endometrial oxytocin receptors; treatment with PGF2 alpha alone had no effect. It is concluded that progesterone promotes the PGFM response to oxytocin while simultaneously suppressing the levels of endometrial oxytocin receptors. PGF2 alpha treatment had no effect on either the uterine secretory response to oxytocin or the levels of oxytocin receptors in the endometrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

De novo synthesis and release of polypeptides from cyclic and early pregnant porcine oviductal tissue in explant culture

The objective of the present study was to identify and characterize in a limited manner the major de novo oviductal secretory proteins (OSP) synthesized and released by the porcine oviduct. Oviductal tissue was collected on various days of the estrous cycle (EC) and early pregnancy (EP) and cultured in a modified minimal essential medium supplemented with 100 muCi L-[3H]-leucine. Oviductal secretory activity, as measured by the rate of incorporation of 3H-leucine (dpm/mg wet tissue weight) into nondialyzable macromolecules, was greatest (P less than .01) between days 0 and 2 and reached its lowest levels on days 10 to 15. There was no difference between left and right side or pregnancy status. This increased rate of incorporation at proestrus and estrus is temporally associated with elevated levels of estrogen. Incorporation rate for ampulla was greater than for the isthmus. Analysis of oviductal culture medium by one-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis and fluorography revealed three protein bands of relative molecular weight (Mr) 335,000, 115,000, and 85,000, which were associated with proestrus, estrus, and metestrus and were not detectable on other days. All three proteins also incorporated 3H-glucosamine. The 115,000 Mr band was the major 3H-glucosamine-labeled protein. Two protein bands (Mr 60,000 and 20,000) were expressed with increasing progesterone during diestrus. Other de novo synthesized protein bands appear to be present throughout the EC and EP with little modulation by estrogen or progesterone. Thus, this study demonstrates that for the porcine oviduct, the increase in the incorporation rate of 3H-leucine into OSP by both whole oviduct and ampulla and de novo synthesis and secretion of three glycoproteins, Mr 335,000, 115,000, and 85,000, were associated with proestrus and estrus when events such as fertilization and early cleavage stages of embryo development occurred.     

Effects of tumor necrosis factor-alpha on secretion of prostaglandins E2 and F2alpha in bovine endometrium throughout the estrous cycle

To determine the physiological significance of tumor necrosis factor-alpha (TNFalpha) in the regulation of endometrial prostaglandin (PG) release in cattle, we investigated the effects of TNFalpha on the secretion of PGE2 and PGF2alpha by bovine endometrium during the estrous cycle. Bovine uteri were classified into six stages (estrus: Day 0, early luteal 1: Days 2 to 3, early luteal 11: Days 5 to 6, mid-luteal: Days 8 to 12, late luteal: Days 15 to 17 and follicular: Days 19 to 21). After 1 h of pre-incubation, endometrial tissues (20 to 30 mg) were exposed to 0 or 0.6 nM TNFalpha for 4 h. The PGE2 concentrations in the medium were higher in the luteal stages than in the follicular stage and in estrus. In contrast, PGF2alpha concentrations were higher in the follicular stage and in estrus than in the luteal stages. The ratio of the basal concentrations of PGE2 and PGF2alpha (PGE2/PGF2alpha ratio) was higher in the luteal stages than in the follicular stage and in estrus. Although TNFalpha stimulated both PGE2 and PGF2alpha secretion during the entire period of the estrous cycle, the level of stimulation of TNFalpha on PGE2 output by the bovine endometrium does not show the same cyclical changes as that shown on PGF2alpha output. The stimulation of TNFalpha resulted in a decrease in the PGE2/PGF2alpha ratio only in the late luteal stage. Furthermore, TNFalpha stimulated PGE2 secretion in stromal, but not epithelial cells. The overall results suggest that TNFalpha is a potent regulator of endometrial PGE2 secretion as well as PGF2alpha secretion during the entire period of estrous cycle, and that TNFalpha plays different roles in the regulation of secretory function of bovine endometrium at different phases of the estrous cycle.     

Aquaporin-2 expression in human endometrium correlates with serum ovarian steroid hormones

The aim of the present study was to examine the expression of aquaporin-2 (AQP2), a member of the water channel family aquaporins (AQPs), in human uterine endometrium and its modulation of ovarian steroid hormone at the proliferative and secretory phases. Western blot, immunohistochemistry, and RT-PCR were employed in the present study. Western blot revealed a 29-kDa band that represented AQP2 in human endometrium. The expression of AQP2 in endometrium was confirmed by RT-PCR and immunohistochemical results. The immunohistochemical analysis demonstrated that AQP2 was prominent in luminal and glandular epithelial cells of endometrium. The levels of endometrial AQP2 expression changed during the menstrual cycle and were higher in the secretory endometrium than in the proliferative endometrium. A significantly high level of AQP2 was detected at the mid-secretory phase. There was a positive correlation between the levels of the endometrial AQP2 expression and the concentrations of the serum 17beta-estradiol (E2) or/and progesterone (P4). These data for the first time corroborate that AQP2 is expressed in human endometrium and that the expression of AQP2 in human endometrium might be regulated by E2 or/and P4. The changed expression of AQP2 at different phases of the menstrual cycle may be essential to reproductive physiology in human. The high level of endometrial AQP2 expression was observed at the mid-secretory phase, the time of embryo implantation, suggesting that AQP2 might play physiological roles in the uterine receptivity.