MUC1, glycans and the cell-surface barrier to embryo implantation

WNK1 regulates uterine homeostasis and its ability to support pregnancy

WNK1 (with no lysine [K] kinase 1) is an atypical kinase protein ubiquitously expressed in humans and mice. A mutation in its encoding gene causes hypertension in humans, which is associated with abnormal ion homeostasis. WNK1 is critical for in vitro decidualization in human endometrial stromal cells, thereby demonstrating its importance in female reproduction. Using a mouse model, WNK1 was ablated in the female reproductive tract to define its in vivo role in uterine biology. Loss of WNK1 altered uterine morphology, causing endometrial epithelial hyperplasia, adenomyotic features, and a delay in embryo implantation, ultimately resulting in compromised fertility. Combining transcriptomic, proteomic, and interactomic analyses revealed a potentially novel regulatory pathway whereby WNK1 represses AKT phosphorylation through protein phosphatase 2A (PP2A) in endometrial cells from both humans and mice. We show that WNK1 interacted with PPP2R1A, the alpha isoform of the PP2A scaffold subunit. This maintained the levels of PP2A subunits and stabilized its activity, which then dephosphorylated AKT. Therefore, loss of WNK1 reduced PP2A activity, causing AKT hypersignaling. Using FOXO1 as a readout of AKT activity, we demonstrate that there was escalated FOXO1 phosphorylation and nuclear exclusion, leading to a disruption in the expression of genes that are crucial for embryo implantation.

Design and evaluation of a novel nanodrug delivery system for reducing the side effects of clomiphene citrate on endometrium

BACKGROUND

Stimulation of ovulation with clomiphene citrate can cause side effects on endometrial receptivity. Formulation with nano-size may be an alternative therapy for women with ovulatory disorders. In this study, we investigated sustained-release clomiphene citrate by using Phosal-based formulation (PBF) and evaluate its decreased side effect on the endometrial receptivity.

METHODS

In the in-vitro study, CC loaded PBF was analyzed using Zetasizer, Fourier-transform infrared spectroscopy (FTIR), and Transmission electron microscopy (TEM). In the in-vivo study, 24 female mice were randomly divided into three groups: CC (5 mg/kg), CC/PBF (5 mg/kg) and SS (1 ml) daily administered and injected with 5 IU HCG and mated after two days. At day 4.5, pregnant mice were euthanized and endometrial tissue was extracted for quantitative polymerase chain reaction (Q-PCR) analysis.

RESULTS

The optimized PBF contained Phosal 50PG/glycerol in a 2:8 ratios (w/w) and the particle size of optimum formulation was 67 ± 0.30551 nm and the release of CC from CC-containing PBF was slightly faster in the first 24 h; wherein, 29% of CC was released, and 76% of CC was released up to 120 h. The mRNA levels of leukemia inhibitory factor (LIF), leukemia inhibitory factor receptor alpha (LIFR), HOXA10, Heparin-binding epidermal growth factor (HB-EGF), and epidermal growth factor (EGF) were significantly upregulated and MUC1 and PGR mRNA levels were significantly downregulated in the CC-containing PBF-treated animals compared with only CC group (P < 0.05).

CONCLUSION

Sustained release formulation of clomiphene citrate increased its targeting efficiency and improved the impact of the CC on implantation. Graphical abstract A new Phosal Based Formulation (PBF) was designed to decrease the side effects of Clomiphene citrate (CC) on endometrium. This drug formulation could react better during implantation by increasing the expression of genes involved in implantation. The in vivo study demonstrated that the CC-containing PBF in mice has a significantly higher endometrial receptivity, compared with the suspension.

Annexin A1/Formyl Peptide Receptor Pathway Controls Uterine Receptivity to the Blastocyst

Embryo implantation into the uterine wall is a highly modulated, complex process. We previously demonstrated that Annexin A1 (AnxA1), which is a protein secreted by epithelial and inflammatory cells in the uterine microenvironment, controls embryo implantation in vivo. Here, we decipher the effects of recombinant AnxA1 in this phenomenon by using human trophoblast cell (BeWo) spheroids and uterine epithelial cells (Ishikawa; IK). AnxA1-treated IK cells demonstrated greater levels of spheroid adherence and upregulation of the tight junction molecules claudin-1 and zona occludens-1, as well as the glycoprotein mucin-1 (Muc-1). The latter effect of AnxA1 was not mediated through IL-6 secreted from IK cells, a known inducer of Muc-1 expression. Rather, these effects of AnxA1 involved activation of the formyl peptide receptors FPR1 and FPR2, as pharmacological blockade of FPR1 or FPR1/FPR2 abrogated such responses. The downstream actions of AnxA1 were mediated through the ERK1/2 phosphorylation pathway and F-actin polymerization in IK cells, as blockade of ERK1/2 phosphorylation reversed AnxA1-induced Muc-1 and claudin-1 expression. Moreover, FPR2 activation by AnxA1 induced vascular endothelial growth factor (VEGF) secretion by IK cells, and the supernatant of AnxA1-treated IK cells evoked angiogenesis in vitro. In conclusion, these data highlight the role of the AnxA1/FPR1/FPR2 pathway in uterine epithelial control of blastocyst implantation.

Evidence from three cohort studies on the expression of MUC16 around the time of implantation suggests it is an inhibitor of implantation

PURPOSE

To examine the expression of MUC16 in the endometrium peri-implantation period in three different cohort studies.

METHODS

This was a retrospective observational cohort study. A total of 245 participants were recruited in three separate cohort studies: (1) women with recurrent miscarriage (n = 50) and fertile controls (n = 29); (2) women who had high (n = 20) or normal (n = 20) progesterone on the day of hCG trigger in ovarian stimulation cycle for IVF; and (3) women who did (n = 95) or did not (n = 31) conceive following frozen embryo transfer in HRT cycles. All subjects had archived endometrial samples precisely taken on LH+7 in natural cycles, or hCG+6 in ovarian stimulation cycles, or P+5 in HRT cycles. The H-score (median, range) of MUC16 in the luminal epithelium and glandular epithelium was determined by using immunohistochemistry.

RESULTS

The median (range) of H-score of MUC16 in the luminal epithelium (1) in women with recurrent pregnancy loss was 23.7 (0-300), which was significantly (P < 0.05) lower than that of 118.4 (7.7-300) in fertile controls; (2) in women with elevated progesterone on the day of hCG administration (147.8, 18.0-230.1), significantly (P < 0.05) higher than that of women with normal progesterone (61.0, 2.3-205.3); (3) in women who conceived (23.1, 0-250.3), significantly (P < 0.001) lower than that in women who did not conceive (58.4, 0-300).

CONCLUSION

The expression of MUC16 in all three cohort studies is consistent with it being an inhibitor of implantation.

The effects of hyaluronate-containing medium on human embryo attachment to endometrial epithelial cells in vitro

STUDY QUESTION

Does embryo transfer medium containing hyaluronate (HA) promote the attachment phase of human embryo implantation?

SUMMARY ANSWER

HA-containing medium does not promote human blastocyst attachment to endometrial epithelial cells in vitro.

WHAT IS KNOWN ALREADY

Embryo transfer media containing high concentrations of HA are being used to increase implantation and live birth rates in IVF treatment, although the mechanism of action is unknown.

STUDY DESIGN, SIZE, DURATION

Expression of HA-interacting genes in frozen-thawed oocytes/embryos was assessed by microarray analysis (n = 21). Fresh and frozen human blastocysts (n = 98) were co-cultured with human endometrial epithelial Ishikawa cell layers. Blastocyst attachment and the effects of a widely used HA-containing medium were measured.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human embryos surplus to treatment requirements were donated with informed consent from several ART centres. Blastocyst-stage embryos were transferred at day 6 to confluent Ishikawa cell layers; some blastocysts were artificially hatched. Blastocyst attachment was monitored from 1 to 48 h, and the effects of blastocyst pre-treatment for 10 min with HA-containing medium were determined.

MAIN RESULTS AND THE ROLE OF CHANCE

Human embryos expressed the HA receptor genes CD44 and HMMR, hyaluronan synthase genes HAS1–3, and hyaluronidase genes HYAL1–3, at all stages of preimplantation development. Attachment of partially hatched blastocysts to Ishikawa cells at 24 and 48 h was related to trophectoderm grade (P = 0.0004 and 0.007, respectively, n = 34). Blastocysts of varying clinical grades that had been artificially hatched were all attached within 48 h (n = 21). Treatment of artificially hatched blastocysts with HA-containing medium did not significantly affect attachment at early (1–6 h) or late (24 and 48 h) time points, compared with control blastocysts (n = 43).

LIMITATIONS, REASONS FOR CAUTION

Using an adenocarcinoma-derived cell line to model embryo-endometrium attachment may not fully recapitulate in vivo interactions. The high levels of blastocyst attachment seen with this in vitro model may limit the sensitivity with which the effects of HA can be observed.

WIDER IMPLICATIONS OF THE FINDINGS

Morphological trophectoderm grade can be correlated with blastocyst attachment in vitro. HA-containing medium may increase pregnancy rates by mechanisms other than promoting blastocyst attachment to endometrium.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by a grant from the Wellbeing of Women, the NIHR Local Comprehensive Research Network and NIHR Manchester Clinical Research Facility, the Department of Health Scientist Practitioner Training Scheme, and the Ministry of Higher Education, The State of Libya. None of the authors has any conflict of interest to declare.

FOXO1 regulates uterine epithelial integrity and progesterone receptor expression critical for embryo implantation

Successful embryo implantation requires a receptive endometrium. Poor uterine receptivity can account for implantation failure in women who experience recurrent pregnancy loss or multiple rounds of unsuccessful in vitro fertilization cycles. Here, we demonstrate that the transcription factor Forkhead Box O1 (FOXO1) is a critical regulator of endometrial receptivity in vivo. Uterine ablation of Foxo1 using the progesterone receptor Cre (PgrCre) mouse model resulted in infertility due to altered epithelial cell polarity and apoptosis, preventing the embryo from penetrating the luminal epithelium. Analysis of the uterine transcriptome after Foxo1 ablation identified alterations in gene expression for transcripts involved in the activation of cell invasion, molecular transport, apoptosis, β-catenin (CTNNB1) signaling pathway, and an increase in PGR signaling. The increase of PGR signaling was due to PGR expression being retained in the uterine epithelium during the window of receptivity. Constitutive expression of epithelial PGR during this receptive period inhibited expression of FOXO1 in the nucleus of the uterine epithelium. The reciprocal expression of PGR and FOXO1 was conserved in human endometrial samples during the proliferative and secretory phase. This demonstrates that expression of FOXO1 and the loss of PGR during the window of receptivity are interrelated and critical for embryo implantation.

Cytokines, growth factors and macromolecules as mediators of implantation in mammalian species

Implantation is one of the most critical steps in mammalian reproduction and implantation failure constitutes a major cause of infertility in both animals and humans. The mechanism of implantation is exclusively under the control of ovarian steroids progesterone and oestrogen whose actions are mediated in a complex phenomenon that involves a number of cytokines and growth factors. According to a plethora of literature on implantation in mammalian species, prominent of these cytokines and growth factor playing crucial roles in implantation include integrin, osteopontin, integrin, insulin-like growth factor and leukaemia inhibitory factor. Others are cluster domain 44, hyaluronan system and many non-adhesive molecules such as glycoprotein mucin 1. In this review, the specific roles played by these molecules are expatiated. Generally, they function as adhesive molecules that facilitate attachment of ligands/proteins on the trophectoderm to their respective receptors on endometrial luminal epithelia or vice versa. Sometimes, they also function as signalling molecules that enhance communication between implanting blastocyst and receptive endometrium. This is of particular importance in embryo culture and embryo transfer where in vitro derived blastocyst unlike the in vivo condition, is not exposed to these substances and hence, their absence may be partly responsible for the low implantation rate observed in the surrogate. Appreciation of the roles played by these cytokines, growth factors and molecules as revealed in this review will spur further research on these topics, facilitate their inclusion in embryo culture media (if positively required) and are considered as vital aspect while developing strategies to improve fertility.

Placental Homing Peptide-microRNA Inhibitor Conjugates for Targeted Enhancement of Intrinsic Placental Growth Signaling

Suboptimal placental growth and development are the underlying cause of many pregnancy complications. No treatments are available, primarily due to the risk of causing fetal teratogenicity. microRNAs (miRNAs) are short, non-coding RNA sequences that regulate multiple downstream genes; miR-145 and miR675 have previously been identified as negative regulators of placental growth. In this proof of principle study, we explored the feasibility of delivering miRNA inhibitors to the placentas of pregnant mice and developed novel placental homing peptide-microRNA inhibitor conjugates for targeted enhancement of intrinsic placental growth signalling. Scrambled-, miR-145- or miR-675 inhibitor sequences were synthesised from peptide nucleic acids and conjugated to the placental homing peptide CCGKRK. Intravenous administration of the miR-145- and miR-675 conjugates to pregnant C57BL/6J mice significantly increased fetal and placental weights compared to controls; the miR-675 conjugate significantly reduced placental miR-675 expression. When applied to human first trimester placental explants, the miR-145 conjugate significantly reduced placental miR-145 expression, and both conjugates induced significant enhancement of cytotrophoblast proliferation; no effect was observed in term placental explants. This study demonstrates that homing peptide-miRNA inhibitor conjugates can be exploited to promote placental growth; these novel therapeutics may represent an innovative strategy for targeted treatment of compromised placental development.

Let-7-mediated suppression of mucin 1 expression in the mouse uterus during embryo implantation

Mucin 1 (Muc1) is an integral transmembrane mucin glycoprotein expressed on the apical surface of most epithelia. It is considered to be a barrier to the regulation of embryo implantation by inhibiting attachment of the embryo to the endometrium. Therefore, loss of Muc1 on the surface of uterine epithelial cells is necessary for embryo implantation. Studies have demonstrated that microRNAs (miRNAs) play a key role in enhancing embryo implantation in mammals. In this study, we investigated the regulatory role of two miRNAs (let-7a and let-7b) on the expression of Muc1 in mouse uteri during implantation. Western blotting indicated that Muc1 expression was highest on day1 of pregnancy and constantly decreased thereafter until day 4. In contrast to Muc1 expression, increased expression of let-7a and let-7b was evident on day 4 of pregnancy as measured by real-time reverse transcription-polymerase chain reaction (real-time RT-PCR). We demonstrated direct binding of let-7a and let-7b to the 3’untranslated region of muc1. Furthermore, Muc1 expression was suppressed after transfection of mouse uterine epithelial cells isolated from day 1 of pregnancy with let-7a and let-7b. In summary, the present study provides evidence that Muc1 is a direct target of let-7a and let-7b. Additionally, the current study suggests that miRNAs are novel targets which can be used to facilitate a successful pregnancy and repair implantation failure.

Osteopontin: a leading candidate adhesion molecule for implantation in pigs and sheep

Osteopontin (OPN; also known as Secreted Phosphoprotein 1, SPP1) is a secreted extra-cellular matrix (ECM) protein that binds to a variety of cell surface integrins to stimulate cell-cell and cell-ECM adhesion and communication. It is generally accepted that OPN interacts with apically expressed integrin receptors on the uterine luminal epithelium (LE) and conceptus trophectoderm to attach the conceptus to the uterus for implantation. Research conducted with pigs and sheep has significantly advanced understanding of the role(s) of OPN during implantation through exploitation of the prolonged peri-implantation period of pregnancy when elongating conceptuses are free within the uterine lumen requiring extensive paracrine signaling between conceptus and endometrium. This is followed by a protracted and incremental attachment cascade of trophectoderm to uterine LE during implantation, and development of a true epitheliochorial or synepitheliochorial placenta exhibited by pigs and sheep, respectively. In pigs, implanting conceptuses secrete estrogens which induce the synthesis and secretion of OPN in adjacent uterine LE. OPN then binds to αvβ6 integrin receptors on trophectoderm, and the αvβ3 integrin receptors on uterine LE to bridge conceptus attachment to uterine LE for implantation. In sheep, implanting conceptuses secrete interferon tau that prolongs the lifespan of CL. Progesterone released by CL then induces OPN synthesis and secretion from the endometrial GE into the uterine lumen where OPN binds integrins expressed on trophectoderm (αvβ3) and uterine LE (identity of specific integrins unknown) to adhere the conceptus to the uterus for implantation. OPN binding to the αvβ3 integrin receptor on ovine trophectoderm cells induces in vitro focal adhesion assembly, a prerequisite for adhesion and migration of trophectoderm, through activation of: 1) P70S6K via crosstalk between FRAP1/MTOR and MAPK pathways; 2) MTOR, PI3K, MAPK3/MAPK1 (Erk1/2) and MAPK14 (p38) signaling to stimulate trohectoderm cell migration; and 3) focal adhesion assembly and myosin II motor activity to induce migration of trophectoderm cells. Further large in vivo focal adhesions assemble at the uterine-placental interface of both pigs and sheep and identify the involvement of sizable mechanical forces at this interface during discrete periods of trophoblast migration, attachment and placentation in both species.

MUC1 Is Expressed by Human Skin Fibroblasts and Plays a Role in Cell Adhesion and Migration

The mucin MUC1 is expressed by normal and cancerous epithelial cells and some nonepithelial cells in which it plays roles in regulating adhesion, migration, and cell signaling. In the present studies we found that MUC1 is expressed by normal human neonatal and adult skin fibroblasts. Fibroblasts are usually considered negative for MUC1 expression. Reverse-transcription polymerase chain reaction and Western blot analyses indicate the presence of full-length MUC1, and immunofluorescence and subcellular fractionation studies show that the protein is expressed on the plasma membrane. Immunohistochemical analyses confirmed the expression of MUC1 by fibroblasts in cryosections of normal human skin. Silencing MUC1 expression in fibroblasts using MUC1 shRNA increased the adhesion of cells to collagen and laminin. Transfection with MUC1 shRNA also increased fibroblast migration on collagen as measured in a wound-healing assay. The expression of α₂-integrin was increased in MUC1 shRNA-transfected fibroblasts in which it was localized to membrane ruffles, providing a possible explanation for the increased cell migration on collagen. These results extend the range of expression of MUC1 to skin fibroblasts and suggest a functional role for MUC1 in fibroblast adhesion and motility.

Effect of VNTR polymorphism of the Muc1 gene on litter size of pigs

An investigation was undertaken to study the association between the variable number of tandem repeats polymorphism of the Muc1 gene and the litter size in pigs. Four different alleles were found in three breeds. The sequence analysis shows that the repetitive region of pig Muc1 gene is an array of 108-bp repeats. A total of 2,430 litter records from 897 sows genotyped at Muc1 gene were used to analyze the total number born (TNB) and number born alive (NBA). The study of the effects on litter size suggests that TNB and NBA of genotype AA are the highest in Large White, and the TNB and NBA of the third to ninth parities are 1.61 and 2.29 piglets per litter higher (P < 0.05) than those of the genotype DD, respectively. In Landrace, TNB and NBA of the genotype AA are 1.68 (P < 0.01) and 1.58 (P < 0.05) piglets per litter higher than those of the BB genotype in the third to ninth parities, but for all parities the TNB of genotype AA were 0.76 piglets per litter (P < 0.05) higher than BB. In Duroc, the TNB and NBA of genotype AA are about 1.5 piglets per litter more than those of DD in the third to ninth parities, though not significantly. The research suggests that the smaller allele tends to have higher litter size. The results indicate that Muc1 gene is significantly associated with litter size in pigs.

The human endometrium expresses the glycoprotein mucin-1 and shows positive correlation for Thomsen-Friedenreich epitope expression and galectin-1 binding

Mucin 1 (MUC1) is a glycoprotein in human endometrium and is abundant at the luminal epithelial surface in the receptive phase. It has a highly glycosylated ecto-domain that contains keratan sulfate chains, that disappears at the time of implantation. In addition, the glycoforms on MUC1 differ in fertile and infertile women. Therefore the aims of this study were investigations on glycosylation of MUC1 with the Thomsen-Friedenreich (TF) epitope on normal human endometrium throughout the menstrual cycle and binding of galectin-1 on the TF epitope in the endometrium and the expression of galectin-1 on the human oocyte. Human endometrial tissue was obtained from 54 premenopausal patients and was immunohistochemically analyzed with monoclonal antibodies against MUC1, TF epitope, galectin-1, and biotinylated galectin-1. In addition, human oocytes were analyzed for TF, galectin-1 expression, and galectin-1 binding. We identified a significant upregulation of MUC1 and TF epitope and, in addition, galectin-1 binding in glandular epithelium and epithelial apical surface tissue from proliferative to secretory phase. With double staining experiments, we identified a coexpression of TF and MUC1 in the early secretory phase and galectin-1 binding to TF during the same period of time. In addition we identified TF epitope and galectin-1 expression plus binding on the human oocyte and irregularly fertilized oocytes. Upregulation of TF epitope on the glandular epithelium and epithelial apical surface tissue in the secretory phase and binding of galectin-1 at the same time show the possibility of galectin-1-mediated trophectoderm binding to the endometrium within the window of implantation.

An integrated view of L-selectin and trophinin function in human embryo implantation

Determining molecular mechanisms of human embryo implantation is an extremely challenging task due to the limitation of materials and significant differences underlying this process among mammalian species. Recently, L-selectin and its ligand carbohydrate have been proposed as a system that mediates initial adhesion of human blastocysts to the uterine epithelia. We have also identified trophinin as a unique apical cell adhesion molecule potentially involved in the initial adhesion of trophectoderm of the human blastocyst to endometrial surface epithelia. In the mouse, the binding between ErbB4 on the blastocyst and heparin-binding epidermal growth factor-like growth factor on the endometrial surface enables the initial step of the blastocyst implantation. The evidence suggests that L-selectin and trophinin are included in human embryo implantation. This review summarizes findings relevant to the functions of L-selectin and trophinin in human embryo implantation, and proposes a model that reconciles these cell adhesion mechanisms.

Neuroendocrine-immune disequilibrium and endometriosis: an interdisciplinary approach

Endometriosis, a chronic disease characterized by endometrial tissue located outside the uterine cavity, affects one fourth of young women and is associated with chronic pelvic pain and infertility. However, an in-depth understanding of the pathophysiology and effective treatment strategies of endometriosis is still largely elusive. Inadequate immune and neuroendocrine responses are significantly involved in the pathophysiology of endometriosis, and key findings are summarized in the present review. We discuss here the role of different immune mechanisms particularly adhesion molecules, proteinglycan interactions, and pro-angiogenic mediators in the development and progression of the disease. Finally, we introduce the concept of endometrial dissemination as result of a neuroendocrine-immune disequilibrium in response to high levels of perceived stress caused by cardinal clinical symptoms of endometriosis.

Trophoblast-uterine interactions at implantation

Implantation of the embryo in the uterus is a critical and complex event and its failure is widely considered an impediment to improved success in assisted reproduction. Depending on whether placentation is invasive or superficial (epitheliochorial), the embryo may interact transiently or undergo a prolonged adhesive interaction with the uterine epithelium. Numerous candidate interactions have been identified, and there is good progress on identifying gene networks required for early placentation. However no molecular mechanisms for the epithelial phase are yet firmly established in any species. It is noteworthy that gene ablation in mice has so far failed to identify obligatory initial molecular events.

In vitro studies on endometrial adhesiveness for trophoblast: cellular dynamics in uterine epithelial cells

Initiation of embryo implantation involves adhesion of trophoblast cells to the epithelial lining of the endometrium. The mechanisms regulating the adhesive properties of the uterine epithelium for trophoblast during initiation of human embryo implantation, however, are still incompletely understood. We report here on model studies that we have performed in our laboratory, and in particular on certain methodological approaches that seem to yield new insight into basic mechanisms involved. Of central interest is the ability of the uterine epithelium to develop an adhesion competence at its apical cell pole. This confronts us with a cell biological paradox in that adhesion must be established at the pole which in simple epithelia is typically specialized to resist adhesion. Gain of apical adhesion competence by uterine epithelial cells should be related to cellular rearrangements, i.e. a modulation of their apicobasal cell polarity. Here, we used monolayer-cultured uterine epithelial RL95-2 cells as an in vitro model for the human receptive uterine epithelium. We demonstrated that formation of stable cell-to-cell bonds between the free (apical) pole of these cells and attaching trophoblast (modelled by JAr cells) depends on a number of structural and functional peculiarities that RL95-2 cells have in contrast to other uterine epithelial cells (HEC-1-A cells) which resist attachment via this cell pole. RL95-2 cells were shown to lack tight junctions and to exhibit only rudimentary adherens junctions and a non-polar organization of the actin cytoskeleton. Using the atomic force microscope in a force spectroscopy mode, we exactly defined the time dependence of adhesive interactions between RL95-2 cells and trophoblast, measured the pressure force needed to initiate this process, and screened the buildup of the adhesive forces between the binding partners. A dynamic interaction between the actin cytoskeleton and integrins (a prerequisite for functional activity of integrins) was shown to be an important aspect of the adhesive properties of RL95-2 cells. In addition, at least two types of calcium channels in the plasma membrane of RL95-2 cells seem to play a role in activation of a variety of calcium-sensitive response mechanisms including adhesiveness for trophoblast, i.e. diltiazem-sensitive channels seem to contribute to the initiation of JAr cell binding and SKF-96365-sensitive channels to participate in a feedback loop that controls the balance of bonds. By extrapolation, these data suggest an active role of the uterine epithelium in the process of embryo implantation which we are just beginning to understand in terms of its cell biology.