Aspects of human fetoplacental vasculogenesis and angiogenesis. II. Changes during normal pregnancy

Moderate maternal nutrient restriction, but not glucocorticoid administration, leads to placental morphological changes in the baboon (Papio sp.)

The aims of the present study were to describe the ontogeny of spatial relationships between placental components in baboons and to investigate alterations in these indices following (1) moderate maternal nutrient restriction and (2) administration of glucocorticoids to pregnant baboons. We investigated the effects of glucocorticoids since they have been shown to play a role in the altered fetal growth that accompanies maternal nutrient restriction. Glucocorticoids are also given to pregnant women who threaten premature labor to accelerate fetal lung maturation. A third aim was to compare our findings to those in similar conditions in human pregnancy. Volumetric placental development in the baboon was similar to that in the human, although growth of fetal capillaries was slower over the second half of gestation in baboon than in human placentas. Intervillous space (IVS) and villous star volumes were halved at the end of gestation compared to the middle of gestation, as described in the human placenta. When mothers were fed 70% of feed eaten by controls fed ad libitum, placental volumetric structure was unchanged at mid-gestation but was altered by the end of gestation when placental weight, but not fetal weight or length, was decreased. At the end of gestation villous volume and surface area, capillary surface area, and the villous isomorphic coefficient were all decreased, In contrast, IVS hydraulic diameter was increased. All parameters were similar in pregnancies with male and female fetuses, with the exception of fetal capillary volume, which was unchanged in pooled samples and those from male fetuses, but decreased in pregnancies with female fetuses. Glucocorticoid administration during the second half of gestation did not produce any changes in the measured indices of placental composition. In summary, these changes in placental structure, associated with maternal nutrient restriction, would all act to decrease placental transport of nutrients. The influence of MNR on villous capillarization depends on fetal gender.

The Guinea pig placenta: model of placental growth dynamics

Placental growth dynamics are an important, but poorly understood aspect of placentation for which suitable animal models are lacking. Although the guinea pig is recognised as a valuable model for human pregnancy, its suitability for this purpose is not apparent, since the developmental and evolutionary processes that led to its lobulated placenta are not fully understood. Therefore, a study on immunohistochemistry has been undertaken, including the antibody mib-1 against protein Ki-67 as a proliferation marker. It was found that cellular trophoblast derived from the placental margin is the major centre of proliferation, supported by a moderate number of proliferating cells inside the labyrinth. In contrast to the ancestral condition of caviomorph rodents, the evolution of significant placental lobulation requires the latter source of proliferation. The distribution pattern of proliferating trophoblast cells revealed for the guinea pig strongly resembles the distribution in the human placenta in that (a) evenly distributed, moderate numbers of cells occur within the exchange area and (b) focal accumulations of proliferating trophoblast cells are present along the fetal mesenchymal border and appear to serve as a growth zone for appositional growth of new exchange tissues. Thus, the guinea pig appears to be an appropriate animal model for placental growth processes in human pregnancies.

Potential roles of the prokineticins in reproduction

Prokineticins are multifunctional secreted proteins that were originally identified as regulators of intestinal contraction but subsequently shown to affect vascular function, hyperalgesia, spermatogenesis, neuronal survival, circadian rhythm, nociception, feeding behaviour, immune responses, haematopoiesis and the development of the olfactory and gonadotropin-releasing hormone systems. Their role in the reproductive tract is still not fully elucidated, although they are reputed to increase microvascular permeability. Expression of prokineticins and their receptors has been reported in the ovary, uterus, placenta, testis and prostate. Their expression has also been reported in various pathologies of the reproductive tract, and future studies will highlight whether inhibition of prokineticin function in these pathologies would be a useful therapeutic target.

Altered gene expression patterns in intrauterine growth restriction: potential role of hypoxia

OBJECTIVE

Placental insufficiency is a primary cause of intrauterine growth restriction (IUGR). In our study, microarray technology was used to identify genes, which may impair placentation resulting in IUGR.

STUDY DESIGN

The RNA was isolated from both IUGR term placentas and normal term placentas. Microarray experiments were used to identify differentially expressed genes between the 2 cohorts. Real-time quantitative reverse transcriptase polymerase chain reaction and immunohistochemistry were used in follow-up experiments.

RESULTS

Microarray experiments identified increased expression of certain genes including leptin, soluble vascular endothelial growth factor receptor, human chorionic gonadotropin, follistatin-like 3, and hypoxia-inducible factor 2alpha in the IUGR. Real-time quantitative polymerase chain reaction confirmed these results.

CONCLUSION

The upregulation of soluble vascular endothelial growth factor receptor and hypoxia-inducible factor 2alpha at this period in pregnancy indicate that placental angiogenesis is altered in IUGR and that hypoxia is a major contributor to maldevelopment of the placental vasculature.

Development of the chorioallantoic placenta inOctodon degus—a model for growth processes in caviomorph rodents?

The degu Octodon degus is one of the very few members of caviomorph or hystricognath Rodentia that possesses a simply arranged chorioallantoic placenta without advanced lobulation. Therefore this species was used as a model to study regional development and growth processes of the placenta, based on the examination of 20 individuals by light and electron microscopy as well as by using markers for proliferation, trophoblast and endometrial stroma. The results were interpreted by comparison with other hystricognaths in the light of their evolutionary history. It was found that trophoblast derived from the trophospongium is essential for extension of the placenta including the labyrinth: extensive proliferation is restricted to trophoblast cells at the outer margin of the placenta and along internally directed, finger-tip like protrusions of fetal mesenchyme towards the labyrinth. This kind of placental development is regarded as part of the stem species pattern of hystricognaths, evolved more than 40 million years ago. It is indicated for the first time that the replenishment of the syncytiotrophoblast is similar to corresponding processes in the human placenta. In conclusion, the degu is a useful model for placental growth dynamics, particularly because of its simply arranged placental architecture, and may also serve as an animal model in comparison to human pregnancies.

Placental growth throughout the last two thirds of pregnancy in sheep: vascular development and angiogenic factor expression

Morphometric methodologies were developed and applied to investigate the patterns of vascular development in maternal (caruncular; CAR) and fetal (cotyledonary; COT) sheep placentas throughout the last two thirds of gestation. We also examined the expression levels of the major angiogenic factors and their receptors in CAR and COT sheep placentas. Although the vascularity of the CAR tissues increased continuously from Day 50 through Day 140 of pregnancy, those of the COT tissues increased at about twice the instantaneous rate (i.e., the proportionate increase/day) of the CAR. For CAR, vascularity increased 2-fold from Day 50 through Day 140 via relatively small increases in capillary number and 2- to 3-fold increases in capillary diameter. For COT, the increased vascularity resulted from a 12-fold increase in capillary number associated with a concomitant 2-fold decrease in capillary diameter. This large increase in fetal placental capillary number, which was due to increased branching, resulted in 6-fold increases in total capillary cross-sectional area and total capillary surface, per unit of COT tissue. Different patterns of expression of the mRNAs for angiogenic factors and their receptors were observed for CAR and COT. The dilation-like angiogenesis of CAR was correlated with the expression of vascular endothelial growth factor receptor-1 (FLT1), angiopoietin-2 (ANGPT2), and soluble guanylate cyclase (GUCY1B3) mRNAs. The branching-like angiogenesis of COT was correlated with the expression of vascular endothelial growth factor (VEGF), FLT1, angiopoietin-1 (ANGPT1), ANGPT2, and FGF2 mRNAs. Monitoring the expression of angiogenic factors and correlating the levels with quantitative measures of vascularity enable one to model angiogenesis in a spatiotemporal fashion.

Allometric studies on growth and development of the human placenta: growth of tissue compartments and diffusive conductances in relation to placental volume and fetal mass

Correlations between placental size and fetal mass during gestation fail to account for changes in composition that accompany placental growth and maturation. This study uses stereological data on the sizes of different tissue compartments in human placentas from 10 weeks of gestation to term and relates them to placental volume and to fetal mass by means of allometric analysis. In addition, tissue dimensions are used to calculate a physiological transport measure (diffusive conductance) for the villous membrane. Histological sections randomly sampled from placentas and analysed stereologically provided estimates of structural quantities (volumes, exchange surface areas, lengths, numbers of nuclei, diffusion distances). These data were combined with a physicochemical quantity (Krogh's diffusion coefficient) in order to estimate oxygen diffusive conductances for the villous membrane and its two components (trophoblast and stroma). Allometric relationships between these quantities and placental volume or fetal mass were obtained by linear regression analyses after log-transformation. Placental tissues had different growth trajectories: most grew more rapidly than placental volume and all grew more slowly than fetal mass. Diffusion distances were inversely related to placental and fetal size. Differential growth impacted on diffusive conductances, which, again, did not improve commensurately with placental volume but did match exactly growth of the fetus. Findings show that successful integration between supply and demand can be achieved by differential tissue growth. Allometric analysis of results from recent studies on the murine placenta suggest further that diffusive conductances may also be matched to fetal mass during gestation and to fetal mass at term across species.

Developmental changes in hemodynamics of uterine artery, utero- and umbilicoplacental, and vitelline circulations in mouse throughout gestation

In human pregnancy, abnormal placental hemodynamics likely contribute to the etiology of early-onset preeclampsia and fetal intrauterine growth restriction. The mouse is increasingly being deployed to study normal and abnormal mammalian placental development, yet the placental hemodynamics in normal pregnancy in mice is currently unknown. We used ultrasound biomicroscopy to noninvasively image and record Doppler blood velocity waveforms from the maternal and embryonic placental circulations in mice throughout gestation. In the uterine artery, peak systolic velocity (PSV) increased significantly from 23 ± 2 (SE) to 59 ± 3 cm/s, and end-diastolic velocity (EDV) increased from 7 ± 1 to 28 ± 2 cm/s in nonpregnant versus full-term females so that the uterine arterial resistance index (RI) decreased from 0.70 ± 0.02 to 0.53 ± 0.02. Velocities in the maternal arterial canal in the placenta were low and nearly steady and increased from 0.9 ± 0.03 cm/s at embryonic day 10.5 (E10.5) to 2.4 ± 0.07 cm/s at E18.5. PSV in the umbilical artery increased steadily from 0.8 ± 0.1 cm/s at E8.5 to 15 ± 0.6 cm/s at E18.5, whereas PSV in the vitelline artery increased from 0.6 ± 0.1 cm/s at E8.5 to 4 ± 0.2 cm/s at E13.5 and then remained stable to term. In the umbilical artery, the EDV detection rate was 0% at ≤E14.5 and 94% at E18.5, and the RI decreased from 1 to 0.82 ± 0.01 during this interval. We conclude that ultrasound biomicroscopy can be used to monitor placental hemodynamics during pregnancy in mice. These results provide novel information concerning the development of the vitelline and placental circulations in mice and reveal strong similarities in placental hemodynamics between mice and humans.

Sequential Steps During Vasculogenesis and Angiogenesis in the Very Early Human Placenta

Development of blood vessels takes place via two subsequent processes, vasculogenesis and angiogenesis. During vasculogenesis, formation of first blood vessels is achieved by differentiation of hemangiogenic stem cells from pluripotent mesenchymal cells, while during angiogenesis new blood vessels form from already existing vessels. The combination of our data with those from the literature leads us to depict the chronological steps of cell differentiation in the mesenchymal core of placental villi during vasculogenesis and angiogenesis. This current opinion will focus on the temporal and spatial expression of VEGF and its receptors VEGFR-1 and VEGFR-2, and the angiopoietin receptors Tie-1 and Tie-2 in parallel to vascular maturation in human placental villi during very early stages of placental development. There is evidence that the interplay of a variety of growth factors secreted from different cell types during development is needed to trigger as well as maintain placental vasculogenesis and angiogenesis.

Pre-eclampsia and Fetal Growth Restriction: How Morphometrically Different is the Placenta?

Both pre-eclampsia (PET) and fetal growth restriction (FGR) pose a heavy burden on fetal and maternal health and may disrupt pregnancy outcome. Using design based stereological techniques, placental vascular and villous morphology were assessed to determine the individual role played by both PET and FGR on placental growth during the third trimester. The following placentas delivered between 25 and 41 weeks of gestation were included into the study; controls (n=16), PET (n=20), FGR (n=17) and PET-FGR (n=16). Each placenta was uniformly randomly sampled and the sampled tissue processed to paraffin. Sections were stained with a CD34 antibody and the following morphometric parameters estimated: volumes, surface areas, length, diameters and the shape factor of the villous (terminal and intermediate) and vascular placental features. For stereologically estimated parameters pure PET had an effect on IVS and terminal villi volume only. FGR alone or when coexisting with PET contributed towards significant reductions in volumetric and surface area terminal villous and vascular features. FGR factors also contributed towards a significant reduction in the lengths of all parameters estimated and in the terminal villi diameter. Additionally, FGR was associated with a significant difference in shape factor indices for both intermediate and terminal villi. This study has shown that PET on its own has limited influence on the placental morphology studied, since the vascular features estimated do not differ stereologically from age matched normal controls. However, placental morphology is different between PET and PET-FGR and between PET-FGR and FGR. PET and FGR may have a cumulative effect on placental villous and vascular morphology as seen in the PET-FGR but there is no synergistic effect. These morphological abnormalities may have major physiological implications in terms of placental function and fetal growth.

Adeno-associated virus-2 (AAV-2) causes trophoblast dysfunction, and placental AAV-2 infection is associated with preeclampsia

Shallow invasion by extravillous trophoblast cells into the uterine wall reduces placental perfusion and causes placental dysfunction, but the one or more causes of shallow placental invasion are unknown. We hypothesized that infection with adeno-associated virus-2 (AAV-2) inhibits trophoblast invasion and is associated with preeclampsia, which is a common obstetric complication resulting from placental dysfunction. We determined that transformed extravillous trophoblast (HTR-8/SVneo) cells were susceptible to AAV-2 infection in the presence or absence of adenovirus, which provides helper function for AAV-2 replication, and that AAV-2 infection reduced invasion of HTR-8/SVneo cells through an extracellular matrix before cytopathic effects were detected. In a case-control study, AAV-2 DNA was found more frequently in trophoblast cells from cases of severe preeclampsia (22/40) than from normal term deliveries (5/27, P = 0.002). These results indicate that AAV-2 infection is a previously unidentified cause of placental dysfunction. Additional studies to determine the susceptibility of extravillous trophoblast to other viruses, and the mechanisms by which viral infection impairs placental function, are warranted.

Differential expression of placental villous angiopoietin-1 and -2 during early, mid and late baboon pregnancy

Although vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1) and Ang-2 have important roles in angiogenesis, very little is known about the regulation of these factors in the villous placenta during human pregnancy. In the present study, to investigate whether placental expression of Ang-1, Ang-2 and VEGF was altered in a cell-specific manner with advancing baboon gestation, the mRNA levels of these growth factors were determined by RT-PCR in cells isolated by Percoll gradient centrifugation from and protein localization assessed by immunocytochemistry in the villous placenta at early (day 60), mid (day 100) and late (day 170, term is 184 days) baboon gestation. Mean (+/-SE) Ang-1 mRNA levels, relative to 18S rRNA, in villous syncytiotrophoblast (3.92+/-0.68) and cytotrophoblast (1.31+/-0.31) cell fractions were highest on day 60 of gestation, then decreased by approximately 2.5-fold (P<0.05) to 1.39+/-0.29 and 0.49+/-0.07, respectively, on day 170. Moreover, Ang-1 mRNA levels in the villous stromal cells and Ang-2 mRNA levels in all placental villous cell fractions were similar on days 60, 100, and 170 of gestation. In contrast to Ang-1 and Ang-2, placental villous cytotrophoblast VEGF mRNA levels were increased 2.94-fold (P<0.05) between mid (0.67+/-0.15) and late (1.97+/-0.49) gestation. A corresponding decrease in Ang-1, absence of change in Ang-2, and increase in VEGF protein immunocytochemical expression were exhibited in placental trophoblast with advancing baboon pregnancy. Ang-1/Ang-2 and the angiopoietin Tie-2 receptor were expressed in vascular endothelial cells of the villous placenta, indicating that these blood vessel cells are a major site of ligand-receptor interaction for angiogenesis during primate pregnancy. We conclude that there is a cell-specific differential change in placental villous trophoblast expression of VEGF, Ang-1, and Ang-2 which we propose is important in regulating angiogenesis in the villous placenta during primate pregnancy.

Increased expression of sFlt-1 in in vivo and in vitro models of human placental hypoxia is mediated by HIF-1

Elevated expression of soluble vascular endothelial growth factor receptor-1 (sFlt-1) in preeclampsia plays a major role in the pathogenesis of this serious disorder of human pregnancy. Although reduced placental oxygenation is thought to be involved in the pathogenesis of preeclampsia, it is unclear how oxygen regulates placental sFlt-1 expression. The aims herein were to investigate sFlt-1 expression in in vivo and in vitro physiological and pathological models of human placental hypoxia and to understand the role of hypoxia inducible factor-1 (HIF-1) in regulating the expression of this molecule. sFlt-1 expression in placental villi was significantly increased under physiological low oxygen conditions in early first-trimester and in high-altitude placentae, as well as in pathological low oxygen conditions, such as preeclampsia. In high-altitude and in preeclamptic tissue, sFlt-1 localized within villi to perivascular regions, the syncytiotrophoblast layer, and syncytial knots. In first-trimester villous explants, low oxygen, but not hypoxia-reoxygenation (HR), increased sFlt-1 expression. Moreover, exposure of villous explants to dimethyloxalyl-glycin, a pharmacological inhibitor of prolyl-hydroxylases, which mimics hypoxia by increasing HIF-1alpha stability, increased sFlt-1 expression. Conversely, HIF-1alpha knockdown using antisense oligonucleotides, decreased sFlt-1 expression. In conclusion, placental sFlt-1 expression is increased by both physiologically and pathologically low levels of oxygen. This oxygen-induced effect is mediated via the transcription factor HIF-1. Low oxygen levels, as opposed to intermittent oxygen tension (HR) changes, play an important role in regulating sFlt-1 expression in the developing human placenta and hence may contribute to the development of preeclampsia.

The Effect of Interleukin-17 on the Proliferation and Invasion of JEG-3 Human Choriocarcinoma Cells

PROBLEM

As there has been a study in mice showing the expression of IL-17 by decidual cells and the status of IL-17 receptor expression in human pregnancy is not known, we hypothesized that IL-17 may regulate human trophoblast proliferation and invasion.

METHOD OF STUDY

JEG-3 cell line was used as a model for human trophoblast. Immunohistochemitry and reverse transcriptase polymerase chain reaction techniques were used to identify IL-17 receptor protein and mRNA, respectively. The effects of IL-17 on JEG-3 cell proliferation and invasion were tested using the BrdU incorporation and the Matrigel invasion assays, respectively.

RESULTS

IL-17 increased the invasive capacity of JEG-3 cells but had no effect on the proliferation and multinucleated formation of JEG-3 cells.

CONCLUSION

In this JEG-3 cell model of human trophoblast, the IL-17R and IL-17 may have a regulatory role in trophoblast invasion.

Evidence for altered placental blood flow and vascularity in compromised pregnancies

The placenta is the organ that transports nutrients, respiratory gases, and wastes between the maternal and fetal systems. Consequently, placental blood flow and vascular development are essential components of normal placental function and are critical to fetal growth and development. Normal fetal growth and development are important to ensure optimum health of offspring throughout their subsequent life course. In numerous sheep models of compromised pregnancy, in which fetal or placental growth, or both, are impaired, utero-placental blood flows are reduced. In the models that have been evaluated, placental vascular development also is altered. Recent studies found that treatments designed to increase placental blood flow can 'rescue' fetal growth that was reduced due to low maternal dietary intake. Placental blood flow and vascular development are thus potential therapeutic targets in compromised pregnancies.

Placental adaptive responses and fetal programming

Fetal programming occurs when the normal pattern of fetal development is disrupted by an abnormal stimulus or 'insult' applied at a critical point in in utero development. This then leads to an effect, for example diabetes or hypertension, which manifests itself in adult life. As the placenta is the regulator of nutrient composition and supply from mother to fetus and the source of hormonal signals that affect maternal and fetal metabolism, appropriate development of the placenta is crucial to normal fetal development. Placental function evolves in a carefully orchestrated developmental cascade throughout gestation. Disruption of this cascade can lead to abnormal development of the placental vasculature or of the trophoblast. Timing of a developmental 'insult' will be critical in consequent placental function and hence programming of the fetus. The 'insults' that alter placental development include hypoxia and abnormal maternal nutrient status, to which the placenta may adapt by alterations in transporter expression and activity to maintain fetal growth or by epigenetic regulation of placental gene expression. Hypoxia is physiological for organogenesis and placental tissue normally exists in a relatively hypoxic environment, but intrauterine growth restriction (IUGR) and pre-eclampsia are associated with a greater degree of trophoblast hypoxia. The metabolic activity of placental mitochondria leads to oxidative stress even in normal pregnancy which is exacerbated further in IUGR, diabetic and pre-eclamptic pregnancies and may also give nitrative stress known to lead to covalent modification and hence altered activity of proteins. Hypoxia, oxidative and nitrative stress all alter placenta development and may be a general underlying mechanism that links altered placental function to fetal programming.

Histomorphometric study of placental villi vascular volume in toxemia and diabetes

The quantitative changes in the vascular tree in placentas from pregnancies complicated by diabetes mellitus and preeclampsia (PE) are not well defined. The purpose of this study was to quantify placental villi cross-sectional area of capillaries assessed by a computerized morphometry system in pregnancies complicated by PE (n = 23), well-controlled pregestational diabetes mellitus (PGDM; n = 10), and healthy controls (n = 13). Our aims were to test whether villous capillarization volume was changed in PE without intrauterine growth restriction or PGDM compared with the control group and to study these effects in 3 different areas of the placenta. Examination of placentas in women with PGDM and PE revealed limited pathological changes on light microscopic examination. However, the morphometric analysis revealed a more than 5-fold decrease of villous vascular volume in PGDM compared with controls (P = .003) and a 1.6-fold decrease in the PE group that did not reach statistical significance. These findings show quantitative changes in the villous vascular tree in PGDM that are not detectable by conventional light microscopy and suggest that morphometric analysis of the capillary tree may have diagnostic importance in this entity. The findings differ significantly from those previously reported in pregestational diabetes and do not differ significantly from those reported in PE without intrauterine growth restriction.

An imbalance between vascular endothelial growth factor and its soluble receptor in placental villous explants of intrauterine growth-restricted pregnancies

OBJECTIVES

Human umbilical vascular endothelial cells (HUVECs), seeded on Matrigel (BD Biosciences, Bedford, UK), undergo an angiogenic-like process. We hypothesized that placental explants from normal pregnancies, maintained in cultures of different oxygen, would liberate factors that could be measured in this system. We further tested the angiogenic potential of placentae from intrauterine growth-restricted (IUGR) pregnancies and the effects of vascular endothelial growth factor (VEGF) blockade.

METHODS

Placental villous explants were maintained in culture at 3% and 20% O2. The resultant media was added to HUVECs seeded on 80% Matrigel. Cells were incubated at 6% O2 in accordance with the natural placental environment. After 6 hours, cells were fixed and stained and the length and number of tubules measured by morphometric imaging. Finally, VEGF and soluble VEGF receptor (sVEGFR-1) were recorded in the explant conditioned media.

RESULTS

Within the angiogenic assay, recombinant human VEGF significantly enhanced tubule outgrowth (branching and elongation) and this effect was blocked with neutralising antibody. Compared to 20% O2, media of placental explants conditioned at 3% O2 significantly encouraged tubule length and numbers. Again this affect was ablated by VEGF blockade. In cases of IUGR, conditioned media at 3% O2 showed a significant reduction in tubule growth. This was paralleled by a decline in available VEGF brought about an exaggeration in liberated sVEGFR-1. Notably, venous cord serum from IUGR pregnancies showed a similar elevation in sVEGFR-1.

CONCLUSION

Under restricted oxygen, placental angiogenic potential is suppressed in IUGR pregnancies through the overproduction of placental sVEGFR-1. This reduction may discourage normal placental vascularization and impact on fetal development.

Animal models of placental angiogenesis

The study of the development of the fetal membranes is an ancient one, and the importance of placental vascular development to placental function has long been recognized. Animal models have been important in these studies, as they allow for controlled experiments and analysis of multiple time-points during pregnancy. Since the demonstration nearly 20 years ago that the placenta produces angiogenic factors, the major factors regulating placental angiogenesis have been identified. These major factors include vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), the angiopoietins (ANG), and their receptors. Recently, sophisticated computerized image analysis methods have been developed to establish the pattern of placental vascular development in sheep. The maternal placental capillary bed develops primarily by increased size of capillaries, with only small increases in capillary number or surface densities. In contrast, the microvasculature of the fetal placenta develops primarily by increased branching, resulting in a large increase in capillary number and surface densities. These observations help to explain the relatively large increase in umbilical blood flow and nutrient delivery to the fetus that occurs during the last half of gestation. In addition, expression of mRNAs for VEGF, bFGF, ANG, and their receptors have recently been correlated with normal placental vascular development in sheep, and further refinement of these mathematical models is warranted. Lastly, the recent development of animal models of compromised pregnancies, including those resulting from maternal nutrition (both restriction and excess), multiple fetuses, environmental stress (heat stress and high altitude), and fetal and maternal breed effects, has already indicated that reductions in placental vascular development and expression of angiogenic factors are probably a root cause of fetal growth restriction. With these methods and models now in place, we should soon be able to establish the mechanisms involved in both normal and abnormal placental angiogenesis.

Profiling of oxygen-modulated gene expression in early human placenta by systematic sequencing of suppressive subtractive hybridization products

Villi from first-trimester human placenta were exposed to oxygen concentrations of either 2 or 20% during 3 h to construct two reciprocally subtracted libraries using the suppressive subtractive hybridization (SSH) methodology. After cloning, sequencing, and gene identification, the genes (1,071 clones corresponding to 822 different sequences) were classified according to 1) the subtracted library from which they originated and 2) within 58 groups of gene functions. We then developed a logarithm of the odds (LOD) test to identify a possible excess of genes in each group. We show that genes involved in angiogenesis are significantly overrepresented in the “hypoxic” condition (2% O2), whereas apoptotic genes are overrepresented in the “normoxic” condition (20% O2). Furthermore, we observed an excess of kinases relative to phosphatases and an excess of genes involved in proliferation over genes involved in cell growth in the hypoxic condition. To validate our results, we used quantitative RT-PCR to analyze the set of eight genes involved in angiogenesis on six independent placentas. Finally, we studied the distribution of gene clusters on human chromosomes to check whether their chromosomal distribution was random or not. We observed on human chromosome 11 a clear clustering of genes regulated similarly by O2tension, and we also discovered indications that such clustering exists on chromosomes 6 and 12.

An Angiopoietin-2 gene polymorphism in unexplained intrauterine fetal death: a multi-center study

OBJECTIVE

Angiopoietin-2 (Ang-2) is a potent regulator of angiogenesis and vascular tone. As vascular processes have been proposed to be involved in the pathogenesis of pregnancy associated complications such as late unexplained intrauterine fetal death (IUFD), we determined whether a common G/A polymorphism of the Ang-2 gene (ANGPT2) is associated with this condition.

METHODS

In a multicenter case-control study, we evaluated the common G/A polymorphism within exon 4 of the ANGPT2 gene using PCR in 90 women with IUFD and 90 healthy women with at least one uncomplicated full term pregnancy and no history of IUFD.

RESULTS

Genotype (p=0.2; OR=1.4 [0.8-2.6]) and allele frequencies (p=0.1; OR=1.4 [0.9-2.1]) of the ANGPT2 polymorphism did not differ between women with IUFD and healthy women. A multivariate regression analysis with smoking habits and preexisting diabetes as covariates did not change the results.

CONCLUSIONS

We are the first to report on a common polymorphism of the ANGPT2 gene in patients with late IUFD. The investigated ANGPT2 poylmorphism does not seem to be a candidate gene for IUFD in Caucasian women.

Placental angiogenesis in sheep models of compromised pregnancy

Because the placenta is the organ that transports nutrients, respiratory gases and wastes between the maternal and fetal systems, development of its vascular beds is essential to normal placental function, and thus in supporting normal fetal growth. Compromised fetal growth and development have adverse health consequences during the neonatal period and throughout adult life. To establish the role of placental angiogenesis in compromised pregnancies, we first evaluated the pattern of placental angiogenesis and expression of angiogenic factors throughout normal pregnancy. In addition, we and others have established a variety of sheep models to evaluate the effects on fetal growth of various factors including maternal nutrient excess or deprivation and specific nutrients, maternal age, maternal and fetal genotype, increased numbers of fetuses, environmental thermal stress, and high altitude (hypobaric) conditions. Although placental angiogenesis is altered in each of these models in which fetal growth is adversely affected, the specific effect on placental angiogenesis depends on the type of 'stress' to which the pregnancy is subjected, and also differs between the fetal and maternal systems and between genotypes. We believe that the models of compromised pregnancy and the methods described in this review will enable us to develop a much better understanding of the mechanisms responsible for alterations in placental vascular development.

Apoptosis contributes to vascular lumen formation and vascular branching in human placental vasculogenesis

Placental vasculogenesis consists of several stages, including appearance of hemangioblasts and angiogenic cell islands, setting up a primitive vascular network, and transition from vasculogenesis to sprouting and nonsprouting angiogenesis. In the present study, we hypothesized that placental vasculogenesis and angiogenesis require apoptosis during the formation of primitive vascular pattern, vessel elongation, and angiogenic branching. Vasculogenesis and apoptotic cells were identified using CD31 immunohistochemistry, hematoxylin-eosin (H-E) staining, CD31-TUNEL double-labeling, and transmission-electron microscopy (TEM). No TUNEL-positive cell was detected in angiogenic cell islands; however, several TUNEL-positive cells were observed during the primitive lumen formation. Interestingly, some of the stromal cells located between vasculogenic areas during the endothelial tube elongation and angiogenic branching also were TUNEL-positive. The presence of morphological aspects of apoptosis, such as nuclear shrinkage and nuclear bodies (apoptotic bodies), also was confirmed in H-E-stained and TEM-depicted sections. Quantitative analysis showed that higher ratios for apoptotic cells were found in the core stroma of villi among the vascular branching areas and in the primitive capillary lumen compared to angiogenic cell cords and vasculatures with advanced lumens (P < 0.05). In conclusion, our results suggest that apoptosis likely is involved in the physiologic mechanisms of placental vasculogenesis and angiogenesis, such as lumen formation and angiogenic branching.

Aspects of human fetoplacental vasculogenesis and angiogenesis. I. Molecular regulation

Patterns of fetoplacental angiogenesis vary during gestation and in association with certain pregnancy pathologies. In a set of three linked reviews, we provide a survey of current knowledge about the molecular regulation, cellular players, qualitative and quantitative morphological features of the vascularization of human placental villi. Here, an account is given of the role played by hypoxia-inducible factor in mediating the effects of oxygen on production of growth factor ligands and receptors which regulate angiogenesis and vessel maturation. However, it should be noted that, for the human placenta early in gestation, the normal (i.e. physiological) partial pressure of O(2)is low but this does not mean that the tissue is hypoxic. Thus, the mechanisms of regulating angiogenic growth factor production may differ at this time in comparison to those found later in gestation or in other tissues or organs. The vasculature in the placenta is plastic and changes markedly as gestation progresses. This is controlled by the complex interplay between physical factors and chemical factors including oxygen, growth factors and growth inhibitors. The companion reviews describe morphological features of normal and pathological development of the human placenta in the context of the factors discussed here.

Aspects of human fetoplacental vasculogenesis and angiogenesis. III. Changes in complicated pregnancies

Patterns of fetoplacental angiogenesis vary not only during the course of a normal pregnancy but also in certain pregnancy pathologies. Here, we review some of the molecular and morphological events which occur in complicated pregnancies. The pregnancy complications are chosen in an attempt to represent the possible different origins (preplacental, uteroplacental, postplacental) of fetal hypoxia. Molecular events focus on reported changes in hypoxia-inducible factors, angiopoietins and the vascular endothelial, basic fibroblast and placenta growth factors and their receptors. Morphological changes focus on patterns of angiogenesis (branching and non-branching) and a consistent set of morphometric descriptors (covering measures of total capillary growth, villous capillarization and capillary size and shape in transverse section). Apart from some uncertainties due to lack of information, or failure to resolve fully the effects of intrauterine growth restriction and pre-eclampsia, alterations in the angiogenic growth factors and morphologies of capillaries and villi in different complicated pregnancies seem to conform reasonably well to those predicted by the fetal hypoxia paradigm. However, it is clear that future studies on the effects of different origins of fetal hypoxia should exercise more care in the choice and interpretation of relevant descriptors and take more account of the parallel effects of possible confounders. In addition, rather than comparing uncomplicated and complicated pregnancies only at term, more information about molecular and morphological events that occur throughout gestation would be extremely valuable. This includes further studies on changes in growth factor receptors, the less-well-documented angiogenic factors (e.g. angiogenin, angiostatin, endostatin) and the associations between endothelial cells and pericytes. A more integrated approach involving also parallel analysis of the effects of erythropoietin and other potential vasoactive factors on the behaviour and morphology of fetal vessels would be beneficial.