Site-specific increases in utero- and fetoplacental arterial vascular resistance in eNOS-deficient mice due to impaired arterial enlargement

Cold Exposures in Relation to Dysmenorrhea among Asian and White Women

Dysmenorrhea is highly prevalent, ranging from 16% to 91% among women, and it can lead to multiple reproductive disorders. However, risk factors associated with dysmenorrhea remain unexamined. Cold exposures can significantly disturb blood circulation and prostaglandin production in the uterus, leading to dysmenorrhea. This study investigated the relationship between cold exposures and dysmenorrhea, as well as potential disparities between Asians and Whites and the potential cultural influences on these associations. This was a cross-sectional survey among 197 Asian and 222 non-Asian women recruited from the U.S., with more than 40% from California. We assessed cold exposures, such as the frequency of consumption of cold water/drinks and ice cream, as well as room temperatures at home and public places, for both summer and winter over the past 12 months. The type of cold exposure associated with dysmenorrhea differs between Asian and White women. We found that among Asian women, a higher frequency of ice cream consumption in winter (beta = 1.19, p = 0.0002 when comparing high to low categories) was associated with dysmenorrhea; however, among White women, increased consumption of cold water/drinks in winter (beta = 0.49, p = 0.04 when comparing high to low categories) was also associated with dysmenorrhea. Higher home room temperatures in winter were associated with reduced severity of dysmenorrhea among White women but not among Asian women. All these associations supported our hypothesis and were stronger among women who lived in states with colder winters. However, there are a few exceptions. For instance, women who drank cold water/drinks less frequently during their menstrual period were more likely to experience more severe dysmenorrhea. In conclusion, this study provides crucial evidence to support the link between cold exposures and dysmenorrhea among Asians and Whites. The associations contradictory to our hypothesis are likely due to reserved causation influenced by Asian cultural practice. This paper sheds light on an understudied area that profoundly affects women's quality of life.

Predicting pregnancy specific uterine vascular reactivity: A data driven computational model of shear-dependent, myogenic, and mechanical radial artery features

The entire maternal circulation adapts to pregnancy, and this adaption is particularly extensive in the uterine circulation where the major vessels double in size to facilitate an approximately 15-fold increase in blood supply to this organ over the course of pregnancy. Several factors may play a role in both the remodelling and biomechanical function of the uterine vasculature including the paracrine microenvironment, passive properties of the vessel wall, and active components of vascular function (incorporating the myogenic response and response to shear stress induced by intravascular blood flow). However, the interplay between these factors, and how this plays out in an organ-specific manner to induce the extent of remodelling observed in the uterus is not well understood. Here we present an integrated assessment of the uterine radial arteries, likely rate-limiters to flow of oxygenated maternal blood to the placental surface, via computational modelling and pressure myography. We show that uterine radial arteries behave differently to other systemic vessels (higher compliance and shear mediated constriction) and that their properties change with the adaptation to pregnancy (higher myogenic tone, higher compliance, and ability to tolerate higher flow rates before constricting). Together, this provides a useful tool to improve our understanding of the role of uterine vascular adaptation in normal and abnormal pregnancies and highlights the need for vascular bed specific investigations of vascular function in health and disease.

Maternal Nanomaterial Inhalation Exposure: Critical Gestational Period in the Uterine Microcirculation is Angiotensin II Dependent

Maternal inhalation exposure to engineered nanomaterials (ENM) has been associated with microvascular dysfunction and adverse cardiovascular responses. Pregnancy requires coordinated vascular adaptation and growth that are imperative for survival. Key events in pregnancy hallmark distinct periods of gestation such as implantation, spiral artery remodeling, placentation, and trophoblast invasion. Angiotensin II (Ang II) is a critical vasoactive mediator responsible for adaptations and is implicated in the pathology of preeclampsia. If perturbations occur during gestation, such as those caused by ENM inhalation exposure, then maternal-fetal health consequences may occur. Our study aimed to identify the period of gestation in which maternal microvascular functional and fetal health are most vulnerable. Additionally, we wanted to determine if Ang II sensitivity and receptor density is altered due to exposure. Dams were exposed to ENM aerosols (nano-titanium dioxide) during three gestational windows: early (EE, gestational day (GD) 2-6), mid (ME, GD 8-12) or late (LE, GD 15-19). Within the EE group dry pup mass decreased by 16.3% and uterine radial artery wall to lumen ratio (WLR) increased by 25.9%. Uterine radial artery response to Ang II sensitivity increased by 40.5% in the EE group. Ang II receptor density was altered in the EE and LE group with decreased levels of AT2R. We conclude that early gestational maternal inhalation exposures resulted in altered vascular anatomy and physiology. Exposure during this time-period results in altered vascular reactivity and changes to uterine radial artery WLR, leading to decreased perfusion to the fetus and resulting in lower pup mass.

Three-dimensional visualisation of the feto-placental vasculature in humans and rodents

Adequate development of the feto-placental circulation is critical for placental exchange function and healthy fetal growth. Understanding the structure of this circulation and how it informs fetal outcomes is important both in the human placenta, and the rodent, a purported comparative experimental model. Vascular casting and micro-CT imaging approaches enable detailed quantification of the complex vascular relationships in the feto-circulation, and provide detailed data to parameterise in silico models. Here, to assist researchers to apply these technically challenging methods we provide detailed approaches to cast and image; 1) human placentas at the cotyledon-level, and 2) whole rodent placentas.

Uteroplacental Circulation in Normal Pregnancy and Preeclampsia: Functional Adaptation and Maladaptation

Uteroplacental blood flow increases as pregnancy advances. Adequate supply of nutrients and oxygen carried by uteroplacental blood flow is essential for the well-being of the mother and growth/development of the fetus. The uteroplacental hemodynamic change is accomplished primarily through uterine vascular adaptation, involving hormonal regulation of myogenic tone, vasoreactivity, release of vasoactive factors and others, in addition to the remodeling of spiral arteries. In preeclampsia, hormonal and angiogenic imbalance, proinflammatory cytokines and autoantibodies cause dysfunction of both endothelium and vascular smooth muscle cells of the uteroplacental vasculature. Consequently, the vascular dysfunction leads to increased vascular resistance and reduced blood flow in the uteroplacental circulation. In this article, the (mal)adaptation of uteroplacental vascular function in normal pregnancy and preeclampsia and underlying mechanisms are reviewed.

No evidence of the unfolded protein response in the placenta of two rodent models of preeclampsia and intrauterine growth restriction

In humans, intrauterine growth restriction (IUGR) and preeclampsia (PE) are associated with induction of the unfolded protein response (UPR) and increased placental endoplasmic reticulum (ER) stress. Especially in PE, oxidative stress occurs relative to the severity of maternal vascular underperfusion (MVU) of the placental bed. On the premise that understanding the mechanisms of placental dysfunction could lead to targeted therapeutic options for human IUGR and PE, we investigated the roles of the placental UPR and oxidative stress in two rodent models of these human gestational pathologies. We employed a rat IUGR model of gestational maternal protein restriction, as well as an endothelial nitric oxide synthase knockout mouse model (eNOS-/-) of PE/IUGR. Placental expression of UPR members was analyzed via qRT-PCR (Grp78, Calnexin, Perk, Chop, Atf6, and Ern1), immunohistochemistry, and Western blotting (Calnexin, ATF6, GRP78, CHOP, phospho-eIF2α, and phospho-IRE1). Oxidative stress was determined via Western blotting (3-nitrotyrosine and 4-hydroxy-2-nonenal). Both animal models showed a significant reduction of fetal and placental weight. These effects did not induce placental UPR. In contrast to human data, results from our rodent models suggest retention of placental plasticity in the setting of ER stress under an adverse gestational environment. Oxidative stress was significantly increased only in female IUGR rat placentas, suggesting a sexually dimorphic response to maternal malnutrition. Our study advances understanding of the involvement of the placental UPR in IUGR and PE. Moreover, it emphasizes the appropriate choice of animal models researching various aspects of these pregnancy complications.

Male fetal sex affects uteroplacental angiogenesis in growth restriction mouse model†

Abnormally increased angiotensin II activity related to maternal angiotensinogen (AGT) genetic variants, or aberrant receptor activation, is associated with small-for-gestational-age babies and abnormal uterine spiral artery remodeling in humans. Our group studies a murine AGT gene titration transgenic (TG; 3-copies of the AGT gene) model, which has a 20% increase in AGT expression mimicking a common human AGT genetic variant (A[-6]G) associated with intrauterine growth restriction (IUGR) and spiral artery pathology. We hypothesized that aberrant maternal AGT expression impacts pregnancy-induced uterine spiral artery angiogenesis in this mouse model leading to IUGR. We controlled for fetal sex and fetal genotype (e.g., only 2-copy wild-type [WT] progeny from WT and TG dams were included). Uteroplacental samples from WT and TG dams from early (days 6.5 and 8.5), mid (d12.5), and late (d16.5) gestation were studied to assess uterine natural killer (uNK) cell phenotypes, decidual metrial triangle angiogenic factors, placental growth and capillary density, placental transcriptomics, and placental nutrient transport. Spiral artery architecture was evaluated at day 16.5 by contrast-perfused three-dimensional microcomputed tomography (3D microCT). Our results suggest that uteroplacental angiogenesis is significantly reduced in TG dams at day 16.5. Males from TG dams are associated with significantly reduced uteroplacental angiogenesis from early to late gestation compared with their female littermates and WT controls. Angiogenesis was not different between fetal sexes from WT dams. We conclude that male fetal sex compounds the pathologic impact of maternal genotype in this mouse model of growth restriction.

Advances in imaging feto-placental vasculature: new tools to elucidate the early life origins of health and disease

Optimal placental function is critical for fetal development, and therefore a crucial consideration for understanding the developmental origins of health and disease (DOHaD). The structure of the fetal side of the placental vasculature is an important determinant of fetal growth and cardiovascular development. There are several imaging modalities for assessing feto-placental structure including stereology, electron microscopy, confocal microscopy, micro-computed tomography, light-sheet microscopy, ultrasonography and magnetic resonance imaging. In this review, we present current methodologies for imaging feto-placental vasculature morphology ex vivo and in vivo in human and experimental models, their advantages and limitations and how these provide insight into placental function and fetal outcomes. These imaging approaches add important perspective to our understanding of placental biology and have potential to be new tools to elucidate a deeper understanding of DOHaD.

Independent influences of maternal obesity and fetal sex on maternal cardiovascular adaptation to pregnancy: a prospective cohort study

Background/Objectives

Successful pregnancy requires the de novo creation of low-resistance utero-placental and feto-placental circulations and incomplete remodeling of this vasculature can lead to maternal or fetal compromise. Maternal BMI and fetal sex are known to influence vascular compliance and placental development, but it is unknown if these are independent or synergistic effects. Here we aim to investigate the impact of maternal obesity, fetal sex, and any interaction thereof on maternal cardiovascular adaptation to pregnancy, by assessing the physiological drop of uterine artery doppler pulsatility (UtA-PI) and umbilical artery doppler pulsatility index (UA-PI) over gestation.

Subjects/Methods

Nulliparous women with a singleton pregnancy participating in a prospective cohort study (n = 4212) underwent serial UtA-PI and UA-PI measurements at 20-, 28- and 36-weeks gestation. Linear mixed regression models were employed to investigate the influence of maternal BMI, fetal sex and interactions thereof on the magnitude of change in UtA-PI and UA-PI.

Results

Throughout gestation, UtA-PI was higher for male fetuses and UA-PI was higher for female fetuses. The physiological drop of UtA-PI was significantly smaller in overweight (change −24.3% [95%CI −22.3, −26.2]) and obese women (change −21.3% [−18.3, −24.3]), compared to normal-weight women (change −25.7% [−24.3, −27.0]) but did not differ by fetal sex. The physiological drop in UA-PI was greater for female than male fetuses (–32.5% [−31.5, −33.5] vs. −30.7% [−29.8, −31.7]) but did not differ by maternal BMI. No interactions between maternal BMI and fetal sex were found.

Conclusions

Maternal cardiovascular adaptation to pregnancy is independently associated with maternal BMI and fetal sex. Our results imply sexual dimorphism in both maternal cardiovascular adaptation and feto-placental resistance.

MicroRNAs in Uteroplacental Vascular Dysfunction

Pregnancy complications of preeclampsia and intrauterine growth restriction (IUGR) are major causes of maternal and perinatal/neonatal morbidity and mortality. Although their etiologies remain elusive, it is generally accepted that they are secondary to placental insufficiency conferred by both failure in spiral artery remodeling and uteroplacental vascular malfunction. MicroRNAs (miRNAs) are small no-coding RNA molecules that regulate gene expression at the post-transcriptional level. Increasing evidence suggests that miRNAs participate in virtually all biological processes and are involved in numerous human diseases. Differentially expressed miRNAs in the placenta are typical features of both preeclampsia and IUGR. Dysregulated miRNAs target genes of various signaling pathways in uteroplacental tissues, contributing to the development of both complications. In this review, we provide an overview of how aberrant miRNA expression in preeclampsia and IUGR impacts the expression of genes involved in trophoblast invasion and uteroplacental vascular adaptation.

Quantitative characterization of rodent feto-placental vasculature morphology in micro-computed tomography images

BACKGROUND AND OBJECTIVE

Optimal development of placental vasculature is critical for fetal growth and health outcomes. Many studies characterizing the vascular structure of the fetal side of the placenta have utilized a range of two-dimensional and three-dimensional (3D) imaging techniques including X-ray micro-computed tomography (micro-CT) following perfusion of the vasculature with a radio-opaque compound. The CT approach has been used to study feto-placental vasculature in rodents and humans. Its inherent advantage is that it reveals the 3D structure in high resolution without destroying the sample. This permits both multiple scanning of the sample and follow-up histological investigations in the same sample. Nevertheless, the applicability of the approach is hampered both by the challenging segmentation of the vasculature and a lack of straightforward methodology to quantitate the feto-placental vascular network. This paper addresses these challenges.

METHODS

An end-to-end methodology is presented for automatically segmenting the vasculature; obtaining a Strahler-ordered rooted-tree representation and extracting quantitative features from its nodes, segments and branches (including volume, length, tortuosity and branching angles). The methodology is demonstrated for rat and mouse placentas at the end of gestation (day 22 and day 18, respectively), perfused with Microfil® and imaged using two different micro-CT scanners.

RESULTS

The 3D visualizations of the resulting vascular trees clearly demonstrate differences between the branching complexity, tree span and tree depth of the mouse and rat placentas. The quantitative characterizations of these trees include not only the fundamental features that have been utilized in other studies of feto-placental vasculature but also several additional features. Boxplots of several of these-tortuosity, number of side branches, number of offspring per branch and branch volume-computed at each Strahler order are presented and interpreted. Differences and similarities between the mouse and rat casts are readily detected.

CONCLUSION

The proposed end-to-end methodology, and the implementation presented using a combination of Amira and Matlab, offers researchers in the field of placental vasculature characterization a straightforward and objective approach for quantifying micro-CT vascular datasets.

High-resolution contrast-enhanced microCT reveals the true three-dimensional morphology of the murine placenta

Genetic engineering of the mouse genome identified many genes that are essential for embryogenesis. Remarkably, the prevalence of concomitant placental defects in embryonic lethal mutants is highly underestimated and indicates the importance of detailed placental analysis when phenotyping new individual gene knockouts. Here we introduce high-resolution contrast-enhanced microfocus computed tomography (CE-CT) as a nondestructive, high-throughput technique to evaluate the 3D placental morphology. Using a contrast agent, zirconium-substituted Keggin polyoxometalate (Zr-POM), the soft tissue of the placenta (i.e., different layers and cell types and its vasculature) was imaged with a resolution of 3.5 µm voxel size. This approach allowed us to visualize and study early and late stages of placental development. Moreover, CE-CT provides a method to precisely quantify placental parameters (i.e., volumes, volume fraction, ratio of different placental layers, and volumes of specific cell populations) that are crucial for statistical comparison studies. The CE-CT assessment of the 3D morphology of the placentas was validated (i) by comparison with standard histological studies; (ii) by evaluating placentas from 2 different mouse strains, 129S6 and C57BL/6J mice; and (iii) by confirming the placental phenotype of mice lacking phosphoinositol 3-kinase (PI3K)-p110α. Finally, the Zr-POM-based CE-CT allowed for inspection of the vasculature structure in the entire placenta, as well as detecting placental defects in pathologies characterized by embryonic resorption and placental fusion. Taken together, Zr-POM-based CE-CT offers a quantitative 3D methodology to investigate placental development or pathologies.

Thyroid hormone signaling and consequences for cardiac development

The fetal heart undergoes its own growth and maturation stages all while supplying blood and nutrients to the growing fetus and its organs. Immature contractile cardiomyocytes proliferate to rapidly increase and establish cardiomyocyte endowment in the perinatal period. Maturational changes in cellular maturation, size and biochemical capabilities occur, and require, a changing hormonal environment as the fetus prepares itself for the transition to extrauterine life. Thyroid hormone has long been known to be important for neuronal development, but also for fetal size and survival. Fetal circulating 3,5,3'-triiodothyronine (T3) levels surge near term in mammals and are responsible for maturation of several organ systems, including the heart. Growth factors like insulin-like growth factor-1 stimulate proliferation of fetal cardiomyocytes, while thyroid hormone has been shown to inhibit proliferation and drive maturation of the cells. Several cell signaling pathways appear to be involved in this complicated and coordinated process. The aim of this review was to discuss the foundational studies of thyroid hormone physiology and the mechanisms responsible for its actions as we speculate on potential fetal programming effects for cardiovascular health.

Intermittent hypoxia impairs uterine artery function in pregnant mice

KEY POINTS

Obstructive sleep apnoea (OSA) is a chronic condition characterized by intermittent hypoxia that induces oxidative stress and inflammation leading to cardiovascular disease. Women can develop OSA during late pregnancy, which is associated with adverse maternal and fetal outcomes. However, the effects of OSA throughout pregnancy on fetoplacental outcomes are unknown. Using a mouse model of intermittent hypoxia, we evaluated main uterine artery function, spiral artery remodelling, circulating angiogenic and anti-angiogenic factors, and placental hypoxia and oxidative stress at gestational day 14.5 in pregnant mice. Gestational intermittent hypoxia increased placental weight but decreased fetal weight, impaired uterine artery function, increased circulating angiogenic and anti-angiogenic factors, and induced placental hypoxia and oxidative stress, but had no impact on spiral artery remodelling. Our results suggest that pregnant women experiencing OSA during pregnancy could be at risk of maternal and fetal complications.

ABSTRACT

Obstructive sleep apnoea (OSA) is characterized by chronic intermittent hypoxia (IH) and is associated with increased inflammation, oxidative stress and endothelial dysfunction. OSA is a common sleep disorder and remains under-diagnosed; it can increase the risk of adverse maternal and fetal outcomes in pregnant women. We investigated the effects of gestational IH (GIH) on uterine artery function, spiral artery remodelling and placental circulating angiogenic and anti-angiogenic factors in pregnant female mice. WT C57BL/6 mice (8 weeks) were exposed to either GIH ( F I O 2 12%) or intermittent air ( F I O 2 21%) for 14.5 days of gestation. Exposure to GIH reduced fetal weight but increased placental weight. GIH dams had higher plasma levels of oxidative stress (8-isoprostane) and inflammatory markers (tumour necrosis factor-α). GIH significantly reduced uterine artery function as indicated by reduced endothelium-dependent vasodilatation and enhanced vasoconstriction. Plasma levels of placental angiogenic and anti-angiogenic markers (soluble fms-like tyrosine kinase-1, soluble endoglin, angiogenic placental growth factor-2 and vascular endothelial growth factor) were higher in pregnant mice exposed to GIH. There was no evidence of impaired spiral artery remodelling based on immunostaining with α-smooth muscle actin and cytokeratin-7, and also by measurements of lumen area. Immunostaining for markers of hypoxia (pimonidazole) and oxidative stress (4-hydroxynonenal) were higher in mice exposed to GIH. Our data show that GIH adversely affects uterine vascular function and may be a mechanism by which gestational OSA leads to adverse maternal and fetal outcomes.

Generation of a nitric oxide signaling pathway in mesenchymal stem cells promotes endothelial lineage commitment

Enhancing differentiation of mesenchymal stem cells (MSCs) to endothelial cells may improve their ability to vascularize tissue and promote wound healing. This study describes a novel role for nitric oxide (NO) in reprogramming MSCs towards an endothelial lineage and highlights the role of Wnt signaling and epigenetic modification by NO. Rat MSCs were transduced with lentiviral vectors expressing endothelial nitric oxide synthase (pLV-eNOS) and a mutated caveolin gene (pLV-CAV-1^F92A ) to enhance NO generation resulting in increased in vitro capillary tubule formation and endothelial marker gene expression. An exogenous source of NO could also stimulate CD31 expression in MSCs. NO was associated with an arterial-specific endothelial gene expression profile of Notch1, Dll4, and Hey2 and significantly reduced expression of venous markers. Wnt signaling associated with NO was evident through increased gene expression of Wnt3a and β-catenin protein, and expression of the endothelial marker Pecam-1 could be significantly reduced by treatment with the Wnt signaling inhibitor Dkk-1. The role of NO as an epigenetic modifier was evident with reduced gene expression of the methyltransferase, DNMT1, and bisulfite sequencing of the endothelial Flt1 promoter region in NO-producing MSCs showed significant demethylation compared to control cells. Finally, subcutaneous implantation of NO-producing MSCs seeded in a biomaterial scaffold (NovoSorb®) resulted in survival of transplanted cells and the formation of blood vessels. In summary, this study describes, NO as a potent endothelial programming factor which acts as an epigenetic modifier in MSCs and may provide a novel platform for vascular regenerative therapy.

Placental vascular abnormalities in the mouse alter umbilical artery wave reflections

Current methods to detect placental vascular pathologies that monitor Doppler ultrasound changes in umbilical artery (UA) pulsatility have only moderate diagnostic utility, particularly in late gestation. In fetal mice, we recently demonstrated that reflected pressure waves propagate counter to the direction of flow in the UA and proposed the measurement of these reflections as a means to detect abnormalities in the placental circulation. In the present study, we used this approach in combination with microcomputed tomography to investigate the relationship between altered placental vascular architecture and changes in UA wave reflection metrics. Fetuses were assessed at embryonic day (E) 15.5 and E17.5 in control C57BL6/J mice and dams treated with combination antiretroviral therapy (cART), a known model of fetal growth restriction. Whereas the reflection coefficient was not different between groups at E15.5, it was 27% higher at E17.5 in cART-treated mice compared with control mice. This increase in reflection coefficient corresponded to a 36% increase in the total number of vessel segments, a measure of overall architectural complexity. Interestingly, there was no difference in UA pulsatility index between groups, suggesting that the wave reflections convey information about vascular architecture that is not captured by conventional ultrasound metrics. The wave reflection parameters were found to be associated with the morphology of the fetoplacental arterial tree, with the area ratio between the UA and first branch points correlating with the reflection coefficient. This study highlights the potential for wave reflection to aid in the noninvasive clinical assessment of placental vascular pathology. NEW & NOTEWORTHY We used a novel ultrasound methodology based on detecting pulse pressure waves that propagate along the umbilical artery to investigate the relationship between changes in wave reflection metrics and altered placental vascular architecture visualized by microcomputed tomography. Using pregnant mice treated with combination antiretroviral therapy, a model of fetal growth restriction, we demonstrated that reflections in the umbilical artery are sensitive to placental vascular abnormalities and associated with the geometry of the fetoplacental tree.

Deletion of fetoplacental Fshr inhibits fetal vessel angiogenesis in the mouse placenta

It has been shown in both human and mouse placentas that follicle stimulating hormone receptor (FSHR) is expressed in fetal vascular endothelium. There are conflicting reports, however, on the role of FSH to stimulate angiogenesis in vitro in cultured endothelial cells from umbilical veins. Therefore, in this study we undertook an in vivo approach utilizing Fshr null mice to definitively address this question. In the context where all pregnant dams have identical Fshr genotypes, we generated fetuses and associated fetal portions of placenta that were Fshr wt or Fshr null and analyzed angiogenesis within the placental labyrinths. Quantitative morphometric analyses of placentas obtained at mid-gestation revealed that the percentage of the placenta composed of labyrinth is significantly decreased in Fshr null placentas relative to wt placentas. Furthermore, data presented demonstrate that within the Fshr null labyrinths, fetal vessel angiogenesis was significantly reduced relative to wt labyrinths. The results obtained with this combination of in vivo and genetic approaches conclusively demonstrate that signaling through endothelial FSHR does indeed stimulate angiogenesis and that placental Fshr is essential for normal angiogenesis of the fetal placental vasculature.

Nitric oxide and the biology of pregnancy

Nitric oxide (NO) is a key regulator of both maternal and fetal homeostasis during pregnancy, facilitating the maternal cardio-vascular changes, fetal development and growth and adaptation to extrauterine life. Dysfunction of the NO system during pregnancy is associated to placental and vascular-related diseases such as hypertensive disorders of pregnancy (HDP) and intrauterine growth restriction (IUGR). Emerging therapeutic strategies involving NO precursors, NO donors, natural derivatives or pharmacological modulators of the NO system seem hold promise for the treatment of such conditions of pregnancy.

Hypoxia and Placental Development

Hemochorial placentation is orchestrated through highly regulated temporal and spatial decisions governing the fate of trophoblast stem/progenitor cells. Trophoblast cell acquisition of specializations facilitating invasion and uterine spiral artery remodeling is a labile process, sensitive to the environment, and represents a process that is vulnerable to dysmorphogenesis in pathologic states. Hypoxia is a signal guiding placental development, and molecular mechanisms directing cellular adaptations to low oxygen tension are integral to trophoblast cell differentiation and placentation. Hypoxia can also be used as an experimental tool to investigate regulatory processes controlling hemochorial placentation. These developmental processes are conserved in mouse, rat, and human placentation. Consequently, elements of these developmental events can be modeled and hypotheses tested in trophoblast stem cells and in genetically manipulated rodents. Hypoxia is also a consequence of a failed placenta, yielding pathologies that can adversely affect maternal adjustments to pregnancy, fetal health, and susceptibility to adult disease. The capacity of the placenta for adaptation to environmental challenges highlights the importance of its plasticity in safeguarding a healthy pregnancy. Birth Defects Research 109:1309-1329, 2017.© 2017 Wiley Periodicals, Inc.

Activated NO pathway in uterine arteries during pregnancy in an IUGR rat model

Insufficient development of the uteroplacental circulation may contribute to the development of intrauterine growth restriction (IUGR). We developed a rat model of IUGR by administering a low-Na⁺ diet. This diet reduces maternal blood volume expansion and uteroplacental perfusion. We hypothesized that an impaired endothelial function in radial arteries decreases vasorelaxation and lowers placental perfusion in this IUGR model. The objective was to assess radial uterine artery responses to vasoactive agents in the IUGR model versus controls. The vasoactive agents included phenylephrine and carbachol, use of a pressurized artery myograph, in the absence or presence of inhibitors of nitric oxide (NO) synthase [ N-nitro-l-arginine methyl ester (l-NAME)], cyclooxygenase (Ibuprofen), and endothelium-dependent hyperpolarization {apamin/1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole}, allowing better characterization of the mechanism implicated in endothelium-dependent relaxation. The results show that 1) the diameter of uterine radial arteries was significantly decreased in the IUGR group; 2) sensitivity to phenylephrine was reduced in IUGR arteries, which could be returned to control group values by inhibition of NO production; 3) the relaxation response to carbachol was increased in IUGR rats, principally mediated by endothelium-dependent hyperpolarization in both groups; 4) NO synthase inhibition by l-NAME decreased the maximum relaxation to carbachol only in the IUGR group; and 5) relaxation response to NO donors is increased in IUGR compared with control radial arteries. Contrary to the hypothesis, results in the IUGR model indicate that the NO pathway is activated in radial uterine arteries, most likely in compensation for the reduction in blood uteroplacental perfusion. NEW & NOTEWORTHY In contrast to genetic or surgical models of intrauterine growth restriction, the diet-induced model of reduced maternal volume expansion shows the nitric oxide pathway to be activated in the uterine artery, possibly from increased shear stress and/or placental factors.

HIV antiretroviral exposure in pregnancy induces detrimental placenta vascular changes that are rescued by progesterone supplementation

Adverse birth outcomes are common in HIV-positive pregnant women receiving combination antiretroviral therapy (cART), especially when cART is initiated in early pregnancy. The mechanisms remain poorly understood. Using a mouse model we demonstrate that protease inhibitor based-cART exposure beginning on day 1 of pregnancy was associated with a pro-angiogenic/pro-branching shift in the placenta driven by lower Flt-1 levels and higher Gcm-1 expression. Micro-CT imaging revealed an increase in the number of arterioles in cART-treated placentas, which correlated with fetal growth restriction. Delaying initiation of cART, or supplementing cART-treated mice with progesterone, prevented the pro-angiogenic/pro-branching shift and the associated placenta vascular changes. In agreement with our mouse findings, we observed an increase in the number of terminal-villi capillaries in placentas from HIV-positive cART-exposed women compared to HIV-negative controls. Capillary number was inversely correlated to maternal progesterone levels. Our study provides evidence that cART exposure during pregnancy influences placenta vascular formation that may in turn contribute to fetal growth restriction. Our findings highlight the need for closer investigation of the placenta in HIV-positive pregnancies, particularly for pregnancies exposed to cART from conception, and suggest that progesterone supplementation could be investigated as a possible intervention to improve placenta function in HIV-positive pregnant women.

Analytical model of the feto-placental vascular system: consideration of placental oxygen transport

The placenta is a transient vascular organ that enables nutrients and blood gases to be exchanged between fetal and maternal circulations. Herein, the structure and oxygen diffusion across the trophoblast membrane between the fetal and maternal red blood cells in the feto-placental vasculature system in both human and mouse placentas are presented together as a functional unit. Previous models have claimed that the most efficient fetal blood flow relies upon structures containing a number of 'conductive' symmetrical branches, offering a path of minimal resistance that maximizes blood flow to the terminal villi, where oxygen diffusion occurs. However, most of these models have disregarded the actual descriptions of the exchange at the level of the intermediate and terminal villi. We are proposing a 'mixed model' whereby both 'conductive' and 'terminal' villi are presumed to be present at the end of single (in human) or multiple (in mouse) pregnancies. We predict an optimal number of 18 and 22 bifurcation levels in the human and the mouse placentas, respectively. Wherever possible, we have compared our model's predictions with experimental results reported in the literature and found close agreement between them.

Feto- and utero-placental vascular adaptations to chronic maternal hypoxia in the mouse

KEY POINTS

Chronic fetal hypoxia is one of the most common complications of pregnancy and is known to cause fetal growth restriction. The structural adaptations of the placental vasculature responsible for growth restriction with chronic hypoxia are not well elucidated. Using a mouse model of chronic maternal hypoxia in combination with micro-computed tomography and scanning electron microscopy, we found several placental adaptations that were beneficial to fetal growth including capillary expansion, thinning of the interhaemal membrane and increased radial artery diameters, resulting in a large drop in total utero-placental vascular resistance. One of the mechanisms used to achieve the rapid increase in capillaries was intussusceptive angiogenesis, a strategy used in human placental development to form terminal gas-exchanging villi. These results contribute to our understanding of the structural mechanisms of the placental vasculature responsible for fetal growth restriction and provide a baseline for understanding adaptive physiological responses of the placenta to chronic hypoxia.

ABSTRACT

The fetus and the placenta in eutherian mammals have a unique set of compensatory mechanisms to respond to several pregnancy complications including chronic maternal hypoxia. This study examined the structural adaptations of the feto- and utero-placental vasculature in an experimental mouse model of chronic maternal hypoxia (11% O₂ from embryonic day (E) 14.5-E17.5). While placental weights were unaffected by exposure to chronic hypoxia, using micro-computed tomography, we found a 44% decrease in the absolute feto-placental arterial vascular volume and a 30% decrease in total vessel segments in the chronic hypoxia group compared to control group. Scanning electron microscopy imaging showed significant expansion of the capillary network; consequently, the interhaemal membrane was 11% thinner to facilitate maternal-fetal exchange in the chronic hypoxia placentas. One of the mechanisms for the rapid capillary expansion was intussusceptive angiogenesis. Analysis of the utero-placental arterial tree showed significant increases (24%) in the diameter of the radial arteries, resulting in a decrease in the total utero-placental resistance by 2.6-fold in the mice exposed to chronic maternal hypoxia. Together these adaptations acted to preserve placental weight whereas fetal weight was decreased.

Arterio-venous fetoplacental vascular geometry and hemodynamics in the mouse placenta

INTRODUCTION

The fetoplacental vasculature network is essential for the exchange of nutrients, gases and wastes with the maternal circulation and for normal fetal development. The present study quantitatively compares arterial and venous morphological and functional differences in the mouse fetoplacental vascular network.

METHODS

High resolution X-ray micro-computed tomography was used to visualize the 3D geometry of the arterial and venous fetoplacental vasculature in embryonic day 15.5 CD-1 mice (n = 5). Automated image analysis was used to measure the vascular geometry of the approximately 4100 arterial segments and 3200 venous segments per specimen to simulate blood flow through these networks.

RESULTS

Both the arterial and venous trees demonstrated a hierarchical branching structure with 8 or 9 (arterial) or 8 (venous) orders. The venous tree was smaller in volume and overall dimensions than the arterial tree. Venous vessel diameters increased more rapidly than arteries with each successive order, leading to lower overall resistance, although the umbilical vein was notably smaller and of higher resistance than these scaling relationships would predict. Simulation of blood flow for these vascular networks showed that 57% of total resistance resides in the umbilical artery and arterial tree, 17% in the capillary bed, and 26% in the venous tree and umbilical vein.

DISCUSSION

A detailed examination of the mouse fetoplacental arterial and venous tree revealed features, such as the distribution of resistance and the dimension of the venous tree, that were both morphologically distinct from other vascular beds and that appeared adapted to the specialized requirements of sustaining a fetus.

Feeding Your Baby In Utero: How the Uteroplacental Circulation Impacts Pregnancy

The utero-placental circulation links the maternal and fetal circulations during pregnancy, ensuring adequate gas and nutrient exchange, and consequently fetal growth. However, our understanding of this circulatory system remains incomplete. Here, we discuss how the utero-placental circulation is established, how it changes dynamically during pregnancy, and how this may impact on pregnancy success, highlighting how we may address knowledge gaps through advances in imaging and computational modeling approaches.

Inflammation-induced fetal growth restriction in rats is associated with increased placental HIF-1α accumulation

Introduction

Hypoxia-inducible factor 1-alpha (HIF-1α) is the oxygen-sensitive subunit of the transcription factor HIF-1, and its expression is increased in placentas from pregnancies complicated by pre-eclampsia (PE). Fetal growth restriction (FGR) and PE often share a common pathophysiology; however, it is unknown whether increased placental HIF-1α occurs in FGR. We previously demonstrated that aberrant maternal inflammation in rats resulted in altered utero-placental perfusion and FGR, both of which were prevented by administration of the nitric oxide mimetic glyceryl trinitrate (GTN). Our aim here was to determine whether abnormal maternal inflammation causing FGR is linked to placental HIF-1α accumulation and whether GTN administration could prevent increases in placental HIF-1α.

Methods

Levels of inflammatory factors in maternal plasma were measured using a multiplex assay after an injection of low-dose lipopolysaccharide (LPS) to rats on gestational day (GD) 13.5. Following three additional daily LPS injections from GD14.5–16.5, GD17.5 placentas were harvested for HIF-1α immunolocalisation; serial sections were also stained for the hypoxia marker pimonidazole. A subset of rats received LPS injections along with GTN delivered continuously (25 μg/h via a transdermal patch) on GD12.5-GD17.5.

Results

Within two hours of LPS administration, levels of maternal pro-inflammatory cytokines were increased compared with saline-treated controls. GD17.5 placentas of growth-restricted fetuses exhibited increased HIF-1α accumulation; however, this did not correlate with pimonidazole staining for which no differences were observed between groups. Furthermore, the LPS-mediated increases in maternal inflammatory cytokine levels and placental HIF-1α accumulation did not occur in rats treated with GTN.

Discussion

Our results demonstrate that inflammation-induced FGR is associated with increased placental HIF-1α accumulation; however, expression of this transcription factor may not correlate with regions of hypoxia in late-gestation placentas. The GTN-mediated attenuation of placental HIF-1α accumulation in LPS-treated rats provides insight into the mechanism by which GTN improves inflammation-induced complications of pregnancy.

Viscosity and haemodynamics in a late gestation rat feto-placental arterial network

The placenta is a transient organ which develops during pregnancy to provide haemotrophic support for healthy fetal growth and development. Fundamental to its function is the healthy development of vascular trees in the feto-placental arterial network. Despite the strong association of haemodynamics with vascular remodelling mechanisms, there is a lack of computational haemodynamic data that may improve our understanding of feto-placental physiology. The aim of this work was to create a comprehensive 3D computational fluid dynamics model of a substructure of the rat feto-placental arterial network and investigate the influence of viscosity on wall shear stress (WSS). Late gestation rat feto-placental arteries were perfused with radiopaque Microfil and scanned via micro-computed tomography to capture the feto-placental arterial geometry in 3D. A detailed description of rat fetal blood viscosity parameters was developed, and three different approaches to feto-placental haemodynamics were simulated in 3D using the finite volume method: Newtonian model, non-Newtonian Carreau-Yasuda model and Fåhræus-Lindqvist effect model. Significant variability in WSS was observed between different viscosity models. The physiologically-realistic simulations using the Fåhræus-Lindqvist effect and rat fetal blood estimates of viscosity revealed detailed patterns of WSS throughout the arterial network. We found WSS gradients at bifurcation regions, which may contribute to vessel enlargement, and sprouting and pruning during angiogenesis. This simulation of feto-placental haemodynamics shows the heterogeneous WSS distribution throughout the network and demonstrates the ability to determine physiologically-relevant WSS magnitudes, patterns and gradients. This model will help advance our understanding of vascular physiology and remodelling in the feto-placental network.

Ehmt2/G9a controls placental vascular maturation by activating the Notch pathway

Defective fetoplacental vascular maturation causes intrauterine growth restriction (IUGR). A transcriptional switch initiates placental maturation where blood vessels elongate. However, cellular mechanisms and regulatory pathways involved are unknown. We show that the histone methyltransferase Ehmt2, also known as G9a, activates the Notch pathway to promote placental vascular maturation. Placental vasculature from embryos with G9a-deficient endothelial progenitor cells failed to expand due to decreased endothelial cell proliferation and increased trophoblast proliferation. Moreover, G9a deficiency altered the transcriptional switch initiating placental maturation and caused downregulation of Notch pathway effectors including Rbpj. Importantly, Notch pathway activation in G9a-deficient endothelial progenitors extended embryonic life and rescued placental vascular expansion. Thus, G9a activates the Notch pathway to balance endothelial cell and trophoblast proliferation and coordinates the transcriptional switch controlling placental vascular maturation. Accordingly, G9A and RBPJ were downregulated in human placentae from IUGR-affected pregnancies, suggesting that G9a is an important regulator in placental diseases caused by defective vascular maturation.

Quantification of Gestational Changes in the Uteroplacental Vascular Tree Reveals Vessel Specific Hemodynamic Roles During Pregnancy in Mice

The purpose of this study was to establish the time course and hemodynamic significance of de novo formed and enlarged uteroplacental arteries during pregnancy. Using x-ray microcomputed tomography (n = 4–7 placentas from 2–4 dams/gestational group), uteroplacental arterial vascular dimensions were measured at individual implantation sites. Dimensions and topology were used to compute total and vessel-specific resistances and cross-sectional areas. Diameter enlargement of the uterine artery (+55% by Embryonic Day 5.5 [E5.5]) and preplacental radial arteries (+30% by E8.5) was significant only in early gestation. Formation of spiral arteries (E9.5–E11.5), maternal canals, and canal branches (E11.5–E13.5) during midgestation was followed by enlargement of these vessels such that, from E9.5 to E17.5 (near term), spiral artery resistance dropped 9-fold, and canal resistance became negligible. A 12-fold increase in terminal vessel cross-sectional area was nearly sufficient to offset known increases in flow so that blood velocity entering the exchange region was predicted to increase by only 2-fold. The calculated 47% decrease in total resistance downstream of the uterine artery, determined from vascular geometry, was in accord with prior uterine blood flow data in vivo and was due to enlarging spiral artery diameters. Interestingly, radial artery resistance was unchanged after E9.5 so that radial arteries accounted for 91% of resistance and pressure drop in the uteroplacental arterial network by E17.5. These findings led us to propose functional roles for the three morphologically defined vessel types: radial arteries to reduce pressure, spiral artery enlargement to increase flow with gestation, and maternal canal elaboration and enlargement to maintain low exit velocities into the exchange region.

The effect of isosorbide dinitrate on uterine and ovarian blood flow in cycling and early pregnant mares: A pilot study

Poor uterine perfusion has been proposed as a cause of infertility in mares. The objective of this study was to investigate the effect of isosorbide dinitrate (ISDN), a nitric oxide donor, on uterine and ovarian blood flow resistance during diestrus and early pregnancy in mares. Six Trotter mares, aged 7 to 14 years, were examined daily during the first 11 days of three diestrous periods, and five of those mares were also examined during the first 11 days of two pregnancies. Six mares randomly received a placebo, a low dose (30 mg, ISDN30), or a high dose of ISDN (60 mg, ISDN60) through three nonconsecutive cycles. The treatments were administered orally, every 12 hours from Day 1 to 11 of the cycle (Day 0 = ovulation). Five of the 6 mares received a placebo or 60 mg of ISDN orally every 12 hours from Day 1 to 11 of pregnancy. The mares were short cycled on Day 12 of each trial. Transrectal color Doppler was used to determine blood flow resistance semiquantitatively and expressed as pulsatility index. Mean pulsatility index of both uterine arteries combined and of the dominant (ipsilateral to the CL) ovarian artery was lower (treatment effects: P ≤ 0.01; time effects: P ≤ 0.002) in mares receiving 30 mg or 60 mg of ISDN compared with placebo-treated mares. Blood flow resistance in the dominant ovarian artery was lower in ISDN-treated pregnant mares than in placebo-treated pregnant and cycling mares (treatment effect: P = 0.04; time effect: P = 0.003). Isosorbide dinitrate increases uterine and ovarian perfusion in cycling mares and ovarian perfusion in early pregnant mares. Further studies are needed to investigate these effects in relation to fertility of the mare.