Control of the fetoplacental circulation

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

AIM

The aim of this study is to investigate the ultra structural difference of placentas in IUGR fetuses that were all found to have abnormal umbilical artery Doppler waveforms.

METHODS

Nine placentas from 7 IUGR fetuses and 2 from healthy normal fetuses were evaluated by scanning electron microscopy.

RESULTS

All of the placentas of IUGR fetuses who had abnormal umbilical artery Doppler flow antenatally were found to have a prominent increase in the amount of total blood vessels; these aberrant blood vessels had a very tortuous course and they showed increased branching when compared with placentas from uncomplicated term pregnancies.

CONCLUSIONS

Our study showed that there is a clear difference in vascular ultra structure of the placentas of pregnancies with fetal growth restriction selected by abnormal umbilical artery Doppler flow test. IUGR seems to be a problem due to placental vascular insufficiency.

Acute increase of umbilical artery vascular flow resistance in compromised fetuses provoked by uterine contractions

BACKGROUND

An impaired placental circulation is involved in the mechanism of late fetal heart rate (FHR) decelerations.

OBJECTIVES

To explore umbilical artery (UA) Doppler velocimetry changes in response to uterine contractions during the oxytocin challenge test (OCT) and assess the potentially clinical value of 'OCT Doppler velocimetry'.

METHODS

111 women with pregnancy complications were subjected to an OCT with simultaneous recordings of FHR and UA pulsatility index (PI). Positive OCT cases (late FHR decelerations) were compared with negative OCT cases (normal FHR patterns). Only negative OCT cases were allowed a trial of vaginal delivery.

RESULTS

The UA PI was higher in OCT positive (N=21) than in negative cases (N=90) during uterine contractions and relaxations (P<0.05), but not during basal measurements. A positive OCT, but not negative, was associated with an increase of PI during contractions and relaxations compared with basal measurements (P<0.05). Fetal growth restriction (N=58), operative delivery for fetal distress (ODFD) in labor (N=10), and birth asphyxia (N=8) were not associated with PI changes significantly different from cases without these complications.

CONCLUSIONS

During uterine contractions and relaxations, but not during basal measurements, a significantly higher UA PI evolved in OCT positive cases compared with OCT negative cases. This indicates a pathophysiological mechanism disclosed only during uterine activity. Although these changes would not have been revealed by traditional Doppler velocimetry, the data suggest a limited predictive value of 'OCT Doppler velocimetry' on the short-term neonatal outcome.

Adrenomedullin in perinatal medicine

This review will consider whether adrenomedullin (AM) plays a role in the different aspects of perinatal medicine: contributing to maternal systemic vasodilatation during pregnancy, regulating uterine and placental blood flow, being involved in the process of implantation and participating in uterine quiescence prior to parturition. In addition, this will also consider whether a modification of AM secretion contributes to some pathological conditions in pregnancy such as preeclampsia and impairment of fetal growth. The biosynthesis of AM increases in gravid rats and in pregnant women, and the placenta represents an important site of AM production during pregnancy. Both the peptide and its receptors have been found in the uterus, placenta, fetal membranes and cord vessels, and fetal membranes and placental tissues in culture secrete AM. AM contributes to maternal systemic vasodilatation, the placental vessels are relaxed by AM in a dose-dependent manner and AM is expressed in the fetoplacental and umbilical vascular endothelium where basal production of AM contributes to low fetoplacental vascular resistances. Controversy exists over the status of circulating and placental AM in preeclampsia and of the relative contribution of AM to impaired fetoplacental circulation and fetal growth. Moreover, the uterus expresses AM mRNA and exogenous AM relaxes the myometrium in a dose-dependent manner; however, clinical studies have shown that AM does not decrease before the onset of parturition. Rather, AM secretion increases during spontaneous labor and in preterm delivery.

Stereologic examination of placentas from mothers who smoke during pregnancy

OBJECTIVE

Structural changes in the placenta might contribute to the lower birth weight seen among infants born to mothers who smoke cigarettes. In this study, a morphologic examination and a stereologic quantitation of placentas from mothers who smoked cigarettes and who did not smoke cigarettes during pregnancy were performed.

STUDY DESIGN

Twenty-five placentas from mothers who did not smoke cigarettes, 15 placentas from mothers who smoked 5 to 10 cigarettes per day, 16 placentas from mothers who smoked 11 to 20 cigarettes per day, and 16 placentas from mothers who smoked >20 cigarettes per day were delivered at term after normal pregnancies and were fixed by dual perfusion. The volume and the surface area of villi, the trophoblast volume, and the volume and the surface area and length of villous capillaries were estimated. A measurement of the concentration of cadmium in serum was used to assess the validity of information concerning smoking habits.

RESULTS

No differences were shown in the total volume of placenta between the groups. The estimated volume and surface area and the calculated lengths for villous capillaries were significantly reduced in all 3 groups of smokers. A significant increase of the trophoblast volume was observed in the mothers who smoked cigarettes.

CONCLUSION

Cigarette smoking during pregnancy influences the placental vasculature. The reduced dimensions of fetal capillaries in villi may affect the placental blood flow, and the diminished area for exchange of gases and nutrients between the mother and the fetus will increase the risk of fetal undernourishment.

Adrenomedullin, a new vasoactive peptide, is increased in preeclampsia

Adrenomedullin is a novel peptide that elicits a long-lasting vasorelaxant activity. Recently, we found high concentrations of adrenomedullin in maternal and umbilical cord plasma and in amniotic fluid in full-term human pregnancy, indicating a role of this peptide during gestation. To investigate the possibility that adrenomedullin is involved in the pathophysiology of preeclampsia, we measured its concentration in maternal and fetoplacental compartments. We studied 12 normotensive nonpregnant women, 13 hypertensive nonpregnant subjects, 29 patients with preeclampsia, and 30 normotensive pregnant women. In all patients, plasma was collected from the cubital vein, and amniotic fluid samples were obtained by transabdominal amniocentesis or at elective cesarean section. Plasma samples from umbilical vein and placental tissues were collected at delivery. Adrenomedullin was assayed on plasma and amniotic fluid samples using a specific radioimmunoassay, and its localization and distribution on placental sections was determined by immunohistochemistry. Adrenomedullin concentrations were higher in hypertensive than in normotensive nonpregnant patients. Pregnant women had higher adrenomedullin levels than nonpregnant subjects, although maternal plasma adrenomedullin concentrations did not differ between normal pregnant and preeclamptic women. Preeclamptic patients showed higher concentrations (P<0.01) than normotensive pregnant women of adrenomedullin in amniotic fluid (252+/-29 versus 112+/-10 fmol/ micromol creatinine) and umbilical vein plasma (18.1+/-2.1 versus 8. 5+/-1.1 fmol/mL). Increased local production of adrenomedullin is associated with preeclampsia. The fetus seems to be responsible for the higher levels of this hormone. Increased adrenomedullin concentrations may be necessary to maintain placental vascular resistance and/or fetal circulation at a physiological level.

Nitric oxide in preeclampsia: lack of evidence for decreased production

The purpose of our study was to determine the involvement of the L-arginine-NO system in preeclampsia. We studied 26 patients with preeclampsia and 27 normotensive pregnancies. Maternal and cord plasma, urine and amniotic fluid were assayed for nitric oxide metabolites (nitrite and nitrate) using the Griess reaction. Sections of placenta and fetal membranes were immunostained with polyclonal anti-endothelial and anti-neuronal nitric oxide synthase antibodies. The concentration of nitrate in the amniotic fluid of preeclamptic patients (median 10.3 mumol/mg creatinine) was significantly higher (P < 0.001) than in the normotensive group (5.6 mumol/mg creatinine). Nitrate concentrations in maternal and cord plasma and in urine were similar in the two groups. Endothelial cells of the villi of preeclamptic placentas showed a higher positivity in endothelial nitric oxide synthase immunostaining with respect to normotensive controls. Our results indicate that feto-placental NO production is not reduced in preeclampsia. In contrast, the increased concentrations of NO metabolites in amniotic fluid and the positive immunostaining of endothelial nitric oxide synthase in the placental villi suggest that the placental L-arginine-NO system is up-regulated in preeclampsia.

Computerized microscope morphometry of umbilical vessels from pregnancies with intrauterine growth retardation and abnormal umbilical artery Doppler

Computerized microscope morphometry was used to study cross sections from the vessels of the umbilical cord in placentas of patients with intrauterine growth retardation (IUGR) that displayed either normal or abnormal umbilical arteries (UA) Doppler flow velocity waveforms (FVW). Cords from 63 eutrophic fetuses with normal Doppler (controls), 47 IUGR fetuses with normal Doppler and 32 IUGR fetuses with abnormal Doppler underwent morphometric analysis using a highly optimized microscope environment (HOME) and "CordHOME" software. IUGR with an accompanying normal Doppler versus control showed a reduction of Wharton jelly and both the total and lumen vein areas. IUGR with an accompanying pathological Doppler showed a comparable reduction in wall thickness and areas of every vessel. These findings indicate that the hypoplastic umbilical vessels are associated with an increase in placental vascular resistance that may be the consequence of underdevelopment in response to a chronic reduction in placental blood flow.

Amniotic fluid nitric oxide and uteroplacental blood flow in pregnancy complicated by intrauterine growth retardation

OBJECTIVE

To examine the correlation between placental nitric oxide production and uteroplacental blood flow.

PARTICIPANTS

Thirty-one pregnant women with fetuses with intrauterine growth retardation and 27 normal pregnancies as controls.

DESIGN

Correlation between amniotic fluid measurements of nitrite metabolite in the third trimester and flow velocimetry waveforms recorded from uterine, umbilical and fetal middle cerebral arteries. Intrauterine growth retarded pregnancies were compared with controls.

MAIN OUTCOME MEASURES

Concentrations of nitric oxide metabolites (NO2- and NO3-) in amniotic fluid were correlated with flow velocimetry waveforms findings by the determination of correlation coefficient.

RESULTS

Overall median nitrite values in amniotic fluid were higher (P < 0.01) in intrauterine growth retarded patients (median 8.6 micromol/mg creatinine) than in controls (5.6 micromol/mg creatinine). Pathologic uterine flow velocimetry waveforms in uterine artery (-2SD) were observed in 12 women of the intrauterine growth retarded group, and the concentration of amniotic fluid nitrite was significantly lower (P < 0.01) in these patients (median 4.45 micromol/mg creatinine) than in those with normal flow velocity waveforms (median 11.43 micromol/mg creatinine). A significant negative correlation was observed between nitrite concentrations and uterine artery resistance index, umbilical artery pulsatility index and umbilical artery pulsatility index:middle cerebral artery pulsatility index ratio.

CONCLUSIONS

We conclude that placental nitric oxide is significantly associated with uteroplacental blood flow and may be important in maintaining adequate uteroplacental perfusion in intrauterine growth retarded pregnancies.

Classification of human placental stem villi: review of structural and functional aspects

The stem villi of the human placenta represent the central branches of the villous trees. They are characterized by a condensed fibrous stroma in which the fetal arteries and veins as well as the arterioles and venules are embedded. Functionally they are accepted as the mechanically supporting structures of the villous trees, and they are supposed to control fetal blood flow to the maternofetal exchange area, which is located in the peripheral villi. To obtain further insights into the functions of the stem villi, the recent literature has been reviewed, and some immunohistochemical, ultrastructural, and reconstruction studies have been added. These new studies were aimed at identifying immunohistochemically different subtypes of stem villi, their branching patterns, the distribution of macrophages, the stromal proliferation patterns, and the differentiation of extravascular stromal cells. Our findings demonstrate that the stem villi and their precursors, the immature intermediate villi, can selectively be identified by anti-gamma-smooth muscle (sm) actin staining. Furthermore, the existence of three different subtypes of stem villi is shown; these differ regarding the presence and distribution of gamma-sm actin-positive cells. These cells were immunohistochemically and ultrastructurally identified as smooth muscle cells and myofibroblasts. Increasingly complex coexpression patterns of cytoskeletal proteins reflect a clearly defined differentiation gradient of extravascular stromal cells, which covers the whole range of an undifferentiated germinative layer beneath the trophoblast to highly differentiated myofibroblasts surrounding the medias of the stem vessels. Possible functions of the extravascular contractile system include the regulation of villous turgor and the control of intervillous blood flow impedance.

Placental expression of vascular endothelial growth factor in placentae from pregnancies complicated by pre-eclampsia and intrauterine growth restriction does not support placental hypoxia at delivery

In view of the pathological placental features of pre-eclampsia and intrauterine growth restriction (IUGR), and the angiogenic effects of vascular endothelial growth factor (VEGF), the aim of this study was to determine the expression of VEGF in placentae from normal pregnancies and to compare the results with placentae from pregnancies complicated by pre-eclampsia and intrauterine growth restriction (IUGR). ELISA was used to measure circulating VEGF immunoreactivity in umbilical vein serum samples and immunohistochemistry was used to determine tissue expression of the protein. Since VEGF is known to be upregulated by hypoxia, the expression pattern of VEGF would provide further clues to the oxygen status in the placentae at the time of sampling, presently a subject under great debate. The geometric mean concentration of VEGF immunoreactivity in umbilical vein serum of normal pregnant women was 112.46 pg/ml, in women with pre-eclampsia 50.23 pg/ml and in IUGR alone 175.35 pg/ml. These values were not statistically different from each other. Immunolocalization of VEGF in normal term villous placenta was observed in the syncytiotrophoblast with less intense staining in stromal cells. No qualitative differences in localization of staining between the groups (normal pregnancies, pre-eclampsia, pre-eclampsia plus IUGR, and IUGR) was found. Intensity of staining in stromal cells was also similar in the groups studied. However, intensity of VEGF immunostaining in syncytiotrophoblast was significantly reduced in the three pathological groups (P < 0.02) compared with the control group. These results suggest that reduced VEGF may be responsible, at least in part, for the impaired vascular development which occurs in these conditions. Our results are therefore not consistent with villous placental hypoxia at the time of sample collection.

The c-ets1 protooncogene is expressed in human trophoblast during the first trimester of pregnancy

Expression of the c-Ets1 protooncogene which codes for a transcription factor is associated with neovascularization and invasive processes. In order to determine c-Ets1 expression at the mRNA level, during the process of implantation during the first trimester of human pregnancy, samples of trophoblast were retrieved at the time of legal abortion and processed for in situ hybridization. We found that c-Ets1 mRNAs are transcribed in the endothelial cells of villous trophoblast and in the extravillous trophoblastic cells invading the uterine vessels. However, no transcript was found in maternal endothelial cells. We conclude that c-Ets1 plays a role in angiogenesis occurring in the development of the villous tree and is involved during the invasive process of the endometrium and maternal vessels by trophoblastic cells; this latter physiological event is crucial for a normal development of the fetus, its failure leading to pathological cases. We suggest that the role of the c-Ets1 protooncogene is related to the regulation of metalloproteinase genes transcription, a gene family which is known to be a target for Ets protein.

Intrauterine growth restriction with absent end-diastolic flow velocity in the umbilical artery is associated with maldevelopment of the placental terminal villous tree

OBJECTIVE

Our purpose was to evaluate the structure of placental terminal villi and their capillaries in pregnancies complicated by intrauterine growth restriction with absent end-diastolic flow velocity in the umbilical artery.

STUDY DESIGN

Glutaraldehyde-perfusion-fixed villous tissue and a plastic cast of the vessels in at least two cotyledons were prepared from 10 cases with intrauterine growth restriction and 9 gestational age-matched control placentas. The structure and dimensions of 20 terminal capillary loops per cast were determined by scanning electron microscopic examination, and their appearances were correlated with the peripheral villi of the perfusion-fixed villous tissue.

RESULTS

Capillary loops in the growth-restricted cases were sparse in number and significantly longer than in the control cases (218 microns [72] vs 137 microns [30], mean and SD, p < 0.05). They exhibited fewer branches (4.0 [1.9] per loop vs 6.1 [2.2], p < 0.05) and a majority of loops were uncoiled (79% vs 18%, p < 0.05). The villous tissues from the growth-restricted cases demonstrated elongated villi, consistent with the cast findings. The trophoblast surface was wrinkled and in some areas covered by fibrin plaques.

CONCLUSIONS

The terminal villous compartment of the placenta appears to be maldeveloped in preterm intrauterine growth restriction pregnancies where absent end-diastolic flow velocity is demonstrated in the umbilical artery before delivery. These findings are consistent with an increase in fetoplacental vascular impedance at the capillary level and may account for the impaired gas and nutrient transfer in this disorder.

Nitric oxide concentrations are increased in the feto-placental circulation in intrauterine growth restriction

The aim of this study was to measure plasma concentrations of total nitrites, as an index of nitric oxide (NO) synthesis, in the fetal circulation of normal pregnancies and in pregnancies complicated by intrauterine growth restriction. Plasma was prepared from umbilical venous blood collected from 13 placentae from normal pregnancies complicated by intrauterine growth restriction. Plasma NO concentrations were determined using the Greiss reaction by measuring combined oxidation products of NO, plasma nitrite (NO2-) and nitrate (NO3-) after reduction with nitrate reductase. Significantly higher NO2-concentrations were found in umbilical venous plasma in the group complicated by intrauterine growth restriction compared to the control group (65.6 mumol/1, P < 0.001. These results support the hypothesis that increased NO production may be a compensatory response to improve blood flow in the placenta and/or may play a role in limiting platelet adhesion and aggregation.

Elaboration of stem villous vessels in growth restricted pregnancies with abnormal umbilical artery Doppler waveforms

OBJECTIVE

To assess the elaboration of placental stem villous vessels from pregnancies complicated by intrauterine growth restriction (IUGR) with absent end-diastolic flow velocity detected prior to delivery in the umbilical artery.

DESIGN

Comparison between IUGR and control groups of the distribution, in 15 microns increments of 600 randomly chosen stem vessel profiles (post-fixation diameter 10-160 microns) identified by immunohistochemical localisation of alpha-smooth muscle actin in the vessel media.

SETTING

Clinical teaching hospital and university anatomy department.

SUBJECTS

Paraffin-fixed blocks obtained from placentas of eight pregnancies complicated by IUGR and eight gestational age-matched controls.

RESULTS

The distribution of the stem villous vessels in the IUGR placentas, as assessed by the mean vessel diameter in each case, did not differ from the controls (mean vessel diameter 31.8 microns [SD 2.4] vs 29.6 microns [2.3]; P = 0.13). In five IUGR cases alpha-smooth muscle actin positive cells (myofibroblasts) were identified within the stroma of nonmuscularised peripheral (mature intermediate and terminal) villi, but in none of the controls.

CONCLUSIONS

Our data do not support the theory that IUGR with absent end-diastolic flow velocity in the umbilical artery is due to a selective loss of small stem villous vessels. The increased impedance in this condition may be conferred more distally within the nonmuscularised capillaries of the peripheral villi.

Nitric oxide synthase activities in placental tissue from normotensive, pre-eclamptic and growth retarded pregnancies

OBJECTIVE

To measure nitric oxide synthase activity in tissues from the placenta, placental bed and umbilical cord at delivery in normal and complicated pregnancies.

DESIGN

A prospective blinded study.

SETTING

The obstetric departments of three London teaching hospitals.

SUBJECTS

Samples of whole placenta, dissected stem villous arteries, umbilical cord vessels and the placental bed of the uterus were collected at delivery and assayed for nitric oxide synthase activity. Samples of placenta were taken from ten normotensive, six pre-eclamptic and eight growth retarded pregnancies, and stem villous arteries from a further seven normotensive pregnancies.

RESULTS

There was minimal placental bed nitric oxide synthase activity in each group. Placental villous homogenates from pregnancies complicated by pre-eclampsia and fetal growth retardation had significantly lower activities of nitric oxide synthase than those from normotensive women with appropriately grown babies. There were no significant differences in calcium dependent or calcium independent nitric oxide synthase activities in the umbilical vein and artery in the normal or in the pre-eclamptic groups. However, there was significantly more calcium dependent than calcium independent nitric oxide synthase in the umbilical veins in all groups.

CONCLUSIONS

Local nitric oxide production in the placental bed of the uterus is unlikely to contribute substantially to the low resting vascular tone in the uteroplacental circulation. However, a relative deficiency of placental nitric oxide in pregnancies complicated by fetal growth retardation and pre-eclampsia may contribute to the development of the high impedance fetoplacental circulation found in these conditions.

Nitric oxide and human umbilical vessels: pharmacological and immunohistochemical studies

Human umbilical vessels are devoid of nerves and therefore endothelial cells may play an important role in the control of fetoplacental blood flow. In this study we examined the pharmacological effects of various substances, known to produce endothelial-mediated vasodilation in many blood vessels, on the human umbilical artery and vein from legal terminations [mean gestational age, 15 (8-17) weeks; n = 12] and normal term vaginal deliveries [mean gestational age, 39 (38-41) weeks; n = 12]. Acetylcholine, adenosine 5'-triphosphate, the calcium ionophore A23187 and substance P had no effect on raised vascular tone, whereas sodium nitroprusside relaxed 5-hydroxytryptamine (5-HT) preconstricted, umbilical artery and vein from both early and late pregnancy. L-NG-Nitroarginine methyl ester (L-NAME) had no effect on basal tone or on high tone, after it was raised by 5-HT. Localization of nitric oxide synthase [NOS, type I (neuronal)] was examined in the same umbilical vessels using electron immunocytochemistry. No NOS-immunoreactive endothelial cells were observed in the umbilical vessels taken during early pregnancy. However, the percentage of NOS-immunoreactive endothelial cells in umbilical artery and vein from late pregnancy was 3 and 10 per cent, respectively. These results suggest that nitric oxide contributes little, if any, to the local control of umbilical blood flow throughout pregnancy, despite the presence of NOS-immunoreactivity in a subpopulation of endothelial cells in late pregnancy.