Diverse subtypes and developmental origins of trophoblast giant cells in the mouse placenta

Trophoblast giant cells (TGCs) are the first terminally differentiated subtype to form in the trophoblast cell lineage in rodents. In addition to mediating implantation, they are the main endocrine cells of the placenta, producing several hormones which regulate the maternal endocrine and immune systems and promote maternal blood flow to the implantation site. Generally considered a homogeneous population, TGCs have been identified by their expression of genes encoding placental lactogen 1 or proliferin. In the present study, we have identified a number of TGC subtypes, based on morphology and molecular criteria and demonstrated a previously underappreciated diversity of TGCs. In addition to TGCs that surround the implantation site and form the interface with the maternal deciduas, we demonstrate at least three other unique TGC subtypes: spiral artery-associated TGCs, maternal blood canal-associated TGCs and a TGC within the sinusoidal spaces of the labyrinth layer of the placenta. All four TGC subtypes could be identified based on the expression patterns of four genes: Pl1, Pl2, Plf (encoded by genes of the prolactin/prolactin-like protein/placental lactogen gene locus), and Ctsq (from a placental-specific cathepsin gene locus). Each of these subtypes was detected in differentiated trophoblast stem cell cultures and can be differentially regulated; treatment with retinoic acid induces Pl1/Plf+ TGCs preferentially. Furthermore, cell lineage tracing studies indicated unique origins for different TGC subtypes, in contrast with previous suggestions that secondary TGCs all arise from Tpbpa+ ectoplacental cone precursors.     

Spatiotemporal expression of Notch receptors and ligands in developing mouse placenta

Notch signaling is involved in cell lineage specification in many developing organs. In mice there are four known Notch receptor genes (Notch1–4) and five ligands genes (Dll1, 3, 4 and Jagged1 and 2). Notch2 is essential for development of placenta, an organ that mediates feto-maternal nutrient and gas exchange as well as maternal adaptations to pregnancy. However the role of other Notch receptors and ligands in placentation is not known. In order to gain better insight into the role of Notch signaling in mouse placenta we thoroughly analyzed mRNA expression of all Notch receptors and ligands in all trophoblast cell types from the embryonic day (E) 7.5 to E12.5, the period during which all of the substructures of the placenta develop. Here we show that Notch receptors and ligands are specifically and dynamically expressed in multiple cell layers of developing placenta. We found that the Notch2 receptor and Jagged1 and Jagged2 ligand genes are complementarily expressed in trophoblast cells of the chorion and its later derivatives in the labyrinth. Dll4 and Notch2 expression complement each other in the ectoplacental cone, while Dll1 and Notch2 are expressed in an ectoplacental cone derivative, the junctional zone. Moreover Dll4 and Notch2 are expressed at the ectoplacental cone–decidua interface at early stages of placentation. Additionally we show that Notch2 is dynamically expressed in all trophoblast giant cell subtypes, which is consistent with previous reports. Overall these expression pattern results suggest that Notch signaling may play several diverse roles during placenta development.     

Notch2 is required for formation of the placental circulatory system, but not for cell-type specification in the developing mouse placenta

We have previously reported that a mutation in the ankyrin repeats of mouse Notch2 results in embryonic lethality by embryonic day 11.5 (E11.5), showing developmental retardation at E10.5. This indicated that Notch2 plays an essential role in postimplantation development in mice. Here, we demonstrate that whole embryo culture can circumvent developmental retardation of Notch2 mutant embryos for up to 1 day, suggesting that the lethality was primarily caused by extraembryonic defects. Histological examinations revealed delayed entry of maternal blood into the mutant placenta and poor blood sinus formation at later stages. Notch2-expressing cells appeared around maternal blood sinuses. Specification of trophoblast subtypes appeared not to be drastically disturbed and expression of presumptive downstream genes of Notch2 signaling was not altered by the Notch2 mutation. Thus, in the developing mouse placenta, Notch2 is unlikely to be involved in cell fate decisions, but rather participates in formation of maternal blood sinuses. In aggregation chimeras with wild-type tetraploid embryos, the mutant embryos developed normally until E12.5, but died before E13.5. The chimeric placentas showed a restored maternal blood sinus formation when compared with the mutant placentas, but not at the level of wild-type diploid placentas. Therefore, it was concluded that the mutant suffers from defects in maternal blood sinus formation. Thus, Notch2 is not cell autonomously required for the early cell fate determination of subtypes of trophoblast cells, but plays an indispensable role in the formation of maternal blood sinuses in the developing mouse placenta.     

Intravital Placenta Imaging Reveals Microcirculatory Dynamics Impact on Sequestration and Phagocytosis of Plasmodium-Infected Erythrocytes

Malaria in pregnancy is exquisitely aggressive, causing a range of adverse maternal and fetal outcomes prominently linked to Plasmodium-infected erythrocyte cytoadherence to fetal trophoblast. To elucidate the physiopathology of infected erythrocytes (IE) sequestration in the placenta we devised an experimental system for intravital placental examination of P. berghei-infected mice. BALB/c females were mated to C57Bl/6 CFP+ male mice and infected with GFP+ P. berghei IE, and at gestational day 18, placentas were exposed for time-lapse imaging acquisition under two-photon microscopy. Real-time images and quantitative measurements revealed that trophoblast conformational changes transiently restrain blood flow in the mouse placental labyrinth. The complex dynamics of placental microcirculation promotes IE accumulation in maternal blood spaces with low blood flow and allows the establishment of stable IE-trophoblast contacts. Further, we show that the fate of sequestered IE includes engulfment by both macrophagic and trophoblastic fetal-derived cells. These findings reinforce the current paradigm that IE interact with the trophoblast and provide definitive evidence on two novel pathogenesis mechanisms: (1) trophoblast layer controls placental microcirculation promoting IE sequestration; and (2) fetal-derived placental cells engulf sequestered IE.     

Interactions between trophoblast cells and the maternal and fetal circulation in the mouse placenta

Mammalian embryos have an intimate relationship with their mothers, particularly with the placental vasculature from which embryos obtain nutrients essential for growth. It is an interesting vascular bed because maternal vessel number and diameter change dramatically during gestation and, in rodents and primates, the terminal blood space becomes lined by placental trophoblast cells rather than endothelial cells. Molecular genetic studies in mice aimed at identifying potential regulators of these processes have been hampered by lack of understanding of the anatomy of the vascular spaces in the placenta and the general nature of maternal-fetal vascular interactions. To address this problem, we examined the anatomy of the mouse placenta by preparing plastic vascular casts and serial histological sections of implantation sites from embryonic day (E) 10.5 to term. We found that each radial artery carrying maternal blood into the uterus branched into 5-10 dilated spiral arteries located within the metrial triangle, populated by uterine natural killer (uNK) cells, and the decidua basalis. The endothelial-lined spiral arteries converged together at the trophoblast giant cell layer and emptied into a few straight, trophoblast-lined "canals" that carried maternal blood to the base of the placenta. Maternal blood then percolated back through the intervillous space of the labyrinth toward the maternal side of the placenta in a direction that is countercurrent to the direction of the fetal capillary blood flow. Trophoblast cells were found invading the uterus in two patterns. Large cells that expressed the trophoblast giant cell-specific gene Plf (encoding Proliferin) invaded during the early postimplantation period in a pattern tightly associated with spiral arteries. These peri/endovascular trophoblast were detected only approximately 150-300 microm upstream of the main giant cell layer. A second type of widespread interstitial invasion in the decidua basalis by glycogen trophoblast cells was detected after E12.5. These cells did not express Plf, but rather expressed the spongiotrophoblast-specific gene Tpbp. Dilation of the spiral arteries was obvious between E10.5 and E14.5 and was associated with a lack of elastic lamina and smooth muscle cells. These features were apparent even in the metrial triangle, a site far away from the invading trophoblast cells. By contrast, the transition from endothelium-lined artery to trophoblast-lined (hemochorial) blood space was associated with trophoblast giant cells. Moreover, the shaping of the maternal blood spaces within the labyrinth was dependent on chorioallantoic morphogenesis and therefore disrupted in Gcm1 mutants. These studies provide important insights into how the fetoplacental unit interacts with the maternal intrauterine vascular system during pregnancy in mice.     

Function and control of human invasive trophoblast subtypes: Intrinsic vs. maternal control

ABSTRACT

The establishment of a functional placenta is pivotal for normal fetal development and the maintenance of pregnancy. In the course of early placentation, trophoblast precursors differentiate into highly invasive trophoblast subtypes. These cells, referred to as extravillous trophoblasts (EVTs), penetrate the maternal uterus reaching as far as the inner third of the myometrium. One of the most fundamental functions of EVTs is the transformation of spiral arteries to establish the uteroplacental blood circulation assuring an adequate nutrient and gas supply to the developing fetus. To achieve this, specialized EVT subpopulations interact with maternal immune cells, provoke elastolysis in the arterial wall and replace the endothelial cells lining the spiral arteries to induce intraluminal vascular remodeling. These and other trophoblast-mediated processes are tightly controlled by paracrine signals from the maternal decidua and furthermore underlie an intrinsic cell-type specific program. Various severe pregnancy complications such as preeclampsia or intrauterine growth retardation are associated with abnormal EVT function, shallow invasion, and decreased blood flow to the placenta. Hence a better understanding of human trophoblast invasion seems mandatory to improve therapeutic intervention. This approach, however, requires a profound knowledge of the human placenta, its various trophoblast subtypes and in particular a better understanding of the regulatory network that controls the invasive phenotype of EVTs.     

Development of the hemochorial maternal vascular spaces in the placenta through endothelial and vasculogenic mimicry

The maternal vasculature within the placenta in primates and rodents is unique because it is lined by fetal cells of the trophoblast lineage and not by maternal endothelial cells. In addition to trophoblast cells that invade the uterine spiral arteries that bring blood into the placenta, other trophoblast subtypes sit at different levels of the vascular space. In mice, at least five distinct subtypes of trophoblast cells have been identified which engage maternal endothelial cells on the arterial and venous frontiers of the placenta, but which also form the channel-like spaces within it through a process analogous to formation of blood vessels (vasculogenic mimicry). These cells are all large, post-mitotic trophoblast giant cells. In addition to assuming endothelial cell-like characteristics (endothelial mimicry), they produce dozens of different hormones that are thought to regulate local and systemic maternal adaptations to pregnancy. Recent work has identified distinct molecular pathways in mice that regulate the morphogenesis of trophoblast cells on the arterial and venous sides of the vascular circuit that may be analogous to specification of arterial and venous endothelial cells.     

Genetic insights into trophoblast differentiation and placental morphogenesis

The placenta is comprised of an inner vascular network covered by an outer epithelium, called trophoblast, all designed to promote the delivery of nutrients to the fetus. Several specialized trophoblast cell subtypes arise during development to promote this function, including cells that invade the uterus to promote maternal blood flow to the implantation site, and other cells that fuse into a syncytium, expand and fold to increase the surface area for efficient transport. Mutation of many genes in mice results in embryonic mortality or fetal growth restriction due to defects in placental development. Several important principles about placental development have emerged from these studies. First, distinct molecular pathways regulate the differentiation of the various trophoblast cell subtypes. Second, trophoblast proliferation, differentiation and morphogenesis are highly regulated by interactions with adjacent cell types. Finally, the specific classes of mutant phenotypes observed in the placenta of knockout mice resemble those seen in humans that are associated with preeclampsia and intrauterine growth restriction.     

Early patterning of the chorion leads to the trilaminar trophoblast cell structure in the placental labyrinth

The labyrinth of the rodent placenta contains villi that are the site of nutrient exchange between mother and fetus. They are covered by three trophoblast cell types that separate the maternal blood sinusoids from fetal capillaries--a single mononuclear cell that is a subtype of trophoblast giant cell (sinusoidal or S-TGC) with endocrine function and two multinucleated syncytiotrophoblast layers, each resulting from cell-cell fusion, that function in nutrient transport. The developmental origins of these cell types have not previously been elucidated. We report here the discovery of cell-layer-restricted genes in the mid-gestation labyrinth (E12.5-14.5) including Ctsq in S-TGCs (also Hand1-positive), Syna in syncytiotrophoblast layer I (SynT-I), and Gcm1, Cebpa and Synb in syncytiotrophoblast layer II (SynT-II). These genes were also expressed in distinct layers in the chorion as early as E8.5, prior to villous formation. Specifically, Hand1 was expressed in apical cells lining maternal blood spaces (Ctsq is not expressed until E12.5), Syna in a layer immediately below, and Gcm1, Cebpa and Synb in basal cells in contact with the allantois. Cebpa and Synb were co-expressed with Gcm1 and were reduced in Gcm1 mutants. By contrast, Hand1 and Syna expression was unaltered in Gcm1 mutants, suggesting that Gcm1-positive cells are not required for the induction of the other chorion layers. These data indicate that the three differentiated trophoblast cell types in the labyrinth arise from distinct and autonomous precursors in the chorion that are patterned before morphogenesis begins.     

Ablation of Tpbpa-positive trophoblast precursors leads to defects in maternal spiral artery remodeling in the mouse placenta

The placenta is composed of multiple trophoblast cell types that have diverse endocrine, vascular and nutrient transport functions. We have developed a transgenic system to investigate the developmental and functional roles of specific cell types using conditional expression of a cytotoxin to induce cell ablation in transgenic mice. The Tpbpa gene is expressed in ectoplacental cone cells starting between embryonic days (E) 7.5 and 8.5, and later in the spongiotrophoblast layer of the mature placenta. Tpbpa-positive cells are progenitors of many trophoblast subtypes including three subtypes of trophoblast giant cells (TGCs) and glycogen trophoblast cells. We used a Cre recombinase transgene driven by the Tpbpa promoter to irreversibly activate a diphtheria toxin A (DTA) transgene. Cre/DTA double transgenic placentas showed dramatic reduction of Tpbpa-positive spongiotrophoblast cells by E10.5 and conceptuses died by ~ E11.5. The number of cells associated with maternal blood spaces, spiral artery TGCs (SpA-TGCs) and canal TGCs, and glycogen trophoblast cells were reduced. The loss of these specific trophoblast subtypes, especially SpA-TGCs, was correlated with a decrease in maternal spiral artery diameters, indicating a critical role of these cells in modulating the maternal vasculature. In contrast, parietal TGCs were not significantly reduced by progenitor cell ablation, suggesting that there is compensatory growth of this population and indeed a population of Ascl2 (Mash2)-positive/Tpbpa-negative cells was increased in the spongiotrophoblast layer in the Cre/DTA double transgenics. Our work demonstrates that the Tpbpa-positive lineage is essential for placental function and particularly critical for maternal vasculature remodeling.     

Determinants of trophoblast lineage and cell subtype specification in the mouse placenta

Cells of the trophoblast lineage make up the epithelial compartment of the placenta, and their rapid development is essential for the establishment and maintenance of pregnancy. A diverse array of specialized trophoblast subtypes form throughout gestation and are responsible for mediating implantation, as well as promotion of blood to the implantation site, changes in maternal physiology, and nutrient and gas exchange between the fetal and maternal blood supplies. Within the last decade, targeted mutations in mice and the study of trophoblast stem cells in vitro have contributed greatly to our understanding of trophoblast lineage development. Here, we review recent insights into the molecular pathways regulating trophoblast lineage segregation, stem cell maintenance, and subtype differentiation.     

Partial loss of Ascl2 function affects all three layers of the mature placenta and causes intrauterine growth restriction

Several imprinted genes have been implicated in the regulation of placental function and embryonic growth. On distal mouse chromosome 7, two clusters of imprinted genes, each regulated by its own imprinting center (IC), are separated by a poorly characterized region of 280 kb (the IC1–IC2 interval). We previously generated a mouse line in which this IC1–IC2 interval has been deleted (Del^7AI allele) and found that maternal inheritance of this allele results in low birth weights in newborns. Here we report that Del^7AI causes a partial loss of Ascl2, a maternally expressed gene in the IC2 cluster, which when knocked out leads to embryonic lethality at midgestation due to a lack of spongiotrophoblast formation. The hypomorphic Ascl2 allele causes embryonic growth restriction and an associated placental phenotype characterized by a reduction in placental weight, reduced spongiotrophoblast population, absence of glycogen cells, and an expanded trophoblast giant cell layer. We also uncovered severe defects in the labyrinth layer of maternal mutants including increased production of the trilaminar labyrinth trophoblast cell types and a disorganized labyrinthine vasculature. Our results have important implications for our understanding of the role played by the spongiotrophoblast layer during placentation and show that regulation of the dosage of the imprinted gene Ascl2 can affect all three layers of the chorio-allantoic placenta.     

Regulation of murine placentogenesis by the retroviral genes Syncytin-A,Syncytin-B and Peg10

The murine placenta has a trichorial structure with two multinucleated syncytiotrophoblast (SCT) layers representing a barrier between the maternal and fetal blood system. Genes of endogenous retroviruses and retrotransposon-derived paternally expressed genes (Peg), remnants of past infections and integrations in the genome, have essential functions in placentogenesis. Previous studies showed that the envelope genes Syncytin-A and Syncytin-B were essential for cell–cell fusion of the SCT. The goal of this study was to analyze the temporal localization and expression of nine genes throughout placental development from embryonic day (E)8.5 to E18.5 using in situ-hybridization and absolute RNA-quantification. These included a comparison of previously characterized genes from the labyrinth Syncytin-A, Syncytin-B, Gcm1, the junctional zone PL-1, PL-2, Plf, Tpbpa with two further characterized genes Peg10 and Tpbpb. Syncytin-A and Syncytin-B RNA localized to SCT-I and SCT-II, respectively. Peg10 RNA localized to all extraembryonic tissues, specifically to the parietal and sinusoidal TGC of the labyrinth layer, which is in contact with SCT-I and the maternal blood. All three retroviral/retrotransposon-derived genes showed the highest expression at E16.5, but Peg10 with 188,917.1 molecules/ng cDNA was 208-fold and 106.8-fold higher expressed than Syncytin-A and Syncytin-B, respectively. Tpbpb localized to the junctional zone and showed the highest expression at E16.5 along with PL-2, Plf, Tpbpa, but not PL-1, which decreased in expression at E10.5. To investigate a role of Syncytin-A, Syncytin-B and Peg10 in cell–cell fusion, we established a cell culture system with fractionated primary trophoblasts from murine placentae. Culturing trophoblasts for up to 72 h partly resembled trophoblast development in vivo according to the nine marker genes. Knockdown of Syncytin-A demonstrated a functional regulation of cell–cell fusion, where knockdown of Peg10 showed no involvement in cell fusion. Due to the expression of Peg10 in TGCs, we propose an essential functional role in the fetal–maternal blood system.     

Comparative electron microscopy of chorio-allantoic placental barrier in some Indian Chiroptera

In the present study the comparative ultrastructure of the definitive chorio-allantoic placental barrier has been studied in considerable detail in six species of bats, representing six different families and both suborders of Chiroptera, by electron microscopy, and these species illustrate different kinds of interhaemal membranes met with among bats. The definitive chorio-allantoic placenta of Rousettus leschenaulti is haemodichorial, since the syncytiotrophoblast and cytotrophoblast layers are present to term. The fine structure of the placental barrier in the labyrinth of the definitive placenta of Rhinopoma hardwickei hardwickei is essentially endotheliomonochorial due to the presence of a single layer of cytotrophoblast and maternal endothelial cells. The placenta of Taphozous melanopogon, examined electron-microscopically in the present study, shows a thick maternal endothelium, a continuous interstitial membrane and the presence of a single layer of syncytiotrophoblast. The placenta of Megaderma comprises a typical endotheliochorial labyrinth and the presence of two layers of trophoblast. In Rhinolophus rouxi, the mature placenta during advanced pregnancy resembles that of Megaderma, its labyrinth containing large maternal capillaries with maternal endothelial cells and the two layers of trophoblast. Finally, the placental barrier of Hipposideros fulvus fulvus is haemodichorial due to the presence of two layers of trophoblast and the absence of maternal endothelial cells.     

Ultrastructural observations on the maturation of the placental labyrinth of the golden hamster (days 10 to 16 of gestation).

Morphogenesis of the labyrinthine part of the chorioallantoic placenta of the golden hamster between day 10 of gestation and term (day 16) was studied by light and electron microscopy. During this period the labyrinth increases greatly in both size and complexity. Trabeculae of the labyrinth, thin partitions composed of trophoblastic tissue and fetal capillaries which delimit the maternal blood spaces, apparently proliferate both by appositional and interstitial growth. From the time of its formation (day 9 of gestation) until term the labyrinth is hemotrichorial in organization (i.e. three layers of trophoblast separate maternal blood from fetal capillaries). Both the inner and intermediate layers of trophoblast (layers III and II, respectively) are syncytial. The outer trophoblastic layer (III), which is in direct contact with maternal blood, remains cellular, although many of its component cells grow to giant cell dimensions ("labyrinthine giant cells"). Between the tenth and fourteenth days of gestation the anatomical barrier to diffusion between maternal and fetal blood is progressively reduced. This is accomplished both by gradual attenuation of the trophoblastic layers and fetal capillary endothelium and by the formation of discontinuities (gaps) in layer I, and diaphragmed fenestrae in fetal capillary endothelium. The labyrinthine placental barrier is fully developed and probably attains maximal functional efficiency by the fourteenth day of gestation. Late in the fifteenth day of gestation, a few hours before parturition, distinct degenerative changes are apparent in the trophoblastic layers and fetal capillaries of the trabeculae. The factors responsible for initiation these degenerative changes and the onset of parturition are still controversial.     

Particulate urban air pollution affects the functional morphology of mouse placenta

In humans, adverse pregnancy outcomes (low birth weight, prematurity, and intrauterine growth retardation) are associated with exposure to urban air pollution. Experimental data have also shown that such exposure elicits adverse reproductive outcomes. We hypothesized that the effects of urban air pollution on pregnancy outcomes could be related to changes in functional morphology of the placenta. To test this, future dams were exposed during pregestational and gestational periods to filtered or nonfiltered air in exposure chambers. Placentas were collected from near-term pregnancies and prepared for microscopical examination. Fields of view on vertical uniform random tissue slices were analyzed using stereological methods. Volumes of placental compartments were estimated, and the labyrinth was analyzed further in terms of its maternal vascular spaces, fetal capillaries, trophoblast, and exchange surface areas. From these primary data, secondary quantities were derived: vessel calibers (expressed as diameters), trophoblast thickness (arithmetic mean), and total and mass-specific morphometric diffusive conductances for oxygen of the intervascular barrier. Two-way analysis of variance showed that both periods of exposure led to significantly smaller fetal weights. Pregestational exposure to nonfiltered air led to significant increases in fetal capillary surface area and in total and mass-specific conductances. However, the calibers of maternal blood spaces were reduced. Gestational exposure to nonfiltered air was associated with reduced volumes, calibers, and surface areas of maternal blood spaces and with greater fetal capillary surfaces and diffusive conductances. The findings indicate that urban air pollution affects placental functional morphology. Fetal weights are compromised despite attempts to improve diffusive transport across the placenta.     

Notch1 and the activated NOTCH1 intracellular domain are expressed in differentiated trophoblast cells

The Notch signalling pathway regulates proliferation, cell death and cell type specification that is critical for organogenesis. Mouse models carrying mutations in the Notch signalling pathway display defects in development of the placenta, suggesting that this pathway is required for placental development. In particular, Notch1 mutant embryos exhibit abnormal placental morphogenesis and arrest early in development. However, expression of Notch1 gene has not been detected during placental development. Trophoblast stem cells are derived from the precursor of the placenta and express Notch1. We report that Notch1 is also expressed in differentiated trophoblast cells. Under standard differentiation conditions, Notch1 expression ceases by day 6. Furthermore, the activated NOTCH1 intracellular domain is enriched at the nucleolus of trophoblast stem cells and differentiated trophoblast cells. Our results suggest that NOTCH1 is active in both trophoblast stem cells and differentiated trophoblast cells.     

PPARγ Regulates Trophoblast Proliferation and Promotes Labyrinthine Trilineage Differentiation

Background

Abnormal trophoblast differentiation and function is the basis of many placenta-based pregnancy disorders, including pre-eclampsia and fetal growth restriction. PPARγ, a ligand-activated nuclear receptor, plays essential roles in placental development; null murine embryos die at midgestation due to abnormalities in all placental layers, in particular, small labyrinth and expanded giant cell layer. Previous studies have focused mostly on the role of PPARγ in trophoblast invasion. Based on the previously reported role of PPARγ in preadipocyte differentiation, we hypothesized that PPARγ also plays a pivotal role in trophoblast differentiation. To test this hypothesis, we report derivation of wild-type and PPARγ-null trophoblast stem (TS) cells.

Methodology/Principal Findings

PPARγ-null TS cells showed defects in both proliferation and differentiation, specifically into labyrinthine trophoblast. Detailed marker analysis and functional studies revealed reduced differentiation of all three labyrinthine lineages, and enhanced giant cell differentiation, particularly the invasive subtypes. In addition, rosiglitazone, a specific PPARγ agonist, reduced giant cell differentiation, while inducing Gcm1, a key regulator in labyrinth. Finally, reintroducing PPARγ into null TS cells, using an adenovirus, normalized invasion and partially reversed defective labyrinthine differentiation, as assessed both by morphology and marker analysis.

Conclusions/Significance

In addition to regulating trophoblast invasion, PPARγ plays a predominant role in differentiation of labyrinthine trophoblast lineages, which, along with fetal endothelium, form the vascular exchange interface with maternal blood. Elucidating cellular and molecular mechanisms mediating PPARγ action will help determine if modulating PPARγ activity, for which clinical pharmacologic agonists already exist, might modify the course of pregnancy disorders associated with placental dysfunction.