Deep placentation

Deep placentation in human pregnancy is realised by deep invasion of the placental bed by the extravillous trophoblast, involving the decidua and the inner (junctional zone) myometrium. Interstitial invasion of the stroma and endovascular trophoblast invasion of the spiral arteries both occur. Deep endovascular trophoblast invasion into the myometrial segments of spiral arteries is important for proper placental functioning. Before this extended vascular invasion begins, decidua-associated vascular remodelling, which includes swelling and disorganisation of the vascular smooth muscle, occurs during a period of rising placental oxygen. This early remodelling step may accommodate the progressively increasing maternal blood flow to the developing placenta. The subsequent trophoblast-associated remodelling step enhances and stabilises the widening of the vessels, whereas the vascular smooth muscle and elastic lamina are replaced by a fibrinoid matrix with embedded trophoblast. Defective deep remodelling contributes to placental malfunctioning in complications of pregnancy.     

Uteroplacental arterial changes related to interstitial trophoblast migration in early human pregnancy

Morphometric and statistical techniques were used to assess the relation of myometrial interstitial trophoblast to the uteroplacental vasculature in 27 intact hysterectomy specimens ranging from 8 to 18 weeks' gestation. It was found that the volume density of cytotrophoblast in the myometrium and in particular the proximity of such trophoblast to the placental bed spiral arteries correlated significantly with morphological alterations in these vessels. The changes included swelling of endothelium, hypertrophy of individual medial smooth muscle cells, and oedema and disruption of the architecture of the vessel wall as a time-related continuum. Some of the changes, such as swollen endothelium and basophilia of medial smooth muscle cells were noted also in spiral arteries in the non-placental bed endometrium but to a considerably less extent than in the placental bed. Intimal vacuolation was common to placental bed and non-placental bed arteries, increased with gestational age and can be considered as a non-specific feature. The migration of endovascular trophoblast into the myometrial spiral arteries in the second trimester occurred only when these arteries had been considerably altered in their morphology. These findings indicate that migratory interstitial cytotrophoblast probably has a role to play in the preparation of the myometrial segments of the uteroplacental arteries for the second wave of endovascular trophoblast migration that occurs in the second trimester of human pregnancy.     

Normal and abnormal transformation of the spiral arteries during pregnancy

This article reviews the anatomy and physiology of the uterine circulation, with emphasis on the remodeling of spiral arteries during normal pregnancy, and the timing and anatomical pathways of trophoblast invasion of the spiral arteries. We review the definitions of the placental bed and basal plate of the placenta, their relevance to the study of the physiologic transformation of the spiral arteries, as well as the methods to obtain and examine placental bed biopsy specimens. We also examine the role of the extravillous trophoblast in normal and abnormal pregnancies, and the criteria used to diagnose failure of physiologic transformation of the spiral arteries. Finally, we comment on the use of uterine artery Doppler velocimetry as a surrogate marker of chronic uteroplacental ischemia.     

Deep trophoblast invasion and spiral artery remodelling in the placental bed of the lowland gorilla

In contrast to baboon or rhesus macaque, trophoblast invasion in the human placental bed occurs by the interstitial as well as the endovascular route and reaches as deep as the inner myometrium. We here describe two rare specimens of gorilla placenta. In the light of recent findings in the chimpanzee, we postulated the occurrence of deep invasion in gorilla pregnancy. Tissues were processed for histology (PAS, orcein), lectin staining (Ulex europaeus agglutinin 1) and immunohistochemistry (cytokeratin 7/17, α-actin). A specimen of young but undetermined gestational age included deep placental bed tissue, showing interstitial and spiral artery invasion of the inner myometrium as well as the decidua. The cell density and depth of trophoblast invasion was equivalent to a human placental bed of 10-14 weeks. Intraluminal trophoblasts were not seen in any of the invaded vessels, allowing no definite conclusions about the origin of the intramural trophoblast and the time-course of spiral artery invasion. A different late second trimester placenta specimen showed scattered extravillous trophoblast in the basal plate and underlying decidua, as well as a remodelled spiral artery containing intramural trophoblast. Absence of inner myometrial tissue precluded assessment of invasion depth in this later specimen. Despite the limited material we can conclude that key aspects of trophoblast invasion are shared by the three hominid species: gorilla, chimpanzee and human.     

Interaction of interstitial trophoblast with placental bed capillaries and venules of normotensive and pre-eclamptic pregnancies

While endovascular trophoblast invasion of the human placental bed spiral arteries has been studied extensively, no information is available on the interaction between interstitially invading trophoblast and uterine capillaries and venules. Placental bed biopsies of eight normotensive and 15 pre-eclamptic patients were double-immunostained for cytokeratin and the endothelial marker CD31, providing satisfactory staining results in six and 10 biopsies, respectively. Interstitial trophoblast tissue density did not differ between the two series of biopsies, implying that this pathway of invasion is not impaired in pre-eclampsia. Both groups showed a similar incidence of approach of non-arterial vascular structures by perivascular trophoblast. Differences in CD31 staining intensity were noticed in different vascular cross-sections. Lower staining intensity was related to the presence of perivascular trophoblast. Because of the identity of CD31 with the platelet-endothelial cell adhesion molecule (PECAM)-1, the trophoblast-dependent downregulation of CD31 may play a role in the control of leukocytic traffic within the placental bed. The phenomena described in this paper did not show any difference between the normotensive and pre-eclamptic patients, implying that interaction of interstitial trophoblast with venous and capillary structures is not related to the pathogenesis of pre-eclampsia.     

Localization of Angiogenic Growth Factors and Their Receptors in the Human Placental Bed Throughout Normal Human Pregnancy

During early human pregnancy invasion of uterine spiral arteries by extravillous trophoblast cells contributes to their remodelling characterised by loss of musculo-elastic media and replacement by fibrinoid containing trophoblast. Despite its importance for successful pregnancy, the mechanisms underlying ‘transformation’ of spiral arteries are not well understood. The aim of this study was to localize expression of members of the angiopoietin (Ang) family (Ang-1, Ang-2 and their receptor Tie-2) and the vascular endothelial growth factor (VEGF) family (VEGF-A, VEGF-C, VEGF-D and their receptors VEGF-R1, VEGF-R2 and VEGF-R3) in the placental bed throughout normal human pregnancy. Placental bed biopsies were obtained from women undergoing elective termination of pregnancy at 8–10, 12–14 and 16–20 weeks' gestation and elective caesarean section at term (n = 6 each group). Paraffin-embedded sections were immunostained for Ang-1, Ang-2, Tie-2, VEGF-A, VEGF-C, VEGF-D, VEGF-R1, VEGF-R2 and VEGF-R3 using an avidin biotin peroxidase technique. Reactivity of endovascular, interstitial, intramural and multinucleate extravillous trophoblast populations in the placental bed was analysed semi-quantitatively. There was an increase in the level of immunostaining of intramural EVT for Tie-2 and VEGF-C with increasing gestational age. In addition, there was a reduction in Ang-1 and Ang-2 expression by multinucleate interstitial EVT and of VEGF-R1 and VEGF-R2 by endovascular EVT with increasing gestational age. At the earlier gestational ages studied, immunostaining for Ang-1, Ang-2, Tie-2, VEGF-C, VEGF-R1 and VEGF-R2 on intramural EVT was reduced compared to both mononuclear interstitial and endovascular EVT. These findings suggest that the Ang and VEGF families may play a role in the process of spiral artery remodelling in normal pregnancy.     

Pathophysiology of preeclampsia

Preeclampsia is a multisystem disorder affecting about 5-10% of all pregnancies. It is a major cause of maternal, fetal and neonatal mortality and morbidity. Despite intensive research the etiology of this disease still remains unknown. Until now the inadequate transformation of the smooth-muscle cells of spiral arteries in the placental bed caused by an insufficient endovascular invasion of the trophoblast has been considered to be the major reason for the development of preeclampsia. Maternal-fetal (paternal) immune maladaptation is implicated in the insufficient trophoblast invasion, which leads to an imbalance of angiogenic and antiangiogenic factors at the maternal-fetal interface. This review summarizes the actual knowledge of important pathophysiological basic principles of preeclampsia.     

Transforming growth factor beta expression in human placenta and placental bed during early pregnancy

Normal human pregnancy depends on physiological transformation of spiral arteries. Pre-eclampsia and fetal growth restriction are associated with impaired trophoblast invasion and spiral artery transformation. Recent data obtained from studies on placenta suggest that temporal changes in expression of TGF-beta3 play a key role in trophoblast invasion and that over-expression of TGF-beta3 in pre-eclampsia is responsible for inadequate trophoblast invasion. There are, however, no studies of specific TGF-betas in the placental bed throughout pregnancy although this is where the invasive trophoblast and spiral arteries are located. In this study we have used immunohistochemistry, Western blot analysis, ELISA and RT-PCR to examine the expression of TGF-beta1, TGF-beta2 and TGF-beta3 in placental bed biopsies and placentas from 7--19 weeks' gestation. The results show that TGF-beta1, 2 and 3 are expressed in the placenta throughout this time but the striking temporal changes in TGF-beta3 expression previously reported were not observed. Extravillous trophoblast within the placental bed expressed TGF-beta2 but not TGF-beta1 or TGF-beta3 while extracellular TGF-beta1 and cytoplasmic TGF-beta2 were detected in decidua. These data suggest that TGF-beta1 and TGF-beta2 but not TGF-beta3 may play a role in trophoblast invasion.     

The role of the placenta in the initiation of spiral artery remodelling in an early pregnant chimpanzee uterus

IntroductionIn this study we evaluated the full extent of placental bed changes (centre to periphery) in a pregnant chimpanzee uterus, kept at the Museum for Central Africa in Tervuren, Belgium. According to placental size the specimen was equivalent to an 8 weeks pregnant human uterus.MethodsHistological sections from central to peripheral tissue blocks of the placental bed were stained to reveal the presence of trophoblast, endothelium, vascular smooth muscle and elastic laminae. As an indicator for early arterial remodelling, we evaluated endothelial nuclear rounding and subendothelial vascular changes within the maternal vasculature in decidua and adjacent inner myometrium.ResultsWhile interstitially invading trophoblasts were present, endovascular trophoblast invasion seemed about to start into one spiral artery outlet at the centre of the placental bed, confirming our previous impression of a later onset of endovascular trophoblast invasion as compared to the human. An early sign of spiral artery remodelling was rounding of the endothelial nuclei. This phenomenon was not related to the local presence of interstitial trophoblast.DiscussionEndothelial nuclear rounding turned out to be a feature of the placental bed as a whole, being significantly less prominent in the adjacent non-placental bed part of the uterus, indicating an effect of the presence of the placenta. The different time-course of early spiral artery remodelling in the chimpanzee as compared to the human may have had a significant impact upon our evolution.     

Immunohistologic identification of trophoblast populations in early human pregnancy with the use of monoclonal antibodies

Three murine monoclonal antibodies (H315, H316, and NDOG1) have been used in a peroxidase-antiperoxidase technique on formalin-fixed paraffin-embedded tissues to identify populations of fetal trophoblast cells by their expression of membrane antigens in chorionic and decidual tissue from the first trimester of normal human pregnancy. H315 and H316 showed comparable staining of placental villous syncytiotrophoblast and cytotrophoblast and were also able to distinguish subpopulations of nonvillous trophoblast in the placental bed, including perivascular and endovascular trophoblastic cells as well as cytotrophoblastic elements within the decidua and myometrium. H315 and H316 also showed cytoplasmic staining of columnar epithelium of endometrial glands throughout the first trimester. In contrast, NDOG1 stained chorionic syncytiotrophoblast but not villous cytotrophoblast and also did not react with any cytotrophoblastic elements in the placental bed. NDOG1 distinguished these different subpopulations of trophoblast as early as 13 to 15 days after ovulation.     

Trophoblast invasion of spiral arteries: a novel in vitro model

Extravillous trophoblasts invade the uterine wall (interstitial invasion) and the spiral arteries (endovascular invasion), replacing the cells of the vessel wall and creating a high-flow low-resistance vessel. We have developed a novel model to allow the interactions between the invading trophoblast cells and the cells of the spiral artery to be directly examined. Unmodified (non-placental bed) spiral arteries were obtained from uterine biopsies at caesarean section. Fluorescently labelled trophoblasts were seeded on top of artery segments embedded in fibrin gels (to study interstitial invasion) or perfused into the lumen of arteries mounted on a pressure myograph (to study endovascular invasion). Trophoblasts were incubated with the vessels for 3-5 days prior to cryo-sectioning. Both interstitial and endovascular interactions/invasion could clearly be detected and a comparison of the extravillous trophoblast cell line, SGHPL-4 and primary first trimester cytotrophoblasts showed both to be invasive in this model. This novel method will prove useful in an area where in vitro studies have been hampered by the lack of suitable models directly examining cellular interactions during invasion.     

Altered protease expression by periarterial trophoblast cells in severe early-onset preeclampsia with IUGR

Adaptation of uteroplacental arteries in patients with early-onset preeclampsia combined with IUGR is compromised due to insufficient invasion of extravillous trophoblast cells (EVT) into the spiral artery wall. The underlying molecular mechanisms are widely unknown. We investigated expression and possible mechanisms of regulation of different matrix-metalloproteases (MMPs) by EVT in placental bed biopsies from patients with early onset preeclampsia combined with IUGR and healthy pregnant women. Expression of MMP-3 and MMP-7 by EVT was markedly reduced in preeclamptic patients, especially close to spiral arteries. In contrast to healthy pregnancies these cells strongly expressed the receptor for leukemia inhibitory factor (LIF). LIF is known to suppress MMP-expression and is produced by uterine natural killer (uNK) cells which we found to be present in higher concentrations in the placental bed of preeclamptic patients, and accumulating aside the spiral arteries. We speculate that in preeclampsia a maternal immune cell network accumulating and interfering in the placental bed leads to an altered cytokine environment, resulting in disturbed trophoblast cell function such as impaired MMP expression and reduced invasiveness.     

Inadequate maternal vascular response to placentation in pregnancies complicated by pre-eclampsia and by small-for-gestational age infants

An examination of the maternal vascular response to placentation shows that physiological changes in the placental bed normally extend from the decidua into the inner myometrium. In pre-eclampsia and in a proportion of pregnancies with small-for-gestational age infants (SGA) the physiological changes are restricted to the decidual segments alone. In addition, complete absence of physiological changes throughout the entire length of some spiral arteries is seen in pre-eclampsia and SGA. This new observation is confirmed in a study of basal plates of placentas from abnormal pregnancies. Intraluminal endovascular trophoblast may be seen in the placental bed spiral arteries in the third trimester in pre-eclampsia and SGA, a feature not seen beyond the second trimester in normal pregnancy. These findings point to a defect in the normal interaction between migratory trophoblast and maternal uterine tissues in pre-eclampsia and in SGA.     

Interstitial trophoblast cells: An enigmatic and variable component of the developing macaque placenta

The distribution of cytokeratin-positive interstitial trophoblast cells in the endometrium of the macaque during placental development was examined. Such cells are moderately abundant only from the 15th through the 22th day of pregnancy, although there is considerable individual variation. During this period of gestation, interstitial trophoblast cells are distributed in the perivascular stroma immediately surrounding spiral arteries, including coils of arteries already invaded by endovascular trophoblast. The interstitial trophoblast cells are not seen to directly intrude into the smooth muscle of the spiral arteries. Very few interstitial trophoblast cells are present from days 12 through 14 of gestation when the arteries are first invaded by endovascular trophoblast. Even fewer interstitial trophoblast cells are seen after day 30 of gestation. The brief time in gestation when interstitial trophoblast cells are abundant coincides with a time when spiral arteries are undergoing rapid modification into patent uteroplacental arteries. It is suggested that during this period, the interstitial trophoblast cells may facilitate changes in shape of the spiral arteries by alterations of the perivascular connective tissue, perhaps through interactions with other cellular constituents rather than by directly modifying the arteries per se.     

The Role of Apoptosis on Trophoblast Cell Invasion in the Placental Bed of Normotensive and Preeclamptic Pregnancies

Background. Placental development depends on careful coordination of trophoblast proliferation and apoptosis; however, the synchrony of its effect on trophoblast invasion is unknown. Objective. To examine the relationship between trophoblast apoptosis and proliferation in placental bed tissue of preeclamptic and normotensive pregnancies. Methods. Serial sections from archived placental bed biopsies of 12 normotensive (group 1) and 12 preeclamptic (group 2) were immunolabeled with a rabbit anti-Ki67 antibody, a mouse anti-cytokeratin 18 and its neo-epitope, and a monoclonal cytodeath M30 antibody. Results. The immunoexpression of Ki67 for all trophoblast cell subpopulations within the myometrium was non-reactive in both study groups. Smooth muscle cells of the microvasculature reflected a moderate degree of proliferation in both groups. Morphometric image analysis of the wall of the spiral artery revealed a mean area of 31,1729?±?51,180?µm² compared to 35,795?±?8045?µm² in groups 1 and 2, respectively. An elevation of intramural trophoblast was evident within the spiral artery of group 1 (13%). Comparative analyses of M30 distribution on corresponding serial sections were 0.06% versus 0% in groups 1 and 2, respectively. The mean field area percentage of interstitial trophoblast invasion was 10.79% versus 2.87% with corresponding areas of apoptosis been 0.8 % versus 1.9 % in groups 1 and 2, respectively. Conclusions. This study demonstrates an increased trophoblast apoptosis in placental bed of preeclamptic compared to normotensive pregnancies with concurrent absence of proliferation at term.     

Pathophysiology of preeclampsia and the role of serotonin

Hypertensive disorders constitute the most common medical complications of pregnancy. In normal pregnancy, impressive physiological changes take place in the maternal cardiovascular system. Morphological changes are the result of invasion of migratory trophoblast cells into the walls of the spiral arteries. After destruction of elastic, muscular and neural tissue in the media, the trophoblast cells get incorporated into the vessel wall and the endothelial lining of the spiral arteries is restored. The physiological changes create a low-resistance, low-pressure, high-flow system with the absence of maternal vasomotor control. Biochemical adaptations in maternal vasculature include changes in the prostaglandin system, the renin-angiotensin-aldosteron system and the kallikrein-kinin system.In preeclampsia, physiological changes in the spiral arteries are confined to the decidual portion of the arteries. Myometrial segments remain anatomically intact and fail to dilate. In addition, the adrenergic nerve supply is left intact. The cause of this impaired endovascular trophoblast invasion is not yet elucidated. But in combination with the imbalance between vasodilator and vasoconstrictor eicosanoids, it gives rise to reduced perfusion of the intervillous space. In the absence of an adequate production of antiaggregatory prostacyclin (PGI(2)), nitric oxide, or both, surface-mediated platelet activation is supposed to occur on the surface of the spiral arteries. Because platelets are the principal source of circulating serotonin, the increased platelet aggregation in preeclampsia causes an increase in serotonin levels. Interaction of serotonin with serotonin(1)- or serotonin(2)-receptors depends on the state of the endovascular trophoblast or endothelium in the spiral arteries and has opposite effects with regard to vasodilating and vasoconstrictive influences.     

Inadequate maternal vascular response to placentation in pregnancies complicated by pre-eclampsia and by small-for-gestational age infants

Summary. An examination of the maternal vascular response to placentation shows that physiological changes in the placental bed normally extend from the decidua into the inner myometrium. In pre-eclampsia and in a proportion of pregnancies with small-for-gestational age infants (SGA) the physiological changes are restricted to the decidual segments alone. In addition, complete absence of physiological changes throughout the entire length of some spiral arteries is seen in pre-eclampsia and SGA. This new observation is confirmed in a study of basal plates of placentas from abnormal pregnancies. Intraluminal endovascular trophoblast may be seen in the placental bed spiral arteries in the third trimester in pre-eclampsia and SGA, a feature not seen beyond the second trimester in normal pregnancy. These findings point to a defect in the normal interaction between migratory trophoblast and maternal uterine tissues in pre-eclampsia and in SGA.     

Structure adaptation and blood flow control in the uterine arterial system after hemochorial placentation

In the course of hemochorial placentation, trophoblast cells build a most conductive placental stream bed for maternal blood with intramural foetal capillaries. The stream bed is adapted to the maternal vascular system, causing local increase in flow and shear stress. Subsequently, local maternal microvascular system vanishes. The remaining upstream arteries, up to the aorta, undergo "physiological changes", i.e. increase in circumference and length, predominantly by growth and remodeling. Peripheral uterine arteries, possibly collapsed due to low intraluminal pressure, show over-dilatation associated with endothelium destruction and trophoblast cell invasion. Overall arterial conductance increases to controlled extent; placental blood flow rate rises moderately. Intraplacental blood pressures are kept low allowing for free perfusion of placental foetal capillaries. According to the generally accepted concept, destructing impact of invading trophoblast cells causes the physiological changes. However, trophoblast invasion does not explain physiological changes occurring before and outside trophoblast invasion. On the other hand, physiological changes may well be induced by shear stress, which operates in extrauterine body regions as signal for vascular adaptation to growth. Thus, it is hypothesized that trophoblast cells control vascular adaptation and placental blood flow rate simply by constructing the hemochorial steam bed, thereby building up shear stress and triggering the general physiological automatism for controlled vascular adaptation to growth.     

Placental oxidative stress: from miscarriage to preeclampsia

OBJECTIVE: To review the role of oxidative stress in two common placental-related disorders of pregnancy, miscarriage and preeclampsia. METHODS: Review of published literature. RESULTS: Miscarriage and preeclampsia manifest at contrasting stages of pregnancy, yet both have their roots in deficient trophoblast invasion during early gestation. Early after implantation, endovascular trophoblast cells migrate down the lumens of spiral arteries, and are associated with their physiological conversion into flaccid conduits. Initially these cells occlude the arteries, limiting maternal blood flow into the placenta. The embryo therefore develops in a low oxygen environment, protecting differentiating cells from damaging free radicals. Once embryogenesis is complete, the maternal intervillous circulation becomes fully established, and intraplacental oxygen concentration rises threefold. Onset of the circulation is normally a progressive periphery-center phenomenon, and high levels of oxidative stress in the periphery may induce formation of the chorion laeve. If trophoblast invasion is severely impaired, plugging of the spiral arteries is incomplete, and onset of the maternal intervillous circulation is premature and widespread throughout the placenta. Syncytiotrophoblastic oxidative damage is extensive, and likely a major contributory factor to miscarriage. Between these two extremes will be found differing degrees of trophoblast invasion compatible with ongoing pregnancy but resulting in deficient conversion of the spiral arteries and an ischemia-reperfusion-type phenomenon. Placental perfusion will be impaired to a greater or lesser extent, generating commensurate placental oxidative stress that is a major contributory factor to preeclampsia. CONCLUSION: Miscarriage, missed miscarriage, and early- and late-onset preeclampsia represent a spectrum of disorders secondary to deficient trophoblast invasion.