Placental microcirculation of intrauterine growth retarded fetuses: scanning electron microscopy of placental vascular casts

Elevated gestational testosterone impacts vascular and uteroplacental function

Maternal vascular adaptations to establish an adequate blood supply to the uterus and placenta are essential for optimal nutrient and oxygen delivery to the developing fetus in eutherian mammals, including humans. Numerous factors contribute to maintaining appropriate hemodynamics and placental vascular development throughout pregnancy. Failure to achieve or sustain these pregnancy-associated changes in women is strongly associated with an increased risk of antenatal complications, such as preeclampsia, a hypertensive disorder of pregnancy. The precise etiology of preeclampsia is unknown, but emerging evidence points to a potential role for androgens. The association between androgens and maternal cardiovascular and placental function merits particular attention due to the notable 2- to 3-fold elevated plasma testosterone (T) levels observed in preeclampsia. T levels in preeclamptic women positively correlate with vascular dysfunction, and preeclampsia is associated with increased androgen receptor (AR) levels in placental tissues. Moreover, animal studies replicating the pattern and magnitude of T increase observed in preeclamptic pregnancies have reproduced key features of preeclampsia, including gestational hypertension, endothelial dysfunction, heightened vasoconstriction to angiotensin II, impaired spiral artery remodeling, placental hypoxia, reduced nutrient transport, and fetal growth restriction. Collectively, these findings suggest that AR-mediated activity plays a significant role in the clinical presentation of preeclampsia. This review critically evaluates this hypothesis, considering both clinical and preclinical evidence.

Fetal Sex and Fetal Environment Interact to Alter Diameter, Myogenic Tone, and Contractile Response to Thromboxane Analog in Rat Umbilical Cord Vessels

Fetal growth needs adequate blood perfusion from both sides of the placenta, on the maternal side through the uterine vessels and on the fetal side through the umbilical cord. In a model of intrauterine growth restriction (IUGR) induced by reduced blood volume expansion, uterine artery remodeling was blunted. The aim of this study is to determine if IUGR and fetus sex alter the functional and mechanical parameters of umbilical cord blood vessels. Pregnant rats were given a low sodium (IUGR) or a control diet for the last 7 days of pregnancy. Umbilical arteries and veins from term (22 day) fetal rats were isolated and set-up in wire myographs. Myogenic tone, diameter, length tension curve and contractile response to thromboxane analog U46619 and serotonin (5-HT) were measured. In arteries from IUGR fetuses, myogenic tone was increased in both sexes while diameter was significantly greater only in male fetuses. In umbilical arteries collected from the control group, the maximal contraction to U46619 was lower in females than males. Compared to the control groups, the maximal response decreased in IUGR male arteries and increased in female ones, thus abolishing the sexual dimorphism observed in the control groups. Reduced contractile response to U46619 was observed in the IUGR vein of both sexes. No difference between groups was observed in response to 5HT in arteries. In conclusion, the change in parameters of the umbilical cord blood vessels in response to a mild insult seems to show adaptation that favors better exchange of deoxygenated and wasted blood from the fetus to the placenta with increased myogenic tone.

Considering intrauterine location in a model of fetal growth restriction after maternal titanium dioxide nanoparticle inhalation

Fetal growth restriction (FGR) is a condition with several underlying etiologies including gestational disease (e.g., preeclampsia, gestational diabetes) and xenobiotic exposure (e.g., environmental contaminants, pharmaceuticals, recreational drugs). Rodent models allow study of FGR pathogenesis. However, given the multiparous rodent pregnancy, fetal growth variability within uterine horns may arise. To ascertain whether intrauterine position is a determinant of fetal growth, we redesigned fetal weight analysis to include litter size and maternal weight. Our FGR model is produced by exposing pregnant Sprague Dawley rats to aerosolized titanium dioxide nanoparticles at 9.44 ± 0.26 mg/m3 on gestational day (GD) 4, GD 12 or GD 17 or 9.53 ± 1.01 mg/m3 between GD 4-GD 19. In this study fetal weight data was reorganized by intrauterine location [i.e., right/left uterine horn and ovarian/middle/vaginal position] and normalized by maternal weight and number of feti per uterine horn. A significant difference in fetal weight in the middle location in controls (0.061g ± 0.001 vs. 0.055g ± 0.002), GD 4 (0.033g ± 0.003 vs. 0.049g ± 0.004), and GD 17 (0.047g ± 0.002 vs. 0.038g ± 0.002) exposed animals was identified. Additionally, GD 4 exposure produced significantly smaller feti in the right uterine horn at the ovarian end (0.052g ± 0.003 vs. 0.029g ± 0.003) and middle of the right uterine horn (0.060g ± 0.001 vs. 0.033g ± 0.003). GD 17 exposure produced significantly smaller feti in the left uterine horn middle location (0.055g ± 0.002 vs. 0.033 ± 0.002). Placental weights were unaffected, and placental efficiency was reduced in the right uterine horn middle location after GD 17 exposure (5.74g ± 0.16 vs. 5.09g ± 0.14). These findings identified: 1) differences in fetal weight of controls between the right and left horns in the middle position, and 2) differential effects of single whole-body pulmonary exposure to titanium dioxide nanoparticles on fetal weight by position and window of maternal exposure. In conclusion, these results indicate that consideration for intrauterine position, maternal weight, and number of feti per horn provides a more sensitive assessment of FGR from rodent reproductive and developmental studies.

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.

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.

Elevated Testosterone Reduces Uterine Blood Flow, Spiral Artery Elongation, and Placental Oxygenation in Pregnant Rats

Elevated maternal testosterone levels are shown to cause fetal growth restriction, eventually culminating in sex-specific adult-onset hypertension that is more pronounced in males than in females. In this study, we tested whether uteroplacental and fetoplacental disturbances underlie fetal growth restriction and if these changes vary in male and female placentas. Pregnant Sprague-Dawley rats were injected with vehicle (n=16) or testosterone propionate (0.5 mg/kg per day from gestation day 15-19; n=16). On gestation day 20, we quantified uterine artery blood flow using microultrasound, visualized placental arterial network using x-ray microcomputed tomography, determined fetoplacental hypoxia using pimonidazole and hypoxia-inducible factor-1α, and used Affymetrix array to determine changes in placental expression of genes involved in vascular development. Plasma testosterone levels increased 2-fold in testosterone-injected rats. Placental and fetal weights were lower in rats with elevated testosterone. Uterine artery blood flow was lower, and resistance index was higher in the testosterone group. Radial and spiral artery diameter and length, the number of fetoplacental arterial branches, and umbilical artery diameter were reduced in the testosterone group. In addition, markers of hypoxia in the placentas and fetuses were elevated in the testosterone group. The magnitude of changes in placental vasculature and hypoxia was greater in males than in females and was associated with sex-specific alteration of unique sets of genes involved in angiogenesis and blood vessel morphogenesis. The results demonstrate that elevated testosterone during gestation induces a decrease in uterine arterial blood flow and fetal sex-related uteroplacental vascular changes, which may set the stage for subsequent sex differences in adult-onset diseases.

Associations of maternal retinal vasculature with subsequent fetal growth and birth size

OBJECTIVE

We aimed to study the maternal retinal microvasculature at mid-trimester and its relationship with subsequent fetal growth and birth size.

METHODS

We recruited 732 pregnant women aged 18-46 years in the first trimester with singleton pregnancies. All had retinal photography and fetal scan performed at 26-28 weeks gestation, and subsequent fetal scan at 32-34 weeks gestation. Infant anthropometric measurements were done at birth. Retinal microvasculature was measured using computer software from the retinal photographs.

RESULTS

In multiple linear regression models, each 10 μm narrowing in maternal retinal arteriolar caliber was associated with decreases of 1.36 mm in fetal head circumference at 32-34 weeks gestation, as well as decreases of 1.50 mm and 2.30 mm in infant head circumference and birth length at delivery, respectively. Each standard deviation decrease in maternal retinal arteriolar fractal dimension was associated with decreases of 1.55 mm in fetal head circumference at 32-34 weeks gestation, as well as decreases of 1.08 mm and 46.42 g in infant head circumference and birth weight at delivery, respectively.

CONCLUSIONS

Narrower retinal arteriolar caliber and a sparser retinal vascular network in mothers, reflecting a suboptimal uteroplacental microvasculature during mid-pregnancy, were associated with poorer fetal growth and birth size.

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

The sites of elevated vascular resistance that impede placental perfusion in pathological pregnancies are unknown. In the current study, we identified these sites in a knockout mouse model (eNOS(-/-)) with reduced uterine (-55%) and umbilical (-29%) artery blood flows caused by endothelial nitric oxide synthase deficiency. Uteroplacental and fetoplacental arterial vascular trees of pregnant mice near term were imaged using x-ray microcomputed tomography (n = 5-10 placentas from 3-5 dams/group). The resulting three-dimensional images were analyzed to assess vessel geometry and vascular resistance. In control and eNOS(-/-) trees, ∼90% of total uteroplacental vascular resistance was located in the radial arteries. Changes in eNOS(-/-) vessel geometry, including 30% reductions in uterine, radial, and spiral artery diameters, were calculated to increase arterial resistance downstream of the uterine artery by 2.3-fold, predicting a 57% decrease in uterine blood flow. Despite large reductions in eNOS(-/-) spiral arteries (-55% by volume) and maternal canals (-67% by volume), these vessels were relatively minor contributors to resistance. In the eNOS(-/-) fetoplacental tree, the number of arterioles (50-75 μm diameter) increased by 26%. Nevertheless, calculated resistance rose by 19%, predominantly because arteries near the periphery of the tree selectively exhibited a 7%-9% diameter reduction. We conclude that previously observed decreases in uterine and umbilical blood flows in eNOS(-/-) pregnancies are associated with markedly divergent structural changes in the uteroplacental versus fetoplacental circulations. Results showed the radial arteries were critical determinants of uteroplacental resistance in mice and therefore warrant greater attention in future studies in pathological human pregnancies.

Magnetic resonance imaging relaxation time measurements of the placenta at 1.5 T

Placental insufficiency is a major cause of fetal growth restriction (FGR) and accumulating evidence indicates several aspects of placental morphology are altered in this condition. MRI provides quantitative indices that may be used in non-invasive assessment of the human placenta, such as relaxation time measurements, T1 and T2. We hypothesised that placental relaxation times relate to alterations in placental tissue morphology and hence may be useful in identifying the changes associated with FGR. We report on the first phase of testing this hypothesis, in a study of women in normal pregnancy.

AIMS

To assess relaxation time measurements in the placenta in normal pregnancy and correlate these with gestational age and stereological analyses of placental morphology following delivery.

METHODS

30 women underwent MRI examination (1.5 T) between 20 and 41 weeks gestation. Placental T1 and T2 measurements were acquired from a mid-depth placental region, co-localised to a structural scan. Fixed, wax-embedded sections of these placentas collected at delivery were stained with hematoxylin/eosin and subjected to stereological analysis.

RESULTS

Placental T1 and T2 show a significant negative correlation with gestation, (Pearson correlation p=0.01, 0.03 respectively). 17 placentas were analysed stereologically. In the group as a whole there was no significant correlation between T1 and T2 and morphological features. However, in a subset of 7 pregnancies scanned within a week of delivery, a significant positive correlation was observed between the fibrin volume density and the ratio of fibrin: villous volume densities and T2 (Spearman correlation p=0.02, 0.03 respectively).

DISCUSSION

The correlations between placental T1 and T2 and gestation show that these variables are clearly influenced by changes in placental structure. Fibrin might be a key component but further work is needed to fully elucidate the major structural influences on placental T1 and T2.

Three-dimensional assessment of the morphology of the umbilical artery in normal and pre-eclamptic placentas

AIM

Pre-eclampsia is one of the main causes of intrauterine growth retardation. Although there are many studies performed in this setting showing the course of the umbilical vessels within the umbilical cord along with its structural changes, studies are lacking with regard to intraplacental vasculature and its structural changes.

METHODS

We investigated whether the vascular pattern differed in preeclamptic placentas in terms of intraplacental course and morphometry of the umbilical artery by using vascular corrosion cast technique. Furthermore, cross-sections taken from umbilical artery branches at different levels within the placenta, were examined with both light and scanning electron microscopy (SEM).

RESULTS

Vascular corrosion casts generated in both pregnancy groups should be defined with the main umbilical artery divided into 2-4 primary branches. No significant difference was seen between normal and pre-eclamptic groups regarding the diameters of primary and secondary branches and the number of cotyledons contained (P > 0.05). However, microscopic studies demonstrated expanded intervillous spaces in the placentas of pre-eclamptic pregnancies. Under SEM, distortion on the endothelial surface of the umbilical artery and an increase in vessel diameter and wall thickness have been determined in pre-eclamptic placentas. Furthermore, the branching pattern and changes affecting the endothelial surface of the umbilical artery have been shown three-dimensionally in pre-eclamptic placentas, using corrosion cast technique and SEM, respectively.

CONCLUSION

While microscopic findings were consistent with the Doppler ultrasonography findings, namely higher systole/diastole ratio and increased resistance (RI) and pulsatile (PI) index in the umbilical artery, no clear morphometric change has been observed.