Effects of magnesium on isolated human fetal and maternal uteroplacental vessels

The effects of Mg2+ were studied in human umbilical arteries, stem villous arteries and maternal intramyometrial arteries. The vessels were dissected and mounted in organ baths, and isometric tension was recorded. In all fetal preparations investigated, Mg2+ (0.5-6.0 mM) in a concentration-related way decreased pD2 values for prostaglandin F2 alpha responses. The maximum response to prostaglandin F2 alpha was depressed in umbilical arteries, but remained unaffected in stem villous artery preparations. In stem villous arteries pretreated in Ca2(+)-free medium, increasing concentrations of Mg2+ markedly depressed the response to Ca2+ after stimulation with K+ or prostaglandin F2 alpha, suggesting that Mg2+ inhibited transmembrane calcium influx and interfered with intracellular calcium effects. In both stem villous and intramyometrial arteries, increasing concentrations of Mg2+ increased EC50 values for responses to K+, whereas Emax values were unaffected. Mg2+ produced relaxation of agonist-induced contractions by up to 60% in stem villous arteries and up to 40% in intramyometrial artery preparations. The relaxant effect of Mg2+ did not seem to be mediated through the endothelium or through changes in the synthesis of prostanoids, since endothelial disruption and treatment with indomethacin left the responses to Mg2+ unaffected. Relaxation of vessels important for resistance regulation in the human uteroplacental vascular bed may be of benefit when uteroplacental blood flow is impaired, and the present results support the established use of magnesium sulphate in the treatment of pre-eclampsia.     

Pregnancy hypertension, placental evidences of low uteroplacental blood flow, and spontaneous premature delivery

This study was undertaken to determine if preeclampsia and low uteroplacental blood flow cause spontaneous preterm births. No non-invasive methods are currently available to accurately measure uteroplacental blood flow, so surrogates that are known to be associated with low uteroplacental flow were used. These are preeclampsia, placental infarcts, abnormally small placental villi, and excessive syncytial knots. Preeclampsia was associated with a frequency of spontaneous preterm births that was 41% greater than expected (P less than .001). Normotensive gestations with placental findings of low uteroplacental blood flow ended preterm 147% more frequently than expected, and when a woman had two such pregnancies in succession, the second had a particularly high frequency of spontaneous preterm delivery (64%). Overall, preeclampsia and placental findings of low uteroplacental blood flow were associated with 26% of the spontaneous preterm deliveries in this study. Preeclampsia and low uteroplacental blood flow may be major causes of preterm birth.     

Fact and fancy. What can we really tell from the placenta?

Considerable interest in the clinical significance of placental abnormalities has been fueled by malpractice claims when perinatal outcome is less than optimal. The pathologist must be aware of those placental lesions that may contribute to perinatal morbidity and mortality. This article reviews the abnormalities of placentation, placental weight, maternal uteroplacental blood flow, villous histological makeup, fetoplacental blood flow, the umbilical cord, and the membranes, as well as the problems of multiple pregnancy. The important contribution of abnormal uteroplacental blood flow to adverse perinatal outcome is emphasized, as is the need to determine the extent of placental damage when evaluating the contribution thereto of particular placental abnormalities.     

Angiotensin II regulation of ovine fetoplacental artery endothelial functions: interactions with nitric oxide

During normal pregnancy, elevated angiotensin II (Ang II) concentrations in the maternal and fetal circulations are associated with dramatic increases in placental angiogenesis and blood flow. Much is known about a local renin–angiotensin system within the uteroplacental vasculature. However, the roles of Ang II in regulating fetoplacental vascular functions are less well defined. In the fetal placenta, the overall in vivo vasoconstrictor responses of the blood vessels to Ang II infusion is thought to be less than that in its maternal counterpart, even though infused Ang II induces vasoconstriction. Recent data from our laboratories suggest that Ang II stimulates cell proliferation and increases endothelial nitric oxide synthase (eNOS) and production of nitric oxide (NO) in ovine fetoplacental artery endothelial cells. These data imply that elevations of the known vasoconstrictor Ang II in the fetal circulation may indeed play a role in the marked increases in fetoplacental angiogenesis and that Ang II-elevated endothelial NO production may partly attenuate Ang II-induced vasoconstriction on vascular smooth muscle. Together with both of these processes, the high levels of Ang II in the fetal circulation may serve to modulate overall fetoplacental vascular resistance. In this article, we review currently available data on the expression of Ang II receptors in the ovine fetal placenta with particular emphasis on the effects of Ang II on ovine fetoplacental endothelium. The potential cellular mechanisms underlying the regulation of Ang II on endothelial growth and vasodilator production are discussed.     

Cytoskeletal proteins in chronic villitis of unestablished etiology.

Cytoskeletal proteins (i.e., cytokeratins, vimentin, actin, and desmin) are normally present in the placental chorionic villi and are related to the maintenance of the villous shape, and to the ability of the villi to contract and permit a normal blood flow. In areas of villitis of unestablished etiology, the normal reactivity of these proteins in the villous core around fetal stem vessels disappears, and an increased number of cytokeratin positive cells is identified. The vascular damage in stem villi with villitis could develop ischemia in the villous tree promoting cytotrophoblast proliferation. Increased numbers of these areas could be related with the appearance of abnormal pregnancies.     

'Cor placentale': placental intervillus/intravillus blood flow mismatch is the pathophysiological mechanism in severe intrauterine growth restriction due to uteroplacental disease

The underlying pathophysiology in most cases of severe intrauterine growth restriction and pre-eclampsia is thought to be abnormal and inadequate conversion of the branches of the uterine arteries into low resistance uteroplacental vessels, due to poor extravillous trophoblastic invasion, leading to reduced intervillous blood flow. Since, in most vascular beds the main site of flow resistance is at the level of the small arteries/arterioles rather than the capillary bed itself it is likely that in cases of intrauterine growth restriction due to uteroplacental dysfunction with abnormal fetal umbilical artery flow velocity waveforms, the underlying pathological mechanism is primarily an initial reduction in intervillus flow leading to relative local hypoxia of some villus territories. This results initially in autocrine/paracrine mediated localized stem artery vasoconstriction to minimize intervillus/intravillus flow mismatch which, when widespread, will result in abnormal umbilical artery Doppler waveforms due to the globally increased resistance to fetoplacental flow. Since, a small reduction in vessel radius will result in an exponential increase in flow resistance and reduction in flow, the magnitude of stem vessel constriction need only be small to result in large changes in fetoplacental vascular haemodynamics. Thus, the underlying progressive pathology in this condition may be cardiac failure, secondary to chronic stem vessel vasoconstriction caused by abnormalities in oxygenation of the fetal respiratory system hence the term 'cor placentale' is proposed.     

Placental fetal thrombotic vasculopathy is associated with neonatal encephalopathy

Neonatal encephalopathy (NE) remains an important cause of morbidity and mortality in the term infant, and many cases have an antepartum, rather than an intrapartum, etiology. Chronic processes such as thrombosis result in changes in the placenta. We sought to determine whether histopathological examination of the placenta in cases of NE, focusing on these changes, could identify significant antenatal processes that are not recognized by clinical assessment alone. Infants born at term with NE were identified retrospectively over a 12-year period. Placental tissue from deliveries during the study period was available for reexamination. Controls were selected from a cohort of 1000 consecutive deliveries on which clinical and pathological data were collected as part of an earlier study. Bivariate and multivariate analyses of clinical and pathological factors for cases and controls were used to test for an independent association with NE. Clinical and placental data was collected on 93 cases of NE and 387 controls. The placental features of fetal thrombotic vasculopathy (FTV), funisitis (signifying a fetal response to infection), and accelerated villous maturation were independently associated with NE. Of the clinical factors studied, meconium-stained liquor and abnormal cardiotocograph were independently associated. There were no independently associated clinical antenatal factors. Placental features of infection, thrombosis, and disturbed uteroplacental flow are significant independent factors in the etiology of NE in this study. Acute and chronic features suggest that NE may result from acute stress in an already compromised infant. The absence of significant clinical antenatal factors supports the value of placental examination in the investigation of infants with NE.     

Hemorrhagic endovasculitis and hemorrhagic villitis of the placenta

A unique vascular lesion has been identified in 32 placentas sent to the Michigan Placental Tissue Registry, East Lansing, Mich. Half of the associated infants are stillborn; 11 of 16 liveborn were in distress or small for gestational age. Placental vessels show thrombosis, endothelial, and medial hyperplasia and narrowing or obliteration of the lumen. A microangiopathic process is suggested by intraluminal fragmentation of erythrocytes with diapedesis of intact and fragmented RBCs through vessel walls. Red blood cell fragments and hemosiderin are present within villous stroma. Nucleated erythrocytes in placental vessels suggest fetal hypoxia. On gross examination, the placentas are meconium stained. The umbilical cords are frequently edematous, redundant, and around a fetal part; gradual narrowing of placental vessels may be related. Chronic villitis is present in 75% of the cases, intranuclear inclusions have been identified in 10%. An infectious agent, possibly viral, is suggested; toxic and chemical substances must be considered.     

The role of intraplacental vascular smooth muscle in the dynamic placenta: a conceptual framework for understanding uteroplacental disease

Although non-innervated, the placenta must continually accommodate changes in uteroplacental pressure (due to spiral artery failure, maternal position, maternal emotional state, etc.), which might otherwise be expected to result in rapid feto-maternal water fluxes in the highly water-permeable human hemochorial placenta. Uteroplacental flow must also be under the same influences, producing temporary, or permanent, regions of poor intervillous flow, yet the reduction of umbilical vein oxygen content that would be expected to be produced by such shunts of feto-placental blood do not occur in the normal fetus. We suggest that there is a local villus tree mechanism matching intravillus flow of fetal blood to local uteroplacental oxygen content. By analogy to ventilation/perfusion (V/Q) matching in the postnatal lung we suggest the term U/Q matching for this mechanism in the placenta. We further suggest that such disturbances in flow matching are compensated for by the fetus, via complementary adjustment of umbilico-chorionic artery and umbilical venous flow resistances, utilizing the differing sensitivities of vascular smooth muscle tissues of embryonic and extra-embryonic origin to vasoactive agents.     

Distal villous immaturity

Distal villous immaturity (DVI) is a placental phenotype characterized by enlarged distal villi with excessive stroma, hypercellular villous trophoblast, paucity of vasculosyncytial membranes, and a decreased fetoplacental weight ratio. It is predominantly observed in term or near-term placentas and is associated with specific maternal conditions including impaired glucose tolerance, obesity, and excessive pregnancy weight gain. A similar phenotype is observed in placentas with hypercoiling of the umbilical cord. DVI is also occasionally seen in the placentas of preterm infants with idiopathic fetal growth restriction, congenital malformations, and genetic or chromosomal abnormalities. In infants of diabetic mothers, DVI may in part be a consequence of excessive maternal glucose leading to release of fetal insulin and other growth factors that promote excessive placental growth at the expense of villous maturation. Adverse outcomes associated with DVI include intrauterine fetal death (IUFD), fetal growth restriction (FGR), and, more speculatively, late gestational hypoxia and chronic diseases of childhood and later adult life.     

Elevated circulating fetal nucleated red blood cells and placental pathology in term infants who develop cerebral palsy

An elevated circulating fetal nucleated red blood cell count has long been recognized as an indicator of significant intrauterine stress. However, the nature of the causative events and their timing remain controversial. In this study, subacute and chronic placental lesions known to be associated with neurodisability were used as surrogates for antenatal stress. Mother-infant pairs with complete blood counts within 2 hours of delivery (n = 81) were drawn from a larger database of 152 term infants with cerebral palsy. An elevated nucleated red blood cell count (2.5 × 10³/mm³) in these infants was associated with a significantly increased prevalence of subacute or chronic placental lesions, whereas clinical findings did not significantly differ. The number of nucleated red blood cells per 10 high-power fields of villous parenchyma was directly correlated with the nucleated red blood cell count, and a threshold of 10 or more nucleated red blood cells predicted a nucleated red blood cell count greater than 2.5 × 10³/mm³. Among individual placental lesions, multiple foci of avascular villi and chronic villitis were significantly associated with an elevated nucleated red blood cell count, whereas meconium-associated vascular necrosis showed a borderline association. Acute chorioamnionitis was the only placental lesion more common in the group without elevated nucleated red blood cell count. The presence of significant placental lesions was associated with an elevated nucleated red blood cell count in infants with or without either acidosis (cord pH <7.0) or birth asphyxia (American College of Obstetrics and Gynecology criteria). Acidosis and birth asphyxia were not significantly related to an elevated nucleated red blood cell count in infants without these placental lesions.     

Placental pathology in pre-eclampsia eclampsia syndrome

Quantitative analysis of placental pathology was carried out on 20 placentae from various grades of pre-eclampsia eclampsia syndrome and 20 placentae from control group. Placental weights were lower in the study group. The gross abnormalities noted were the placental infarcts, retroplacental haematoma and calcification. The striking villous lesions observed in the study group were cytotrophoblastic cell proliferation, thickening of villous basement membrane and paucity of vasculosyncytial membrane and these findings correlated well with the severity of maternal disease. These vascular villous lesions were considered secondary to uteroplacental ischaemia.     

Expression of class II MHC gene products by macrophages in human uteroplacental tissue.

The expression of class II MHC determinants by fetal and maternal macrophages in human uteroplacental tissues was examined with monoclonal antibodies directed against HLA-DR, DP and DQ antigens. Maternal macrophages in early and full-term pregnancy decidua were HLA-DR positive, and a substantial proportion also expressed DP and DQ antigens. Fetal macrophages in chorionic villous stroma in first-trimester pregnancy were rarely HLA-DR positive, and DQ and DP antigens were never expressed. In term placental tissues, groups of villous stromal macrophages were HLA-DR positive, as were fetal macrophages within amniotic mesenchyme. In contrast, DP and DQ antigens were detected on a small minority of fetal macrophages in term placental tissues. DP and DQ antigens were not detected in the absence of DR antigens.     

Human physiological adaptation to pregnancy: Inter‐ and intraspecific perspectives

Reproductive success requires successful maternal physiological adaptation to pregnancy. An interspecific perspective reveals that the human species has modified features of our haplorhine heritage affecting the uteroplacental circulation. We speculate that such modifications — including early implantation and deep, widespread invasion of fetal (trophoblast cells) into and resultant remodeling of maternal uterine vessels — are responses to or compensation for the biomechanical constraints imposed by bipedalism which, in turn, render our species susceptible to the pregnancy complication of preeclampsia. Preeclampsia is characterized by incomplete remodeling of maternal uterine vessels as the result of shallow trophoblast invasion, which in turn reduces uteroplacental blood flow and frequently leads to intrauterine growth restriction (IUGR). Using an intraspecific perspective, we consider the fitness‐related consequences of variation in uteroplacental blood flow during high‐altitude pregnancy. Although birth weights are reduced at high altitudes in Bolivia, multigenerational Andean residents are relatively protected from altitude‐associated IUGR. Our preliminary data suggest that Andean women have greater uteroplacental oxygen delivery than European high‐altitude residents due to more complete growth and remodeling of maternal uterine vessels. Identification of the physiological and genetic mechanisms involved in such inter‐ and intraspecific variations in pregnancy physiology will likely be useful for understanding human evolution and contemporary challenges to successful reproduction. Am. J. Hum. Biol. 15:330–341, 2003. © 2003 Wiley‐Liss, Inc.     

Fas and Fas-ligand are expressed in the uteroplacental unit of first-trimester pregnancy

PROBLEM: Fas and Fas-ligand (FasL) are thought to provide a strategy for reducing graft rejection in immunologically 'privileged' tissues by controlling injurious lymphocyte reactions. As the uteroplacental unit is often defined as an immune-privileged site, we investigated the expression of Fas and FasL in this tissue in the first trimester of pregnancy. METHOD OF STUDY: Western blotting, immunohistochemistry, and double immunofluorescence were used for this examination. RESULTS: Western blotting with purified first-trimester trophoblast cells revealed one specific band for FasL. The presence of FasL on different trophoblast populations could be confirmed by immunohistochemistry and double immunofluorescence. In the villous part of the placenta, FasL is mostly located on cytotrophoblast cells with no access to maternal blood flow, whereas in trophoblast-invaded uterine tissue, interstitial trophoblast cells, which are in close contact with maternal leukocytes, revealed a strong signal for FasL, but no staining for Fas on these cells. However, Fas was found on CD45+ maternal leukocytes. CONCLUSION: Based on our experimental findings, we speculate that the abundant presence of FasL on trophoblast cells within the maternal decidua may play an important role in the maintenance of immune privilege in the pregnant uterus by endowing fetal trophoblast cells with a defense mechanism against activated maternal leukocytes, whereas in the villous part of the placenta, the Fas FasL system seems to be involved in the regulation of placental growth.     

A stereological perspective on placental morphology in normal and complicated pregnancies

Stereology applied to randomly-generated thin sections allows minimally-biased and economical quantitation of the 3D structure of the placenta from molecular to whole-organ levels. With these sampling and estimation tools, it is possible to derive global quantities (tissue volumes, interface surface areas, tubule lengths and particle numbers), average values (e.g. mean cell size or membrane thickness), spatial relationships (e.g. between compartments and immunoprobes) and functional potential (e.g. diffusive conductance). This review indicates ways in which stereology has been used to interpret the morphology of human and murine placentas including the processes of villous growth, trophoblast differentiation, vascular morphogenesis and diffusive transport. In human placenta, global quantities have shown that villous maturation involves differential growth of fetal capillaries and increases in endothelial cell number. Villous trophoblast is a continuously renewing epithelium and, through much of gestation, exhibits a steady state between increasing numbers of nuclei in cytotrophoblast (CT) and syncytiotrophoblast (ST). The epithelium gradually becomes thinner because its surface expands at a faster rate than its volume. These changes help to ensure that placental diffusing capacity matches the growth in fetal mass. Comparable events occur in the murine placenta. Some of these processes are perturbed in complicated pregnancies: 1) fetoplacental vascular growth is compromised in pregnancies accompanied by maternal asthma, 2) changes in trophoblast turnover occur in pre-eclampsia and intrauterine growth restriction, and 3) uteroplacental vascular development is impoverished, but diffusive transport increases, in pregnant mice exposed to particulate urban air pollution. Finally, quantitative immunoelectron microscopy now permits more rigorous analysis of the spatial distributions of interesting molecules between subcellular compartments or shifts in distributions following experimental manipulation.     

Preeclampsia-related inflammatory cytokines regulate interleukin-6 expression in human decidual cells

Preeclampsia, a common pregnancy disorder associated with an increase in systemic inflammation, is the leading cause of maternal and fetal morbidity and mortality throughout the world. It is associated with shallow extravillous trophoblast invasion of the decidua, leading to uteroplacental blood flow that is inadequate for the developing fetal-placental unit. In preeclamptic women, interleukin-6 (IL-6) levels in plasma, but not placenta, are elevated, prompting evaluation of the decidua as a potential source of this excess, circulating IL-6. The current study found significantly higher immunohistochemical staining for IL-6 in decidual cells from preeclamptic versus preterm, gestational age-matched control placentas. Pro-inflammatory cytokines associated with the genesis of preeclampsia (i.e., tumor necrosis factor-alpha and interleukin-1beta) enhanced IL-6 mRNA levels and increased secreted IL-6 levels in first trimester leukocyte-free decidual cell incubations, as measured by real time quantitative RT-PCR, ELISA, and Western blotting. Therefore, decidual cell-derived IL-6 may contribute to excess circulating IL-6 levels that can promote both endothelial cell dysfunction (and subsequent vascular dysfunction) and the pathogenesis of preeclampsia whereas locally elevated IL-6 levels may contribute to an excess of decidual macrophages implicated in shallow extravillous trophoblast invasion of the decidua.     

Intramural Blood Flow and Blood Volume in the Small Intestine of the Gat as Analyzed by an Indicator-Dilution Technique

The mucosal red cell and plasma flow and volume in the cat small intestine were studied at various levels of smooth muscle vascular tone by means of an indicator-dilution technique. ³²P-labelled red cells and plasma particles as well as ¹⁹⁸Au-labelled colloid particles were used. The monitored tissue region includes when using ³²P most of the mucosa, while only the villi were “seen” with ¹⁹⁸Au. During “rest” (total intestinal blood flow about 25 ml/min × 100 g) “mucosal” plasma flow was somewhat lower than total intestinal blood flow while “villous” plasma flow was of the same order of magnitude. “Mucosal” and “villous” plasma volumes measured about 2.0 and 1.5 ml/100 ml, respectively. As the intestinal vascular bed was maximally dilated (total intestinal blood flow 200–250 ml/min × 100 g) by an i.a. infusion of isopropylnoradrenaline, “villous” plasma flow increased to 275 ml/min × 100 ml tissue while “mucosal” blood flow was the same as total intestinal flow. Concomitantly, “mucosal” and “villous” plasma volumes increased to 3.0 and 4.0 ml/100 ml, respectively. Mean transit time through the villous hairpin loop decreased from 5–6 s at rest to about 1 s at maximal vasodilatation.     

Correlation of placental pathology with prenatal ultrasound findings

There have been recent major advances in obstetric ultrasound, regarding both improved technologies and sonographer expertise, which have resulted in changes in antenatal obstetric management. The placenta is routinely examined to some extent at the time of the second trimester fetal anomaly sonogram, timing of delivery in pregnancies complicated by intrauterine growth restriction is primarily dependent on Doppler sonographic assessment of umbilical and uterine artery blood flow, and an increasing number of specific placental lesions have been described. Many non-specialist diagnostic histopathologists may be unfamiliar with these obstetric advances, but they are an increasingly common indication for submission of placentas for histological examination. Since the aims of pathological examination of the placenta are to determine the pathological basis for the clinical findings and advance understanding of the pathophysiology of pregnancy complications, this review therefore provides an overview of the most common prenatal sonographic techniques and their clinical relevance to the diagnostic pathologist, primarily focusing on conditions with specific placental implications. These range from abnormalities of placental site and cord insertion, to obstetric complications such as antepartum haemorrhage, through sonographic placental parenchymal lesions such as subchorionic and intervillous thrombi, or chorioangiomata. In addition, the pathophysiological basis of abnormal maternal and fetal maternal Doppler indices and intrauterine growth restriction are now described, being associated with decidual vasculopathy and villous changes associated with reduced intervillous blood flow respectively. Finally, rare but characteristic, sonographic appearances of villous cystic or hydropic change, may be associated with intrinsic developmental placental abnormalities such as hydatidiform mole and placental mesenchymal dysplasia, which require histological examination for their specific diagnosis.     

The contribution of placental oxidative stress to early pregnancy failure

In cases of miscarriage, onset of the maternal blood flow to the placenta is precocious and disorganized compared with this event in normal pregnancy. We sought to determine whether this difference is associated with excessive levels of oxidative damage and stress in the placental tissues. Morphological and immunohistochemical markers of cellular stress and damage, including expression of heat shock protein 70, formation of N-Tyr residues, and lipid peroxidation, were increased in tissues obtained from missed miscarriages compared with controls. The effect was greatest in those pregnancies of shorter than 77 days' duration and with evidence of recent fetal demise. It was associated with increased apoptosis and decreased numbers of mitotic cells, indicating that oxidative stress overwhelms cellular antioxidant defense systems. No differences were observed between miscarriages with normal and abnormal karyotypes. The spectrum of villous changes occurring after fetal demise indicates that the duration of placental retention in utero after fetal demise is a critical determinant of villous histology. The causes of many miscarriages remain unclear; however, our findings indicate that placental oxidative stress with resultant damage to the syncytiotrophoblast, secondary to early onset of the maternal circulation, may provide a final common mechanism.