Adriana and Luisa Castellucci Award Lecture 1997. Placental pathology in obstetrics: adaptation or failure of the villous tree?

The role of β-HCG and VEGF-MEK/ERK signaling pathway in villi angiogenesis in patients with missed abortion

OBJECTIVE

To analyze the effects of the Human Chorionic Gonadotropin beta (β-hCG) and the VEGF-MEK/ERK signaling pathway on villi angiogenesis in early missed abortion.

METHODS

A total of 12 cases of women with missed abortion and 12 cases of women who had induced abortion voluntarily without any disease were included in the present study. The age, pregnancy time and gestation period in the control group corresponded to the missed abortion group. Wes Simple Western system and qRT-PCR were used to detect the expression of VEGF-MEK/ERK signaling pathway related proteins and genes in villous. Radioimmunoassay and Enzyme-linked immunosorbent assay were used to detect β-hCG and VEGF levels in serum. The microvascular density (MVD) in villous tissue was analyzed by immunohistochemical staining.

RESULTS

The levels of β-hCG and VEGF in serum, the expression of VEGF-MEK/ERK signaling pathway and MVD in villous tissue of the missed abortion group were lower than those of the control group. In addition, compared with the control group, the layers of trophoblasts of the villous tissue in the missed abortion group became thinner significantly, the number of cells reduced, the cell structures were disorganized, and parts of the trophoblast cells were absent. Correlational analysis showed that the protein expression of ERK1/2 was positively correlated with MVD in missed abortion group.

CONCLUSIONS

Our results reveal that decreased production of β-hCG in early pregnant women could down-regulate the expression of VEGF-MEK/ERK signal pathway, then reduce angiogenesis and eventually leading to the abnormal angiogenesis of villous, which may be an important mechanism of missed abortion.

The Impact of Infection in Pregnancy on Placental Vascular Development and Adverse Birth Outcomes

Healthy fetal development is dependent on nutrient and oxygen transfer via the placenta. Optimal growth and function of placental vasculature is therefore essential to support in utero development. Vasculogenesis, the de novo formation of blood vessels, and angiogenesis, the branching and remodeling of existing vasculature, mediate the development and maturation of placental villi, which form the materno-fetal interface. Several lines of evidence indicate that systemic maternal infection and consequent inflammation can disrupt placental vasculogenesis and angiogenesis. The resulting alterations in placental hemodynamics impact fetal growth and contribute to poor birth outcomes including preterm delivery, small-for-gestational age (SGA), stillbirth, and low birth weight (LBW). Furthermore, pathways involved in maternal immune activation and placental vascularization parallel those involved in normal fetal development, notably neurovascular development. Therefore, immune-mediated disruption of angiogenic pathways at the materno-fetal interface may also have long-term neurological consequences for offspring. Here, we review current literature evaluating the influence of maternal infection and immune activation at the materno-fetal interface and the subsequent impact on placental vascular function and birth outcome. Immunomodulatory pathways, including chemokines and cytokines released in response to maternal infection, interact closely with the principal pathways regulating placental vascular development, including the angiopoietin-Tie-2, vascular endothelial growth factor (VEGF), and placental growth factor (PlGF) pathways. A detailed mechanistic understanding of how maternal infections impact placental and fetal development is critical to the design of effective interventions to promote placental growth and function and thereby reduce adverse birth outcomes.

Emerging concepts of shear stress in placental development and function

Blood flow, and the force it generates, is critical to placental development and function throughout pregnancy. This mechanical stimulation of cells by the friction generated from flow is called shear stress (SS) and is a fundamental determinant of vascular homeostasis, regulating remodelling and vasomotor tone. This review describes how SS is fundamental to the establishment and regulation of the blood flow through the uteroplacental and fetoplacental circulations. Amongst the most recent findings is that alongside the endothelium, embryonic stem cells and the villous trophoblast are mechanically sensitive. A complex balance of forces is required to enable effective establishment of the uteroplacental circulation, while protecting the embryo and placental villi. SS also generates flow-mediated vasodilatation through the release of endothelial nitric oxide, a process vital for adequate placental blood flow. The identification of SS sensors and the mechanisms governing how the force is converted into biochemical signals is a fast-paced area of research, with multiple cellular components under investigation. For example, the Piezo1 ion channel is mechanosensitive in a variety of tissues including the fetoplacental endothelium. Enhanced Piezo1 activity has been demonstrated in response to the Yoda1 agonist molecule, suggesting the possibility for developing tools to manipulate these channels. Whether such agents might progress to novel therapeutics to improve blood flow through the placenta requires further consideration and research.

Extracellular vesicles generated by placental tissues ex vivo: A transport system for immune mediators and growth factors

PROBLEM

To study the mechanisms of placenta function and the role of extracellular vesicles (EVs) in pregnancy, it is necessary to develop an ex vivo system that retains placental cytoarchitecture and the primary metabolic aspects, in particular the release of EVs and soluble factors. Here, we developed such a system and investigated the pattern of secretion of cytokines, growth factors, and extracellular vesicles by placental villous and amnion tissues ex vivo.

METHODS OF STUDY

Placental villous and amnion explants were cultured for 2 weeks at the air/liquid interface and their morphology and the released cytokines and EVs were analyzed. Cytokines were analyzed with multiplexed bead assays, and individual EVs were analyzed with recently developed techniques that involved EV capture with magnetic nanoparticles coupled to anti-EV antibodies and flow cytometry.

RESULTS

Ex vivo tissues (i) remained viable and preserved their cytoarchitecture; (ii) maintained secretion of cytokines and growth factors; (iii) released EVs of syncytiotrophoblast and amnion epithelial cell origins that contain cytokines and growth factors.

CONCLUSION

A system of ex vivo placental villous and amnion tissues can be used as an adequate model to study placenta metabolic activity in normal and complicated pregnancies, in particular to characterize EVs by their surface markers and by encapsulated proteins. Establishment and benchmarking the placenta ex vivo system may provide new insight in the functional status of this organ in various placental disorders, particularly regarding the release of EVs and cytokines. Such EVs may have a prognostic value for pregnancy complications.

Evaluation of placenta in foetal demise and foetal growth restriction

AIM

The study objective was to evaluate the pathological changes of the placenta in foetal death and foetal growth restriction and to find correlation of the findings with clinical causes.

SETTING AND DESIGN

Prospective study at a tertiary care hospital.

MATERIAL AND METHODS

Gross and histopathological examinations of the placentae were carried out in pregnancies with foetal demise (IUD) and Foetal Growth Restriction (FGR).

STATISTICAL ANALYSIS

SPSS, version 11.5.

RESULTS

Placentae of twenty seven women with foetal demise and of equal number of women with foetal growth restriction were studied. Placental weight was less than 10(th) percentile in 61.5% women in IUD group and in 93% women in the FGR group. Gross examination of placentae showed abnormalities in 12 (44%) women of IUD group and in 16 (59%) women of FGR group. Histopathological abnormalities were observed in 74.1% women of the IUD group and in 66.7% women of FGR group. Placental histopathology correlated with clinical risk factors in 60% women of IUD group and in 40% women of FGR group. Among the women with no clinically explainable cause for IUD and FGR, 86% and 57% had placental histopathological abnormalities respectively.

CONCLUSION

The histopathological abnormalities of the placenta can be used to document the clinical causes of foetal demise and growth restriction; it may explain the causes in cases of clinically unexplained foetal demise and foetal growth restriction.

Placental Pathology and Blood Pressure's Level in Women with Hypertensive Disorders in Pregnancy

Objective. The aim of this study was to investigate the extent of placental lesions associated with blood pressure (BP) levels in pregnancies complicated by hypertension. Methods. 55 singleton pregnancies complicated by mild hypertension were recruited and compared to 55 pregnancies complicated by severe hypertension. The histological assessment was carried out with regard to the following aspects: vessels number/field of vision, infarction, villous fibrinoid necrosis, villous hypermaturity, avascular villi, calcifications, lymphohistiocytic villitis, and thickened vessels. Statistical analysis was performed by SPSS. Results. All placental lesions were observed more often in the severe hypertension group. Vessels number was significantly decreased, and infarction and villous fibrinoid necrosis were significantly increased in the placentas of the severe hypertension group compared to the mild hypertension group (P < 0.001). Conclusion. This study supports that the extent of placental lesions in hypertensive pregnancies is correlated with hypertension level and so highlights blood pressure level as a mirror of placental function.

Late-onset preeclampsia is associated with an imbalance of angiogenic and anti-angiogenic factors in patients with and without placental lesions consistent with maternal underperfusion

OBJECTIVE

An imbalance between maternal angiogenic/anti-angiogenic factors concentrations has been observed in preeclampsia (PE) and other obstetrical syndromes. However, the frequency of pathologic findings in the placenta and the changes in maternal plasma angiogenic/anti-angiogenic factor concentrations differ between late- and early-onset PE. The aim of this study was to determine if the maternal plasma concentrations of placental growth factor (PlGF), soluble endoglin (sEng), and soluble vascular endothelial growth factor receptor-1 and 2 (sVEGFR-1 and sVEGFR-2) are different in late-onset PE with and without placental pathologic findings consistent with maternal underperfusion.

STUDY DESIGN

A cross-sectional study was conducted including 64 uncomplicated women and 66 women with late-onset PE (>34 weeks) who had blood samples and placenta available for pathologic examination. Patients with late-onset PE were divided into those with and without placental histologic findings consistent with maternal underperfusion as proposed by the Society for Pediatric Pathology. Maternal plasma concentrations of PlGF, sEng, sVEGFR-1 and sVEGRF-2 were determined by ELISA. Non-parametric statistics were used for analysis.

RESULTS

1) the prevalence of placental histological findings consistent with maternal underperfusion among women with late-onset PE was higher than that of those with an uncomplicated pregnancy (47% (31/66) vs. 7.8% (5/64), respectively; p < 0.01); 2) patients with late-onset PE and histological findings consistent with maternal underperfusion had a significantly lower median plasma concentration of PlGF, plasma PlGF/sVEGFR-1 ratio and plasma PlGF/sEng ratio than those with late-onset PE without placental underperfusion lesions (each p < 0.05); 3) the most common pathological findings in the placenta of patient with PE were lesions consistent with villous changes (77%, 24/31); and 4) isolated vascular lesions in the placenta were found only in 2 cases (6.5%), and the rest had a combination of villous and vascular lesions.

CONCLUSIONS

Nearly half of the patients with late-onset PE have placental lesions consistent with maternal underperfusion. These lesions are associated with an imbalance in the maternal concentration of angiogenic/anti-angiogenic factors. We propose that there is a link between maternal underperfusion and an anti-angiogenic state characterized by the changes in the concentrations of angiogenic and anti-angiogenic factors in women with late onset PE.

Compromised chorionic villous vascularization in idiopathic second trimester fetal loss

BACKGROUND

For normal fetal growth and development a well-developed chorionic villous vascularization is essential.

AIM

The aim of this study is to investigate whether idiopathic second trimester fetal loss is associated with an underdeveloped chorionic villous vascularization.

METHODS

38 placentas after late miscarriage, classified as idiopathic fetal loss (IFL, n=16) or as fetal loss due to intrauterine infection (IUI, n=22) were collected. After CD34 immunohistochemical staining the villous stromal area, number of villous vessels, vascular area and vascular area density (central, peripheral and total) were measured in randomly selected immature intermediate villi.

RESULTS

The mean gestational age was 19+4 weeks for the IFL group and 20+6 weeks for the IUI group. After controlling for gestational age, we found no differences in fetal weight, placental weight, villous stromal area, number of vessels and central vascular features. The mean peripheral vascular area and peripheral vascular area density were, after adjusting for gestational age, reduced in the IFL group.

CONCLUSION

Idiopathic second trimester fetal loss is associated with a reduced peripheral chorionic villous vascularization. We hypothesize that in these cases, placentation is already disturbed in first trimester of pregnancy, leading to a reduced materno-fetal interface in second trimester, thus to early postplacental fetal hypoxia and fetal death.

Evaluation of placental maturity by the sonographic textures

PURPOSE

Routine ultrasound screening to predict gestational age is important for risk assessment of pregnancy complications among pregnant women. We explored a quantitative method for sonographic analysis of placental texture, with the objective of reproducible measurement.

METHODS

We studied 151 pregnant women; the gestational ages of their fetuses ranged from 10 to 38 weeks. Three experienced sonographers delineated the placental contour to define the region of interest (ROI). From these sonograms, 72 texture features were derived from the spatial gray-level dependence matrices and gray-level difference matrices. We used these as input variables in a multiple linear regression analysis.

RESULTS

A significant positive correlation (P < 0.01) was found between the multiple linear regression results and the corresponding gestation ages by the three assessors (r (A) = 0.755, r (B) = 0.851, and r (C) = 0.832). We also found good agreement between multiple linear regression results for the three observers. Their κ statistic values were 0.685 between assessors A and B, 0.679 between A and C, and 0.804 between B and C.

CONCLUSION

Quantitative sonography using texture analysis of the placenta was useful in practice to determine gestational age.

Vascularization in first-trimester chorionic villi in complicated and uncomplicated pregnancies

OBJECTIVE

The purpose of this study was to investigate possible altered chorionic vascularization patterns that are seen already in the first trimester of pregnancies that are complicated by hypertensive disorders or intrauterine growth restriction (IUGR) in the third trimester of pregnancy.

STUDY DESIGN

After chorionic villous sampling, surplus of villi were stored, and a selection was made of pregnancies that were complicated further by hypertensive disorders (n = 26), normotensive IUGR (n = 13), and matched control subjects (n = 60). Vascular parameters of these villi were analyzed with a video-image-analysis system.

RESULTS

In pregnancies that are complicated by early-onset hypertensive disorders and IUGR, the mean distance of the peripheral vessels to the intervillous space and the total of the distances (central and peripheral) are significantly smaller, compared with control subjects (9.3% and 13.8% for hypertensive disorders and 12.2% and 16.1% for IUGR, respectively).

CONCLUSION

Differences in vascularization patterns in the placenta already in the first trimester of pregnancies that are complicated later by hypertensive disorders or IUGR confirm the hypothesis of early changes by means of more vessels and more peripheral vessels that are located in chorionic villi.

Gross placental structure in a low-risk population of singleton, term, first-born infants

Suboptimal fetal growth has been associated with an increased risk of adult disease, which may be exacerbated by an increased placental weight-to-fetal weight ratio. Placental weight is a summary measure of placental growth and development throughout pregnancy. However, measures of placental structure, including the chorionic disk surface area and thickness and eccentricity of the umbilical cord insertion, have been shown to account for additional variance in birth weight beyond that explained by placental weight. Little is known of the variability of these placental parameters in low-risk populations; their association with maternal, pregnancy, and neonatal characteristics; and the agreement between manual and digital measures. This study used manual and digital image analysis techniques to examine gross placental anatomy in 513 low-risk, singleton, term, first-born infants. Parametric methods compared groups and examined relationships among variables. Maternal birth weight, prepregnancy weight, and body mass index were associated with increased placental and birth weight (all P < 0.005), but only maternal birth weight was associated with increased placental surface area (P < 0.0005) and thickness (P = 0.005). Smoking during pregnancy reduced birth weight and increased the eccentricity of umbilical cord insertion (P = 0.012 and 0.034, respectively). The variability in these placental parameters was consistently lower than that reported in the literature, and correlations between digital and manual measurements were reasonable (r = .87-.71). Detailed analyses of gross placental structure can provide biologically relevant information regarding placental growth and development and, potentially, their consequences.

Hypoxia in human trophoblasts stimulates the expression and secretion of connective tissue growth factor

The mechanisms underlying cellular injury when human placental trophoblasts are exposed to hypoxia are unclear. Connective tissue growth factor (CTGF) mediates cell injury and fibrosis in diverse tissues. We hypothesized that hypoxia enhances the production of CTGF in primary term human trophoblasts. Using cultured term primary human trophoblasts as well as villous biopsies from term human placentas, we showed that CTGF protein is expressed in trophoblasts. When compared with cells cultured in standard conditions (FiO2 = 20%), exposure of primary human trophoblasts to low oxygen concentration (FiO2 = 8% or <or= 1%) enhanced the expression of CTGF mRNA in a time-dependent manner, with a significant increase in CTGF levels after 16 h (2.7 +/- 0.7-fold; P < 0.01), reaching a maximum of 10.9 +/- 3.2-fold at 72 h. Whereas exposure to hypoxia had no effect on cellular CTGF protein levels, secretion of CTGF to the medium was increased after 16 h in hypoxia and remained elevated through 72 h. The increase in cellular CTGF transcript levels and CTGF protein secretion was recapitulated by exposure of trophoblasts to agents that enhance the activity of hypoxia-inducible factor (HIF)1alpha, including cobalt chloride or the proline hydroxylase inhibitor dimethyloxaloylglycine, and attenuated using the HIF1alpha inhibitor 2-methoxyestradiol. Although all TGFbeta isoforms stimulated the expression of CTGF in trophoblasts, only the expression of TGFbeta1 mRNA was enhanced by hypoxia. We conclude that hypoxia increases cellular CTGF mRNA levels and CTGF protein secretion from cultured trophoblasts, likely in a HIF1alpha-dependent manner.

Reduction of Maternal Adrenal Steroids Results in Increased VEGF Protein Without Increased eNOS in the Ovine Placenta

Fetal sheep studies have shown that reduced maternal cortisol or aldosterone levels alter placental morphology, with a reduction in placental blood flow. We have now tested the hypothesis that changes in placental morphology with relative adrenal hypoadrenalism are associated with changes in vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS). Four groups of late gestation pregnant ewes with singleton fetuses were studied; controls (intact adrenals), normal cortisol and aldosterone (ewes adrenalectomized and replaced with normal cortisol and aldosterone levels), low cortisol (ewes adrenalectomized and replaced with low cortisol levels), and low aldosterone (ewes adrenalectomized and replaced with low aldosterone levels). The placenta was categorized into A, B, C or D type placentomes. There were significantly more B and C type placentomes in the adrenalectomized groups than in controls. Overall, B types had more VEGF mRNA than A types. VEGF protein levels corresponding to a 23 kDa band were highest in low aldosterone animals in A and C type placentomes. VEGF protein levels corresponding to a 47 kDa band were higher in C type placentomes than A types; protein levels were also higher overall in low cortisol animals compared to controls. Fetoplacental eNOS protein levels were lower in the adrenalectomized groups than in controls. In conclusion, our results indicate that increases in cotyledonary VEGF(164) protein were associated with fetal tissue overgrowth in the placenta when the pregnancy-induced increase in adrenal steroids was prevented in the ewe. However, cotyledonary eNOS protein was suppressed with reduced maternal adrenal steroids, which is consistent with the reduced placental perfusion previously observed in this model.

Angiogenic growth factors in maternal and fetal serum in pregnancies complicated by intrauterine growth restriction

The present study was performed to compare serum concentrations of maternal and fetal angiogenic growth factors in IUGR (intrauterine growth restriction) and normal pregnancy at the time of delivery. VEGF (vascular endothelial growth factor), PlGF (placental growth factor), sFlt-1 (soluble fms-like tyrosine kinase 1), sKDR (soluble kinase domain receptor) and bFGF (basic fibroblast growth factor) were measured by ELISA in serum from a maternal peripheral vein, the umbilical vein and the umbilical arteries in 15 women with pregnancies complicated by IUGR and 16 controls (women with normal pregnancies). In IUGR, sFlt-1 was increased, and PlGF and sKDR were decreased, in both maternal serum and serum from the umbilical vein. Additionally, bFGF was increased in serum from the umbilical vein of women with pregnancies complicated by IUGR. No significant differences in growth factor concentrations between the groups were found in serum from the umbilical artery. In both groups, levels of VEGF were higher and levels of sFlt-1 were lower in serum from the umbilical vein and umbilical artery compared with maternal serum. PlGF levels were found to be lower in serum from the umbilical vein compared with maternal serum in both groups, whereas PlGF levels in serum from the umbilical artery were significantly lower only in the control group. These findings suggest an imbalance of angiogenic and anti-angiogenic factors in IUGR, with formation of an anti-angiogenic state in maternal and, to a lesser extent, umbilical vein blood. The placenta appears to play a central role in the release of sFlt-1 into maternal and umbilical blood. Umbilical artery blood was unaffected in IUGR, indicating that the fetus does not contribute to changes in angiogenic growth factor concentrations.

Developmental biology of the placenta and the origins of placental insufficiency

Defects in all the trophoblast-differentiating pathways--endovascular, interstitial and chorionic villous--play a role in the pathogenesis of early-onset intra-uterine growth restriction (IUGR). There are two types of extravillous trophoblast: endovascular trophoblast, that forms the definitive placenta by occlusion of the spiral arteriole at the implantation site, and interstitial extravillous trophoblast, responsible for the anatomical erosion of the distal spiral arteriole and the secretion of angiogenic and vasodilator signals to improve uterine blood flow. Defective endovascular erosion may render the basal plate inadequate to meet the demands of the fetus. Failed interstitial invasion of spiral arterioles could lead to failure of local angiogenic and systemic cardiovascular adaptation signals that could be the underlying basis for early-onset IUGR and pre-eclampsia. As debate persists regarding the relative importance of cord, stem and terminal villous pathology, the study of factors controlling trophoblast turnover from immature intermediate villi to conductance stem villi and gas-exchanging terminal villi, translation of our knowledge from mouse placental genetics into human placental development, and defining causes of thrombo-occlusive damage to the placenta would help our understanding of the pathophysiology of early-onset IUGR.

Development of the utero-placental circulation: the role of carbon monoxide and nitric oxide in trophoblast invasion and spiral artery transformation

It is now well known that in order to establish human hemochorial placentation and to provide a progressive increase in blood supply to the growing fetus, the uterine spiral arteries must undergo considerable alterations. This physiological modification is thought to be brought about by the interaction of invasive cytotrophoblast with the spiral artery vessel wall. Despite intensive research our understanding of the mechanisms that control human trophoblast invasion in normal, let alone abnormal pregnancy, are sill poorly understood. This is partly due to difficulties in obtaining "true" placental bed biopsies and most investigators have relied on in vitro models of trophoblast invasion. This article describes the morphological changes that occur within the placental bed throughout human pregnancy along with a review of the various studies which have attempted to sample the placental bed. Thereafter, follows a review of the evidence that invasive trophoblast can release the vasoactive agents nitric oxide and/or carbon monoxide which, in turn, could contribute to early physiological changes in spiral arteries prior to destruction of the smooth muscle within the vessel wall. Current evidence supports the idea that trophoblast-derived carbon monoxide may contribute to spiral artery modification. In contrast there is no evidence for a similar role by nitric oxide.

Heme oxygenase expression in human placenta and placental bed: reduced expression of placenta endothelial HO-2 in preeclampsia and fetal growth restriction

In this study we tested the hypothesis that expression of heme oxygenases HO-1 and HO-2, which are responsible for the production of carbon monoxide, are reduced in the placenta and placental bed of pregnancies complicated by preeclampsia (PE) and fetal growth restriction (FGR) compared with control third-trimester pregnancies. Placental protein expression was determined by Western blotting (n=10 in each group) and immunohistochemistry (controls n=18, PE n=19, FGR n=10). Extravillous trophoblast expression was determined by immunohistochemistry of placental bed biopsy samples (controls n=17, PE n=19, FGR n=10). Western blot analysis of placental homogenates showed no overall differences in HO-2 among groups. However, immunohistochemical analysis showed a reduction in HO-2 expression in endothelial cells in both abnormal groups (PE P<0.01; FGR P<0.0005 vs. control group) but no differences in villous trophoblast staining. HO-1 was undetectable by Western blotting in control and abnormal pregnancies and immunoreactivity was very low, suggesting that there is little HO-1 in the placenta. Within the placental bed, HO-2 but not HO-1 was detected on all populations of extravillous trophoblast, but expression of HO-2 or HO-1 did not change in PE or FGR. The reduced expression of HO-2 on endothelial cells in PE and FGR may be responsible for reduced placental blood flow in these conditions. The data do not show changes in HO in the placental bed in PE or FGR.

Development of the placental villous tree and its consequences for fetal growth

Co-ordinated development of the fetal villous tree of the placenta is necessary for continued fetal growth and well-being. Before fetal viability, blood vessel development within the developing immature intermediate villi (IIV) is characterized by branching angiogenesis, such that the placenta expands to produce 10-16 generations of stem villi. Once fetal viability is attained, a developmental switch occurs to form large numbers of gas-exchanging terminal villi (TV) by non-branching angiogenesis in mature intermediate villi (MIV). Several growth factors, including vascular endothelial growth factor (VEGF), placenta growth factor (PlGF), angiopoietins, and angiostatins are produced within the villi and act locally, via their receptors, to control angiogenesis. Their relative contributions to placental vascular development are not fully understood at the present time. Severe early-onset intrauterine growth restriction (IUGR) is characterized by absent/reversed end-diastolic flow velocity (ARED) in the umbilical arteries, leading to fetal hypoxia, acidosis and a substantial rise in perinatal mortality and morbidity. The placentas from such cases show a deficit in peripheral villous development, which may be perpetuated by the effects of oxygen (delivered by maternal blood into the intervillous space) upon VEGF-directed angiogenesis, the so-called 'placental hyperoxia' theory of villous maldevelopment. Trophoblast apoptosis is a significant feature of early-onset IUGR and may explain poor flow-independent transfer of nutrients to the fetus. Finally, since transgenic mouse studies highlight the importance of trophoblast-derived transcription factors for placental villous (labyrinth) development, it is possible that the villous trophoblast controls the orderly development of the underlying mesoderm and blood vessels into the fetal villi.

Oxygen and placental vascular development

Human embryogenesis takes place in a hypoxic environment because the trophoblast shell excludes entry of maternal blood. The first fetal-placental villi develop as trophoblast sprouts. These are invaded by allantoic mesoderm to form secondary villi and are transformed, by vasculogenesis, into tertiary villi. The placental barrier to maternal blood is gradually breached between 8-12 weeks of gestation, due to invasion of placental-bed uteroplacental spiral arteries by the extravillous trophoblast (EVT). Placental oxygen tension thus rises and a phase of branching angiogenesis continues until 24 weeks. Thereafter a gradual shift takes place favoring non-branching angiogenesis. Gas-exchanging terminal villi thus form which are essential for rapid fetal growth and development of a high-flow, low-resistance fetal-placental circulation. Inadequate invasion of the uteroplacental spiral arteries by EVT results in placental ischemia and the development of obstetrical complications--preeclampsia and/or intrauterine growth restriction (IUGR). Placental villi often show evidence of continued branching angiogenesis, as is the case with anemic pregnancy, and pregnancy at high altitude. These structural alterations may reflect continued hypoxia-driven activity of vascular endothelial growth factor (VEGF). By contrast, a minority of severe early-onset IUGR pregnancies exhibit reduced fetal-placental blood flow with elongated maldeveloped villous capillaries. Placenta-like growth factor (PIGF) expression is increased while trophoblast proliferation is reduced, suggesting "hyperoxia" in the placental villous tree. IUGR may thus have two phenotypes--a more common hypoxic and a rarer hyperoxic type. While this concept is gaining acceptance, we have no insight as to the initiating mechanism(s).

Hemeoxygenase expression in human placenta and placental bed implies a role in regulation of trophoblast invasion and placental function

The purpose of this study was to examine the expression of hemeoxygenases HO-1 and HO-2, which are responsible for the production of carbon monoxide (CO), in the human placenta and placental bed and to determine the role of inhibitors of HO on placental perfusion pressure. We hypothesized that HO is expressed within the placenta and that invading cytotrophoblast cells (CTB) express HO isoforms. The expression of HO-1 and HO-2 was studied on placenta and placental bed biopsies, obtained using a transcervical sampling technique, from normal human pregnancies between 8 and 19 wk gestation and at term. In the placenta, HO-2 immunostaining was prominent in syncytiotrophoblast in the first trimester and reduced toward term (P<0.0005). HO-2 endothelial immunostaining was weak in the first trimester, but increased by term (P<0.0005). Within the placental bed, HO-2 was expressed by CTB in cell columns, the cytotrophoblast shell, and cell islands. Both intravascular CTB and interstitial CTB expressed HO-2. HO-1 immunostaining was low in the placenta but intense on the CTB within the placental bed. A striking feature was the absence of HO-1 from the proximal layers of cell columns, with strong expression on the more distal CTB layers of the cell columns. In placental perfusion studies, a significant dose-dependent increase in perfusion pressure was observed in the presence of zinc protoporphyrin, an inhibitor of HO. These results suggest a role for CO in placental function, trophoblast invasion, and spiral artery transformation. Hemeoxygenase expression in human placenta and placental bed implies a role in regulation of trophoblast invasion and placental function.