Effects of luteal phase administration of mifepristone (RU486) and prostaglandin analogue or inhibitor on endometrium in the rhesus monkey

Early luteal phase administration of a potent anti-progestin like mifepristone (RU486) inhibits blastocyst implantation and the establishment of pregnancy without marked changes in menstrual cyclicity and ovarian steroid hormone profiles; however, the underlying mechanism is not very clear. In the present study, a hypothesis that prostaglandins (PG) are involved in the anti-gestatory action of luteal phase mifepristone was tested. Endometrial changes in rhesus monkeys were examined following luteal phase administration of mifepristone, a prostaglandin synthesis inhibitor (diclofenac) and a prostaglandin analogue (misoprostol) either alone or in combination. Twenty-five monkeys were randomly assigned to six groups: group 1 (n = 4), normal control group; group 2 (n = 4), mifepristone (2 mg, daily, s.c.) treated group; group 3 (n = 4), diclofenac (25 mg, daily, i.m.) treated group; group 4 (n = 4), misoprostol (100 microg, daily, oral) treated group; group 5 (n = 5), mifepristone and diclofenac (same dosages as for groups 2 and 3) treated group; group 6 (n = 4), mifepristone and misoprostol (same dosages as for groups 2 and 4) treated group. All treatments were given to monkeys on days 16-18 of mated cycles and endometrial tissue samples were collected on day 20. With diclofenac alone (group 3), marginal changes were observed in glandular, stromal and vascular compartments, and there were few apoptotic bodies in gland cells; partial inhibition and delay in implantation was earlier reported. Significantly higher oestrogen receptor expression in glandular epithelial cells as compared with all other treatment groups was found after treatment with misoprostol alone (group 4) and was associated with normal fecundity. The anti-nidatory action of luteal phase antiprogestin treatment alone or in combination with diclofenac or misoprostol was associated with altered endometrial histometric features characterized by glandular apoptosis, regression in secretory functions, decreased oedema, extravasation and a higher degree of stromal leukocytic infiltration. In these three groups (groups 2, 5 and 6) receptors for oestrogen and progesterone receptors were significantly higher in stromal cells, and lower in vascular cells, while glandular cells showed significantly higher progesterone receptors compared with the control group. The anti-nidatory activity of mifepristone and associated endometrial changes could not be accentuated or attenuated with co-administration of PGE or diclofenac, nor could these be mimicked by these agents alone.     

Immunohistochemical localization of extracellular matrix proteins in luteal phase endometrium of fertile and infertile patients

The lack of expression of certain components involved in cell adhesion and migration is believed to contribute to endometrial dysfunction and implantation failure. The purpose of this study was to investigate whether luteal phase endometrium in women with unexplained infertility differs, with respect to specific extracellular matrix (ECM) proteins, from endometrium of normal fertile women. A panel of monoclonal antibodies to collagen type IV, fibronectin and laminin was used to characterize the localization of ECM components in the different endometrial compartments. Precisely timed endometrial biopsies obtained at 4, 7, 10 and 13 days following the luteinizing hormone surge were obtained from 22 normal fertile women (group 1) and 24 women suffering from unexplained infertility (group 2). Paraffin-embedded sections were labelled using the streptavidin-biotin alkaline phosphatase technique. In group 1, collagen type IV, fibronectin and laminin were absent from the luminal epithelium but present in stromal cells and the basement membrane of glands and blood vessels. In group 2, these components were absent from all endometrial regions using equivalent titres of antibody to those used in group 1. This suggests that the endometrium of women with unexplained infertility demonstrates defects in the distribution of certain ECM glycoproteins. A possible consequence of this defect may be implantation failure.     

Isolation of rat chromosome-specific paint probes by bivariate flow sorting followed by degenerate oligonucleotide primed-PCR

Angiogenesis is an essential component of endometrial regeneration after menses in preparation for implantation. Vascular endothelial growth factor (VEGF) is a secreted angiogenic peptide with mitogenic activity specific for endothelial and trophoblast cells. VEGF-immunoreactivity was detected in glandular epithelium throughout the menstrual cycle by immunohistochemistry, but, showed cyclic variation in the stroma and the blood vessels. During the early proliferative phase, strong staining was seen in the glandular epithelial cells while staining in the stroma was confined to a subpopulation of stromal cells and endometrial blood vessels appeared negative. In contrast, very intense staining of the endometrial stromal cells was seen in the mid proliferative endometrium possibly due to increased synthesis of VEGF by oestrogen. In the late proliferative endometrium, staining was seen in the endothelial cells and the perivascular stromal cells around the endometrial blood vessels. The greatest degree of immunostaining of stromal cells was observed in the mid to late proliferative endometrium. Throughout the secretory phase no staining was seen around the endometrial blood vessels and staining of endometrial stromal cells was confined to early secretory endometrium. In the late secretory endometrium only the glands were positive to VEGF antibody. The observed increase in the immunostaining of stroma suggests increased production of VEGF from early to mid and late proliferative endometrium which parallels the increase in the oestradiol levels in the proliferative phase of the menstrual cycle. It is proposed that VEGF may serve as a paracrine mediator of the effects of ovarian steroids on endometrial vascular development.     

Response of the primate secretory endometrium to subchronic hypercortisolemia

OBJECTIVE: To assess the impact of subchronic and moderate hypercortisolism on the secretory endometrium of the cynomolgus monkey. METHODS: Osmotic pumps containing hydrocortisone phosphate (HP) were implanted subcutaneously in each monkey on the first day of the menstrual cycle; each monkey also received pumps containing saline in another cycle. Blood was obtained three times per week and urine was collected daily for hormone analyses. Endometriectomy was performed 13 +/- 1 days after the serum estradiol (E2) peak in each study cycle. RESULTS: Infusion of HP elevated serum cortisol levels by an average of 70%. Mean serum progesterone (P) levels were decreased by 50% during the secretory phase of HP-treatment cycles by comparison with self-control cycles (P < .01); as a result, the mean endometrial glycogen concentration was reduced by 30% (P < .05) and the activity of 17 beta-hydroxysteroid dehydrogenase was decreased by 70% (P < .05). Serum E2 levels were not consistently elevated by HP treatment, but cytosolic estrogen receptor levels of the endometrium were decreased by 50% (P < .01), indicating increased estrogenic stimulation. Histologic development of the secretory endometrium was retarded, but the length of the secretory phase was not affected by the treatment. CONCLUSION: A moderate elevation of serum cortisol levels over one menstrual cycle consistently produced a reduction in serum P and a hypoprogestogenic-hyperestrogenic response of the secretory endometrium in the cynomolgus monkey.     

Identification and characterization of cytosolic sulfotransferases in normal human endometrium

Understanding the factors which alter estrogen metabolism and activity in endometrial tissue is important because unopposed estrogen stimulation is an important risk factor in the development of endometrial carcinoma. The cyclic progression of the endometrium through proliferative and secretory phases is normally under the control of the ovarian hormones beta-estradiol (E2) and progesterone. One mechanism by which progesterone inhibits the activity of E2 in secretory endometrium is by elevating the degree of E2 sulfation, thereby reducing its ability to bind to the estrogen receptor and elicit a cellular response. Our laboratories have investigated the cytosolic sulfotransferases (STs) found in biopsies of both proliferative and secretory endometrium obtained from five normal pre-menopausal women who were not taking any drugs or steroids. Two of the human cytosolic STs were detected in human endometrial tissues. The phenol-sulfating form of phenol ST (P-PST) was found at varying levels in cytosol from both proliferative and secretory endometrium in all of the women studied but with no consistent correlation to the phase of the menstrual cycle. In contrast, estrogen ST (EST) was not detected in the proliferative endometrial cytosol of any of the women studied but was consistently found in all of the secretory endometrial cytosols. The presence and levels of these STs was confirmed by ST activity studies, immunoblot analysis and Northern blot analysis. These results indicate that the expression of EST in human endometrial tissues varies with the phase of the menstrual cycle and is most likely regulated by progesterone secreted from the ovaries.     

Adhesion of human endometrium to the epithelial lining and extracellular matrix of amnion in vitro: an electron microscopic study

One of the first steps in the pathogenesis of endometriosis is the attachment of the endometrium to the peritoneal lining. Since the peritoneum is extremely fragile and hard to obtain, amnion has been used as an in-vitro model to study adhesion. Scanning and transmission electron microscopy was applied to evaluate the adhesion of endometrial cells isolated in the proliferative and secretory phases of the menstrual cycle. Endometrial fragments obtained in either phase of the cycle were able to adhere to the extracellular matrix of the amnion. Fragments from proliferative phase endometrium showed active spreading and growth over the matrix surface, whereas fragments from secretory phase endometrium did not. Fragments from proliferative as well as secretory phase endometrium were able to adhere to the epithelial side of the amnion, but only at locations where the amniotic epithelium was damaged or partly absent. These observations indicate that the basement membrane and extracellular matrix provide a suitable substrate for endometrial cell attachment and growth and that endometrial cell adhesion occurs preferentially to subepithelial structures, whereas an intact epithelium prevents the adhesion of endometrial fragments to the amnion.     

Studies on the mechanism of embryo implantation

Implantation is a complex process accomplished by synchronization and interactions between embryo and endometrium by local exchange of signals including a number of cytokines and growth factors and direct cell-cell and cell-matrix contact. However, the research in early events of human implantation is still in its infancy. This presentation comprises the results of our attempts to investigate the mechanisms of human implantation process in its early stage by cell-biological method, including establishment of experimental implantation model in vitro. 1. Human trophoblast of early stage of gestation showed active cell locomotion, active endocytosis, and invasion of endometrial cell monolayer in mixed cultures. Trophoblast invasion was later arrested by transformed endometrial cells similar to decidual cells in vivo. These results appeared to indicate the interactions between trophoblast and endometrial cells in implantation. 2. Coculture system of rabbit preimplantation blastocyst and endometrial epithelium reformed from isolated endometrial epithelial cells on basement membrane matrix (Matrigel) simulated the in vivo rabbit implantation processes. This coculture system may provide a useful experimental implantation model. 3. A human trophoblast cell line was established from chorionic tissues of normal early pregnancy. These cells were cytotrophoblast-like morphology and endocrine functions. They formed the villous structures similar to those in vivo in culture on Matrigel and invasion of Matrigel was observed. These indicated the extracellular matrix may affect the morphology and function of invading trophoblast in implantation site. 4. Human endometrial epithelial single cells were cultured on Matrigel. Reconstruction of gland followed by epithelium formation quite similar to in vivo structures by migration and proliferation of isolated cells was demonstrated. Height of gland was promoted by estrogen and initiation of epithelization was upregulated by platelet-derived growth factors. This system revealed the extracellular matrix regulated morphogenesis of endometrial epithelium in vivo and is an essential substrate in experimental implantation model of endometrial epithelium. 5. Parallel cultures of endometrial epithelial cells on Matrigel were carried out with the IVF. ET patients to evaluate the endometrial morphology at time of ET. Endometrial cultures were initiated in previous cycles on Matrigel and the sera of patients were added to her own cultures from 1st day of IVF treatment cycle. Evaluation of reformed epithelium revealed the apparently unsuitable morphology for implantation in group of patients who eventually failed in pregnancy. This system may provide a useful measures in evaluation of endometrial receptivity and modality of treatment for endometrial aberrations. 6. Cyclic changes of extracellular matrix components in endometrium were investigated. Collagen I, III, IV, V were immunohistochemically estimated. Relative levels of all types of collagen except for collagen V declined at early secretory phase. In rodents, not only collagen but also laminin and fibronectin levels declined at early secretory phase. These changes may facilitate trophoblast invasion of endometrium. Collagen V distributed in myometrial surface was found to consist of subunit (alpha 1)2 alpha 2 and trophoblast growth was inhibited on substrate of alpha 1 subunit. Collagen V in myometrial surface may have a role in blocking trophoblast invasion. 7. HGF (hepatocyte growth factor) mRNA was demonstrated to be expressed during menstruation and secretory phase in endometrium distinctly and its receptor in endometrial epithelial cells and decidual cells. Positive correlation between plasma HGF levels and ultrasonographic thickness of endometrium was observed at late secretory phase. Recombinant HGF promoted proliferation of endometrial epithelial cells and decidual cells and upregulated initiation of endometrial epithelization of Matrigel.     

Cyclic changes in genital organs and vaginal cytology in cynomolgus monkeys (Macaca fascicularis)

Vaginal cytology was evaluated weekly over 12 months in 20 adult female Cynomolgus monkeys (Macaca fascicularis). After sacrifice of the animals the histology of the ovaries, uterus and vagina were studied in different phases of the menstrual cycle. The cytological examination of the vaginal smears showed that the superficial cells increased in number towards the middle of the cycle and the number of intermediate cells declined gradually. Parabasal cells were observed mainly at the beginning of the cycle; they disappeared towards the middle of the menstrual cycle. During the early follicular phase, the cells were moderately separated from each other, and during the second half of the proliferative or follicular phase, the superficial cells appeared clumped together. Leucocytes were usually absent except for at the beginning of the cycle and in the last few days of the late secretory or luteal phase. The maturation index of the vaginal smears can be considered as a tool for distinguishing the different phases of the menstrual cycle. The microscopic examination of the genital organs showed that during the proliferative or follicular phase of the cycle, which corresponds to the development of the ovarian follicles, the uterus showed growth of endometrial glands, stroma and endothelial cell proliferation with capillary sprouts. Shortly after ovulation and parallel to the formation of the corpora lutea, the endometrium enters the secretory or luteal phase, which is characterized by coiling of endometrial glands, glandular secretion and the differentiation of the spiral artery. The most striking changes in the vagina, is the marked basal cell proliferation and thickening of the stratum granulosum during the follicular phase of the menstrual cycle. The histological changes observed in the vagina demonstrated a good correlation with the observation on cytological examination of the smears. The present study demonstrated that the process of angiogenesis in the uterus during the different phases of the menstrual cycle is a multiple phenomenon involving proliferation, maturation and differentiation.     

Immunolocalization of inhibin and activin subunits in human endometrium across the menstrual cycle

Inhibin/activin alphaC/alphaN and betaA subunits were localized immunohistochemically in the human endometrium throughout the menstrual cycle using an affinity-purified sheep polyclonal antibody raised against the alphaC/alphaN subunit and an affinity-purified rabbit polyclonal antibody raised against the betaA subunit. The betaB subunit was below the level of detection in all human endometrial samples tested. Immunoreactive inhibin alphaC/alphaN subunit was localized in the luminal epithelium, glandular epithelium, stromal tissues and vascular endothelium with no significant variation across the normal menstrual cycle. Immunoreactive betaA subunit, common to inhibin A and activins AA and AB was localized in the luminal and glandular epithelium and in migratory cells while the endometrial stromal cells, decidua, vascular smooth muscle and endothelium were devoid of immunoreactivity. A significant variation of immunoreactive betaA subunit was observed in glandular and luminal epithelium across the normal menstrual cycle. In proliferative endometrium, only a very low level of betaA immunostaining was seen in luminal and glandular epithelium, while the luminal epithelial staining increased significantly in the early secretory phase and remained relatively constant over the rest of the menstrual cycle. A progressive increase in betaA immunoreactivity was observed also in the glandular epithelium during the secretory phase reaching a maximum in the late secretory phases, and decreasing at menstruation. Co-localization studies on serial sections suggested that the migratory cells expressing strong betaA immunoreactivity were macrophages and neutrophils but not eosinophils or mast cells. Thus, cells within the human endometrium are capable of expressing inhibin/activin molecules in vivo. The variation in the pattern of secretion of the betaA subunit across the menstrual cycle suggests that activin peptides may have a physiological role in endometrial function.     

Prostaglandin F and progesterone secretion by porcine endometrium and corpus luteum in vitro

Slices of porcine endometrium and corpus luteum tissue obtained from mature sows throughout the luteal phase of the oestrous cycle were incubated in culture medium which was analysed at regular intervals over a period of 8 hours for prostaglandin F and progesterone. Prostaglandin F secretion was greatest by endometrium obtained during the mid III to late I luteal stage of the cycle and the increased levels secreted by this tissue were paralleled by high levels of secretion from corpus luteum tissue. The addition of indomethacin (10 mug/ml) to the culture medium completely abolished prostaglandin F secretion by both endometrium and luteal tissue indicating that the high levels of the prostaglandin were due to synthesis. Progesterone secretion by the corpus luteum was maximal from early luteal tissue and had declined to considerably lower levels by late stage tissue when prostaglandin secretion was greatest. The possible physiological significance of luteal prostaglandin F secretion is discussed.     

Immunohistochemical characterization of proliferation, oestrogen receptor and progesterone receptor expression in endometriosis: comparison of eutopic and ectopic endometrium with normal cycling endometrium

Recent studies examining oestrogen and progesterone receptor status and the proliferative activity of endometriotic lesions have produced conflicting reports. This study aimed to clarify the receptor status and proliferative activity of eutopic and ectopic endometrium from women with endometriosis and endometrium from normal women. Progesterone and oestrogen receptor expression and proliferative activity were studied in eutopic and ectopic endometrium from 30 women with endometriosis and in endometrium from 30 normal cycling women using microwave-pretreated paraffin-embedded sections stained with an avidin-biotin peroxidase technique. Progesterone and oestrogen receptor expression in the control endometrium did not differ from that of eutopic endometrium from women with endometriosis. Oestrogen receptor expression in ectopic endometrium increased from the proliferative to the late secretory phase. Epithelial progesterone receptor expression decreased during the cycle. Oestrogen receptor expression in both epithelium and stroma of ectopic endometrium was significantly higher than in eutopic endometrium throughout the cycle. In contrast, stromal progesterone receptor expression tended to be reduced in ectopic endometrium compared with eutopic tissue. Epithelial progesterone receptor expression was increased in ectopic endometrium but only in the late secretory phase. Although proliferative activity in the epithelium of control and eutopic endometrium was reduced from the proliferative to the late secretory phase, stromal activity did not vary. The proliferative activity in ectopic endometrium remained low and constant throughout the cycle. In the proliferative and early secretory phases, the proliferative activity of eutopic endometrium was increased compared with ectopic endometrium, but in the late secretory phase, levels were comparable. These findings challenge previous reports which have suggested that oestrogen receptors are reduced in ectopic tissue. This may have clinical implications for the development of novel treatments for endometriosis.     

The effect of a single dose of mifepristone (RU486) on the fine structure of the human endometrium during the early luteal phase

This study examined the fine structure of the human endometrial glandular epithelium after the administration of a single dose of RU486 (mifepristone), given in the early luteal phase. The drug was administered on days 2, 3, 5 and 6 after the luteinizing hormone peak (LH + 0). Biopsies were performed on days LH + 5, 6, 8 and 9. These were compared with control biopsies taken on corresponding days. Qualitatively, the main cytological effect of the RU486 was on the secretory apparatus and on the polarity of the cell. The formation of the nuclear channel system was also affected by the drug. A two-way analysis of variance on cell and nuclear volume data revealed no significant effect of day of biopsy, condition or interaction. Mitochondrial volume and secretory apparatus volume data revealed a significant effect of day of biopsy and interaction term; mitochondrial volume at LH + 5 was 95.9 +/- 25.3 microm3 (mean +/- SD) for control and 57.7 +/- 31.9 microm3 for RU486-treated epithelium. The volume of the secretory apparatus in the treated group was smaller on days LH + 5 and 6 (14.6 +/- 4.2 microm3, 6.41 +/- microm3) when compared to day-matched control biopsies (35.9 +/- 10.5 microm3, 41.7 +/- 26.4 microm3). RU486 administration in the early luteal phase disrupted the secretory activity of the cells. These findings provide an insight into the cellular mechanisms of progesterone receptor blockade in the peri-implantation period.     

Progesterone induces calcitonin gene expression in human endometrium within the putative window of implantation

The human endometrium acquires the ability to implant the developing embryo within a specific time window that is thought to open between days 19-24 of the secretory phase of the menstrual cycle. During this period the endometrium undergoes pronounced structural and functional changes induced by the ovarian steroids, estrogen and progesterone, that prepare it to be receptive to invasion by the embryo. The identification of reliable biochemical markers to assess this critical receptive phase in the context of the natural cycle remains one of the major challenges in the study of human reproduction. Our previous studies in a rat model system demonstrated that the expression of calcitonin, a peptide hormone involved in calcium homeostasis, is transiently induced by progesterone in the glandular epithelium at the onset of implantation. Attenuation of calcitonin synthesis in the uterus during the preimplantation phase by administration of calcitonin antisense oligodeoxynucleotides severely impairs implantation of rat embryos, suggesting that this peptide hormone plays a critical role in uterine receptivity. To investigate whether calcitonin is also expressed in the human endometrium during implantation, we monitored the spatio-temporal expression of calcitonin on various days of the menstrual cycle. Our studies employing RT-PCR showed that calcitonin messenger ribonucleic acid is expressed in human endometrium during the postovulatory midsecretory phase (days 17-25) of the menstrual cycle, with maximal expression occurring between days 19-21. Very little calcitonin expression was detected in the endometrium in either the preovulatory proliferative (days 5-14) or the late secretory (days 26-28) phase. In situ hybridization and immunocytochemical analyses localized the calcitonin expression predominantly in the glandular epithelial cells of the endometrium. Our studies further showed that calcitonin expression in the human endometrium is under progesterone regulation. Treatment of women with an antiprogestin, mifepristone (RU-486), drastically reduced calcitonin expression in the endometrium. Collectively, these findings reveal that progesterone-induced expression of calcitonin in the secretory endometrium temporally coincides with the putative window of implantation in the human.     

A prospective randomized controlled study comparing the morphological and biochemical responses of the endometrium to two different forms of 'period-free' hormone replacement therapy

Thirty postmenopausal women were randomized to receive either continuous combined (cc) 2 mg oestradiol valerate and 0.7 mg norethisterone acetate hormone replacement therapy (HRT) daily (15 women) or tibolone 2.5 mg daily (15 women) and were monitored to determine the relationship between the two biochemical markers placental protein 14 (PP14) and the glycoprotein CA125, endometrial histology and occurrence of irregular bleeding after 12 months of treatment. The concentrations of PP14 and CA125 in plasma and uterine flushings before and after therapy were measured and their concentrations were associated with the histology of endometrial biopsies obtained on the same day as venesection and endometrial flushing. The levels of PP14 in uterine flushings were significantly increased after the administration of both types of HRT (P < 0.05 for tibolone and P < 0.001 for ccHRT). However, the concentrations of PP14 found in flushings after ccHRT were considerably greater than those found in flushings after tibolone; levels were increased about 150-fold by ccHRT and 6-fold by tibolone (P < 0.001). Plasma concentration of PP14 after both types of HRT were also significantly raised to a similar degree (P < 0.01). In contrast, the concentration of plasma and uterine CA125 were unchanged by either treatment. Histological analysis of the endometrium from women after 12 months of HRT treatment showed that 86% (6/7) of women on ccHRT had secretory activity as compared to 44% (4/9) women on tibolone (P < 0.05). Women with higher post-HRT uterine PP14 concentration were more likely to have irregular bleeding (P < 0.05). Our studies have shown that endometrial PP14 but not CA125 concentrations are raised to a significant degree by two different forms of period-free HRT regimens. Increased PP14 concentrations in uterine flushing may suggest endometrial stimulation of some form and predict the predilection to irregular bleeding. Thus uterine PP14 concentrations may be used to monitor endometrial responses in women on HRT.     

Iron deposition at mineralization fronts and osteoid formation following chronic cadmium exposure in ovariectomized rats

Integrins are heterodimeric glycoproteins that have been found to undergo dynamic temporal and spatial changes in distribution in the endometrium during the menstrual cycle in women. Likewise the extracellular matrix (ECM) ligands for these receptors are likely to play a role in the establishment of a receptive endometrium. To develop primate models to study the role of these molecules in the cascade of molecular events leading to implantation, integrin expression and associated changes in ECM were investigated during the menstrual cycle and in early pregnancy in the baboon. Antibodies specific for the integrins (alpha(1-6) and alpha(v); beta1, beta3, and beta4) and ECM (laminin, collagen IV, fibronectin) were utilized. In addition, cytokeratin and alpha-smooth muscle actin were used as epithelial, stromal, and smooth muscle cell markers, respectively. Endometrium was obtained in duplicate or triplicate during the menstrual cycle and early pregnancy. Changes observed during the natural menstrual cycle were confirmed using ovariectomized, steroid-treated animals. Constitutively expressed integrins on the endometrial epithelium included the collagen/laminin receptors: alpha2, alpha3, alpha6, and beta4. The pattern of expression correlated well with the distribution of ECM in this tissue. Collagen IV was confined to the basement membrane of glandular epithelium and blood vessels. Laminin immunostaining was found in the basement membrane, mostly in the stroma of the basal region, in the glandular endometrium and vasculature. Fibronectin was present throughout the stroma but not in the basement membrane. The collagen receptor alpha1 beta1 and fibronectin receptor alpha4 beta1 appeared in the glandular epithelium in the luteal phase. As in the human, alpha1 and alpha4 disappeared from the glandular epithelium with the establishment of pregnancy. In contrast, the alpha4 beta3 vitronectin receptor appeared in the glandular epithelium only in pregnancy or following long-term steroid treatment with estrogen and progesterone but not during the time of uterine receptivity associated with the initial period of embryo attachment. Osteopontin, an ECM ligand for alpha(v) beta3, was coexpressed with this integrin in invading cytotrophoblasts, glandular epithelium, and decidualizing stromal cells. Decidualization in the baboon was associated with changes in integrin expression similar to those found in humans: there was an increase in alpha1, alpha3, alpha6, beta1, and alpha(v) beta3 in the decidualized stromal cells. Laminin and collagen IV expression also increased at the implantation site and throughout the endometrium. In contrast, fibronectin expression was most evident at the implantation site and corresponded to alpha5 expression on the invading cytotrophoblasts. In summary, marked similarities were found in the expression of ECM and the integrin receptors between the baboon and the human endometrium throughout the menstrual cycle and in pregnancy. Cycle-specific integrins, alpha1, and alpha4, were present on epithelial cells during the secretory phase. Delayed expression of alpha(v) beta3 in baboon endometrial glands correlated closely with the time of enhanced glandular secretory activity in this primate. The baboon appears to be an excellent model for the investigation of the role of integrins and ECM leading to successful implantation.