Uterine Luminal Proteins Research Articles

Dynamic intrauterine crosstalk promotes porcine embryo implantation during early pregnancy.

Dynamic crosstalk between the embryo and mother is crucial during implantation. Here, we comprehensively profile the single-cell transcriptome of pig peri-implantation embryos and corresponding maternal endometrium, identifying 4 different lineages in embryos and 13 cell types in the endometrium. Cell-specific gene expression characterizes 4 distinct trophectoderm subpopulations, showing development from undifferentiated trophectoderm to polar and mural trophectoderm. Dynamic expression of genes in different types of endometrial cells illustrates their molecular response to embryos during implantation. Then, we developed a novel tool, ExtraCellTalk, generating an overall dynamic map of maternal-foetal crosstalk using uterine luminal proteins as bridges. Through cross-species comparisons, we identified a conserved RBP4/STRA6 pathway in which embryonic-derived RBP4 could target the STRA6 receptor on stromal cells to regulate the interaction with other endometrial cells. These results provide insight into the maternal-foetal crosstalk during embryo implantation and represent a valuable resource for further studies to improve embryo implantation.

Uterine luminal fluid proteins in goats during various reproductive stages (Follicular stage, luteal stage, and early pregnancy)

Uterine luminal fluid proteins in goats during various reproductive stages (Follicular stage, luteal stage, and early pregnancy)

ITRAQ-based quantitative proteomic analysis of porcine uterine fluid during pre-implantation period of pregnancy

Proteins in the uterine luminal fluid are essential for embryo development and regulation of embryo-maternal interaction in porcine. However, little is known about the profile of proteins in uterine luminal fluid of porcine during the pre-implantation period. The present study, applied iTRAQ proteomics technology to identify and analyze uterine luminal fluid proteins on day 9 of estrus cycle and days 9, 12, and 15 of pregnancy. A total of 964 proteins were identified in the present study. Principal component analysis and hierarchical clustering revealed the dynamic developmental characteristics of embryo implantation, which indicated significant differences on day 12 or 15 of pregnancy. In addition, further analysis conducted in the present study identified 279 differentially abundance proteins among the three groups, five clusters were generated using SOTA clustering to examine changes in of the differentially abundant proteins. Results of the current study also found that the proteins in the cluster are involved in some important processes such as regulation of low-density lipoproteins and regulation of TGF-β secretion. Notably, it was found that regulation of TGF-β is essential for porcine embryonic morphological transformation. Furthermore, proteins that play vital roles in implantation, such as CTSC, CTSB, and ACP5 were identified through protein-protein interaction network. Therefore, these findings of the present study provide a basis for understanding embryo development mechanisms and implantation in pigs. SignificanceProteins are directly acting molecules for the functioning of organisms. It is important to study the regulation mechanism of embryo implantation from the perspective of protein function. In the current study, iTRAQ proteomics technology was employed to identify and explore uterine luminal fluid proteins on day 9 of estrus cycle and days 9, 12, and 15 of pregnancy. The findings provide novel insights into the process of porcine early embryo implantation. Furthermore, it is also helpful to clarify the mechanism of embryonic development and implantation.

Influence of estradiol on bovine trophectoderm and uterine gene transcripts around maternal recognition of pregnancy†.

Embryo survival and pregnancy success is increased among animals that exhibit estrus prior to fixed time-artificial insemination, but there are no differences in conceptus survival to d16. The objective of this study was to determine effects of preovulatory estradiol on uterine transcriptomes, select trophectoderm (TE) transcripts, and uterine luminal fluid proteins. Beef cows/heifers were synchronized, artificially inseminated (d0), and grouped into either high (highE2) or low (lowE2) preovulatory estradiol. Uteri were flushed (d16); conceptuses and endometrial biopsies (n = 29) were collected. RNA sequencing was performed on endometrium. Real-time polymerase chain reaction (RT-PCR) was performed on TE (n = 21) RNA to measure relative abundance of IFNT, PTGS2, TM4SF1, C3, FGFR2, and GAPDH. Uterine fluid was analyzed using 2D Liquid Chromatography with tandem mass spectrometry-based Isobaric tags for relative and absolute quantitation (iTRAQ) method. RT-PCR data were analyzed using the MIXED procedure in SAS. There were no differences in messenger RNA (mRNA) abundances in TE, but there were 432 differentially expressed genes (253 downregulated, 179 upregulated) in highE2/conceptus versus lowE2/conceptus groups. There were also 48 differentially expressed proteins (19 upregulated, 29 downregulated); 6 of these were differentially expressed (FDR < 0.10) at the mRNA level. Similar pathways for mRNA and proteins included: calcium signaling, protein kinase A signaling, and corticotropin-releasing hormone signaling. These differences in uterine function may be preparing the conceptus for improved likelihood of survival after d16 among highE2 animals.

Uterine Luminal Fluid Proteins in Buffalo During Follicular and Luteal Stages and Early Pregnancy

Uterine Luminal Fluid Proteins in Buffalo During Follicular and Luteal Stages and Early Pregnancy

Proteomic Profile of Uterine Luminal Fluid from Early Pregnant Ewes.

Embryonic development is a time-sensitive period that requires a synchronized uterine environment, which is created by the secretion of proteins from both the embryo and uterus. Numerous studies have identified uterine luminal proteins and related these to specific adaptations during early pregnancy (EP). However, no study has yet utilized LC-MS/MS to identify the signature profile of proteins in the uterine lumen during EP. In this study, uterine luminal fluid from nonpregnant (NP; n = 3) and EP (n = 3; gestational day 16) ewes were analyzed by LC-MS/MS and validated by Western immunoblotting. We identified a unique signature profile for EP luminal fluid; 15 proteins related to specific aspects of embryonic development including growth and remodeling, immune system regulation, oxidative stress balance, and nutrition were significantly altered (up to 65-fold of NP) in EP profile. Specific uterine remodeling proteins such as transgelin (P = 0.008) and placental proteins like PP9 (P = 0.02) were present in EP luminal fluid but were barely detectable in the NP flushings. Direct correlations (R(2) = 0.84, P = 0.01) were observed between proteomics and immunoblotting. These data provide information on dynamic physiological processes associated with EP at the level of the uterus and conceptus and may potentially demonstrate a signature profile associated with embryonic well-being.

Proteomic Profile of Uterine Luminal Fluid from Early Pregnant Ewes

Embryonic development is a time-sensitive period that requires a synchronized uterine environment, which is created by the secretion of proteins from both the embryo and uterus. Numerous studies have identified uterine luminal proteins and related these to specific adaptations during early pregnancy (EP). However, no study has yet utilized LC-MS/MS to identify the signature profile of proteins in the uterine lumen during EP. In this study, uterine luminal fluid from nonpregnant (NP; n = 3) and EP (n = 3; gestational day 16) ewes were analyzed by LC-MS/MS and validated by Western immunoblotting. We identified a unique signature profile for EP luminal fluid; 15 proteins related to specific aspects of embryonic development including growth and remodeling, immune system regulation, oxidative stress balance, and nutrition were significantly altered (up to 65-fold of NP) in EP profile. Specific uterine remodeling proteins such as transgelin (P = 0.008) and placental proteins like PP9 (P = 0.02) were present in EP luminal fluid but were barely detectable in the NP flushings. Direct correlations (R(2) = 0.84, P = 0.01) were observed between proteomics and immunoblotting. These data provide information on dynamic physiological processes associated with EP at the level of the uterus and conceptus and may potentially demonstrate a signature profile associated with embryonic well-being.

Uterine luminal proteins and estrogens in gilts on a normal nutritional plane during the estrous cycle and on a normal or high nutritional plane during early pregnancy

In gilts, a high plane of nutrition during early pregnancy often results in increased embryo mortality, possibly related to changes in embryo-uterine asynchrony at a critical stage of pregnancy (around Day 11). Therefore, in the present study, uterine luminal proteins and estrogens were studied between Days 5 and 16 after the onset of estrus in gilts on either a normal (2.5 kg/d, cyclic and pregnant gilts) or a high (4.0 kg/d, pregnant gilts only) feeding level. Conceptus recovery rate between Days 5 and 12 was not affected by the feeding level during early pregnancy, neither were systemic progesterone levels. Between Days 9 and 11, dramatic changes took place in the protein composition of the uterine luminal 10kD+ proteins, shifting from most (90%) of the acidic proteins at Day 5 and 7 to approximately 50% at Day 11/12, especially due to an increase in basic proteins with an iso-electrical point of more than 8. This shift occurred most rapidly for the pregnant gilts at the high feeding level and least rapidly in the cyclic gilts, resulting in significant differences in the relative amount of acidic proteins at Day 10 and 11 after the onset of estrus (P < 0.05). Similarly, levels of estrogens in the uterine flushings at Days 10, 11 and 12 were always highest for the pregnant gilts on the high feeding level and were always lowest in the cyclic gilts (P < 0.05); pregnant gilts on the normal feeding level showed intermediate estrogen levels. The fact that gilts on a high feeding level during early pregnancy show more rapid changes in the uterine luminal protein composition and embryonic estrogen production seems to suggest that the rate of these changes may be related to embryo survival.

Isolation of immunologically active uterine luminal proteins associated with follicular and luteal phases of the ovary in buffalo (Bubalus bubalus)

Uterine luminal proteins (ULP) collected from the genital tract of buffalo during the follicular (Group F) and luteal (Group L) phases of the estrous cycle were chromatographed using sephacryl S-200 gel. Five peaks were detected in each group. Different protein concentrations (10 to 200 μg) from Peaks I and V in each group were examined for immunological activity on polymorph nuclear leukocytic cells (PMNL) in vitro. All concentrations except 10 μg of ULP Peak I (≥ 250 kDa) in Group F enhanced phagocytic activity of PMNL. Peak V (56 kDa) in the same group enhanced phagocytic activity of PMNL only at low protein concentrations (10, 20 and 40 μg protein), while at greater concentrations (80, 150 and 200 μg protein) PMNL activity was suppressed. On the other hand, all protein concentrations from Peak I (≥ 250 kDa) in Group L suppressed PMNL activity in a dose-dependent manner. Proteins from Peak V (31 kDa) in Group L suppressed PMNL phagocytic activity at all concentrations but not to the same extent as in Peak I. Electrophoretic analysis of Peaks I and V in both groups revealed only 3 detectable protein bands (subunits) in Peak I and 1 detectable subunit in Peak V. Several additional proteins were probably not detected. The molecular weights of the detected subunits in Peaks I and V in Group F were greater than those in Group L as indicated by SDS-PAGE analysis. The results of this study show that ULP collected from buffalo possessed proteins that modulated phagocytic activity of PMNL in vitro. Proteins collected during the follicular phase, especially Peak I, enhanced phagocytic activity of the PMNL, whereas those collected during the luteal phase (Peaks I and V) suppressed activity. Changes in the molecular weights of ULP detected in this experiment may be related to the changes in phagocytic activity of PMNL tested in vitro.

Isolation of immunologically active uterine luminal proteins associated with follicular and luteal phases of the ovary in buffalo (Bubalus bubalus)

Uterine luminal proteins (ULP) collected from the genital tract of buffalo during the follicular (Group F) and luteal (Group L) phases of the estrous cycle were chromatographed using sephacryl S-200 gel. Five peaks were detected in each group. Different protein concentrations (10 to 200 μg) from Peaks I and V in each group were examined for immunological activity on polymorph nuclear leukocytic cells (PMNL) in vitro. All concentrations except 10 μg of ULP Peak I (≥ 250 kDa) in Group F enhanced phagocytic activity of PMNL. Peak V (56 kDa) in the same group enhanced phagocytic activity of PMNL only at low protein concentrations (10, 20 and 40 μg protein), while at greater concentrations (80, 150 and 200 μg protein) PMNL activity was suppressed. On the other hand, all protein concentrations from Peak I (≥ 250 kDa) in Group L suppressed PMNL activity in a dose-dependent manner. Proteins from Peak V (31 kDa) in Group L suppressed PMNL phagocytic activity at all concentrations but not to the same extent as in Peak I. Electrophoretic analysis of Peaks I and V in both groups revealed only 3 detectable protein bands (subunits) in Peak I and 1 detectable subunit in Peak V. Several additional proteins were probably not detected. The molecular weights of the detected subunits in Peaks I and V in Group F were greater than those in Group L as indicated by SDS-PAGE analysis. The results of this study show that ULP collected from buffalo possessed proteins that modulated phagocytic activity of PMNL in vitro. Proteins collected during the follicular phase, especially Peak I, enhanced phagocytic activity of the PMNL, whereas those collected during the luteal phase (Peaks I and V) suppressed activity. Changes in the molecular weights of ULP detected in this experiment may be related to the changes in phagocytic activity of PMNL tested in vitro.

Evidence of juxtacrine signaling for transforming growth factor alpha in human endometrium.

To determine the mechanism of signaling for transforming growth factor alpha (TGFalpha) in human endometrium, uterine luminal fluid proteins were retrieved by lavage followed by collection of the adjacent endometrium at hysterectomy. In the endometrium we observed the presence of the full-length transmembrane TGFalpha protein and the phosphorylation of its only known receptor, the epidermal growth factor receptor (EGFR), by immunoprecipitation-Western blot; TGFalpha mRNA via reverse transcription-polymerase chain reaction; and immunolocalization of TGFalpha to the surface endometrium adjacent to the uterine lumen. Despite this demonstration of TGFalpha in functional endometrium, we could not detect measurable amounts of TGFalpha in any of the 16 endometrial washings by either immunoprecipitation-Western blot or by ELISA. Recovery rate for intraluminal fluid spiked with TGFalpha control peptide was 93.4-97%. The inability to detect TGFalpha in intraluminal fluid despite its high concentration in cells directly adjacent to the uterine lumen, along with the absence of any cleaved TGFalpha species identified in the endometrium, suggests that TGFalpha signals its receptor as a transmembrane ligand. Since the EGFR is present in the endometrium and on the surface of embryos, these data are consistent with a juxtacrine mode of signaling for TGFalpha between endometrial cells, and between the luminal surface epithelium and preimplantation embryos.

Protein secretions in mouse uterus after methoxychlor or estradiol exposure

Four-month-old, ovariectomized mice were treated with 14 daily ip injections of either 10.0 μg of 17β-estradiol or 1.0 mg technical grade methoxychlor. Uterine luminal proteins were radiolabeled with 35S-methionine and run on 2-dimensional electrophoresis gels. The influence of methoxychlor or estradiol was compared to untreated controls by examining reproductive tract weights, uterine histology, and the patterns of uterine protein secretions. The data indicate that the stimulation of the uterus in ovariectomized, adult mice by methoxychlor is indistinguishable from that caused by estradiol. Both substances alter reproductive tract weights, promote cellular hypertrophy, stimulate uterine development, and induce the secretion of comparable levels of incorporation of methionine into a large number of proteins. The data further indicate that the alterations in gene expression promoted by estradiol in terms of secreted proteins appear to be qualitatively similar to those caused by the pesticide methoxychlor.

Interaction of Bovine Uterine Luminal Protein with Interleukin-2 and the Interleukin-2 Receptor of T Lymphocytes1

Bovine uterine luminal proteins (ULP) collected on Day 17 of pregnancy were tested for inhibition of binding of interleukin-2 (IL-2) to the IL-2 receptor (IL-2R) of bovine (CLC) and human (HLC) T lymphocytes and for binding to IL-2. Additional experiments assessed IL-2 binding to the p55 alpha chain (Tac protein) of the IL-2R of HLC. High- and low-molecular weight (Mr) ULP components (H-ULP greater than 248,000 Mr and L-ULP 21,000 Mr, respectively) inhibited (p less than 0.05 and 0.01, respectively) the binding of 125I-IL-2 to the IL-2R of CLC, whereas only H-ULP inhibited (p less than 0.05) binding to the IL-2R (presumably, the p75 beta chain) of HLC. H-ULP failed (p greater than 0.05) to bind to the p55 alpha chain of the IL-2R of HLC. For IL-2 binding, L-ULP failed (p greater than 0.05) to bind 125I-IL-2 in short (2 h)-term and long (45 h)-term experiments, whereas binding was evident (p less than 0.05) for H-ULP at 2 h of incubation. For H-ULP, mean (+/- SEM) percentages for bound and unbound 125I-IL-2 were 70.1 +/- 11.4 and 29.9 +/- 11.4, respectively. Further purification of H-ULP yielded a component (1.76 x 10(6) Mr) that bound 11.7% of 125I-IL-2 and inhibited (p less than 0.01) thymidine uptake and binding of 125I-IL-2 to the IL-2R of CLC. H-ULP-mediated suppression of lymphocyte proliferation may result from blocking IL-2R recognition of IL-2 as well as binding to IL-2, whereas suppression by L-ULP may predominantly result from blocking IL-2R.

Molecular effects of antiestrogens (tamoxifen and LY117018) on estrogen-dependent glycoprotein (USP-1) synthesized and secreted by rat uterine epithelial cells.

We have previously identified and characterized a 97K secretory glycoprotein (uterine secretory protein-1:USP-1) synthesized and secreted by rat uterine epithelial cells under estrogen stimulation. We now have analyzed the qualitative and quantitative effects of antiestrogens (tamoxifen and LY117018) on the induction of USP-1 biosynthesis when administered alone or combined with 17 beta-estradiol (E2). By radioimmune precipitation assay of [35S]methionine-labeled uterine luminal fluid proteins, it was shown that tamoxifen and LY117018 could weakly induce USP-1 compared with E2. Concomitant administration of either tamoxifen or LY117018 with E2 significantly diminished the effects of E2 on USP-1 induction. Hence, both tamoxifen and LY117018 possess agonistic as well as antagonistic properties affecting the induction of USP-1. These agonistic effects of antiestrogens were also evident from the presence of USP-1 in rat uterine epithelial cells treated with antiestrogens, as revealed by immunohistochemical staining. Sodium dodecylsulfate-polyacrylamide gel analysis of immunoprecipitable [35S]methionine-labeled protein revealed that USP-1 induced by tamoxifen is larger (110K) than that induced by estrogen or LY117018 (97K). Peptide-N-glycosidase treatment of USP-1 induced by E2 or tamoxifen removed asparagine-linked carbohydrate chains and resulted in the appearance of polypeptides with apparent mol wt of 91K and 105K, respectively. Thus, the higher mol wt of tamoxifen-induced USP-1 is not due to changes in asparagine-linked carbohydrates. Since LY117018 could not induce any qualitative change in the USP-1 molecule, tamoxifen may act on uterine epithelial cells through a different molecular mechanism than LY117018.

Progesterone and antiprogesterone (RU 38486) modulation of estrogen-inducible glycoprotein (USP-1) synthesis and secretion in rat uterine epithelial cells.

Previous study in this laboratory revealed that estrogen stimulates synthesis and secretion of 97K glycoprotein [uterine secretory protein-1 (USP-1)] in rat uterine epithelial cells. In the present communication, the effects of progesterone and antiprogesterone (RU 38486) on estrogen-stimulated USP-1 biosynthesis were investigated. Ovariectomized rats were treated with estrogen for 4 days to induce USP-1 synthesis and secretion. Then, progesterone and RU 38486 were injected sc. After 6 and 12 h of treatment, uterine luminal fluid proteins were labeled for 6 h by direct administration of [35S]methionine into uterine lumen. By radioimmune precipitation assay using specific antibody against USP-1, it was shown that progesterone can reduce the USP-1 synthesis and secretion, which were prestimulated by estrogen. This antagonistic effect of progesterone on estrogen-induced USP-1 biosynthesis was also evident by the decreased staining intensity of USP-1 in uterine epithelial cells, as revealed by the immunohistochemical method. Although RU 38486 itself did not manifest any effect on estrogen-stimulated USP-1 biosynthesis, it completely inhibited the progesterone-induced decrease in synthesis and secretion. This system is believed to provide a useful model to analyze the progesterone and antiprogesterone actions on rat uterine epithelial cells.

Uterine luminal proteins during the preimplantation period in the ferret.

The pattern of synthesis of uterine luminal proteins during the 6-day preimplantation period of the ferret has been analyzed by polyacrylamide gel electrophoresis and fluorography. The results demonstrate that several classes of protein, in particular those of molecular weights of 75,000, 58,000, 50,000, 18,000, and 8,000, are actively synthesized and appear in the uterine lumen during this period. Quantitative changes in the amount of radioactivity incorporated by specific proteins as viewed on autoradiograms are also suggested in association with the observed qualitative changes and both become more prevalent as implantation approaches.

Metastasis-stimulating activity in the mouse uterus.

Mouse uterine luminal proteins are thought to play important roles in inducing diapausing blastocysts to implant into the uterine wall. Employing a syngeneic teratocarcinoma cell line (402AX), we demonstrate that neoplastic cells are better able to invade and metastasize if they are coinjected with uterine fluid from pregnant or estrogen-primed mice. This metastasizing activity of uterine fluid was partially purified by using disc polyacrylamide electrophoresis and gel filtration chromatography. Preliminary experiments indicate that the post-albumin and albumin bands contain most of the bioactivity. Furthermore, these bands contain smaller molecular weight proteins (less than 14,000) than can be separated by detergent and mild acetic acid (0.1 N) treatment.

Effects of the estrous cycle and early pregnancy on bovine uterine, luteal, and follicular responses.

Uterine, luteal, and follicular responses associated with the estrous cycle and early pregnancy in cattle were examined. Dairy (n = 19) and beef (n 19) cattle were slaughtered either on Day 4, 8, 12, 14, 16, or 19 postestrus (estrus = Day o). Corpus Iuteum (CL) weight, specific PGF20 binding by the luteal particulate fraction (100,000 X g pellet), in vitro estradiol (E2) production by the two largest follicles, total recoverable uterine luminal protein (TP), total recoverable iinmunoreactive uterine luminal PGF (TPGF), and peripheral plasma steroids were evaluated. In a parallel study, beef cattle (n 22) were slaughtered either on Day 8, 12, 14, 16, or 19 of pregnancy for measurements of TP, TPGF, and plasma steroids. Uterine luminal proteins, from cyclic and pregnant cattle (Day 19), were subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) for determination of protein molecular weights and protein profile characterization. In cyclic cattle, CL regression was not completed by Day 19. Follicle E2 secretion varied among animals within day (,i = 4.0-24.0 ng E2 /follicle/3.5 h; P<0.O1), but not among days. Total PGF2a binding (fmole/CL) for Days 4, 8, 12, 14, 16, and 19 was 14.29, 145.79, 177.34, 111.82, 174.51, and 199.17 (P<O.ol). TPGF (ng) varied among cycle Days 4 (14.4), 8 (13.9), 12 (19.7), 14 (47.7), 16 (17.4), and 19 (111.0 ng: P<0.01). In contrast, TPGF (ng) from pregnant cattle was 481.6 and 1187.8 ng on Day 16 (n 5) and 19 (n = 6). TP (mg) in cyclic cattle varied (P<0.05) among Days 4 (7.34), 8 (7.03), 12 (4.14), 14(15.92), 16 (5.88), and 19 (11.35). Mean TP (mg) for pregnant cattle ranged from 2.73 to 12.09. Thirty-two protein categories were identified in cyclic cattle uterine flushings by SDS-PAGE (Mr range X i(T� = 18.7 to 292.0). Proteins appeared with greater frequency later in the cycle than earlier (Days 14, 16, 19: 60% vs Days 4, 8, 12: 45%). Composite SDS-PAGE profiles from Days 8-12 and Days 14-16 differed (P<0.10) suggesting luteal phase stimulation of protein secretion. Protein profiles (SDS-PAGE) from Day 19 of pregnancy differed from Day 19 of the estrous cycle, resembled those of midluteal phase, and revealed four protein constituents possibly unique to early pregnancy. In summary, uterine luminal PGF increased with luteal phase of the estrous cycle at a time when there appeared to be a stable population of specific PGF2a binding sites in the CL. Higher PGF in utero at Days 16 and 19 of pregnancy and changes in TP and SDS-PAGE protein profiles may reflect responsiveness of endometrium to changes in ovarian status (cyclic) and/or conceptus activity (pregnancy comparisons).

In-vitro binding of prostaglandin F-2 alpha to uterine luminal proteins of pregnant and non-pregnant ewes.

Pregnant and non-pregnant ewes were utilized to determine whether the presence of the embryo affected the binding of prostaglandin (PG) F-2 alpha to a uterine luminal protein. The uterine horn adjacent to the corpus luteum was flushed on Day 13 of gestation or the oestrous cycle. Flushings were incubated with [3H]PGF-2 alpha and subsequently eluted through a Sephadex column. Uterine luminal proteins of pregnant and non-pregnant ewes eluted with the void volume and failed to bind PGF-2 alpha.

Human uterine luminal fluid proteins associated with secretory phase endometrium: progesterone-induced products?

Molecular sieve chromatography over Sephadex G-200 columns of human uterine washings from patients in the secretory phase of the menstrual cycle revealed the presence of a protein peak that was absent from serum and, with one exception, was not found in similarly treated washes of proliferative endometria. Ten of 11 secretory washes contained this material, whereas it could not be demonstrated in seven of eight of the proliferative washes and each of two washes of inactive endometria. None of the nine sera obtained simultaneously (4 proliferative, 4 secretory, and 1 inactive) contained this protein peak. This secretory-phase-specific material was shown to contain one major and several minor bands of stained protein on polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The major protein fraction could also be stained with the periodic acid-Schiff reagent, suggesting its glycoprotein nature. The possibility that these proteins are secreted into the lumen of the uterus as a direct result of progesterone action on this tissue is currently under investigation.