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

INTRODUCTION: In 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. METHODS: Histological 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. RESULTS: While 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. DISCUSSION: Endothelial 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.

Emil Selenka on the embryonic membranes of the mouse and placentation in gibbons and orangutans.

BACKGROUND: Emil Selenka made important contributions to embryology in marsupials, rodents and primates that deserve wider recognition. Here we review his work on early development of the mouse and placentation in the great apes. FINDINGS: Selenka was intrigued by germ layer theory, which led him to study inversion of the germ layers in the mouse and other rodents. He found it was growth of the ectoplacental cone that caused a downward shift in the position of the underlying ectoderm and endoderm, leading to an inside-outside inversion of these layers. In primates he made the important discovery that the embryos of gibbons and orangutans develop under a decidua capsularis. Thus all great apes, including humans, exhibit interstitial implantation; this is in contrast to other primates where implantation is superficial. CONCLUSIONS: Selenka's work was thorough and brilliantly illustrated. It was an important influence on his contemporaries and was well known to scientists of the following generation. Embryologists continue to advance our knowledge of fetal membranes and placentation in the mouse, but Selenka's work on gibbons is unique and our knowledge of orangutan placentation is restricted to his specimens.

A.A.W. Hubrecht and the naming of the trophoblast.

The term 'trophoblast' was introduced in 1889 by the Dutch embryologist A.W.W. Hubrecht during his study of early postimplantation stages in the hedgehog. He had selected this species because of its supposed relatedness to ancestral early mammals, which might provide clues to the evolution of viviparity. Although the new term referred in the first place to the nutritive activity of these cells, Hubrecht was well aware of other possible functions, including a protective role by their positioning at the interface between mother and conceptus. He also paid attention to the invasive activity of the trophoblast into the decidua, but was confused about the nature of the trophospongial layer of the placenta. Because of the structural analogy between early postimplantation stages in hedgehogs and humans, he speculated that, in contrast to other primate groups, anthropoids might have been derived from an evolutionary line separate from ancestral insectivore-like mammals. Although the latter idea has obviously become out of date, Hubrecht's writings provide a fascinating insight in the evolutionary thinking at that time, and therefore still merit a close study.

Defective myometrial spiral artery remodelling as a cause of major obstetrical syndromes in endometriosis and adenomyosis.

Endometriosis and adenomyosis are characterized by the presence of ectopic endometrium, but are also associated with functional and structural changes in the eutopic endometrium and inner myometrium. Alterations in the inner myometrium occurring in women with endometriosis and adenomyosis may be at the root of a defective remodelling of the myometrial spiral arteries from the onset of decidualization and result in vascular resistance and increased risk of defective deep placentation. The association of major obstetrical syndromes and different types of defective remodelling of the myometrial spiral arteries has been well documented. The possibility of a link between both endometriosis and adenomyosis and some major obstetric syndromes remains controversial because of at least two factors: first, changes of the inner myometrium are frequently present in women with endometriosis but the diagnosis requires high-resolution imaging such as magnetic resonance which is not routinely performed and second, patients with endometriosis are frequently subjected to prolonged hormone suppressive therapy. Indeed, there is evidence that pre-treatment with a Gonadotropin Releasing-Hormone analogue can improve the uterine microenvironment and implantation rate following IVF in infertile patients with endometriosis.

IFPA Meeting 2011 workshop report II: Angiogenic signaling and regulation of fetal endothelial function; placental and fetal circulation and growth; spiral artery remodeling.

Workshops are an important part of the IFPA annual meeting as they allow for discussion of specialized topics. At IFPA meeting 2011 there were twelve themed workshops, three of which are summarized in this report. These workshops related to vascular systems and circulation in the mother, placenta and fetus, and were divided in to 1) angiogenic signaling and regulation of fetal endothelial function; 2) placental and fetal circulation and growth; 3) spiral artery remodeling.

PP085. Angiotensin II type 1 receptor antibodies increase angiotensin II sensitivity in pregnant rats.

INTRODUCTION: Pregnant women who subsequently develop preeclampsia are highly sensitive to infused angiotensin (Ang) II; the sensitivity persists postpartum. Activating autoantibodies against the Ang II type 1 (AT1) receptor are present in preeclampsia. In vitro and in vivo data suggest that they could be involved in the disease process. OBJECTIVES: The aim of the study was to show if AT1-AB generated by immunisation alters Ang II sensitivity in pregnant rats. METHODS: We generated and purified activating antibodies against the AT1 receptor (AT1-AB) by immunizing rabbits against the AFHYESQ epitope of the second extracellular loop, which is the binding epitope of endogenous activating autoantibodies against AT1 from patients with preeclampsia. We then purified AT1-AB using affinity chromatography with the AFHYESQ peptide. RESULTS: We were able to detect AT1-AB both by ELISA and a functional bioassay. We then passively transferred AT1-AB into pregnant rats, alone or combined with Ang II. AT1-AB activated protein kinase C-alpha and extracellular-related kinase 1/2. Passive transfer of AT1-AB alone or Ang II (435ng/kg per minute) infused alone did not induce a preeclampsia-like syndrome in pregnant rats. However, the combination (AT1-AB plus Ang II) induced hypertension, proteinuria, intrauterine growth retardation, and arteriolosclerosis in the uteroplacental unit. We next performed gene-array profiling of the uteroplacental unit and found that hypoxia-inducible factor 1alpha was upregulated by Ang II plus AT1-AB, which we then confirmed by Western blotting in villous explants. Furthermore, endothelin 1 was upregulated in endothelial cells by Ang II plus AT1-AB. We show that AT1-AB induces Ang II sensitivity. CONCLUSION: Our mechanistic study supports the existence of an "autoimmune-activating receptor" that could contribute to Ang II sensitivity and possibly to preeclampsia.

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.

Deep trophoblast invasion and spiral artery remodelling in the placental bed of the chimpanzee.

Deep trophoblast invasion is usually considered to be a unique feature of human placentation as compared to other primates. Because of the occasional occurrence of preeclampsia in great apes, which in the human is associated with impaired deep invasion, this uniqueness may be questioned. The availability of two well-documented pregnant chimpanzee uteri in the Hubrecht Collection (Museum für Naturkunde, Berlin) allowed us to evaluate the extent of trophoblast invasion in this species. By adjusting currently used protocols, we obtained successful immunohistochemical staining for cytokeratin and α-actin, as well as Ulex europaeus agglutinin 1 (UEA1) lectin staining, in this archival material. In both specimens interstitial trophoblast invasion had occurred in both decidua and myometrium. Because of a lack of published data on fetal growth for this species, fetal sizes (7cm and 13cm) could not be strictly related to gestational ages and thus be compared with the time-course of human trophoblast invasion. However, since the earlier specimen did not show any endovascular trophoblast invasion in spiral arteries - in contrast to pregnant human uteri with equivalent fetal sizes - endovascular migration seems to begin at a different gestational age in the chimpanzee. In the later specimen endovascular trophoblast was associated with spiral artery remodelling in the inner myometrium, and this invasion was extended to include a radial artery, which at that stage still showed relatively intact vascular smooth muscle and elastic lamina. We conclude that invasion depth and spiral artery remodelling are basically similar in chimpanzees and humans, although the seemingly different time of onset may have implications for uteroplacental oxygen supply and fetal development.

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.

Placental effects of systemic tumour necrosis factor-α in an animal model of gestational diabetes mellitus.

BACKGROUND: Gestational diabetes mellitus (GDM) may adversely affect fetoplacental interaction. Numerous reports demonstrate that GDM women have increased circulating tumour necrosis factor-α (TNF), a pro-apoptotic peptide. OBJECTIVE: To examine whether implantation site apoptosis is increased by exogenous TNF in mice heterozygous for a defective leptin receptor (db/+), a GDM animal model. STUDY DESIGN: Implantation sites were studied at gestational day (gd)18.5 in 3 groups: saline-treated wild-type (wt) and db/+ mice, and TNF-treated db/+ mice. Saline or TNF (total dose 4 µg) was administered by miniosmotic pump from gd11.5. Immunostaining for cleaved caspase-3, PAS and cytokeratin was performed for quantification of apoptotic cells, uterine natural killer (uNK) cells, and trophoblast invasion, respectively. The mRNA expression of TNF and TNF-induced apoptotic markers in placenta and mesometrial triangle (MT) was measured by quantitative RT-PCR. RESULTS: The implantation sites from saline-treated wt and db/+ mice showed comparable numbers of apoptotic cells and uNK cells. Compared with the saline-treated groups, TNF-treated db/+ dams had less fetuses; the placental labyrinth and trophospongium contained more apoptotic cells; and the MT contained a higher total number of uNK cells including more cells intensely stained for cleaved caspase-3 as well as cells with negative staining. Trophoblast invasion was shallower in db/+ than in wt mice (14% and 30% of total invasion into MT, respectively) but this was not affected by TNF. The mRNA expression of TNF and apoptotic markers was comparable in the 3 groups. CONCLUSIONS: TNF treatment in db/+ mice raises the number of apoptotic cells in the placenta, and appears to increase the retention of uNK cells in the MT. Db/+ mice demonstrate shallower trophoblast invasion which is unaffected by exogenous TNF.

Maternal apelin physiology during rat pregnancy: the role of the placenta.

OBJECTIVE: Apelin is a multifunctional peptide which is catabolized by the angiotensin-converting enzyme-related carboxypeptidase-2 (ACE2). The peptide is well known for its hemodynamic effects and its role in energy and fluid homeostasis. Pregnancy is a state of dramatically altered maternal hemodynamics and metabolism, but the role of apelin is unknown. To gain further insight in apelin physiology, we investigated relative tissue expression, plasma clearance and metabolic pathways of apelin in pregnant rats. METHODS: We measured maternal plasma apelin levels throughout normal rat gestation and examined relative apelin gene expression in several tissues, including the placenta. We documented apelin clearance using radiolabeled apelin and assessed maternal plasma levels in rats that underwent surgical reduction of the fetoplacental mass, thereby further examining the role of the placenta in apelin clearance. Finally, we localized apelin and ACE2 in the placenta and mesometrial triangle using immunohistochemistry. RESULTS: Maternal apelin plasma concentrations dropped by 50% between mid- and late gestation. Apelin expression was comparable between non-pregnant and late-pregnant rats in non-reproductive tissues. The placenta showed low apelin gene expression compared to brain tissue. Apelin clearance was enhanced in term gestation as evidenced by a steeper decline of the slow phase of the elimination curve of radiolabeled apelin. Compared to sham-operated dams, maternal plasma apelin was raised by 23% in late-pregnant rats in which half of the fetoplacental units were removed at day 16 of gestation. ACE2 mRNA expression was detectable in late- but not mid-pregnancy placental tissue; immunohistochemically, ACE2 was primarily localized in the smooth muscle layer of fetal arterioles in the labyrinth. CONCLUSION: Maternal circulating apelin drops considerably between mid- and late- pregnancy owing to faster clearance. The current data suggest a role for placental ACE2 in the accelerated apelin metabolism.

Changes in endovascular trophoblast invasion and spiral artery remodelling at term in a transgenic preeclamptic rat model.

As a follow-up to our previous study which revealed a surprisingly deeper endovascular trophoblast (ET) invasion on day 18 in a transgenic preeclamptic (PE) rat model (hAngiotensinogen female symbol x hRenin male symbol) compared to non-PE controls, we examined further changes in ET invasion and associated spiral artery (SA) remodelling at term (day 21). PE transgenic rats and non-PE reversely mated (RM) transgenic rats were compared to normal SD rats (C). Sections were stained to visualize trophoblast, fibrinoid, vascular smooth muscle (VSM) and endothelium. SA were evaluated in three depth levels in the mesometrial triangle (MT) using the KS-400 image analysis system. In separate transgenic rats, Doppler ultrasound was performed in uterine arteries, and the resistance indices (RI) were calculated. Although for the whole MT differences in ET invasion were no longer significant between the PE and C, indicating a partial catching up in C rats, there was still significantly more ET in the deepest level in the PE group as compared to the C and RM groups. At the same time the SA walls in PE rats contained significantly more fibrinoid (versus RM and C) and VSM (versus C). In all SA cross-sections, re-endothelialisation was prominent, but significantly different between PE and C group. The Doppler results showed a significantly lower RI in the arcuate uterine artery of the PE group compared to the C group. There was no evidence of elimination of deeply invaded ET at term, previously considered as a possible mechanism for restriction of vascular remodelling in human PE. The differences in vascular remodelling, previously described on day 18 by histology and Doppler data, were maintained on day 21, but there was extensive endothelial repair in the three groups. Atherosis-like lesions were observed in the three groups, most frequently in the RM group, but were never associated with placental infarcts.

Gestational age-dependent repair kinetics of microsurgical defects in monolayers of human amniocytes.

OBJECTIVES: There is paucity of data on the capacity of fetal membranes to repair surgical defects following trauma. We aimed at developing an in vitro model using monolayers of human amnion epithelial cells to study fetal membrane healing. METHODS: Term (n = 6) and preterm (n = 3) fetal membranes were collected at caesarean section. The amnion was digested twice in a trypsin solution. Amniocytes were seeded (250,000-750,000/ml) and incubated at 37 degrees C and 5% CO(2) and 21 or 5% O(2). A microsurgical injury was made centrally in the monolayers and the cultures were incubated for 48 h. Every 6 h, slides were fixed and immunohistochemical staining was performed to quantify proliferation at the site of the defect and centrally in the monolayer. The closure rate was evaluated by measuring the defect size every 6 h. RESULTS: The closure rate of the defects was higher in preterm versus term cultures. Proliferation was significantly higher in the defect zone versus the peripheral zone, and also higher in the preterm group. CONCLUSION: We describe a new model for the study of fetal membrane healing and observed gestational age-dependent repair capacity of the amnion.

Interstitial trophoblastic cell fusion and E-cadherin immunostaining in the placental bed of normal and hypertensive pregnancies.

During their invasion of the placental bed, interstitial trophoblasts fuse to multinuclear giant cells which are thought to have lost their invasive properties. Trophoblast fusion is associated with downregulation of E-cadherin, and persistent E-cadherin expression has been linked to defective placentation in preeclampsia. Since a previous study suggested 'premature' giant cell formation in preeclampsia, we started with the working hypotheses that fusion is increased in hypertensive pregnancies, and that the intensity of fusion correlates with the severity of disease. Using double immunostaining for E-cadherin and cytokeratin 7/17, nuclei in interstitially invasive trophoblasts (IT) in the myometrial compartment of the placental bed from normotensive pregnancies (NT, n=8), gestational hypertension (GH, n=4), preeclampsia (PE, n=9), and HELLP syndrome (n=5) were categorised according to the E-cadherin staining of the cell and their occurrence in single, clustered or multinuclear cells. GH and PE patients showed a higher percentage of nuclei in clustered non-fused E-cadherin-positive cells (P<0.01 and P<0.05), and in smaller (bi- and trinuclear) placental bed giant cells (P<0.05) compared to NT pregnancies, suggesting defective IT fusion. In contrast, in HELLP syndrome no such failed fusion could be discerned, which may support the idea of a heterogeneous aetiology of different hypertensive diseases of pregnancy. Since we are still ignorant about the specific role of mononuclear and multinuclear trophoblasts in the placental bed, it is not yet possible to relate the present findings to the pathogenesis of different categories of hypertensive pregnancies.

Endovascular trophoblast invasion, spiral artery remodelling and uteroplacental haemodynamics in a transgenic rat model of pre-eclampsia.

The aim of the present study was to evaluate the depth of endovascular trophoblast invasion and associated remodelling of spiral arteries in a transgenic model of pre-eclampsia in the rat, a species showing a comparable deep invasion during normal pregnancy as the human. Pre-eclamptic (PE) transgenic rats (TGR) (hAngiotensinogen female x hRenin male) and non-PE reversely mated (RM) TGR rats were compared to normal Sprague-Dawley rats (C). Day 18 implantation sites were collected and the presence of endovascular trophoblast, fibrinoid, endothelial and smooth muscle cells were evaluated in spiral arteries in three parallel layers in the mesometrial triangle using an image analysis system (KS-400). In a separate group of animals peak-systolic and end-diastolic velocities were measured by Doppler in uterine and arcuate arteries, and the resistance indices (RI) were calculated. In PE and RM rats, the entire mesometrial triangle contained significantly more endovascular trophoblast and vascular fibrinoid deposits than the C group. No difference was found between the groups in the overall amount of smooth muscle surrounding the lumen, but in the PE and RM groups significantly more muscle was present in parts of the contours covered by trophoblast. There was significantly less CD31-positive endothelium in the total lumen contours of the PE and RM groups than in the C group, but in parts of the contours covered by trophoblast more residual endothelium was present in both TGR groups. Comparison of the three layers indicated deeper invasion in both the PE and RM groups than in the C group. By Doppler analysis of the proximal uterine artery the RI was found to be significantly lower in the PE and the RM group than in the C group. In the arcuate artery, the RI was significantly lower in the PE group as compared to the RM and C groups. We conclude that in this transgenic PE rat model there is deeper endovascular invasion of spiral arteries and decreased RI of uterine arteries at day 18 of pregnancy.

Shifting concepts of the fetal-maternal interface: a historical perspective.

The first microscopic images of the human placenta, obtained in the 1830s, revealed the presence of an epithelial lining separating fetal capillaries from maternal blood, which was in later years successively interpreted as maternal endothelial, decidual and finally as "trophoblastic". With this new term, introduced by Hubrecht in 1889, its embryonic/fetal origin was recognized as well as its role in nutrient uptake from maternal blood. Thomas Huxley considered the presence of a decidua as an important feature for mammalian classification, but still mixed up maternal and trophoblastic tissue. Mathias Duval recognized invasive activities by trophoblast in rodents, but over-interpreted the arterial invasion observed in rats. In the human, unusual endovascular cells were first described by Carl Friedländer, but their trophoblastic nature was only recognized in the early 20th century. Nitabuch's description of a continuous fibrinoid layer underneath the basal plate led to the erroneous concept of a borderline separating the trophoblast-invaded upper decidua from the deeper non-invaded uterine tissue. This concept - based on the study of one pregnant uterus - has been made obsolete by later studies of trophoblast invasion. Many erroneous interpretations of placental histology in the past were logical in the context of then current knowledge. A better understanding depended on improved technology which allowed tracing of histological continuity of structural features in space and time. Although identification of cell types increasingly relies on molecular markers, classical histological principles should still be applied in conjunction with newer techniques in order to arrive at a broad understanding of placental development. Understanding past errors in interpreting placental histology should guard us against overconfidence in so-called breakthrough discoveries.

Erasmus Darwin's enlightened views on placental function.

In his major work "Zoonomia", Erasmus Darwin (1731-1802) devoted one chapter to the placenta, in which the new knowledge of the recently discovered element oxygen was applied to the functioning of this organ. He considered the "cavities" or "lacunae" in the placenta as the main areas for oxygenation of the fetal blood, as he thought them to be structurally comparable to the lungs and the gills of fish. He obviously was aware of species differences in the uterine arterial blood supply to the placenta between humans and cows, assuming a higher contractility of the vasculature in the latter species. The new evidence for a primarily respiratory role overshadowed ideas of a possible nutritive function of the placenta. Since Hunter's definitive demonstration of separate maternal and fetal blood circulations, nutritive functions of the placenta needed to be explained by transmembrane transport processes, which were unknown at that time. Instead Erasmus Darwin erroneously considered the amniotic fluid as the main source of nutrients for the fetus. His understanding of placental respiration found expression in his long poem on the history of life on earth.

The use of Alamar Blue assay for quantitative analysis of viability, migration and invasion of choriocarcinoma cells.

BACKGROUND: The current techniques for quantifying trophoblast viability, migration and invasion are mainly limited by the need to sacrifice the cells during the test procedure. In this study, the vital dye AB (AB) was used to quantify cell number and viability of BeWo and JEG-3 choriocarcinoma cells, as well as their migration and invasion through fibronectin-coated filters. METHODS: AB was directly added to culture medium of incubated test and control cells. At various time intervals, the redox reaction, in which AB is reduced by the cells, was measured by absorbance readings at 540 and 630 nm. For cell migration and invasion, cells were cultured onto uncoated or fibronectin-coated inserts, respectively. AB reduction of migrated cells was normalized to that of control cells to calculate percentages of migration. This model was also tested in the presence of a reported inhibitor, transforming growth factor (TGF) beta. RESULTS: The curve of %AB reduction versus cell number was linear, with intra- and inter-assay Coefficient of Variations of 1.88%and 2.94%, respectively. AB reduction increased with both seeding concentrations and incubation time with AB. TGFbeta treatment caused a modest decrease in AB reduction in both JEG-3 and BeWo cells. TGFbeta treatment also decreased migration in BeWo, but not in JEG-3, cells. CONCLUSIONS: AB assay is a simple and reliable method for quantifying trophoblast viability, migration and invasion.

The uterine spiral arteries in human pregnancy: facts and controversies.

Uterine spiral arteries play a vital role in supplying nutrients to the placenta and fetus, and for this purpose they are remodelled into highly dilated vessels by the action of invading trophoblast (physiological change). Knowledge of the mechanisms of these changes is relevant for a better understanding of pre-eclampsia and other pregnancy complications which show incomplete spiral artery remodelling. Controversies still abound concerning different steps in these physiological changes, and several of these disagreements are highlighted in this review, thereby suggesting directions for further research. First, a better definition of the degree of decidua- versus trophoblast-associated remodelling may help to devise a more adequate terminology. Other contestable issues are the vascular plugging and its relation with oxygen, trophoblast invasion from the outside or the inside of the vessels (intravasation versus extravasation), the impact of haemodynamics on endovascular migration, the replacement of arterial components by trophoblast, maternal tissue repair mechanisms and the role of uterine natural killer (NK) cells. Several of these features may be disturbed in complicated pregnancies, including the early decidua-associated vascular remodelling, vascular plugging and haemodynamics. The hyperinflammatory condition of pre-eclampsia may be responsible for vasculopathies such as acute atherosis, although the overall impact of such lesions on placental function is far from clear. Several features of the human placental bed are mirrored by processes in other species with haemochorial placentation, and studying such models may help to illuminate poorly understood aspects of human placentation.