Analysis of osteopontin at the maternal-placental interface in pigs

Noninvasive, epitheliochorial placentation in the pig follows a prolonged preimplantation period characterized by migration, spacing and elongation of conceptuses, and secretion of estrogen for maternal recognition of pregnancy. Osteopontin (OPN) is an extracellular matrix protein that binds integrins to promote cell-cell attachment and communication. OPN appears to play a key role in conceptus implantation and maintenance of pregnancy in sheep; however, a role for OPN in the porcine uterus has not been established. Therefore, this study examined OPN expression and function in the porcine uterus and conceptus (embryo/fetus and associated extraembryonic membranes). Northern and slot blot hybridization detected an increase in endometrial OPN expression between Days 25 and 30, and levels remained elevated through Day 85 of pregnancy. In situ hybridization localized OPN mRNA to discrete regions of the uterine luminal epithelium (LE) on Day 15 of pregnancy and to the entire LE thereafter. Glandular epithelial (GE) expression of OPN mRNA was first detected on Day 35 of pregnancy and increased through Day 85. Both 70- and 45-kDa forms of OPN protein were detected in cyclic and pregnant endometrium by Western blotting. OPN protein was localized to the LE and GE by immunofluorescence; however, only the 70-kDa OPN was detected in uterine flushings. OPN protein was present along the entire uterine-placental interface after Day 30 of pregnancy. In addition, OPN mRNA and protein were localized to immune-like cells within the stratum compactum of the endometrium in both Day 9 cyclic and pregnant gilts. Incubation of OPN-coated microbeads with porcine trophectoderm and uterine luminal epithelial cells induced Arg-Gly-Asp (RGD)-dependent integrin activation and transmembrane accumulation of cytoskeletal molecules at the apical cell surface as assessed by immunofluorescence detection of talin or alpha-actinin as markers for focal adhesions. These results suggest that OPN, expressed by uterine epithelium and immune cells, may interact with receptors (i.e., integrins) on conceptus and uterus to promote conceptus development and signaling between these tissues as key contributors to attachment and placentation in the pig.     

Muc-1, integrin, and osteopontin expression during the implantation cascade in sheep

The extracellular matrix protein osteopontin (OPN) is a component of histotroph that increases in uterine flushings from pregnant ewes during the peri-implantation period and is localized on the apical surfaces of the uterine luminal epithelium (LE) and conceptus trophectoderm (Tr). The potential involvement of OPN in the implantation adhesion cascade in sheep was investigated by examining temporal, spatial, and potential functional relationships between OPN, Muc-1, and integrin subunits during the estrous cycle and early pregnancy. Immunoreactive Muc-1 was highly expressed at the apical surfaces of uterine luminal (LE) and glandular epithelium (GE) in both cycling and pregnant ewes but was decreased dramatically on LE by Day 9 and was nearly undetectable by Day 17 of pregnancy when intimate contact between LE and Tr begins. In contrast, integrin subunits alpha(v), alpha(4), alpha(5), beta(1), beta(3), and beta(5) were constitutively expressed on conceptus Tr and at the apical surface of uterine LE and GE in both cyclic and early pregnant ewes. The apical expression of these subunits could contribute to the apical assembly of several OPN receptors including the alpha(v)beta(3), alpha(v)beta(1), alpha(v)beta(5), alpha(4)beta(1), and alpha(5)beta(1) heterodimers on endometrial LE and GE, and conceptus Tr in sheep. Functional analysis of potential OPN interactions with conceptus and endometrial integrins was performed on LE and Tr cells in vitro using beads coated with OPN, poly-L-lysine, or recombinant OPN in which the Arg-Gly-Asp sequence was replaced with RGE or RAD. Transmembrane accumulation of talin or alpha-actinin at the apical surface of uterine LE and conceptus Tr cells in contact with OPN-coated beads revealed functional integrin activation and cytoskeletal reorganization in response to OPN binding. These results provide a physiological framework for the role of OPN, a potential mediator of implantation in sheep, as a bridge between integrin heterodimers expressed by Tr and uterine LE responsible for adhesion for initial conceptus attachment.     

Ovine osteopontin: II. Osteopontin and alpha(v)beta(3) integrin expression in the uterus and conceptus during the periimplantation period.

Osteopontin (OPN) is an acidic 70-kDa glycoprotein that is cleaved by proteases to yield 45-kDa and 24-kDa fragments. The 70-kDa and 45-kDa proteins contain a Gly-Arg-Gly-Asp-Ser (GRGDS) sequence that binds to cell surface integrins (primarily alpha(v)beta(3) heterodimer) to promote cell-cell attachment and cell spreading. A 70-kDa acidic protein was previously detected by two-dimensional (2D) PAGE in Day 17 pregnant endometrial cytosolic extracts using Stainsall and identified as immunoreactive OPN using Western blotting. Three forms of immunoreactive OPN proteins (70, 45, and 24 kDa) were detected by 1D PAGE and Western blot analysis of endometrial extracts. OPN protein in endometrial extracts did not differ between cyclic and pregnant ewes. However, the amount of 45-kDa OPN increased in uterine flushings from pregnant ewes between Days 11 and 17. Immunoreactive OPN was localized to luminal and glandular epithelia of both cyclic and pregnant ewes, and to trophectoderm of Day 19 conceptuses. The alpha(v) and beta(3) integrins were detected on Day 19 endometrium and conceptuses by immunofluorescence. It was reported that OPN mRNA increases in the uterine glands of pregnant ewes and secretion of OPN protein into the uterine lumen increases during early pregnancy. The present results demonstrate accumulation of OPN protein on endometrial LE and conceptus trophectoderm. Therefore, it is hypothesized that progesterone and/or interferon-tau induce expression, secretion and/or proteolytic cleavage of OPN by uterine epithelium. Secreted OPN is then available as ligand for alpha(v)beta(3) integrin heterodimer on trophectoderm and uterus to 1) stimulate changes in morphology of conceptus trophectoderm and 2) induce adhesion between luminal epithelium and trophectoderm essential for implantation and placentation.     

Osteopontin expression in uterine stroma indicates a decidualization-like differentiation during ovine pregnancy

Osteopontin (OPN) is a component of the extracellular matrix that interacts with cell surface receptors, including integrins, to mediate cell adhesion, migration, differentiation, survival, and immune function. In pregnant mice and primates, OPN has been detected in decidualized stroma and is considered to be a gene marker for decidualization. Decidualization involves transformation of spindle-like fibroblasts into polygonal epithelial-like cells that are hypothesized to limit conceptus trophoblast invasion through the uterine wall during invasive implantation. Decidualization is not considered characteristic of species with noninvasive implantation, such as domestic animals. However, the extent of trophoblast invasion between sheep and pigs differs, with sheep exhibiting erosion of the uterine luminal epithelium (LE) and fusion of trophectoderm with LE to form syncytia, and pigs maintaining an intact LE throughout pregnancy. Therefore, the present study measured changes in the decidualization marker genes OPN, desmin, and alpha smooth muscle actin (alphaSMA) in ovine and porcine uterine stroma throughout pregnancy. The morphology of endometrial stromal cells in pregnant ewes changes following conceptus attachment, with cells increasing in size and becoming polyhedral in shape by Day 35 of pregnancy. Expression of OPN mRNA and protein, as well as desmin and alphaSMA proteins, was observed in this same uterine stromal compartment. In contrast, no morphological changes in uterine stroma nor induction of OPN mRNA and protein, or desmin protein, were detected during porcine pregnancy. Interestingly, alphaSMA protein was absent on Day 20, but prominent in uterine stroma of pregnant pigs on Day 45. Collectively, these results indicate that the uterine stroma of sheep undergoes a program of differentiation similar to decidualization in invasive implanting species, whereas porcine stroma exhibits differentiation that is more limited than that in sheep, rodents, or primates. Results suggest that uterine stromal decidualization is common to species with different types of placentation, but the extent is variable and correlates with the depth of trophoblast invasion during implantation.     

Pregnancy and interferon tau regulate major histocompatibility complex class I and beta2-microglobulin expression in the ovine uterus

Major histocompatibility complex (MHC) class I molecules, consisting of an alpha chain and beta2-microglobulin (beta2MG), play an important role in immune rejection responses by discriminating self and nonself and are increased by type I interferons during antiviral responses. Interferon tau (IFNtau), the pregnancy-recognition signal in ruminants, is a type I interferon produced by the ovine conceptus between Days 11 and 21 of gestation. In study 1, expression of MHC class I alpha chain and beta2MG mRNA and protein was detected primarily in endometrial luminal epithelium (LE) and glandular epithelium (GE) on Days 10 and 12 of the estrous cycle and pregnancy. On Days 14-20 of pregnancy, MHC class I and beta2MG expression increased only in endometrial stroma and GE and, concurrently, was absent in LE and superficial ductal GE (sGE). Although neither MHC class I nor beta2MG proteins were detected in Day 20 trophectoderm, beta2MG mRNA was detected in conceptus trophectoderm. In study 2, cyclic ewes were ovariectomized on Day 5, treated daily with progesterone to Day 16, received intrauterine infusions between Days 11 and 16 of either control serum proteins or recombinant ovine IFNtau, and were hysterectomized on Day 17. The IFNtau increased MHC class I and beta2MG expression only in endometrial stroma and GE. During pregnancy, MHC class I and beta2MG gene expression is inhibited in endometrial LE and sGE but, paradoxically, is stimulated by IFNtau in the stroma and GE. The silencing of MHC class I alpha chain and beta2MG genes in the endometrial LE and sGE during pregnancy recognition and establishment may be a critical mechanism preventing immune rejection of the conceptus allograft.     

Progesterone and placentation increase secreted phosphoprotein one (SPP1 or osteopontin) in uterine glands and stroma for histotrophic and hematotrophic support of ovine pregnancy

Secreted phosphoprotein one (SPP1, osteopontin) may regulate conceptus implantation and placentation. We investigated effects of progesterone (P(4)) and the conceptus on expression and localization of SPP1 in the ovine uterus. Steady-state levels of SPP1 mRNA in the endometrium of unilaterally pregnant ewes did not differ significantly between nongravid and gravid horns within their respective days of pregnancy; however, levels did increase as pregnancy progressed. SPP1 mRNA was detectable in the glandular epithelium (GE) of both nongravid and gravid horns via in situ hybridization. SPP1 protein was localized to the apical surface of the luminal epithelium of both nongravid and gravid uterine horns. Gravid horns exhibited extensive stromal SPP1 on Days 40 through 120, whereas SPP1 was markedly lower in the stroma of nongravid uterine horns through Day 80 of pregnancy. By Day 120, stromal expression of SPP1 between nongravid and gravid horns was similar. Long-term P(4) treatment of ovariectomized ewes induced SPP1 in the uterine stroma and GE. A bioactive 45-kDa SPP1 fragment was purified from uterine secretions and promoted ovine trophectoderm cell attachment in vitro. Interestingly, increased stromal cell expression of SPP1 was positively associated with vascularization as assessed by von Willebrand factor staining. Finally, ovine uterine artery endothelial cells produced SPP1 during outgrowth into three-dimensional collagen matrices in an in vitro model system that recapitulates angiogenesis. Collectively, P(4) induces and the conceptus further stimulates SPP1 in uterine GE and stroma, where SPP1 likely influences histotrophic and hematotrophic support of conceptus development.     

Galectin 15 (LGALS15): a gene uniquely expressed in the uteri of sheep and goats that functions in trophoblast attachment

Galectins are a family of secreted animal lectins with biological roles in cell adhesion and migration. In sheep, galectin 15 (LGALS15) is expressed specifically in the endometrial luminal (LE) and superficial glandular (sGE) epithelia of the uterus in concert with blastocyst elongation during the peri-implantation period. The present study examined LGALS15 expression in the uterus of cattle, goats, and pigs. Although the bovine genome contains an LGALS15-like gene, expressed sequence tags encoding LGALS15 mRNA were found only for sheep, and full-length LGALS15 cDNAs were cloned only from endometrial total RNA isolated from pregnant sheep and goats, but not pregnant cattle or pigs. Ovine and caprine LGALS15 were highly homologous at the mRNA (95%) and protein (91%) levels, and all contained a conserved carbohydrate recognition domain and RGD recognition sequence for integrin binding. Endometrial LGALS15 mRNA levels increased after Day 11 of both the estrous cycle and pregnancy, and were considerably increased after Day 15 of pregnancy in goats. In situ hybridization detected abundant LGALS15 mRNA in endometrial LE and sGE of early pregnant goats, but not in cattle or pigs. Immunoreactive LGALS15 protein was present in endometrial epithelia and conceptus trophectoderm of goat uteri and detected within intracellular crystal structures in trophectoderm and LE. Recombinant ovine and caprine LGALS15 proteins elicited a dose-dependent increase in ovine trophectoderm cell attachment in vitro that was comparable to bovine fibronectin. These results support the hypothesis that LGALS15 is uniquely expressed in Caprinae endometria and functions as an attachment factor important for peri-implantation blastocyst elongation.     

Progesterone modulation of osteopontin gene expression in the ovine uterus

Osteopontin (OPN) is an acidic phosphorylated glycoprotein component of the extracellular matrix that binds to integrins at the cell surface to promote cell-cell attachment and cell spreading. This matrix constituent is a ligand that could potentially bind integrins on trophectoderm and endometrium to facilitate superficial implantation and placentation. OPN mRNA increases in the endometrial glandular epithelium (GE) of early-pregnant ewes, and OPN protein is secreted into the uterine lumen. Therefore, progesterone and/or interferon-tau (IFNtau) may regulate OPN expression in the uterine GE. Cyclic ewes were ovariectomized and fitted with intrauterine (i. u.) catheters on Day 5 and treated daily with steroids (i.m.) and protein (i.u.) as follows: 1) progesterone (P, Days 5-24) and control serum proteins (CX, Days 11-24); 2) P and ZK 136.317 (ZK; progesterone receptor [PR] antagonist, Days 11-24) and CX proteins; 3) P and recombinant ovine IFNtau (roIFNtau, Days 11-24); or 4) P and ZK and roIFNtau. All ewes were hysterectomized on Day 25. Progesterone induced the expression of endometrial OPN mRNA in the GE and increased secretion of a 45-kDa OPN protein from endometrial explants maintained in culture for 24 h. Administration of ZK ablated progesterone effects. Intrauterine infusion of roIFNtau did not affect OPN gene expression or secretion in any of the steroid treatments. Interestingly, OPN mRNA-positive GE cells lacked detectable PR expression, although PR were detected in the stroma. Results indicate that progesterone regulates OPN expression in GE through a complex mechanism that includes PR down-regulation, and we suggest the possible involvement of a progesterone-induced stromal cell-derived growth factor(s) that acts as a progestamedin.     

Select nutrients in the ovine uterine lumen. ii. glucose transporters in the uterus and peri-implantation conceptuses

Total glucose in ovine uterine lumenal fluid increases 6-fold between Days 10 and 15 of gestation, but not the estrous cycle; however, mechanisms for glucose transport into the uterine lumen and uptake by conceptuses (embryo/fetus and associated membranes) are not established. This study determined the effects of the estrous cycle, pregnancy, progesterone (P4), and interferon tau (IFNT) on expression of both facilitative (SLC2A1, SLC2A3, and SLC2A4) and sodium-dependent (SLC5A1 and SLC5A11) glucose transporters in ovine uterine endometria from Days 10 to 16 of the estrous cycle and Days 10 to 20 of pregnancy, as well as in conceptuses from Days 10 to 20 of pregnancy. The SLC2A1 and SLC5A1 mRNAs and proteins were most abundant in uterine luminal epithelia and superficial glandular epithelia (LE/sGE), whereas SLC2A4 was present in stromal cells and glandular epithelia (GE). SLC5A11 mRNA was most abundant in endometrial GE, whereas SLC2A3 mRNA was not detectable in endometria. SLC2A1, SLC2A3, SLC2A4, SLC5A1, and SLC5A11 were expressed in the trophectoderm and endoderm of conceptuses. Steady-state levels of SLC2A1, SLC5A1, and SLC5A11 mRNAs, but not SLC2A4 mRNA, were greater in endometria from pregnant than from cyclic ewes. Progesterone increased SLC2A1, SLC5A11, and SLC2A4 mRNAs in the LE/sGE and SLC5A1 in the GE of ovariectomized ewes. Expression of SLC5A1 was inhibited by ZK136,317 (progesterone receptor antagonist), and the combination of ZK136,317 and IFNT further decreased expression in GE. In constrast, P4 induced and IFNT stimulated expression of SLC2A1 and SLC5A11, and these effects were blocked by ZK136,317. Results of this study indicate differential expression of facilitative and sodium-dependent glucose transporters in ovine uteri and conceptuses for transport and uptake of glucose, and that P4 or P4 and IFNT regulate their expression during the peri-implantation period of pregnancy.     

Osteopontin: roles in implantation and placentation

Osteopontin (OPN) is an acidic member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family of extracellular matrix proteins/cytokines that undergoes extensive posttranslational modification, including phosphorylation, glycosylation, and cleavage, yielding molecular mass variants ranging in size from 25 to 75 kDa. The result is a versatile protein(s) with multiple functions arising from its role as a mediator of cell-cell and cell-extracellular matrix (ECM) communication that encompass both normal and tumorigenic developmental processes, immunological responses during inflammation and wound healing, and biomineralization. Studies in primates, pigs, sheep, and rodents have revealed that OPN is a major constituent of the uterine-placental microenvironment with influence as 1) a component of histotroph required for adhesion and signal transduction at the uterine-placental interface throughout pregnancy, 2) a gene product expressed by uterine stroma contributing to a decidualization-like transformation that correlates with the degree of conceptus invasiveness, and 3) a product of resident uterine and placental immune cells that may regulate their behavior and cytokine production. This minireview summarizes information regarding uterine and placental expression of OPN that has accumulated over the past 15 yr, and we briefly describe structural/functional properties of this protein that are likely relevant to its role(s) during pregnancy. Comparative studies have offered insights into the potential hormonal/cytokine, cellular, and molecular mechanisms underlying OPN-mediated adhesion, remodeling, and cell-cell/cell-ECM communication within the uterus and placenta. OPN has the potential to profoundly impact pregnancy, and investigators are now challenged to focus on the mechanistic nature of the functions of this multifaceted and major component of the uterine-placental microenvironment.     

Ovine osteopontin: I. Cloning and expression of messenger ribonucleic acid in the uterus during the periimplantation period.

Trophoblast-derived interferon tau (IFNtau) acts on the endometrium to increase secretion of several proteins during the pregnancy recognition period in ruminants. One of these is a 70-kDa acidic protein that has not been identified. Our hypothesis was that the 70-kDa acidic protein is osteopontin (OPN). OPN is an acidic glycoprotein that fragments upon freezing and thawing or treatment with proteases including thrombin. OPN contains a Gly-Arg-Gly-Asp-Ser (GRGDS) sequence that binds to cell surface integrins to promote cell-cell attachment and cell spreading. Using antisera to recombinant human OPN, both 70-kDa and 45-kDa proteins were identified in uterine flushings from pregnant ewes by Western blotting. A clone containing the entire ovine OPN cDNA coding sequence was isolated by screening a Day 15 pregnant ovine endometrial cDNA library with a partial ovine OPN cDNA. In pregnant ewes, steady-state levels of OPN endometrial mRNA increased (P < 0. 01) after Day 17. In both cyclic and pregnant ewes, in situ hybridization analysis showed that OPN mRNA was localized on unidentified immune cells within the stratum compactum of the endometrium. In pregnant ewes, OPN mRNA was also expressed by the glandular epithelium. Results suggest that progesterone and/or IFNtau induce expression and secretion of OPN by uterine glands during the periimplantation period and that OPN may induce adhesion between luminal epithelium and trophectoderm to facilitate superficial implantation.     

Expression of the interferon tau inducible ubiquitin cross-reactive protein in the ovine uterus.

Ubiquitin cross-reactive protein (UCRP) is a 17-kDa protein that shows cross-reactivity with ubiquitin antisera and retains the carboxyl-terminal Leu-Arg-Gly-Gly amino acid sequence of ubiquitin that ligates to, and directs degradation of, cytosolic proteins. It has been reported that bovine endometrial UCRP is synthesized and secreted in response to conceptus-derived interferon-tau (IFNtau). In the present studies, UCRP mRNA and protein were detected in ovine endometrium. Ovine UCRP mRNA was detectable on Day 13, peaked at Day 15, and remained high through Day 19 of pregnancy. The UCRP mRNA was localized to the luminal epithelium (LE), stromal cells (ST) immediately beneath the LE, and shallow glandular epithelium (GE) on Day 13, but it extended to the deep GE, deep ST, and myometrium of uterine tissues by Day 15 of pregnancy. Western blotting revealed induction of UCRP in the endometrial extracts from pregnant, but not cyclic, ewes. Ovine UCRP was also detected in uterine flushings from Days 15 and 17 of pregnancy and immunoprecipitated from Day 17 pregnant endometrial explant-conditioned medium. Treatment of immortalized ovine LE cells with recombinant ovine (ro) IFNtau induced cytosolic expression of UCRP, and intrauterine injection of roIFNtau into ovariectomized cyclic ewes induced endometrial expression of UCRP mRNA. These results are the first to describe temporal and spatial alterations in the cellular localization of UCRP in the ruminant uterus. Collectively, UCRP is synthesized and secreted by the ovine endometrium in response to IFNtau during early pregnancy. Because UCRP is present in the uterus and uterine flushings, it may regulate endometrial proteins associated with establishment and maintenance of early pregnancy in ruminants.     

Uterine histotroph and conceptus development: select nutrients and secreted phosphoprotein 1 affect mechanistic target of rapamycin cell signaling in ewes

Interferon tau (IFNT), the pregnancy recognition signal in ruminants, abrogates the uterine luteolytic mechanism to ensure maintenance of function for the corpora lutea to produce progesterone (P4). IFNT also suppresses expression of classical IFN-stimulated genes by uterine lumenal epithelium (LE) and superficial glandular (sGE) epithelium but, acting in concert with progesterone, affects expression of a multitude of genes critical to growth and development of the conceptus. The LE and sGE secrete proteins and transport nutrients into the uterine lumen necessary for conceptus development, pregnancy recognition signaling, and implantation. Secretions include arginine and secreted phosphoprotein 1 (SPP1). Arginine can be metabolized to nitric oxide and to polyamines or act directly to activate the mechanistic target of rapamycin cell signaling pathway to stimulate proliferation, migration, and mRNA translation in trophectoderm cells. SPP1 binds alphavbeta3 and alpha5beta1 integrins to induce focal adhesion assembly, adhesion, and migration of conceptus trophectoderm cells during implantation. Thus, arginine and SPP1 mediate growth, migration, cytoskeletal remodeling, and adhesion of trophectoderm essential for pregnancy recognition signaling and implantation. This minireview focuses on components of histotroph that affect conceptus development in the ewe.     

Steroid regulation of cell specific secreted phosphoprotein 1 (osteopontin) expression in the pregnant porcine uterus

Secreted phosphoprotein 1 (SPP1, commonly referred to as osteopontin and formerly known as bone sialoprotein 1, early T-lymphocyte activation 1) is an extracellular matrix/adhesion molecule that is upregulated in the pregnant uterus of all mammals examined to date. This study focused on the pig, which has true epitheliochorial placentation and exhibits induction of SPP1 mRNA in luminal epithelium (LE) just before conceptus attachment and in glandular epithelium (GE) after Day 30 of pregnancy. The objective of this study was to determine steroid regulation of SPP1 mRNA and protein in porcine uterine epithelium. To examine the effect of estrogen, cyclic gilts were treated daily (Days 11-14) with 5 mg estradiol benzoate (i.m.) and hysterectomized on Day 15. To evaluate the long-term effect of pseudopregnancy, cyclic gilts were given daily injections (Days 11-15) with steroid as above and hysterectomized on Day 90. In situ hybridization showed high expression of SPP1 mRNA only in LE contiguous with apposing conceptus tissue on Day 15 of pregnancy. In contrast, estrogen injection resulted in moderate but uniform SPP1 mRNA in all LE of Day 15 nonpregnant gilts, with expression maintained through Day 90 of pseudopregnancy. SPP1 mRNA also localized to the GE of Day 90 pseudopregnant gilts, similar to expression in late gestation. Consistent with in situ hybridization results, SPP1 protein localized to the apical surface of LE in all estrogen-treated gilts and in the GE on Day 90 of pseudopregnancy. We conclude that, in pregnant pigs, SPP1 is induced by conceptus estrogen in uterine LE and is regulated in GE in a manner coincident with CL/placental progesterone production.     

Stanniocalcin (STC) in the endometrial glands of the ovine uterus: regulation by progesterone and placental hormones

Stanniocalcin (STC) is a hormone in fish that regulates calcium levels. Mammals have two orthologs of STC with roles in calcium and phosphate metabolism and perhaps cell differentiation. In the kidney and gut, STC regulates calcium and phosphate homeostasis. In the mouse uterus, Stc1 increases in the mesometrial decidua during implantation. These studies determined the effects of pregnancy and related hormones on STC expression in the ovine uterus. In Days 10-16 cyclic and pregnant ewes, STC1 mRNA was not detected in the uterus. Intriguingly, STC1 mRNA appeared on Day 18 of pregnancy, specifically in the endometrial glands, increased from Day 18 to Day 80, and remained abundant to Day 120 of gestation. STC1 mRNA was not detected in the placenta, whereas STC2 mRNA was detected at low abundance in conceptus trophectoderm and endometrial glands during later pregnancy. Immunoreactive STC1 protein was detected predominantly in the endometrial glands after Day 16 of pregnancy and in areolae that transport uterine gland secretions across the placenta. In ovariectomized ewes, long-term progesterone therapy induced STC1 mRNA. Although interferon tau had no effect on endometrial STC1, intrauterine infusions of ovine placental lactogen (PL) increased endometrial gland STC1 mRNA abundance in progestinized ewes. These studies demonstrate that STC1 is induced by progesterone and increased by a placental hormone (PL) in endometrial glands of the ovine uterus during conceptus (embryo/fetus and extraembryonic membranes) implantation and placentation. Western blot analyses revealed the presence of a 25-kDa STC1 protein in the endometrium, uterine luminal fluid, and allantoic fluid. The data suggest that STC1 secreted by the endometrial glands is transported into the fetal circulation and allantoic fluid, where it is hypothesized to regulate growth and differentiation of the fetus and placenta, by placental areolae.