Vascular biology in implantation and placentation

Ó Springer 2005 Angiogenesis (2005) 8: 157--167 DOI 10.1007/s10456-005-9007-8 Review Vascular biology in implantation and placentation Berthold Huppertz1 & Louis L.H. Peeters2 1 Department of Anatomy II, University Hospital RWTH Aachen, Aachen, Germany; 2Department of Obstetrics & Gynecology, University Hospital Maastricht, Maastricht, The Netherlands Received 14 October 2004; accepted in revised form 12 December 2004 Key words: extravillous trophoblast, invasion, IUGR, preeclampsia, spiral artery Abstract Pregnancy leads to dramatic changes of the vascular system of the mother and enables the development of a completely new vascular system within the growing embryo including the formation of the placenta as the exchange organ between both circulations. Besides a general adaptation of the maternal blood system, the uterine spiral arteries display the greatest changes. Within placental villi angiogenesis as well as vasculogenesis can be found already a few weeks after implantation. Both systems in parallel will determine the blood flow within the placental villi and the intervillous space. Finally, compromised blood flow on either side of the placental membrane will not only lead to fetal malnutrition, but will also trigger morphological changes of the villous trees. This review tries to cover all the abovementioned topics and will try to depict the consequences of poor placentation on mother and fetus. Why do we need a placenta -- and why is there this huge diversity? An important characteristic of evolution is the neverending rise in diversity and complexity of species, an aspect implicating that also maturation of a fertilized egg can be expected to become more complex and with it slower and more vulnerable to the dangers encountered in the environment (fluctuations in temperature, pressure and humidity, the presence of predators, etc). To be able to cope with the latter, complex species such as mammals have developed systems for their immature offsprings to provide them with protection and environmental stability. In conjunction with these systems the placenta has evolved to become the key regulatory organ to enable the fertilized egg to grow and mature for a certain period in a secure environment at the expense of the maternal organism. There are three major groups of mammals: the egglaying monotremes, the viviparous marsupials and the eutherians. Eutherians, which include rodents, domestic animals and humans, are classified as ‘placental mammals’ (placentalia), because they have an allantoic placenta from a certain stage of pregnancy until birth. The placenta couples a wide structural diversity beCorrespondence to: Berthold Huppertz, Department of Anatomy II, University Hospital RWTH Aachen, Germany. Tel: +49-241-8089975; E-mail: bhuppertz@ukaachen.de tween species with a remarkably uniform function. The latter includes (1) regulating the maternal adaptation to pregnancy, and (2) providing the means for optimal exchange of oxygen, nutrients and waste products between mother and fetus [1]. Because of the wide inter-species variety in structure, extrapolation of animal data to the human should be done with caution, but the first comparisons between mouse and human have been successful [2, 3]. Placental morphology and its structural arrangements to perform its functions in the uterus have been used as a basis to assess the phylogenetic relations between mammalian species [4--6]. The most primitive form appears to be the choriovitelline placenta, which is found in most marsupials, and is only a transient structure in most eutherian mammalians. The chorioallantoic placenta has a more sophisticated structure with a similar set of functions. However, the evolutionary link between different types of chorioallantoic placentas and between epitheliochorial, endotheliochorial and hemochorial placentas remains unclear. Data from molecular analyses of large data sets suggest that placental mammalians can be divided into different categories or superorders. The early divergence of ancestral groups was followed by periods of isolation, and this in turn led to the convergent evolution of equivalent features, both between and within the different categories. Thus, although the global organization of the placental morphology appears to differ 158 little between species, the detailed organization and the developmental pathways by which this is achieved vary widely [7]. In the last two decades, the analysis of the genetic control of placental development has provided new and important insights into the evolutionary pathways of placental development. Thus, it has become clear that the number of placenta-specific genes is relatively small [3]. Most of the genes that are crucial for normal development of the placenta also play important roles in the development of other organs. Therefore, evolution of the placenta does not only rely on the appearance of new genes, but rather existing and functioning genes were restructured and used to provide new functions. Early villous vasculogenesis When describing vascular biology in implantation and placentation two different aspects have to be considered. The changes of the maternal vasculature will be the topics of the next chapters whereas the development and changes of the fetal placental vascular system will be described in this chapter. About a week after implantation (days 12--15 post conception, p.c.) the placenta is composed of a layer of extra-embryonic mesoderm and two trophoblast populations. Branches of the syncytiotrophoblast trabeculae start as mere trophoblastic structures with a cover of syncytiotrophoblast and a core of cytotrophoblast cells (primary villi). At around days 15--20 p.c. the cores of the primary villi are filled with mesenchymal cells thus generating secondary villi. At the beginning of the third week of development (day 21 p.c.) mesenchymal cells inside the villi transform into first hemangiogenic precursor cells (tertiary villi). The villous trophoblast of this early stage of development is considered to be paramount in regulating the development of the placental vasculature. Several components of the key signaling pathways are only found in the trophoblast and studies in mouse have shown that the expression and function of specific signaling proteins is only required in the trophoblast [8]. It is speculated that both tissues, trophoblast and mesenchyme, modulate each other’s function and development [9]. Vascularization of the human placenta is achieved by a local de-novo formation of small vessels derived from pluripotent mesenchymal precursor cells inside the placental villous core, rather than angiogenesis with sprouting from already existing vessels of the embryo into the placenta. At the time of vascularization of the placenta, around day 21 p.c. or 35 days amenorrhea, the embryo is in the four-somite stage [10, 11]. At this stage, within the core of the secondary villi, progenitors of hemangiogenic cells differentiate and form first vessels inside the mesenchyme. Since at that stage no infiltration of fetal vessels and blood into the placenta has taken place, the progenitor cells are directly derived from placental mesenchymal cells [11] B. Huppertz & L.L.H. Peeters rather than originating from fetal blood cells. Only slightly later placental macrophages (Hofbauer cells) appear inside the villous core in direct vicinity to the vasculogenic precursor cells indicating a putative paracrine role for these cells during the early stages of placental vasculogenesis [12]. The expression patterns of angiogenic growth factors have been described mostly for later stages of pregnancy, including acidic and basic fibroblast growth factor (aFGF, bFGF) [13, 14], VEGF [15--17], and placental growth factor (PlGF) [18--20]. The receptors of VEGF and PlGF, VEGF-R1 (Flt-1) and VEGFR-2 (Flk-1 or KDR) have also been identified in the placenta [15, 21]. Specific functions that can be ascribed to both receptors have been identified using knockout experiments in mice. These experiments have highlighted that VEGFR-2 (Flk-1/KDR) is crucial for specification and early differentiation of the hemangioblastic precursor cells into umbilical capillaries [22], whereas VEGFR-1 (Flt-1) is essential for the subsequent organization and arrangement of early endothelial cells to form tube-like structures covered by endothelial cells [23--25]. During very early development of the placenta, VEGF is highly expressed in cytotrophoblast cells and secondly also in Hofbauer cells. On the other hand, the respective receptors, Flt-1 and Flk-1 are expressed on the vasculogenic and angiogenic precursor cells [26]. With this constellation an increase in the expression of VEGF and its receptors may orchestrate the temporal and spacial regulation of the differentiation and maturation of villous vascularization [27, 28]. At present, there is growing detailed information concerning the presence and function of VEGF, its two receptors and their relation to angiogenesis and vasculogenesis in the very early stages of human placental vasculogenesis [26, 29--31]. Decidual vascularization after implantation Normal development and function of the placenta requires invasion of the maternal decidua by extravillous trophoblast (EVT) (see section ‘Spiral artery remodeling’), followed by abundant and organized vascular growth. As VEGF combines restricted specificity with the ability to raise vascular permeability, it may also represent, together with its receptors, a key regulator of vascular growth and permeability in the decidua. In the presence of the proper endocrine environment, decidual expression of VEGF and its receptors appears to be locally orchestrated by the interplay between stimulating and inhibiting cytokines, a balance, which changes with advancing placentation [32--34]. In this process, the uterine natural killer cells (uNKs) seem to play an important mediating role, as they (1) make up for 70--80% of the maternal cells in the decidua during early gestation [35], (2) are capable of producing large amounts of angiogenic factors [36], and (3) share with Vascular biology in implantation and placentation Figure 1. Schematic representation of the regional distribution of decidual angiogenesis in the 8-day pregnant mouse. Note the avascular primary decidual zone (PDZ) and the hypervascularized secondary decidual zone (SDZ) with patchy distribution of the vasculature, which is positive for VEGF [37]. the decidual neovascularization a patchy distribution throughout the decidua (Figure 1). However, the invasive EVT cells are to be considered the key regulators of decidual angiogenesis, as they modulate the expression pattern of vasoactive proteins of adjacent decidual cells [38, 39]. Although the local nature of the regulation of decidual angiogenesis complicates its exploration, its advantages in early placentation are obvious, as pro-angiogenic events such as new vessel formation can take place in concert with anti-angiogenic events in the immediate vicinity, such as endothelial cell apoptosis during spiral arteries remodeling. During the first 2 weeks of human implantation, metabolic exchange between mother and embryo depends entirely upon diffusion across a thin layer of avascular decidua surrounding the syncytiotrophoblastic mass of the invading human embryo. This interface has been well described in the mouse, where it is referred to as the ‘primary decidual zone’ (PDZ) [33] (Figure 1). The PDZ seems to coat the trophoblast as if it serves as a barrier for the embryo to protect it against maternal insults [40]. The PDZ contains tight junctions and is avascular, which is likely -- at least in part -- an effect of anti-angiogenic stimuli unidirectionally released by the trophoblast. As the PDZ develops, 159 the surrounding decidual layer becomes densely vascularized and develops into the so-called ‘secondary decidual zone’ (SDZ) (Figure 1). This temporospatial regulation of vascular density in the endometrium adjacent to the invading trophoblast is thought to serve two purposes: (1) The PDZ provides protection of the conceptus against immune rejection in the early postimplantation period, and (2) the PDZ and SDZ together contribute to the creation of an oxygen gradient perpendicular to the decidual-trophoblastic interface [41]. As human trophoblast cells exposed to a hypoxic environment proliferate and expand to create invasive trophoblastic cell columns [42], an oxygen gradient would promote trophoblast mitogenesis centripetally and induce trophoblast differentiation centrifugally. It also guides trophoblast differentiation and protects the mother by restricting invasiveness to a limited, predefined layer of decidua around the conceptus. Key regulators in the processes of neovascularization in the decidua are the angiogenic growth factors VEGF and PlGF and their cognate receptors VEGFR1, VEGFR2 and neuropilin-1 [41, 43]. These proteins are sensitive to endocrine and metabolic cues, in particular hypoxia [44], each of which is highly relevant to the placental and decidual microenvironments. Furthermore, in the interplay between decidua and trophoblast, the latter has been found to produce a soluble form of the VEGFR1 protein, which antagonizes some of the VEGF actions and therefore, contributes to the preservation of the avascular status of the PDZ [43, 45]. While the soluble VEGFR1 probably impedes neovascularization, it does not interfere with the positive effect of VEGF on local vascular permeability and fluid extravasation. Although these observations were obtained in mice, morphological studies of the implantation site [46] and measurements of the oxygen gradient perpendicular to the implantation site [47] provide indirect evidence for a comparable situation in early human pregnancy. Spiral artery remodeling During early placental development cytotrophoblast cells penetrate the syncytiotrophoblast and come into direct contact with maternal uterine tissues at sites called the anchoring villi. Here the trophoblastic cell columns develop with a subset of proliferative trophoblast cells in direct contact to the basement membrane of the villus. The subsequent rows of cells comprise postproliferative daughter cells that shortly later start to actively invade the maternal endometrium and the inner third of the myometrium. From this interstitial route of invasion, a subset of extravillous trophoblast cells changes route. These cells reach the walls of spiral arteries and as soon as they invade into the walls they are termed endovascular trophoblast [48]. This leads to a subdivision of extravillous trophoblast into two subpopulations. (1) The interstitial 160 trophoblast that comprises all those extravillous trophoblast cells of the junctional zone inside the placental bed that are not located inside vessel walls or lumens. (2) The endovascular trophoblast that comprises all those extravillous trophoblast cells that -after interstitial invasion -- have reached the walls or even the lumens of spiral arteries. In early gestation, up to about 12 weeks of pregnancy, endovascular trophoblast partly or even completely occludes the lumens of spiral arteries [for literature, see 48, 49]. The normal growth and development of the fetus strongly depends on the adequate remodeling of the maternal uterine spiral arteries. This process of remodeling is called the ‘physiological conversion of utero- B. Huppertz & L.L.H. Peeters placental arteries’ and is subdivided into three phases (Figure 2). In the first phase, the uteroplacental arterial changes include a generalized perturbation of the arterial walls such as endothelial basophilia and vacuolation, disorganized vascular smooth muscle cells, and at the very end, lumen dilation [50]. It is important to note that these initial changes, which occur during very early pregnancy, are independent from direct trophoblast invasion (Figure 2a). Rather it is considered that a complex paracrine-autocrine set of interactions between smooth muscle cells and endothelial cells is involved, which may evolve independent of trophoblast invasion [50]. These authors demonstrated that during Figure 2. Schematic representation of the changes of uterine spiral arteries from implantation to midgestation. (a) Changes in the walls of spiral arteries can be detected within the whole uterine wall (I). These changes are apparent in the uterine wall opposite to the implantation site during intra-uterine pregnancies (upper left scheme) as well as in the uterus during tubal pregnancies (lower left scheme). (b) During intra-uterine pregnancy the second step of spiral arterial changes can be detected prior to invasion of trophoblast into the wall of the arteries (II). Only later extravillous trophoblast cells invade the wall of spiral arteries, enter the lumen and generate plugs (IIIa). Finally endovascular trophoblast cells remodel the arteries to wide inelastic tubes and guaranty a sufficient blood supply to the placenta (IIIb). Modified and adapted from [48]. Vascular biology in implantation and placentation intra-uterine pregnancies, spiral arteries from both non-implantation and implantation sites display the ‘physiological’ changes. Moreover, in ectopic pregnancies located in the fallopian tube, uterine endometrial spiral arteries undergo these very same physiological vascular modifications [50] (Figure 2a). The second phase is characterized by changes in the vessel wall prior to the invasion of spiral arteries by extravillous trophoblast cells. Before entering the vessel wall, interstitial extravillous trophoblast cells in direct vicinity of the spiral arteries are associated with further dilation of the vessels. Analysis of the setting in the guinea pig reveals that a reduction in the number of smooth muscle cells is accompanied by both accelerated NO production by extravillous trophoblast and deposition of fibrinoid material inside the vessel wall, a process that precedes infiltration with extravillous trophoblast cells [51--53]. The clear description of the respective mechanisms in human beings is still missing, but similar structural changes have been found. For a more detailed description of events see [48]. The main feature of the third phase is the infiltration of the vessel wall from the interstitium by endovascular trophoblast cells (Figures 2bIIIa, b). As a consequence, further dilation of the arterial wall takes place finally reaching an approx. threefold increase in the original diameter of the vessel lumen (Figure 2BIIIb) [49, 54]. At the same time the number of elastic fibers [55] and smooth muscle cells are reduced. It is still unclear whether the smooth muscle cells temporarily de-differentiate as in guinea pig [53] or whether they are completely replaced by endovascular trophoblast [56]. Uterine artery remodeling in mouse pregnancy is characterized by transient de-differentiation in concert with accelerated proliferation of smooth muscle cells resulting in a rise in arterial wall thickness [57]. This process of arterial remodeling can be expected to extend downstream to at least the (proximal portion of the) spiral artery. As this ‘proximal’ remodeling appears to take place in the second half of pregnancy, its potential role in spiral artery remodeling by cytotrophoblast in human beings is probably irrelevant. The heterogeneity of trophoblast invasion into the walls of spiral arteries can be identified by a three dimensional analysis of the depth and width of invasion. Then it becomes clear that the number of invading cells as well as the depth of invasion of uteroplacental arteries is most pronounced in the center of the placental bed, normally the primary site of implantation. Towards the periphery, both number of invading cells and depth of invasion diminish [58] (Figure 3). Pregnancy pathologies such as IUGR are associated with impaired trophoblast invasion of uteroplacental arteries (Figure 3). It is the precise sequence and exact summation of all three phases that guarantees a degree of arterial dilation to adequately and sufficiently per- 161 fuse the placenta throughout gestation. Spontaneous miscarriage is often accompanied by a complete absence of trophoblast invasion [60, 61], while less severe deficiencies are associated with IUGR and early onset preeclampsia [62--64]. This raises the possibility that all three syndromes are part of a spectrum of placental pathologies somehow linked to impaired conversion of the spiral arteries [65]. Maternal blood supply to the placenta Onset of maternal blood supply to the placenta Although extravillous trophoblast cells invade the maternal endometrium and reach the spiral arteries throughout the first trimester of pregnancy, during this period the placenta is not perfused with maternal blood. This is due to the fact that the extravillous trophoblast cells invade the endometrial interstitium, reach the walls of spiral arteries, penetrate the endothelium and generate plugs within the lumens of the spiral arteries, thus hindering maternal blood cells to reach the intervillous space (Figure 2BIIIa) [48]. It has been suggested that it is the enormous extent of invasion that leads to the formation of trophoblast cell aggregates within the spiral arteries that plug their distal segments [46, 66]. The trophoblast plugs behave like filters and only a plasma filtrate reaches the intervillous space (Figure 2BIIIa). On the other hand, these plugs can also be expected to alter blood flow dynamics in the proximal parts of the spiral arteries secondary to hemoconcentration and raised downstream resistance. On the consequences of the latter for regional shunt flow, the rheological properties of the blood in the spiral arteries and the endothelial function proximal to the plugs can only be speculated. Measurements of the oxygen tension within the intervillous space during early gestation revealed oxygen concentrations of less than 20 mm Hg until 10 weeks of gestation [47, 67). This implies that the embryonic development takes place in a low oxygen environment. A reason for this may be that the principal differentiation of organ systems is highly vulnerable to disturbances due to free oxygen radicals [68]. Thus, plugging of spiral arteries may serve as a defense mechanism to protect the embryo from oxygenmediated teratogenesis [69, 70]. Only at the end of the first trimester, around weeks 10--12 of gestation, the plugs start to dislocate to enable maternal arterial blood to reach the intervillous space (Figure 2BIIIb). In normal pregnancies this blood flow is first seen in the periphery of the placenta and only later extends towards the center [60]. After onset of maternal blood supply the oxygen tension within the intervillous space rises three-fold [47, 67], promoting vascular regression of peripheral villous capillaries and finally leading to the generation of stem vessels in the center of villi. Thus, oxygen provides an 162 B. Huppertz & L.L.H. Peeters Figure 3. Schematic representation of the changes of spiral arteries in normal pregnancies and pregnancies complicated by intra-uterine growth retardations (IUGR). Even in a normal placental bed there is a gradient of invasive depth and luminal diameter from the center to the periphery of the placenta. In IUGR cases not only the depth of invasion is reduced but additionally the number of invaded arteries is reduced. Severe early onset IUGR placentas are typically smaller with an eccentric cord due to early loss of a large part of the chorion [59]. Cases with IUGR may have a normal (middle panel) or abnormal (lower panel) uterine artery Doppler waveform. In IUGR cases with a preserved end diastolic flow in the umbilical arteries (PED) the number of villous branches increases due to a relative lowering of the oxygen tension within the placenta (middle panel) (see also Figure 5b). In IUGR cases with an absent or even reversed end diastolic flow in the umbilical arteries (ARED) the number of villous branches decreases due to a relative increase in oxygen tension within the placenta (lower panel) (see also Figure 5c). The blue arrows point to alteration compared to normal pregnancy. Inspired by Kaufmann. important stimulus for re-directing the development of villi. It has to be kept in mind, though that the rise in oxygen tension may also lead to placental oxidative stress and subsequent damage of the syncytiotrophoblast [47, 71]. Flow patterns of maternal blood in the placenta To understand the directional relationship between the flows of maternal and fetal blood within the placenta, knowledge of the organization of the intervillous space is needed. Originating from a main stem villus that arises from the chorionic plate, placental villi are arranged in tree-like structures, placentomes. Even at term the central regions of these lobules comprise the zones with the least developed villi (Figure 4) [72]. Into these regions with only loosely packed immature villi [73, 74] the arterial blood is delivered from the openings of the maternal spiral arteries breaching through the basal plate (Figure 4) [75, 76]. In contrast, the openings of the uterine veins draining the maternal blood from the intervillous space towards the maternal venous compartment are positioned opposite to the more peripheral parts of the villous trees (Figure 4). This organization of arterial inflow and venous outflow results in a maternal blood flow pattern consisting of sprinkle-like blood dispersion from the spiral-artery outlet radially through the villous trees, pushing the earlier-entered intervillous blood to the veins that begin in the cotelydonary periphery. It Vascular biology in implantation and placentation 163 Figure 4. Schematic representation of a villous tree, a placentome, that arises from the chorionic plate. The central region of this tree comprises the zone with the least developed villi (thick immature villi in center of tree). Into this region the arterial blood is delivered from the openings of uterine spiral arteries that breach through the basal plate. The openings of the uterine veins are positioned opposite to the more peripheral parts of the villous tree and drain the maternal blood from the intervillous space back into the maternal blood system. The positions of arterial inflow and venous outflow result in blood flow within the intervillous space that disperses radially through the villous tree in a centrifugal fashion. Modified and adapted from [49]. is likely that the oxygen tension follows this flow pattern with the highest concentrations in the centers of the lobules and a reduction towards the periphery of the villous trees. Vascular changes in pathological pregnancies Poor placentation in IUGR Clinical as well as basic research data suggest that in IUGR maladaptation and malinvasion of uteroplacental arteries result from intrinsic factors, i.e. abnormal biology of the extravillous trophoblast, in parallel with extrinsic, maternal uterine factors, i.e. changes of the surrounding of the uteroplacental arteries, such as impaired decidual remodeling [77, 78], macrophagebased defense mechanisms [79, 80], impaired function of uterine NK cells [81] and maternal endothelial failure to express selectins [82, 83]. All the above factors may interact and generate a cascade of events finally leading to the malinvasion observed in IUGR. For further details see [48]. Villous capillarization under different oxygen conditions The organization and shape of the villous tree is altered in different placental pathologies and is thought to reflect contrasting placental oxygenations [84]. The three major possibilities of constant oxygen concentrations are hypoxia, normoxia and hyperoxia. While the development of hypoxia is easy to explain, it is more difficult to elucidate how hyperoxia is possible within a certain organ. The placenta is a unique organ in that the maternal blood stream supplies oxygen to the organ and the fetal blood stream extracts oxygen from the organ in parallel. Alterations at both ends will lead to respective changes of the oxygen tension within the placenta and will impair the delicate balance of the intervillous PO2 (Figure 5). Hypoxia. Reduced oxygen supply to the maternal side can be due to a preplacental reduction in PO2 because of e.g. hypobaric hypoxia (high altitude) or chronic maternal anemia. Also uteroplacental factors may cause reduced supply, e.g. defective transformation of the spiral arteries. Irrespective of cause, 164 Figure 5. H&E stained sections of placental villous tissues from preterm deliveries. (a) Section from a normal placenta at 32+3 weeks of gestation. The tissue reveals a homogeneous distribution of immature (I) and mature (M) intermediate villi combined with an increasing amount of terminal (T) villi. (b) Section from a case with IUGR and preserved end diastolic flow velocities in the umbilical arteries (31 weeks). The dense package of villi with an increased number of side branches reduces the width of the intervillous space and increases the risk of placental infarctions. The increased number of cross sections of villous branches leads to the clearly visible Tenney-Parker changes (arrows). (c) Section from a case with IUGR and reversed end diastolic flow velocities in the umbilical arteries (REDF since 11 days; 29 weeks). The morphological changes of this case comprise a reduction in villous branching leading to long slender villi with only a few side branches. This is combined with a wide intervillous space and a very low number of terminal villi. B. Huppertz & L.L.H. Peeters reduced supply of blood to the placenta will lead to intervillous hypoxia, provided unaltered umbilical oxygen extraction. This in turn will change the morphology of the villous tree. In mature intermediate villi, lower oxygen will promote proliferation of endothelial cells much more than growth of the surrounding villus. This will increase the number of endothelial protrusions covered by trophoblast, i.e. terminal villi. With an enhanced branching and an increased number of terminal villi, these placentas present clusters of terminal villi with a densely packed intervillous space (Figure 5b). Obviously, the impact of these adaptive changes on the intervillous perfusion differs from that in a capillary microcirculation. An adequate steady-state flow at a constant perfusion pressure of approx. 23 mm Hg at the inlet of a capillary bed is the resultant of resistance regulation in the arterioles and precapillary sphincters proximal to that capillary bed. Oxygen shortage induces a rise in capillary flow by capillary recruitment, which also increases exchange area. Capillary recruitment also tends to reduce capillary resistance as they are positioned in parallel with the existing capillaries. The resulting fall in the perfusion pressure triggers the precapillary sphincters and arterioles to dilate so as to preserve perfusion pressure. In contrast to a capillary microcirculation, the placental microcirculation has neither proximal regulatory arterioles/precapillary sphincters, nor the possibility to recruit parallel channels. As mentioned above, oxygen shortage in the intervillous space induces villous sprouting, which increases surface area at the expense of villous crowding. The latter can be expected to increase resistance to flow and to have a negative impact on the rheological properties of the blood traveling across the intervillous space [85]. The latter only magnifies the problems related to hypoxia. From these inferences it is clear that the intervillous microcirculation has virtually no defense mechanism against hypoxic conditions caused by inadequacy of the spiral artery blood flow capacity. Hyperoxia. On the other hand, due to an increased resistance in the umbilical microcirculation or due to fetal circulatory insufficiency, the extraction of oxygen from the placenta by the fetus may be impaired. If the maternal supply in such a condition is not impaired or less impaired than at the fetal side, this setting will result in an increase in oxygen tension within the placenta and thus relative ‘hyperoxia’. At the same time this will lead to a postplacental fetal hypoxia since the transfer from the placenta to the fetus is reduced. Examples for this situation are IUGR cases with absent or reversed end diastolic flow velocities in the umbilical arteries (ARED cases). In ARED cases where this increased oxygen tension in the placenta is persisting for more than 10 days, structural changes of the villous tree become obvious. Higher oxygen reduces proliferation of endothelial cells in mature intermediate villi resulting in long slender villi with few terminal branches and thus a reduced number of Vascular biology in implantation and placentation terminal villi in a wide intervillous space (Figure 5c). Since branching and number of terminal villi are due to elongation of vessels within mature intermediate villi, the index of branching is a direct measure of the effect of oxygen on endothelial cells. Consequences of poor placentation for mother and fetus Defective placental development leads to a progressively deteriorating placental function. Therefore, clinical symptoms, such as fetal growth restriction and/or hypertensive complications in the mother, occur mostly in the second half of pregnancy, when absolute fetal growth rate per unit placenta is most rapid. Defective placental growth and development can be expected to lead to the following inadequacies in its exchange, metabolic and endocrine functions: Inadequate transplacental exchange. The placental exchange capacity depends on its membrane characteristics (thickness, surface area, diffusion coefficient) and arrangement and rates of fetal and maternal placental blood flows [86]. Generally, the exchange rate of a substrate is limited either by the membrane characteristics or by the uterine and umbilical blood flows, but these interact in a complex fashion. Membrane characteristics are most important, when the exchange occurs by relatively slow passive diffusion as applies to the transfer of certain nutrients (glucose and fat precursors) or when the exchange requires the use of specific carriers (amino acids) [87]. Conversely, the transfer of small molecules such as oxygen and carbon dioxide is primarily dependent upon the blood flow rates at both sides of the placental membrane. Placental insufficiency may develop gradually thus enabling the fetus to adapt to the insufficient transplacental nutrient supply by reducing its growth rate. As long as this process develops slowly, subnormal supply and demand will be in balance at the expense of impaired fetal growth. In contrast, the fetus will have no ‘escape options’, when transplacental exchange becomes acutely compromised, for instance in case of placental infarction or (partial) abruption. Other acute lesions comprise massive peri-villous fibrin depositions during maternal floor infarction [88] and acute decompensation via inter-villus thrombosis resulting in ‘fetoplacental hemorrhage’ [59]. Obviously, acute oxygen deprivation will initiate a vicious circle in the fetus that eventually leads to fetal death due to asphyxia. Inadequate placental metabolic function. Placental oxygen uptake constitutes approx. 40% of total uteroplacental oxygen consumption [89]. Placental metabolism is probably high on the one hand, because of its enormous production of a wide range of hormones, and on the other hand, possibly also in conjunction with high energy costs of actively transferred compounds. Moreover, the placenta breaks down large amounts of glucose into lactate, which serves as an 165 important substrate for fetal growth and oxidative metabolism [90]. Finally, the placenta also regulates fetal amino acid availability, as indicated by the high metabolization/conversion rate of glutamate and serine into glutamine and glycine, respectively, which are subsequently excreted into the umbilical circulation [91]. These inferences suggest that subnormal placental oxygen availability can be expected to interfere with placental nutrient modulation giving rise to a less optimal composition of the ‘fetal diet’. Inadequate endocrine functions. Compromised placental oxygen availability is likely to affect placental hormone production. Theoretically, the latter can be expected to be translated into both subnormal maternal hemodynamic adaptation and subnormal plasma volume expansion [92]. The latter effect, though, may also result from inability of the maternal cardiovascular system to respond to an adequate vasodilatory stimulus generated by the conceptus. At any rate, inadequate maternal circulatory adaptation to an early pregnancy vasodilatory stimulus, irrespective cause, can be expected to affect the preparation of the maternal heart, kidneys and vasculature for the accelerated growth of the uteroplacental circulation in advanced pregnancy. A poorly adapted maternal cardiovascular system will be more vulnerable to the insults exerted upon the endothelial lining by endothelial stressors such as increased shear stress, increased circulating levels of lipid peroxides and free oxygen radicals [93]. If the placenta is poorly developed, the placental release of the latter two stressors is enhanced thus increasing the strain put upon the maternal cardiovascular system, which is already more vulnerable as a result of the defective adaptation to the state of pregnancy. In this context it should be emphasized that endothelial activation plays a central role in the pathogenesis of hypertensive disorders of pregnancy. 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