Trophoblast-decidual cell interactions and establishment of maternal blood circulation in the parietal yolk sac placenta of the rat

Placenta: an old organ with new functions

The transition from oviparity to viviparity and the establishment of feto-maternal communications introduced the placenta as the major anatomical site to provide nutrients, gases, and hormones to the developing fetus. The placenta has endocrine functions, orchestrates maternal adaptations to pregnancy at different periods of pregnancy, and acts as a selective barrier to minimize exposure of developing fetus to xenobiotics, pathogens, and parasites. Despite the fact that this ancient organ is central for establishment of a normal pregnancy in eutherians, the placenta remains one of the least studied organs. The first step of pregnancy, embryo implantation, is finely regulated by the trophoectoderm, the precursor of all trophoblast cells. There is a bidirectional communication between placenta and endometrium leading to decidualization, a critical step for maintenance of pregnancy. There are three-direction interactions between the placenta, maternal immune cells, and the endometrium for adaptation of endometrial immune system to the allogeneic fetus. While 65% of all systemically expressed human proteins have been found in the placenta tissues, it expresses numerous placenta-specific proteins, whose expression are dramatically changed in gestational diseases and could serve as biomarkers for early detection of gestational diseases. Surprisingly, placentation and carcinogenesis exhibit numerous shared features in metabolism and cell behavior, proteins and molecular signatures, signaling pathways, and tissue microenvironment, which proposes the concept of "cancer as ectopic trophoblastic cells". By extensive researches in this novel field, a handful of cancer biomarkers has been discovered. This review paper, which has been inspired in part by our extensive experiences during the past couple of years, highlights new aspects of placental functions with emphasis on its immunomodulatory role in establishment of a successful pregnancy and on a potential link between placentation and carcinogenesis.

Wingless ligands and beta-catenin expression in the rat endometrium: The role of Wnt3 and Wnt7a/beta-catenin pathway at the embryo-uterine interface

Wnt/beta-catenin signaling may play an essential role in endometrial decidualization, placentation, and the establishment of pregnancy. We investigate here the possible roles, immunolocalizations, and synthesis of the Wnt3, Wnt7a, and beta-catenin proteins in the rat endometrium during the estrous cycle and early postimplantation period. Wnt3 and Wnt7a had a similar localization and dynamic expression relative to the endometrial stages. Wnt7a immunostaining was not limited only to the luminal epithelial cells, but also to strong stainings in the stromal and endothelial cells. Wnt3, Wnt7a, and beta-catenin were highly synthesized and colocalized at the trophoblast-decidual interface; and were more obvious in the primary decidual zone, the GTCs, and the ectoplacental cone. Beta-catenin was strongly localized at the borders of the mature decidual cells; however, Wnt3 and Wnt7a immunolocalizations were decreased in those cells. As such, the immunolocalization of Wnt3, Wnt7a, and beta-catenin shifted with decidualization and placentation. The expression level of Wnt3, Wnt7a, and beta-catenin messenger RNAs increased in early pregnancy, and especially between Days 8.5 and 9.5. The dramatic changes in the expression of Wnt3, Wnt7a, and beta-catenin observed during the early days of pregnancy and the estrous cycle may indicate their roles in decidualization, stromal cell proliferation, and trophoblast invasion.

Bushen Huoxue Recipe Alleviates Implantation Loss in Mice by Enhancing Estrogen-Progesterone Signals and Promoting Decidual Angiogenesis Through FGF2 During Early Pregnancy

Bushen Huoxue recipe (BSHXR) is a classic Chinese herbal prescription for nourishing the kidney and activating blood circulation. It consists of six herbs: Astragali radix, Angelicae sinensis radix, Ligustici Chuanxiong Rhizoma, Cuscutae semen, Taxilli Herba, and Dipsaci Radix, and the main active constituents of BSHXR are ferulic acid, calycosin-7-glucopyranoside, hyperoside, quercitrin, and asperosaponin VI. In clinical practice, BSHXR is traditionally used to treat failed pregnancy and its complications. However, little is known about the underlying mechanism of BSHXR for the treatment of implantation loss during early pregnancy. In the current study, controlled ovarian hyperstimulation was induced in mice as our implantation loss model, and we evaluated the effects of BSHXR on implantation, decidualization, decidual angiogenesis, and reproductive outcome. We showed that BSHXR could regulate the supraphysiological levels of serum estrogen and progesterone observed in these mice, and also act on estrogen and progesterone receptors in the stroma and epithelium. BSHXR also enhanced FGF2 expression in the vascular sinus folding area of the decidua, thus potentially reducing implantation loss during early pregnancy and contributing to placentation and survival of the fetuses. Taken together, our findings provide scientific evidence for the application of BSHXR in the clinic as a treatment for implantation loss during early pregnancy, and warrant further investigation of BSHXR as an effective treatment for failed pregnancy and its complications.

[Placental Development and Endogenous Retroviruses]

A placenta as we know now is a relatively new invention in mammals. Data accumulated indicates that a major cell type of the placenta is trophoblast, in which elevated expression of genes derived from various endogenous retroviruses (ERVs) as well as LTR retrotransposons is seen. However, evolutionally significance of ERV expression in placental development has not been well characterized or sorted out. In this review, we describe diversity of placental structures among mammalian species, of which morphological and cells types are far more diverse than those expected from the lines of mammalian orders. We then describe paternally expressed gene 10 (Peg10/Sirh1) and Peg11/Sirh2 as ERVs associated with ancient placenta development, followed by env-related genes such as Syncytin-1, -2, -A, -B, -Rum1, and Fematrin-1 responsible for trophoblast cells fusion, resulting in multinucleate syncytiotrophoblast formation. Because the endogenization of retroviral infections has occurred multiple times in different mammalian lineages, and some of them use similar molecules in their transcriptional activation, we speculate that ERV gene variants integrated into mammalian genomes in a locus specific manner have replaced the genes previously responsible for cell fusion. The role of cell fusion achieved by multiple successive ERV integrations is now called ''baton pass'' hypothesis, possibly resulting in increased trophoblast cell fusion, morphological diversity in placental structures, and survivability of fetuses and/or reproductive advantage in placental mammals.

Inactivation of the MAPK signaling pathway by Listeria monocytogenes infection promotes trophoblast giant cell death

Listeria monocytogenes has a well-characterized ability to cross the placental barrier, resulting in spontaneous abortion and fetal infections. However, the mechanisms resulting in infection-associated abortion are not fully understood. In this study, we demonstrate that the dephosphorylation of MAPK family proteins caused by L. monocytogenes infection of trophoblast giant (TG) cells, which are placental immune cells, contributes to infectious abortion. Dephosphorylation of c-Jun, p38, and ERK1/2 was observed in infected TG cells, causing the downregulation of cytoprotective heme oxygenase (HO)-1. Blocking the dephosphorylation of proteins, including MAPK family proteins, inhibited the decrease in HO-1 expression. Treatment with MAPK inhibitors inhibited bacterial internalization into TG cells. Moreover, Toll-like receptor 2 involved in the expression of MAPK family proteins. Infection with a listeriolysin O-deleted mutant impaired dephosphorylation of MAPK family proteins in TG cells and did not induce infectious abortion in a mouse model. These results suggest that inactivation of the MAPK pathway by L. monocytogenes induces TG cell death and causes infectious abortion.

Baton pass hypothesis: successive incorporation of unconserved endogenous retroviral genes for placentation during mammalian evolution

It is well accepted that numerous RNAs derived from endogenous retroviruses (ERVs) are expressed in mammalian reproductive structures, particularly in the uterus, trophoblast, and placenta. Syncytin 1 and syncytin 2 in humans and syncytin A and syncytin B in mice are membrane proteins originating from Env genes of ERVs. These ERVs are involved in the fusion of trophoblast cells, resulting in multinucleated syncytiotrophoblast formation. Evidence accumulated indicates that syncytin-like fusogenic proteins are expressed in the placenta of rabbits, dogs/cats, ruminant ungulates, tenrecs, and opossums. The syncytin genes so far characterized are known to be endogenized to the host genome only within the past 12-80 million years, more recently than the appearance of mammalian placentas, estimated to be 160-180 million years ago. We speculate that ERVs including syncytin-like gene variants integrated into mammalian genomes in a locus-specific manner have replaced the genes previously responsible for cell fusion. We therefore propose the 'baton pass' hypothesis, in which multiple successive ERV variants 'take over' cell-fusion roles, resulting in increased trophoblast cell fusion, morphological variations in placental structures, and enhanced reproductive success in placental mammals.

Conceptus implantation and placentation: molecules related to epithelial-mesenchymal transition, lymphocyte homing, endogenous retroviruses, and exosomes

Processes of conceptus implantation and placentation, unique to mammalian reproduction, have been extensively studied. It was once thought that processes of these events varied greatly, notably between invasive and noninvasive modes of implantation and/or placentation. Regardless of the mode of implantation, however, physiological and biochemical processes in conceptus implantation to the maternal endometrium including the kinds of gene expression and their products are now considered not to differ so much. Recent progress has identified that in addition to the hormones, cytokines, proteases and cell adhesion molecules classically characterized, epithelial-mesenchymal transition, molecules related to lymphocyte homing, the expression of endogenous retroviruses and possibly exosomes are all required for the progression of conceptus implantation to placentation. In this review, therefore, new findings related to these events are integrated into the context of conceptus implantation to the maternal endometrium.

Interferon γ-induced GTPase promotes invasion of Listeria monocytogenes into trophoblast giant cells

Listeria monocytogenes is well known for having the ability to cross the placental barrier, leading to fetal infections and abortion. However, the mechanisms leading to infectious abortion are poorly understood. In this study, we demonstrate that interferon γ-induced GTPase (IGTP) contributes to the invasion of L. monocytogenes into trophoblast giant (TG) cells, which are placental immune cells. Knockdown of IGTP in TG cells decreased the relative efficiencies of L. monocytogenes invasion. Moreover, IGTP accumulated around infected L. monocytogenes in TG cells. Treatment of TG cells with phosphatidylinositol 3-kinase (PI3K)/Akt inhibitors also reduced bacterial invasion. PI3K/Akt inhibitor or IGTP knockdown reduced the amount of phosphorylated Akt. Monosialotetrahexosylganglioside (GM1) gangliosides, lipid raft markers, accumulated in the membrane of L. monocytogenes-containing vacuoles in TG cells. Furthermore, treatment with a lipid raft inhibitor reduced bacterial invasion. These results suggest that IGTP-induced activation of the PI3K/Akt signaling pathway promotes bacterial invasion into TG cells.

Mannose receptor, C type 1 contributes to bacterial uptake by placental trophoblast giant cells

During pregnancy, maternal immune function is strictly controlled and immune tolerance is induced. Trophoblast giant (TG) cells exhibit phagocytic activity and show macrophage-like activity against microorganisms in the placenta. However, details of molecular receptors and mechanisms for uptake by TG cells have not been clarified. In this study, we investigated the involvement of the mannose receptor, C type 1 (MRC1), in the uptake of the abortion-inducible bacterium Listeria monocytogenes and abortion-uninducible bacteria Bacillus subtilis and Escherichia coli by TG cells differentiated from a mouse trophoblast stem cell line in vitro. Knockdown of MRC1 inhibited the uptake of all of these bacteria, as did the blocking of MRC1 by MRC1 ligands. The uptake of bacteria by MRC1 delayed the maturation of phagolysosomes. These findings suggest that MRC1 plays an important role in the uptake of various bacteria by TG cells and may provide an opportunity for those bacteria to escape from phagosomes.

Physiological and molecular determinants of embryo implantation

Embryo implantation involves the intimate interaction between an implantation-competent blastocyst and a receptive uterus, which occurs in a limited time period known as the window of implantation. Emerging evidence shows that defects originating during embryo implantation induce ripple effects with adverse consequences on later gestation events, highlighting the significance of this event for pregnancy success. Although a multitude of cellular events and molecular pathways involved in embryo-uterine crosstalk during implantation have been identified through gene expression studies and genetically engineered mouse models, a comprehensive understanding of the nature of embryo implantation is still missing. This review focuses on recent progress with particular attention to physiological and molecular determinants of blastocyst activation, uterine receptivity, blastocyst attachment and uterine decidualization. A better understanding of underlying mechanisms governing embryo implantation should generate new strategies to rectify implantation failure and improve pregnancy rates in women.

Toll-like receptor 2 and class B scavenger receptor type I are required for bacterial uptake by trophoblast giant cells

Trophoblast giant (TG) cells, components cells of the mouse placenta, exhibit phagocytic activity, and participate in the placental defense system by extracellular bacterial antigen uptake via phagocytosis. However, the bacterial uptake mechanisms by TG cells remain to be entirely understood. In an attempt to understand these mechanisms, in this study, we investigated the pattern recognition receptors (PRRs) involved in phagocytosis by TG cells. PRRs such as Toll-like receptors (TLRs) and scavenger receptors play a critical role in the immune response to bacterial pathogens. Among these, we selected TLR2 and class B scavenger receptor type I (SR-BI) and then evaluated their properties in TG cells. TLR2 and SR-BI expression is higher in TG cells than in trophoblast stem (TS) cells. Although interferon-gamma treatment activated bacterial uptake in a concentration-dependent manner, it did not induce TLR2 or SR-BI expression. Depletion of TLR2 and SR-BI by siRNA reduced the bacterial uptake ability of TG cells, which was also affected by treatment with the TLR2 agonist triacylated lipopeptide. These results suggested that the phagocytic activity of TG cells is mediated by both TLR2 and SR-BI.

Research on Blastocyst Implantation Essential Factors (BIEFs)

Blastocyst implantation is a process of interaction between embryo and the uterus. To understand this process, this review tries to summarize what blastocyst implantation essential factors (BIEFs) play what roles, as well as where in the uterus and at what stage of implantation process. Addition of more new data to this kind of compilation of information will help the development of diagnosis and treatment of infertility caused by implantation failure. The major, important cells of the endometrial cells that interact with invading blastocyst (trophoblast) are luminal epithelial cells, stromal cells (decidual cells) and resident immune cells. BIEFs regulate these cells to successfully maintain pregnancy.

EEVD motif of heat shock cognate protein 70 contributes to bacterial uptake by trophoblast giant cells

Background

The uptake of abortion-inducing pathogens by trophoblast giant (TG) cells is a key event in infectious abortion. However, little is known about phagocytic functions of TG cells against the pathogens. Here we show that heat shock cognate protein 70 (Hsc70) contributes to bacterial uptake by TG cells and the EEVD motif of Hsc70 plays an important role in this.

Methods

Brucella abortus and Listeria monocytogenes were used as the bacterial antigen in this study. Recombinant proteins containing tetratricopeptide repeat (TPR) domains were constructed and confirmation of the binding capacity to Hsc70 was assessed by ELISA. The recombinant TPR proteins were used for investigation of the effect of TPR proteins on bacterial uptake by TG cells and on pregnancy in mice.

Results

The monoclonal antibody that inhibits bacterial uptake by TG cells reacted with the EEVD motif of Hsc70. Bacterial TPR proteins bound to the C-terminal of Hsc70 through its EEVD motif and this binding inhibited bacterial uptake by TG cells. Infectious abortion was also prevented by blocking the EEVD motif of Hsc70.

Conclusions

Our results demonstrate that surface located Hsc70 on TG cells mediates the uptake of pathogenic bacteria and proteins containing the TPR domain inhibit the function of Hsc70 by binding to its EEVD motif. These molecules may be useful in the development of methods for preventing infectious abortion.

Participation of ezrin in bacterial uptake by trophoblast giant cells

BACKGROUND

Trophoblast giant (TG) cells are involved in systematic removal of bacterial pathogens from the maternal-fetal interface of the placenta. In particular, TG cells have the ability to take up extracellular antigens by active phagocytosis induced by interferon-gamma (IFN-gamma). We previously reported that heat shock cognate protein 70 (Hsc70) present on the surface of TG cells mediated the uptake of Brucella abortus. However, the mechanism of bacterial uptake by TG cells is not completely understood. Here we identified ezrin, a member of ezrin-radixin-moesin (ERM) protein family, as a molecule associated with Hsc70.

METHODS

Mouse TG cells were employed in all experiments, and B. abortus was used as the bacterial antigen. Confirmation of the binding capacity of ERM protein was assessed by pull-down assay and ELISA using recombinant Hsc70 and ERM proteins. Ezrin was depleted using siRNA and the depletion examined by immunoblotting or immunofluorescence staining.

RESULTS

The expression level of ezrin was higher in TG cells than in trophoblast stem (TS) cells, and ezrin knockdown TG cells showed a reduction in bacterial uptake ability. Although tyrosine phosphorylation of ezrin was not related to bacterial uptake activity, localization of Hsc70 on the membrane was affected by the depletion of ezrin in TG cells.

CONCLUSION

Ezrin associates with Hsc70 that locates on the membrane of TG cells and participates in the bacterial uptake by TG cells.

Cyclosporin A improves murine pregnancy outcome in abortion-prone matings: involvement of CD80/86 and CD28/CTLA-4

Immune regulation during pregnancy is complex, and thus an optimal therapy for pregnancy complications is always a big challenge to reproductive medicine. Cyclosporin A (CsA), a potent immunosuppressant, prevents rejection of allografts by hosts, but little is known about the modulating effect of CsA on the materno-fetal relationship. Here, pregnant CBA/J females mated with DBA/2 males as an abortion-prone model were administered with CsA on day 4.5 of gestation, and the pregnant CBA/J females mated with BALB/c males were established as successful pregnancy control. It was demonstrated that administration of CsA at the window of implantation significantly up-regulated the expression of CTLA-4, while down-regulating the levels of CD80, CD86, and CD28 at the materno-fetal interface in the CBA/J×DBA/2 abortion-prone matings, and the embryo resorption rate of the abortion-prone matings reduced significantly after CsA treatment, implying that modulation of costimulatory molecule expression by CsA might contribute to preventing the fetus from maternal immune attack. In addition, treatment with CsA induced enhanced growth and reduced cell apoptosis of the murine trophoblast cells. Together, these findings indicate that CsA has a beneficial effect on the materno-fetal interface in abortion-prone matings, leading to a pregnancy outcome improvement, which might provide new therapeutics for spontaneous pregnancy wastage.

Vascular endothelial growth factor production by rat granulated metrial gland cells and their morphological features in normal and pathological conditions

Granulated metrial gland (GMG) cells are pregnancy-specific cells that may have many functions in successful placentation and pregnancy. In the present study, changes in the rat GMG cell structure, distribution and vascular endothelial growth factor (VEGF) expression during early pregnancy were evaluated by light microscopy. Implantation sites taken from females with spontaneous abortion were also investigated. On Day 7 of pregnancy, GMG cells were distributed through the implantation and interimplantation sites. They formed metrial glands in the mesometrial triangle on Day 9, and were observed in the decidua basalis on Day 14 of pregnancy. Avidin–biotin complex immunohistochemistry revealed that GMG cells showed moderate staining for VEGF at the beginning of pregnancy and intense staining on Days 9 and 10 of pregnancy. They were localised mostly near the newly formed blood vessels. The implantation sites from spontaneously aborting females showed numerous leucocytes in the lumen of mesometrial blood vessels. In spontaneously aborting females, GMG cells showed a distinct morphology, increased in number and volume, their granules were denser and degranulation was observed. These results suggest that rat GMG cells might be a guide for placental angiogenesis and they might share a role with leucocytes in pathological conditions.

Trophoblast cells exhibit differential responses to laminin isoforms

Extracellular matrix (ECM) has specific effects on cell behavior that influence many aspects of early development. In the early postimplantation mouse embryo the ECM component laminin promotes polarization and survival of the embryonic ectoderm and formation of Reichert's membrane. In addition, dynamic patterns of laminins 1 and 10/11 expression in the embryo and the uterus correlate with the progression of implantation. In the implanting blastocyst, laminin 1 is strongly expressed in the trophectoderm basement membrane, whereas laminin 10/11 is expressed only in the inner cell mass and polar trophectoderm. In the uterus, laminin 10/11 is strongly expressed in the decidualizing matrix of the stroma. We show here that laminins 1 and 10/11 have distinct effects on trophoblast cell behavior that influence the process of implantation. Laminin 1 promotes random migration and decreases spreading, whereas laminin 10/11 promotes both spreading and persistent migration. When presented as adjacent substrates, cells stop at the boundary and do not enter the region containing laminin 1. Laminin 1 also affects cell-cell adhesion through changes in the localization of vascular endothelial (VE) cadherin. Cultured cells and primary trophoblast explants become single cells or very small groups on laminin 1 and VE-cadherin localization at regions of cell-cell contact decreases dramatically. In contrast, trophoblast cells maintain strong cell-cell contacts on substrates of laminins 10/11, and exhibit strong staining of VE-cadherin in all regions of cell-cell contact. These effects, and the localization of laminin 1 in Reichert's membrane and laminin 10/11 in the surrounding decidual matrix, suggest that these laminin isoforms influence the direction and quality of invasion of trophoblast cells during implantation, and provide epigenetic cues that drive the morphogenesis of the yolk sac placenta.

The effect of experimental Bothrops jararaca envenomation on pregnant mice

The injury caused by the intramuscular injection of a single dose of Bothrops jararaca venom (0.24 mg/kg body weight) to mice on day 8 of pregnancy and examined on day 9 was investigated. Macroscopic and histological examination showed that the bothropic venom caused an increase in the incidence of fetal resorptions. Histologically, a characteristic involution of mature decidua was noticed in saline-treated mice; however, necrotic trophoblast giant cells and decidual cells were also present in this region of mice treated with B. jararaca venom, mainly close to the embryo. Hemorrhagic areas were also observed at maternal-fetal interface, which contained maternal erythrocytes and polymorphonuclears. Plasma fibrinogen levels were lower in envenomed group (p < or = 0.0001), but prothrombin time and activated partial thromboplastin time remained unaltered. Total and differential white blood cell counts were not statistically different between groups. Thus, B. jararaca venom causes injuries not only to the fetus, but also to decidual tissue and blood coagulation of pregnant mice. It is not clear, nonetheless, whether disturbances during the development of pregnancy are due to a direct effect of venom on uterus/fetus or to homeostatic changes in dams, such as clotting disturbances, or to both of them.

System B0,+ amino acid transport regulates the penetration stage of blastocyst implantation with possible long-term developmental consequences through adulthood

Amino acid transport system B(0,+) was first characterized in detail in mouse blastocysts over two decades ago. Since then, this system has been shown to be involved in a wide array of developmental processes from blastocyst implantation in the uterus to adult obesity. Leucine uptake through system B(0,+) in blastocysts triggers mammalian target of rapamycin (mTOR) signalling. This signalling pathway selectively regulates development of trophoblast motility and the onset of the penetration stage of blastocyst implantation about 20 h later. Meanwhile, system B(0,+) becomes inactive in blastocysts a few hours before implantation in vivo. System B(0,+) can, however, be activated in preimplantation blastocysts by physical stimuli. The onset of trophoblast motility should provide the physiological physical stimulus activating system B(0,+) in blastocysts in vivo. Activation of system B(0,+) when trophoblast cells begin to penetrate the uterine epithelium would cause it to accumulate its preferred substrates, which include tryptophan, from uterine secretions. A low tryptophan concentration in external secretions next to trophoblast cells inhibits T-cell proliferation and rejection of the conceptus. Suboptimal system B(0,+) regulation of these developmental processes likely influences placentation and subsequent embryo nutrition, birth weight and risk of developing metabolic syndrome and obesity.

Serine protease HtrA1 is developmentally regulated in trophoblast and uterine decidual cells during placental formation in the mouse

Development of a hemochorial placenta involves trophoblast proliferation, differentiation, and invasion into the uterus to promote blood flow to the embryo. Trophoblast invasion is tightly controlled by expression of specific proteases in the trophoblast and highly coordinated activities in the uterus. One uterine event essential for placentation is the developmentally regulated formation and regression of the decidua. In mice, decidual regression takes place in a temporal- and spatial-specific manner that is coordinated with placental development. In this study, we identified that the serine protease HtrA1 (high temperature requirement factor A1) was specifically expressed in differentiated trophoblast cells, especially the giant cells, during the early stages of placental development. A high level of HtrA1 expression was also detected in decidua capsularis specifically at the decidual-trophoblast interface where active involution occurs. Thus, we have identified a previously unknown role for HtrA1 as a protease potentially important for trophoblast differentiation/invasion and uterine decidual regression during placental development.

Phagocytosis as a potential mechanism for microbial defense of mouse placental trophoblast cells

Trophoblast giant cells are active phagocytes during implantation and post-implantation. Phagocytosis decreases during placental maturation as the phagocytic function of nutrition is gradually replaced by the direct uptake of nutrients by the labyrinth zone trophoblast. We hypothesize that, after placental maturation, trophoblast cells maintain phagocytic functions for purposes other than nutrition. This study employs histological techniques to examine the ability of trophoblast cells to phagocytose microorganisms (yeast or bacteria)--in vivo in females receiving thioglycolate to activate macrophages and in vitro in the presence of phagocytic promoters such as interferon-gamma and complement component C3. Placental trophoblast cells from the second half of gestation show basal phagocytosis that can be dramatically up-regulated by these promoters when microorganisms are inoculated into pregnant animals or introduced into culture systems. Stimulated trophoblast cells phagocytosed organisms more rapidly and in greater numbers than non-stimulated trophoblast exposed to the same numbers of organisms. Taken together, our results indicate that trophoblast cells do not lose their ability to phagocytose during the placentation process, which may imply that trophoblast cells participate in embryonic and fetal innate immune defense through elimination of microorganisms present at the maternal-fetal interface.

What Can Comparative Studies of Placental Structure Tell Us?—A Review

The diversity of placental structures in Eutherian mammals is such that drawing generalizations from the definitive forms is problematic. There are always areas of reduced interhaemal distance whether the placenta is epitheliochorial, synepitheliochorial, endotheliochorial or haemochorial. However, the thinning may be achieved by different means. The presence of a haemophagous area as an iron transport facilitator is generally associated with endotheliochorial placentae but is also found in sheep and goats (synepitheliochorial) and in tenrecs and hyaenas (haemochorial). Although similar chorioallantoic placentae are found within families, structure begins to diverge at the ordinal level and there is little correlation at the supraordinal level of phylogeny. Differences in formation and function of the yolk sac provide additional variation. There would appear to be considerable adaptive pressure for development or retention of the haemochorial type of chorioallantoic placenta. This type of placenta has several possible drawbacks including more ready passage of fetal cells to the maternal organism and, should the haemochorial condition be achieved early, oxidative stress. At any rate no animal larger than the human and gorilla has this type of placenta. The endotheliochorial condition is found in animals as large as the bears, manatee and elephants. In addition to the ungulates, the epitheliochorial condition is present in the largest animals with the longest gestation periods, the whales. Considering the length of time since the early stages of mammalian evolution, it is probable that few unmodified structural features are present in any currently surviving mammal. Nevertheless, more complete studies of divergent types of mammalian placenta should help our understanding of mammalian interrelationships as well as placental function.

Amino acid transport regulates blastocyst implantation

Mouse blastocyst outgrowth in vitro and probably implantation in vivo require amino acid signaling via the target of rapamycin (TOR) pathway. This signaling does not simply support protein synthesis and trophoblast differentiation. Rather, it regulates development of trophoblast protrusive activity and may act as a developmental checkpoint for implantation. Moreover, intracellular amino acids per se are insufficient to elicit TOR signaling. Instead, de novo transport of amino acids, and particularly of leucine, stimulate mTOR activity at the blastocyst stage. The activity of the broad-scope and yet leucine-selective amino acid transport system B0,+ could produce such increases in intracellular amino acid concentrations. For example, system B0,+ uses a Na+ gradient to drive amino acid uptake, and the Na+ concentration in uterine secretions increases by nearly two-fold about 18 h before implantation. The resultant mTOR signaling could trigger polyamine, insulin-like growth factor II, and nitric oxide production in blastocysts and the increased cell motility sometimes associated with synthesis of these bioactive molecules.

Early maternal changes contributing to the formation of the chorioallantoic and yolk sac placentas in rat: a morphological study

In order to determine the maternal changes contributing to the formation of the chorioallantoic and yolk-sac placentas, rat gestation sites were examined by light and electron microscopy on days 7 through 10 of pregnancy. On day 7, the implantation chamber showed different compartments and contained the blastocyst in the antimesometrial chamber. The epithelial lining of the implantation chamber disappeared at the antimesometrial chamber, transformed into disintegrated cells in the mesometrial chamber, and showed signs of the programmed cell death in the decidual crypt. On day 8, the mesometrial chamber lumen contained red blood cells and it was continuous with subepithelial sinusoids. The endothelial cells lining the mesometrial sinusoids also showed some characteristics of the sprouting type angiogenesis such as hypertrophy and cell proliferation. While the yolk-sac placental circulation was more obvious with participation of the giant trophoblasts at the antimesometrial pole of the conceptus on day 9, the antimesometrial cells showed autophagic degeneration after the formation of the chorioallantoic placenta on day 10. The contribution of the regional cell death and angiogenesis to form both of the two placentas are discussed.

Developmental changes in the ploidy of mouse implanting trophoblast cells in vitro

Shortly after the onset of implantation, polar mouse trophoblast cells proliferate and give rise to the ectoplacental cone, constituted by two distinct cell populations: undifferentiated, diploid cells and giant cells. Giant cells characteristically exhibit exaggerated dimensions and polyploid nuclei. In this study, we employ ectoplacental cones as a dynamic source of trophoblast giant cells to analyze cell proliferation, cell death, and ploidy under in vitro conditions. Our results show that DNA synthesis and the increase in the cell number are relevant only during the first 24 h of culture. Subsequently, DNA synthesis still occurs, mainly in the giant cell compartment, while the number of cells gradually decreases. Cell death by injury and apoptosis was also observed in the non-giant cell compartment of the ectoplacental cone. These findings suggest that the first 24 h of culture are crucial to the mitotic activity of the ectoplacental cone cells that gradually ceases, favoring the endoreduplication process. The DNA synthesis index during the subsequent experimental intervals emphasizes accumulation of DNA for the polyploidization. There was clear correlation between DNA content and nuclear dimension. The ploidy values for the trophoblast giant cells varied from 2C up to 368C in the giant cells, but were not as expressive as those known from in vivo conditions, probably due to the absence of regulatory factors specific to the embryonic-maternal interface. In situ hybridization and histochemistry for the nucleolus-organizing region showed that trophoblast nuclei have only two marker signals, indicative of a typical polytenic process. This present study elucidates important aspects of trophoblast behavior and provides new information on trophoblast physiology in vivo and in vitro.

Trophoblast-specific expression and function of the integrin alpha 7 subunit in the peri-implantation mouse embryo

For implantation and placentation to occur, mouse embryo trophoblast cells must penetrate the uterine stroma to make contact with maternal blood vessels. A major component of the uterine epithelial basement membrane and underlying stromal matrix with which they interact is the extracellular matrix protein laminin. We have identified integrin alpha 7 beta 1 as a major receptor for trophoblast-laminin interactions during implantation and yolk sac placenta formation. It is first expressed by trophectoderm cells of the late blastocyst and by all trophectoderm descendants in the early postimplantation embryo through E8.5, then disappears except in cells at the interface between the allantois and the ectoplacental plate. Integrin alpha 7 expression is a general characteristic of the early differentiation stages of rodent trophoblast, given that two different cultured trophoblast cell lines also express this integrin. Trophoblast cells interact with at least three different laminin isoforms (laminins 1, 2/4, and 10/11) in the blastocyst and in the uterus at the time of implantation. Outgrowth assays using function-blocking antibodies show that alpha 7 beta 1 is the major trophoblast receptor for laminin 1 and a functional receptor for laminins 2/4 and 10/11. When trophoblast cells are cultured on substrates of these three laminins, they attach and spread on all three, but show decreased proliferation on laminin 1. These results show that the alpha 7 beta 1 integrin is expressed by trophoblast cells and acts as receptor for several isoforms of laminin during implantation. These interactions are not only important for trophoblast adhesion and spreading but may also play a role in regulating trophectoderm proliferation and differentiation.

NADPH-diaphorase activity and nitric oxide synthase isoforms in the trophoblast of Calomys callosus

The pattern of expression of a variety of placental nitric oxide synthase isoforms has contributed to elucidating the regulatory mechanisms of nitric oxide (NO) synthesis during gestation. The maintenance of vascular tone, attenuation of vasoconstriction, prevention of platelet and leukocyte adhesion to the trophoblast surface, and possible participation in uterine blood flow seem to be the main functions of NO generated at the fetal-maternal interface in humans and mice. Extending this knowledge to other rodent species commonly used as laboratory animals, in this study we focus on NADPH-diaphorase activity and the distribution of nitric oxide synthase isoforms (NOS) in the trophoblast cells of Calomys callosus during different phases of pregnancy. NADPH-diaphorase activity was evaluated cytochemically and the presence of NOS isoforms detected by immunohistochemistry. These techniques were performed on pre- and postimplantation embryos in situ and in vitro, as well as in placentae on d 14 and 18 of pregnancy. Neither NADPH-diaphorase activity nor inducible or endothelial NOS isoforms were found in pre-implanting embryos except after culturing for at least 48 h, when some of the embryonic cells were positive for the diaphorase reaction. On d 6.5 of pregnancy, trophoblast cells showed intense diaphorase activity both in situ and under in vitro conditions. A positive reaction was also found in the different placental trophoblast cells on d 14 and 18 of pregnancy. The inducible NOS (iNOS) isoform, but not the endothelial isoform, was immunodetected in trophoblast cells from the placenta and from postimplantation embryos in situ and under in vitro conditions. These results strongly suggest the production of NO by the iNOS isoform in the trophoblast of Calomys callosus after embryo implantation. The data also emphasise a possible role for the trophoblast in producing and releasing cytotoxic molecules at the fetal-maternal interface.

Trophoblast giant-cell differentiation involves changes in cytoskeleton and cell motility

Trophoblast giant-cell differentiation is well-characterized at the molecular level, yet very little is known about how molecular changes affect the cellular functions of trophoblast in embryo implantation. We have found, using both explanted E7.5 mouse embryo ectoplacental cone and the rat choriocarcinoma (Rcho-1) cell line, that trophoblast differentiation is distinguished by dramatic changes in cytoarchitecture and cell behavior. Undifferentiated trophoblast cells contain little organized actin and few small, peripheral focal complexes and exhibit high membrane protrusive activity, while differentiated trophoblast giant cells contain prominent stress fibers, large internal as well as peripheral focal adhesions, and become immotile. The dramatic changes in cell behavior and cytoskeletal organization of giant cells correlate with changes in the activities of the Rho family of small GTPases and a decrease in tyrosine phosphorylation of focal adhesion kinase. Together, these data provide detailed insight into the cellular properties of trophoblast giant cells and suggest that giant-cell differentiation is characterized by a transition from a motile to a specialized epithelial phenotype. Furthermore, our data support a phagocytic erosion, rather than a migratory infiltration, mechanism for trophoblast giant-cell invasion of the uterine stroma.

Molecular genetics of implantation in the mouse

Embryo implantation is a complex developmental process requiring precise coordination between mother and offspring to ensure success. Implantation failure is clinically relevant to in vitro fertilization programs and to an understanding of diseases of pregnancy like preeclampsia. Basic and clinical research have identified a number of proteins involved in peri-implantation development, but an understanding of the implantation process and its cellular and molecular components is just beginning. This review will focus on the implantation and development of the murine embryo and placenta. The significance of ectopic expression and targeted mutagenesis models to these processes will be discussed.

Comparative placental structure

The primary function of all placentas is to act as an interface between the mother and fetus that allows, and even promotes, appropriate metabolic exchanges. This function is accomplished by bringing maternal and fetal blood into close apposition while maintaining separation of the maternal and fetal circulatory systems. Despite the common physiological functions shared by placentas, however, examination of placental morphology from different animal groups reveals a remarkable diversity of species-specific structural organization.The separation of fetal and maternal blood is always maintained by an elaboration of extraembryonic fetal tissues that cover fetal blood vessels. In some species the outermost layer of this fetal tissue, the trophoblast, is in direct contact with maternal blood. In many other species uterine tissues also contribute to the selective barrier separating the two blood systems. In addition to morphological variation among placentas of different animal groups, placentas undergo substantial structural modifications during pregnancy in a single species. In some animals different types of placentas function successively, or concurrently during a single pregnancy.As a result of these myriad details of placental structure, effective evaluation of fetal-maternal transfer of drugs must consider not only the components of the interhemal barrier of the fully developed placenta characteristic for each species, but also the placental structures functioning at each gestational stage of the fetus.

Apoptotic cell death in artificially induced deciduoma of pseudopregnant mice

Deciduoma induced by mechanical stimulation in pseudopregnant mice is similar to the decidua in normal pregnancy and it undergoes regression after a certain period. Therefore, we examined cell death in deciduomas which were induced by artificial stimulation. To analyze the regression mechanism of artificially induced deciduoma, DNA fragmentation, in situ 3'-DNA nick end labeling, and RT-PCR were performed on day 6 to 14 of pseudopregnancy. DNA fragmentation appeared on day 8 and it increased to day 10 of pseudopregnancy in the traumatized uterine horn. A large number of apoptotic cells were found on day 10 in the periphery of deciduoma at the antimesometrial side. Deciduoma underwent degeneration on day 11 of pseudopregnancy. Expression of tumor necrosis factor-alpha (TNF-alpha) mRNA was high on days 8 and 10, then decreased, whereas the expression increased again on day 14. TNF-alpha protein was expressed from day 8 to day 12, showing a peak expression on day 10 when deciduoma reached maximum weight. Serum progesterone level was high in the traumatized pseudopregnant mice on day 6, then it gradually decreased. Life span of deciduoma was prolonged 4 days more by daily injection of progesterone. A reduction in serum progesterone coincides with TNF-alpha increase, resulting in an increase of apoptotic deciduomal cells at the regression period, and that the life span of deciduoma is prolonged by additive supply of progesterone.

Phospholipid containing choline histochemistry of mouse uterine epithelia during preimplantation stage

The physiological role of high lipid content in endometrial cells during pregnancy has not been well established. In the present work we used histochemical techniques to analyze the total lipids and phospholipid containing choline (PCC) in the mouse uterine glandular and luminal epithelia during preimplantation stage. Sudan black histochemistry showed the highest intensity during the second day of pregnancy in both the basal and apical portions of luminal epithelium. Peaks of PCC staining were seen both in the luminal and glandular epithelia at the second and fifth days of pregnancy. Changes in localization and in the amount of lipid in the uterine epithelia suggest high mobility and metabolic rates of this substance, which may be related to morphological and/or functional changes occurring at the same time in the pregnant uterus. The increase and depletion timing of PCC content in the uterine epithelia during preimplantation stage, when uterine prostaglandin is also oscillating, suggest a possible involvement of PCC in prostaglandin biosynthesis. Therefore, the fate of lipid droplets found in the uterine epithelia may be related to critical changes of the pregnant endometrium, rather than the nourishment of developing embryos alone.

Spatiotemporal expression of C-CAM in the rat placenta

We investigated the expression of the immunoglobulin superfamily cell adhesion molecule, C-CAM, in developing and mature rat placenta. By immunohistochemical staining at the light microscopic level, no C-CAM-expression was seen before Day 9 of gestation, when it appeared in the trophoblasts of ectoplacental cones. On Day 10.5, spongiotrophoblasts and invasive trophoblasts around the maternal vessels of the decidua basalis were stained positively. On Day 12.5, C-CAM was detected in the spongiotrophoblasts of the junctional layer, but labyrinth trophoblasts and secondary giant trophoblasts were not stained. On Day 17.5, C-CAM was found only in the labyrinth and lacunae of the junctional layer. At this stage, both the labyrinth cytotrophoblasts of the maternal blood vessels and the endothelial cells of the embryonic capillaries were strongly stained. Placental tissues from gestational Days 12.5 and 17.5 were analyzed by immunoelectron microscopy to determine the location of C-CAM at the subcellular level. On Day 12.5, positive staining of the spongiotrophoblasts was observed, mainly on surface membranes and microvilli between loosely associated cells. On Day 17.5, staining was found primarily on the microvilli of the maternal luminal surfaces of the labyrinth cytotrophoblasts, and both on the luminal surface and in the cytoplasm of endothelial cells of the embryonic vessels. RT-PCR analysis and Southern blotting of the PCR products revealed expression of mRNA species for both of the major isoforms, C-CAM1 and C-CAM2. Immunoblotting analysis of C-CAM isolated from 12.5-day and 14.5-day placentae showed that it appeared as a broad band with an apparent molecular mass of 110-170 kD. In summary, C-CAM was strongly expressed in a specific spatiotemporal pattern in trophoblasts actively involved in formation of the placental tissue, suggesting an important role in placental development. In the mature placenta, C-CAM expression was confined to the trophoblastic and endothelial cells lining the maternal and embryonic vessels, respectively, suggesting important functions in placental physiology.

Differential expression of transcripts encoding retinoid binding proteins and retinoic acid receptors during placentation of the mouse

We report the distribution of transcripts from genes encoding the retinol binding protein (RBP), the cellular retinol binding proteins (CRBP I, II) and retinoic acid binding proteins (CRABP I, II), the retinaldehyde dehydrogenase type 2 (RALDH-2), the retinoic acid receptors (RARs), and the retinoid X receptors (RXRs) in mouse placental tissues from 6.5 to 19.5 days postcoitum (dpc). During early placentation, RBP and RALDH-2 gene expression are restricted to the endoderm of the visceral yolk sac and the outer uterine epithelium, respectively, whereas CRBP I transcripts are detected in the visceral yolk sac and in the presumptive chorioallantoic placenta. By 15.5 dpc, CRBP I expression is down-regulated in the yolk sac where CRBP II becomes strongly expressed. Expression of CRBP II is also detected in the trophoblastic giant cells. Throughout placentation, the expression patterns of the CRABP I and II genes partly overlap in the decidual tissue and the vacuolar zones of the decidua, suggesting a role for these binding proteins in sequestering free retinoic acid from maternal blood, thus regulating its availability to the embryo. RAR alpha is ubiquitously expressed in all placental tissues, except in trophoblastic giant cells, at all stages studied. During early placentation, RAR beta and RAR gamma are co-expressed in the decidua but differentially expressed in the chorionic region (RAR beta, 10.5 to 12.5 dpc) and the presumptive labyrinth (RAR gamma, 7.5 to 12.5 dpc). During the same stages, RXR alpha is strongly expressed in the presumptive placenta. RAR gamma remains weakly expressed in the labyrinth until 15.5 dpc, whereas RXR alpha exhibits a strong expression in this zone until birth, suggesting a role for these receptors in the development and function of the definitive placenta.

Placental effects of lead in mice

Although a number of studies in animal models have shown embryolethal and teratogenic lead effects when this element is administered by a parenteral route, the mechanism of the embryonary changes is well not established. In this study, the embryonic effects of parenteral lead exposure on day 9 of gestation were assessed in the Swiss mouse. Lead acetate trihydrate was injected intraperitoneally at 14, 28, 56 and 112 mg/kg. There was no maternal toxicity evidenced by death, reduced body weight gain or reduced food consumption. However, absolute placental weight at 112 mg/kg and relative placental weight at 14, 56 and 112 mg/kg were diminished significantly. The number of total implants, live and dead fetuses, sex ratio and fetal body weight were unaffected by lead exposure. Most sections of placenta showed vascular congestion, an increase of intracellular spaces and deposits of hyaline material of perivascular predominance. Trophoblast hyperplasia was also observed, whereas there was a reinforcement of the fibrovascular network in the labyrinth. It is concluded that the trophoblast hyperplasia observed in the placenta of pregnant mice after parenteral lead exposure at doses that are not toxic for the dam could act as a repairing mechanism of the extraembryonary tissues.

Expression and function of matrix metalloproteinases and their inhibitors at the maternal-embryonic boundary during mouse embryo implantation

Gelatinase B, a matrix metalloproteinase (MMP) of high specific activity, is highly expressed and activated by mouse blastocysts in culture, and inhibition of this enzyme activity inhibits lysis of extracellular matrix (Behrendtsen, O., Alexander, C. M. and Werb, Z. (1992) Development 114, 447–456). Because gelatinase B expression is linked to invasive potential, we studied the expression of gelatinase B mRNA and protein in vivo, in implanting trophoblast giant cells, and found that it was expressed and activated during colonization of the maternal decidua. mRNAs for several other MMPs (stromelysin-1, stromelysin-3 and gelatinase A) and MMP inhibitors (TIMP-1 and TIMP-2) were expressed in the undifferentiated stroma toward the outside of the decidua, and TIMP-3 mRNA was expressed in primary and some mature decidual cells during their differentiation. Both mRNA and TIMP-3 protein were present at high concentrations transiently, and declined from 6.5 days post coitum onward, as the cells underwent apoptosis during the main period of gelatinase B expression and ectoplacental growth and expansion. To assess the function of MMPs during implantation and decidual development, we either injected a peptide hydroxamate MMP inhibitor into normal mice or studied transgenic mice overexpressing TIMP-1. In both cases, decidual length and overall size were reduced, and the embryo was displaced mesometrially. Embryo orientation was less strictly regulated in inhibitor-treated deciduae than in control deciduae. Morphogenesis and development of oil-induced deciduomas were also slowed in the presence of the inhibitor. We conclude that administration of MMP inhibitors retards decidual remodeling and growth, and we suggest that the MMPs expressed in precursor stromal cells promote their differentiation and expansion.

Polytene chromosomes in mammalian cells

This article deals with the structural and functional organization of polytene chromosomes in mammals. Based on cytophotometric, autoradiographic, and electron microscopic data, the authors put forward a concept of nonclassic polytene chromosomes, with special reference to polytene chromosomes in the mammalian placenta. In cells with nonclassic polytene chromosomes, two phases of the polytene nucleus cycle are described, such as the endointerphase (S phase) and endoprophase (G phase). The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes. The final step of the trophoblast giant cell differentiation is characterized by a transition from polyteny to polyploidy, with subsequent fragmentation of the highly polyploid nucleus into fragments of low ploidy. Similarities and dissimilarities of pathways of formation and rearrangement of nonclassic polytene chromosomes in mammals, insects, plants, and protozoans are compared. The authors discuss the significance of polyteny as one of the intrinsic conditions for performance of the fixed genetic program of trophoblast giant cell development, a program that provides for the possibility of a long coexistence between maternal and fetal allogenic organisms during pregnancy.

Cytological aspects of vascular invasion by the trophoblast of Calomys callosus in hepatic tissues

Ectoplacental cones isolated from embryos on the seventh day of pregnancy were transplanted beneath the hepatic capsule of recipient adult animals to document the morphological patterns of vascular invasion by the trophoblast in the absence of the maternal environment and the influence of its peculiar vasculature. Females, and females and males of Calomys callosus, a cricetid rodent, were used, respectively, as embryo donors and recipient animals. Three to 5 days later, the grafted regions were excised and processed for light and electron microscopy. Invasion of the liver parenchyma by the trophoblast progressed along the vascular beds, associated with gradual phagocytosis of hepatic cells, greatly favoring the morphological characterization of invasive steps exhibited by the trophoblast to access the different kinds of vessels, to trespass the various vascular components and the different levels of the surrounding hepatic parenchyma. It is possible that either in utero during the establishment of embryomaternal circulation in early pregnancy or ex utero under experimental conditions, the trophoblast exhibits similar vascular invasion behavior. In view of this, our findings may contribute to a better understanding of trophoblast cell migration to the maternal blood supply as well as the role of the trophoblast in the establishment of the placental circulation during pregnancy.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

The endometrial cell surface and implantation. Expression of the polymorphic mucin MUC-1 and adhesion molecules during the endometrial cycle

The cell surface mucin MUC-1 is present in endometrial epithelial cells and their associated apical glycocalyx and is also released into gland lumens as a secretory product. MUC-1 mRNA and core protein are found at low levels in the proliferative phase of the cycle, but their abundance increases after ovulation. Endometrial MUC-1 has been found to carry sialokeratan sulphate chains and these show a dramatically increased abundance in cells and secretions in the post-ovulatory phase of the cycle, reaching a maximum in secretions 6-7 days after the LH peak. The apical epithelium also contains adhesion receptor molecules of the integrin and CD44 families. MUC-1 is large and highly glycosylated and probably extends farther from the cell surface than these 'conventional' glycoprotein receptors. It has the potential to inhibit sterically receptor-mediated cell-cell adhesion. However, it is also possible that MUC-1 displays specific (e.g., glycan) recognition structures for the initial attachment of the blastocyst or that the embryo may create a specialised microenvironment in which to implant.

Changes in the pattern of expression of alkaline phosphatase in the mouse uterus and placenta during gestation

The development of the rodent chorio-allantoic placenta is a complicated process that results in the formation of a transport system capable of sustaining embryonic and fetal growth and development. Intimately linked to this process is alkaline phosphatase (AP), a cell-surface glycoprotein that possibly functions as a transport protein. In the present study, we have mapped the location of AP-expressing cells in the mouse utero-placental unit during the development of the chorio-allantoic placenta by use of enzyme histochemistry and in situ hybridization histochemistry. We found that at implantation the expression of the tissue non-specific AP (TNAP) gene is located exclusively in the decidua and that most of this decidual expression ceases as the placenta starts to form. One exception is a mesometrially located marginal zone of the decidua, which continues to express the TNAP gene until day 12 and the active protein until at least day 16. Trophoblasts of the chorion already express AP before the time of fusion with the ectoplacental cone, after which AP is expressed by trophoblasts of the resulting ectoplacental plate. AP expression in the mature chorio-allantoic placenta is localized in the placental labyrinth and spongy zones. In the latter zone, expression ceases on about day 14. Giant trophoblasts start to express AP on about day 10, with some cells still positive for AP at day 16. The yolk sac does not express AP at any developmental stage. The results show that AP expression during placental development is neither restricted to cells known to be involved in transport, nor expressed in all cells thought to be involved in this transport. This may indicate that AP is not merely a transport protein but has additional functions.

Expanding roles for alpha 4 integrin and its ligands in development

Interaction of alpha 4 integrins with vascular cell adhesion molecule-1 (VCAM-1) is classically important for immune function. However, we found recently that these receptors have a second role, in embryogenesis, where they mediate cell-cell interactions that are important for skeletal muscle differentiation. Here, we present evidence of an expanding role for these receptors in murine development. alpha 4 and VCAM-1 were found at embryonic sites of hematopoiesis, suggesting a role for these receptors during embryogenesis that parallels their hematopoietic function in adult bone marrow. During angiogenesis in the lung, alpha 4 and VCAM-1 were found on mesenchyme that gives rise to vascular endothelium and smooth muscle. alpha 4 persisted on the smooth muscle and the endothelium of newly forming vessels where it colocalized with its extracellular matrix ligand, fibronectin (FN). These patterns suggest several roles for alpha 4 integrins and their ligands in angiogenesis. alpha 4 was also found on neural crest derivatives where it colocalized with FN. alpha 4 was expressed selectively on cells in the dorsal root ganglia: it was apparent along ventral projections, but absent from dorsal projections, suggesting that alpha 4 integrins could be involved in defining neuronal fates. Although VCAM-1 was not expressed on most neural crest derivatives, it was found in the neural crest-derived outflow tract of the embryonic heart, where it colocalized with alpha 4. These results imply that alpha 4 integrins and their ligands could be important for migration or differentiation of neural crest. alpha 4 was also expressed on embryonic retina and FN was found on inductive mesenchyme surrounding the eye, suggesting a role for these proteins in eye development. Finally, based on their patterns of expression, we conclude that VCAM-1 only participates in a subset of interactions involving alpha 4 integrins, whereas FN appears to be the more general ligand.

Complex patterns of expression suggest extensive roles for the alpha 2 beta 1 integrin in murine development

The extracellular matrix plays important roles in embryogenesis. The integrin family of adhesion receptors may mediate critical cellular interactions with the extracellular matrix during development. In this study, we elucidated the developmental spatial and temporal expression pattern of the alpha 2 beta 1 integrin heterodimer, a cell surface receptor for collagens and laminin. We generated reagents for studying the alpha 2 beta 1 integrin and examined the developmental expression of the integrin in postimplantation mice. A partial length murine alpha 2 cDNA was isolated and the protein encoding region was found to be 82% homologous to that of the human alpha 2 cDNA. A synthetic peptide corresponding to the carboxy-terminus of murine alpha 2 was used to generate alpha 2-specific antiserum. The antiserum and riboprobes derived from both the alpha 2 cDNA and the previously characterized murine beta 1 subunit cDNA were used to determine the spatiotemporal expression of the alpha 2 subunit by immunocytochemistry and of the alpha 2 and beta 1 mRNAs by in situ hybridization. Both approaches gave concordant results. Expression of the alpha 2 integrin subunit was observed in both the maternal and embryonic components of the placenta, namely the perivascular and basal zone decidual cells and decidual cells and spongiotrophoblasts at the maternal/embryonic junction. Expression was also observed in cells actively producing and remodeling the extracellular matrix in the maternal uterus and in the developing gut, lens, cartilage, bone, and tooth of the embryo. Generally, expression of the alpha 2 integrin subunit was found in cells entering their later stages of differentiation such as in chondrocytes as they became hypertrophic, ameloblasts and odontoblasts as they became columnar and began to secrete the matrix of the tooth, endothelial cells after they formed tubules, in the lens just prior to and during lens fiber production, and in the collecting ducts of the kidney only after full gestation. Throughout embryogenesis, beta 1 mRNA was widely distributed and present in cell types expressing alpha 2 mRNA and protein. The developmental expression pattern of the alpha 2 beta 1 integrin suggests roles for the integrin in placental development and matrix assembly and remodeling.

The immunolocalization of basic fibroblast growth factor in the mouse uterus during the initial stages of embryo implantation

Mammalian embryo implantation involves a series of complex interactions between maternal and embryonic cells. Uterine polypeptide growth factors may play critical roles in these cell interactions. Basic fibroblast growth factor (basic FGF) is a member of a family of growth factors. This growth factor may be potentially important for the process of embryo implantation because it (a) is stored within the extracellular matrix and is thus easily available during embryo invasion, (b) is a potent modulator of cell proliferation and differentiation and (c) stimulates angiogenesis. The immunolocalization of basic FGF in the uterus during the peri-implantation period of pregnancy is presented in this study. Uterine tissue samples were obtained on days 6-9 of pregnancy with day 1 of pregnancy being the day of a vaginal copulatory plug. Uterine samples were fixed in Bouin's fluid for no longer than 18 h. Following fixation and paraffin embedding, sections were exposed to primary antisera made in rabbits against either (a) human recombinant basic FGF or (b) 1-24 synthetic fragment of bovine basic FGF. The primary antibody was followed by biotinylated goat anti-rabbit IgG and a biotin-avidin-peroxidase complex. There were no differences in the immunolocalization of basic FGF using either source of primary antibody. Our results demonstrated both temporal and spatial changes in the localization of immunoreactive basic FGF within the implantation chamber during days 6-9 of pregnancy. Inter-implantation sites resembled the non-pregnant uterus with basic FGF present in extracellular matrices including basal laminae. On day 6 of pregnancy, decidual cells within the primary decidual zone lacked both intracellular and pericellular basic FGF while non-decidualized uterine stroma resembled inter-implantation sites. By days 7-8 of pregnancy, the secondary decidual zone had formed and was characterized by the distinct pericellular localization of basic FGF around individual decidual cells. By day 9 of pregnancy, the mesometrial region was forming and contained cords of decidual cells and a labyrinth of maternal blood vessels. The decidual cells contained diffuse intracellular basic FGF. Trophoblast cells were devoid of basic FGF at all times examined. These results indicate that basic FGF is present within the implantation chamber on days 6-9 of pregnancy and may be involved in the decidual cell response, trophoblast cell invasion and angiogenesis.