Fine structure of the paracellular junctions of terminal villous capillaries in the perfused human placenta

Trophoblast stem cell-based organoid models of the human placental barrier

Human placental villi have essential roles in producing hormones, mediating nutrient and waste exchange, and protecting the fetus from exposure to xenobiotics. Human trophoblast organoids that recapitulate the structure of villi could provide an important in vitro tool to understand placental development and the transplacental passage of xenobiotics. However, such organoids do not currently exist. Here we describe the generation of trophoblast organoids using human trophoblast stem (TS) cells. Following treatment with three kinds of culture medium, TS cells form spherical organoids with a single outer layer of syncytiotrophoblast (ST) cells that display a barrier function. Furthermore, we develop a column-type ST barrier model based on the culture condition of the trophoblast organoids. The bottom membrane of the column is almost entirely covered with syndecan 1-positive ST cells. The barrier integrity and maturation levels of the model are confirmed by measuring transepithelial/transendothelial electrical resistance (TEER) and the amount of human chorionic gonadotropin. Further analysis reveals that the model can be used to derive the apparent permeability coefficients of model compounds. In addition to providing a suite of tools for the study of placental development, our trophoblast models allow the evaluation of compound transfer and toxicity, which will facilitate drug development.

Umbilical cord mesenchymal stem cells from gestational diabetes show impaired ability to up-regulate paracellular permeability from sub-endothelial niche

In vitro studies have shown that Wharton's jelly mesenchymal stem cells (WJ-MSCs) can cross umbilical and uterine endothelial barriers and up-regulate endothelial junctional integrity from sub-endothelial niches. This pericytic behaviour may be lost in pregnancies complicated by gestational diabetes (GDM), where increased vascular permeability and junctional disruption are reported. The aim of the present study was to investigate whether WJ-MSCs isolated from GDM pregnancies displayed any changes in morphology, proliferation, VEGF-A secretion, and their ability to influence paracellular junctional composition and permeability. WJ-MSCs were isolated from human umbilical cords from normal pregnancies (nWJ-MSCs, n=13) and those complicated by GDM (gWJ-MSCs), either diet-controlled (d-GDM, n=13) or metformin-treated (m-GDM, n=9). We recorded that 4-fold more WJ-MSCs migrated from m-GDM, and 2.5-fold from d-GDM cord samples compared with the normal pregnancy. gWJ-MSCs showed a less predominance of spindle-shaped morphology and secreted 3.8-fold more VEGF-A compared with nWJ-MSCs. The number of cells expressing CD105 (Endoglin) was higher in gWJ-MSCs compared with nWJ-MSCs (17%) at P-2. The tracer leakage after 24 h across the HUVEC + gWJ-MSCs bilayer was 22.13% and 11.2% higher in the m-GDM and d-GDM, respectively, HUVEC + nWJ-MSCs. Transfection studies with siRNAs that target Endoglin were performed in n-WJ-MSCs; transfected cells were co-cultured with HUVEC followed by permeability studies and VE-cadherin analyses. Loss of Endoglin also led to increased VEGF-A secretion, increased permeability and affected endothelial stabilization. These results reinforce the pericytic role of nWJ-MSCs to promote vascular repair and the deficient ability of gWJ-MSCs to maintain endothelial barrier integrity.

Size-dependent placental retention effect of liposomes in ICR pregnant mice: Potential superiority in placenta-derived disease therapy

The great challenge in developing safe medications for placenta-derived diseases is to reduce or eliminate fetal drug exposure while still providing the necessary therapeutic effect. Rapid advances in nanotechnology have brought opportunities for the therapy of placenta-derived disease through accumulating the drug in the placenta while reducing its placental penetration. Among various nanocarriers, liposomes are regarded as an ideal type of carrier for placental drug delivery due to their biosafety and biodegradability. However, their placental retention effect with different particle sizes has not been studied. This research aimed to explore a suitable size of liposomes for placenta drug delivery. Cy 5 dye was chosen as a model molecule for tracing the distribution of three different-sized liposomes (∼80 nm, 200 nm, and 500 nm) in ICR pregnant mice. The stability, cytotoxicity, and cellular uptake study of Cy 5-loaded liposomes were performed. The in vivo fluorescence studies on ICR pregnant mice suggested that the particle size of liposomes was positively correlated with the degree of liposome aggregation in the placenta. The ratio of fluorescence in the placenta and fetus section (P/F value) was proposed to evaluate the placental retention effect of different-sized liposomes. The results showed that the liposomes with 500 nm had the highest P/F value and thus exhibited the strongest placental retention effect and the weakest placental penetration ability. Moreover, liquid chromatography-mass spectrometry analysis confirmed the reliability of the fluorescence section analysis in exploring the placental retention effect of nanovehicles. In general, this study introduced a simple and intuitive method to evaluate the placental retention effect of nanoplatforms and defined a suitable size of liposomes for placenta-derived disease drug delivery.

Compromised barrier integrity of human feto-placental vessels from gestational diabetic pregnancies is related to downregulation of occludin expression

AIMS/HYPOTHESIS

Reduced occupancy of junctional occludin is a feature of human placental vessels in the diabetic milieu. However, the functional consequence of this and whether this loss is due to differential expression of occludin splice variants is not known. Our study aimed to investigate the effects of gestational diabetes mellitus (GDM), and its treatment, on endothelial junctional integrity, gene and protein expression of occludin splice variants, and potential regulation of expression by microRNAs (miRNAs).

METHODS

Term placentas were obtained from normal pregnancies (n = 21), and pregnancies complicated by GDM where glucose levels were controlled by diet (n = 11) or metformin (n = 6). Gene and microRNA (miRNA) expression were determined by quantitative real-time PCR; protein expression by immunoblotting; endothelial junctional occupancy by fluorescence microscopy and systematic sampling; and paracellular leakage by perfusion of placental microvascular beds with 76 M_r dextran. Transfection studies of miRNAs that target OCLN were performed in HUVECs, and the trans-endothelial electrical resistance and tracer permeability of the HUVECs were measured.

RESULTS

All three predicted OCLN gene splice variants and two occludin protein isoforms were found in human placental samples. In placental samples from diet-controlled GDM (d-GDM) pregnancies we found a lower percentage of conduit vessels showing occludin immunoreactivity (12%, p < 0.01), decreased levels of the fully functional occludin isoform-A protein (29%), and differential gene expression of OCLN variant 2 (33% decrease), variant 3 (3.3-fold increase). These changes were not seen in samples from the group with metformin-controlled GDM. In d-GDM placentas, increased numbers of conduit microvessels demonstrated extravasation of 76 M_r dextran (2.0-fold). In d-GDM expression of one of the five potential miRNAs targeting OCLN, miR-181a-5p, expression was 2.1-fold that in normal pregnancies. Experimental overexpression of miR-181a-5p in HUVECs from normal pregnancies resulted in a highly significant downregulation of OCLN variant 1 (69%) and variant 2 (46%) gene expression, with decreased trans-endothelial resistance (78%) and increase in tracer permeability (1.3-fold).

CONCLUSIONS/INTERPRETATION

Downregulation of expression of OCLN variant 2 and the fully functional occludin isoform-A protein are a feature of placentas in d-GDM pregnancies. These may be behind the loss of junctional occludin and the increased extravasation of exogenous dextran observed. miR-181a-5p was in part responsible for the downregulation of occludin in placentas from d-GDM pregnancies. Induced overexpression of miR-181a-5p compromised the integrity of the endothelial barrier. Our data suggest that, despite good glucose control, the adoption of lifestyle changes alone during a GDM pregnancy may not be enough to prevent an alteration in the expression of occludin and the subsequent functional consequences in placentas and impaired vascular barrier function in offspring. Graphical abstract.

Imaging the placental glycocalyx with transmission electron microscopy

There is a significant glycocalyx present at the maternal-fetal interface of the human placenta, with increasing evidence to suggest it has an important role in placental function. Glycocalyx is adversely affected by traditional tissue processing and fixation techniques. Using transmission electron microscopy, we present methodologies for reliably imaging and measuring glycocalyx of both the syncytiotrophoblast and fetal capillary endothelium in term healthy placentae. These techniques can be used to study the role of the placental glycocalyx in both health and disease, including pre-eclampsia.

Ex Vivo Dual Perfusion of the Human Placenta: Disease Simulation, Therapeutic Pharmacokinetics and Analysis of Off-Target Effects

In recent years ex vivo dual perfusion of the human placental lobule is seeing an international renaissance in its application to understanding fetal health and development. Here, we discuss the methods and uses of this technique in the evaluation of (1) vascular function, (2) transplacental clearance, (3) hemodynamic and oxygenation changes associated with pregnancy complications on placental structure and function, and (4) placental toxicology and post-perfusion evaluation of tissue architecture.

Regulation of human feto-placental endothelial barrier integrity by vascular endothelial growth factors: competitive interplay between VEGF-A165a, VEGF-A165b, PIGF and VE-cadherin

The human placenta nourishes and protects the developing foetus whilst influencing maternal physiology for fetal advantage. It expresses several members of the vascular endothelial growth factor (VEGF) family including the pro-angiogenic/pro-permeability VEGF-A₁₆₅a isoform, the anti-angiogenic VEGF-A₁₆₅b, placental growth factor (PIGF) and their receptors, VEGFR1 and VEGFR2. Alterations in the ratio of these factors during gestation and in complicated pregnancies have been reported; however, the impact of this on feto-placental endothelial barrier integrity is unknown. The present study investigated the interplay of these factors on junctional occupancy of VE-cadherin and macromolecular leakage in human endothelial monolayers and the perfused placental microvascular bed. Whilst VEGF-A₁₆₅a (50 ng/ml) increased endothelial monolayer albumin permeability (P<0.0001), equimolar concentrations of VEGF-A₁₆₅b (P>0.05) or PlGF (P>0.05) did not. Moreover, VEGF-A₁₆₅b (100 ng/ml; P<0.001) but not PlGF (100 ng/ml; P>0.05) inhibited VEGF-A₁₆₅a-induced permeability when added singly. PlGF abolished the VEGF-A₁₆₅b-induced reduction in VEGF-A₁₆₅a-mediated permeability (P>0.05); PlGF was found to compete with VEGF-A₁₆₅b for binding to Flt-1 at equimolar affinity. Junctional occupancy of VE-cadherin matched alterations in permeability. In the perfused microvascular bed, VEGF-A₁₆₅b did not induce microvascular leakage but inhibited and reversed VEGF-A₁₆₅a-induced loss of junctional VE-cadherin and tracer leakage. These results indicate that the anti-angiogenic VEGF-A₁₆₅b isoform does not increase permeability in human placental microvessels or HUVEC primary cells and can interrupt VEGF-A₁₆₅a-induced permeability. Moreover, the interplay of these isoforms with PIGF (and s-flt1) suggests that the ratio of these three factors may be important in determining the placental and endothelial barrier in normal and complicated pregnancies.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Effect of lipid composition of cationic SUV liposomes on materno-fetal transfer of warfarin across the perfused human term placenta

INTRODUCTION

Use of drugs that cross the placenta freely are currently avoided during pregnancy. We investigated whether cationic small unilamellar (SUV) liposomes of different lipid compositions could prevent the transfer and uptake of warfarin across human term placenta.

METHODS

Cationic liposomes encapsulated warfarin was prepared by using lecithin (F-SUV) or sterylamine (S-SUV) with cholesterol and stearylamine. The size distribution, encapsulation efficiency, and stability were determined in blood-based media. The transfer kinetics of free and liposomally encapsulated warfarin were studied in a dually perfused isolated lobule of human term placenta with creatinine. Concentrations of warfarin were measured by fluorimetry. Data are expressed as % of initial dose added and given as mean ± sd.

RESULTS

Warfarin crossed the placenta freely (14.9 ± 1.1%). Trans placental transfer of warfarin was significantly reduced by F-SUV (6.4 ± 0.6%; P < 0.001) and S-SUV liposomes (5.0 ± 0.8%; P < 0.001). Placental uptake of F-SUV (6.3 ± 1.7%; P < 0.001) was greater than that of S-SUV liposomes (2.2 ± 0.5%; P < 0.001).

CONCLUSION

Our data suggest that cationic liposomes reduce trans placental transfer of warfarin. If confirmed "in vivo", liposomes might provide an alternative non-invasive method of drug delivery to the mother.

Placental vascular dysfunction in diabetic pregnancies: intimations of fetal cardiovascular disease?

In the human placenta, the angioarchitecture of fetal vessels lying in maternal blood is useful for nutrient uptake, but it makes the synthesis, maturation and functioning of placental vessels vulnerable to any alterations in the fetal and maternal environment. This review discusses how the maternal diabetic milieu, and the resultant fetal hyperglycemia and hyperinsulinemia, may act together to produce an altered placental vascular phenotype, which includes increased angiogenesis, altered junctional maturity, increased vascular endothelial-like growth factor (VEGF), altered VEGF and insulin receptor profiles, and upregulation of genes involved in signal transduction, transcription and mitosis in placental endothelial cells. The placental vascular dysfunction does extend to other fetal vascular beds including endothelial cells from umbilical vessels, where there are reports of elevated basal iNOS activity and altered sensitivity to insulin. There is emerging evidence of epigenetic modulation of fetal endothelial genes in diabetes and long-term vascular consequences of this. Thus, placental vascular dysfunction in diabetes may be contributing to and describing disturbances in the fetal vasculature, which may produce an overt pathological response in later life if challenged with additional cardiovascular stresses.

Isolation and properties of an in vitro human outer blood-retinal barrier model

The outer blood-retinal barrier is composed of a monolayer of retinal pigment epithelium (RPE), Bruch's membrane, and the choriocapillaris, which is fenestrated. An in vitro model that includes all these layers within a 3-D architecture confers a clear advantage over traditional monolayer cultures. Cells here, whether endothelial or epithelial, reside in conditions resembling that in vivo and can participate in cell-cell and cell-matrix cross talk. This chapter describes how a human trilayer culture model was generated with RPE (ARPE-19) cells cultured on the epithelial surface of amniotic membrane and with human umbilical vein derived endothelial cells (HUVEC) on the interstitial surface. This model resembles the outer retinal barrier both in restricting transport of small molecules (<4 kDa), possession of occludin-rich tight junctions in the RPE and fenestrated endothelial cells. Techniques used to test the generated trilayer properties are also described and these include imaging of structure and molecular occupancy of tight and adherens junctions, estimation of the barrier efficiency of trilayer by measurement of fluorescein and fluorescein-conjugated tracers under flow, measurement of secreted vascular endothelial growth factor-A and ultrastructural studies, which allow analyses of the fine structure of the tight junctions in the RPE, and the endothelial fenestra.

Vascular dysfunction in the diabetic placenta: causes and consequences

The development and functioning of the human fetoplacental vascular system are vulnerable to the maternal diabetic milieu. These vessels are in direct continuum with the fetal vascular system and are therefore also vulnerable to fetal endocrine derangements. Increased angiogenesis, altered junctional maturity and molecular occupancy, together with increased leakiness, constitute a well-described phenotype of vessels in the Type 1 diabetic human placenta and can be related to increased levels of placental vascular endothelial growth factor. The causes of these observed changes, whether maternal hyperglycaemia or fetal hyperinsulinaemia, still remain to be shown in the human placenta. Mechanistic studies using different vascular systems have shown high glucose and insulin to have profound vascular effects, with elevations in vascular endothelial growth factor, nitric oxide and protein kinase C being behind alterations in junctional adhesion molecules such as occludin and vascular endothelial-cadherin and vascular leakage of albumin. The role of advanced glycation products and oxidative stress in this vascular pathology is also discussed. The altered molecular mechanisms underlying the vascular changes in the diabetic human placenta may reflect similar consequences of high glucose and hyperinsulinaemia.

The presence of kinesin superfamily motor proteins KIFC1 and KIF17 in normal and pathological human placenta

Kinesin superfamily proteins (KIFs) are motor proteins that participate in chromosomal and spindle movements during mitosis and meiosis, and transport membranous organelles and macromolecules fundamental for cellular functions. Although the roles of KIFs in axonal and dendritic transports have been studied extensively, their role in intracellular transport in general is less well known. The diversity of kinesins suggests that each kinesin may have a specific function. Therefore, in this study we aimed to investigate the presence and cellular localization of KIFC1 and KIF17 in normal and pathological human placentas. First-trimester (22-56 days) and normal, preeclamptic (PE), and diabetic-term placental tissues were obtained and further studied by immunohistochemistry (IHC) and Western blot methods. KIFC1 was mainly localized to the syncytiotrophoblast both in early and term placental samples. However, a stronger immunoreactivity was observed both in PE and diabetic placentas compared to normal-term placentas. KIF17 was most intensively localized in developing vascular endothelium in early pregnancy. Even though KIF17 was moderately stained in the endothelium of villi from normal human-term placentas, stronger immunoreactivity was observed in all types of villi of both PE and diabetic placentas. Western blotting of tissue extracts confirmed the IHC results. Here, we demonstrate the presence of KIFC1 and KIF17 in human placenta for the first time. The intense expression of KIFC1 in syncytiotrophoblast and KIF17 in vascular endothelium suggests that both the proteins might be important in a cargo-transport system. An increased expression pattern of both KIFC1 and KIF17 in PE and diabetes might suggest that these proteins may be involved in complex trophoblast functions and placental pathologies. Further studies will clarify the physiological role of KIFs in human placental transport and development.

The mechanisms and regulation of placental amino acid transport to the human foetus

The mechanisms by which amino acids are transferred across the human placenta are fundamental to our understanding of foetal nutrition. Amino acid transfer across the human placenta is dependent on transport across both the microvillous and basal plasma membranes of the placental syncytiotrophoblast, and on metabolism within the syncytiotrophoblast. Although the principles underlying uptake of amino acids across the microvillous plasma membrane are well understood, the extent to which amino acids are metabolised within human placenta and the mechanisms by which amino acids are transported out of the placenta across the basal plasma membrane are not well understood. Understanding the mechanisms and regulation of amino acid transport is necessary to understand the causes of intrauterine growth restriction in human pregnancy.

Endothelia of term human placentae display diminished expression of tight junction proteins during preeclampsia

This study explores the molecular composition of the tight junction (TJ) in human term placenta from normal women and from patients with preeclampsia, a hypertensive disorder of pregnancy. Maternal endothelial dysfunction is a critical characteristic of preeclampsia; hence, we have analyzed its impact on placental vessels. The study concentrates on the TJ because this structure regulates the sealing of the paracellular route. We have found that, in placental endothelial vessels, TJ components include the peripheral protein ZO-1 and the integral proteins occludin and claudins 1, 3, and 5. During preeclampsia, the amounts of occludin and ZO-1 exhibit no significant variation, whereas those of claudins 1, 3, and 5 diminish, suggesting the presence of leakier TJs in the endothelia of the preeclamptic placenta, possibly in response to the decreased perfusion of this organ during preeclampsia. We have unexpectedly found that, in normal placentae, the multinucleated syncytiotrophoblast layer displays claudin 4 at the basal surface of the plasma membrane, and claudin 16 along the apical and basolateral surfaces. The presence of membrane-lined channels that cross the syncytiotrophoblast constituting a paracellular pathway has been determined by transmission electron microscopy and by the co-immunolocalization of claudin 16 with the plasma membrane proteins Na+K+-ATPase and GP135. Since claudin 16 functions as a paracellular channel for Mg2+, its diffuse pattern in preeclamptic placentae suggests the altered paracellular transport of Mg2+ between the maternal blood and the placental tissue.

Critical Role of Tight Junctions in Drug Delivery across Epithelial and Endothelial Cell Layers

Epithelia in multicellular organisms constitute the frontier that separates the individual from the environment. Epithelia are sites of exchange as well as barriers, for the transit of ions and molecules from and into the organism. Therapeutic agents, in order to reach their target, frequently need to cross epithelial and endothelial sheets. Two routes are available for such purpose: the transcellular and the paracellular pathways. The former is employed by lipophilic drugs and by molecules selectively transported by channels, pumps and carriers present in the plasma membrane. Hydrophilic molecules cannot cross biological membranes, therefore their transepithelial transport could be significantly enhanced if they moved through the paracellular pathway. Transit through this route is regulated by tight junctions (TJs). The discovery in recent years of the molecular mechanisms of the TJ has allowed the design of different procedures to open the paracellular route in a reversible manner. These strategies could be used to enhance drug delivery across epithelial and endothelial barriers. The procedures employed include the use of peptides homologous to external loops of integral TJ proteins, silencing the expression of TJ proteins with antisense oligonucleotides and siRNAs as well as the use of toxins and proteins derived from microorganisms that target TJ proteins.

Vascular endothelial cadherin and beta-catenin in human fetoplacental vessels of pregnancies complicated by Type 1 diabetes: associations with angiogenesis and perturbed barrier function

AIMS/HYPOTHESIS

Increased angiogenesis of fetoplacental vessels is a feature of pregnancies complicated by Type 1 diabetes mellitus, but the underlying molecular mechanisms are unknown. This investigation tests whether the diabetic maternal environment alters the phenotypic expression of placental vascular endothelial cadherin and beta-catenin, which have been implicated as key molecules in barrier formation and angiogenesis in the endothelium.

METHODS

Term placental microvessels from normal pregnancies (n=8) and from those complicated by Type 1 diabetes (n=8) were perfused with 76-Mr dextran tracers (1 mg/ml) and subjected to immunocytochemistry, immunoblotting and microscopy. Junctional integrity, localisation and phosphorylation were investigated along with total protein levels of vascular endothelial cadherin, beta-catenin and vascular endothelial growth factor. Stereological sampling and estimation tools were used to quantify aspects of angiogenesis and endothelial proliferation.

RESULTS

In the Type 1 diabetic placentae, junctional localisations of vascular endothelial cadherin and beta-catenin altered significantly, with more than 50% of microvessels showing complete loss of immunoreactivity and with no overall loss of total protein. Tracer leakage was associated with these vessels. There was a two- to three-fold increase in vessels showing junctional phospho-tyrosine immunoreactivity and hyperphosphorylated beta-catenin. Vascular endothelial growth factor levels were higher in these placentae. A four-fold increase in endothelial proliferation was observed, along with an increase in total length of capillaries without any change in luminal diameter.

CONCLUSIONS/INTERPRETATION

Molecular perturbations of vascular endothelial cadherin and beta-catenin occur in fetoplacental vessels of pregnancies complicated by Type 1 diabetes. Phosphorylation and loss of these molecules from the adherens junctional domains may be influenced in part by the elevated levels of vascular endothelial growth factor in the placenta. Perturbations of the junctional proteins may explain the observed breach in barrier integrity and may contribute to the mechanisms that drive proliferation and increases in capillary length.

The effect of oscillating low intensity magnetic field on the Na+, K+, Ca++, and Mg++ concentrations in the maternal and fetal circulation of the dually perfused human placental cotyledon

Dual-sided perfusions of the human placental cotyledon in vitro were used to study effects of low intensity magnetic fields (MFs) of 2 mT, 50 Hz (E1, 10 perfusions) and 5 mT, 50 Hz (E2, 10 perfusions). In the control group C (10 experiments) no field was used. Perfusions lasted 180 min each. Increased release of calcium ions from the placental cotyledon was found in the fetal circulation during perfusion when the 2 mT, 50 Hz MF was used. No changes in the release of sodium and magnesium ions were observed compared to the control group. The 5 mT, 50 Hz oscillating MF intensified the release of sodium ions from the perfused cotyledon both to the fetal and maternal circulation up to the 150th min of the experiment. Increased release of magnesium ions was observed only to the fetal circulation between 120 and 180 min and of calcium ions to the fetal circulation between 60 and 180 min. No significant differences in K concentrations were found between the control and MF exposed cotyledons under conditions of these experiments.

Materno-fetal passage of nutritive and inhalant allergens across placentas of term and pre-term deliveries perfused in vitro

BACKGROUND

The pre- and postnatal environment appears to be of crucial importance for the manifestation of allergic diseases, which often begin during infancy. Although T cell reactivity of fetal origin to a range of common allergens is present in most cord blood samples, the immunological basis remains unclear.

OBJECTIVE

In order to test the hypothesis of transplacental allergen transfer we studied double-sided open ex vivo perfusion experiments of isolated placental cotyledons with the nutritive allergens beta-lactoglobulin (BLG) and ovalbumin (OVA) and the inhalant major birch pollen allergen Bet v1.

METHODS

Placentas of full-term and pre-term newborns were obtained immediately after delivery to recover functionally active maternal and fetal circulations. Thus, a fetal artery and a fetal vein were cannulated and perfused with pure medium (fetoplacental circulation), whereas the intervillous space of placentas was flushed with allergen containing medium by puncture of the basal plate (maternoplacental circulation). Samples that were collected throughout the perfusion experiment from fetal venous outflow were tested by allergen-specific enzyme-linked immunosorbent assays (ELISA) for the presence of allergens indicative of materno-fetal transplacental passage.

RESULTS

We observed transplacental transfer of BLG, OVA and Bet v1 in placentas of term as well as premature deliveries. The respective allergen was readily detectable in fetal effluent at the beginning of the perfusion experiment and allergen levels reached a plateau after about 2 h. The steady state transfer rate of BLG and OVA in term placentas was 0.012% +/- 0.001 and 0.013% +/- 0.001 of total dose, i.e. 130.21 +/- 7.41 ng/mL and 115.83 +/- 6.07 ng/mL, respectively. The observed transfer rate of Bet v1 after 2h of perfusion was 0.155% +/- 0.034 of total dose, that is 2.41 +/- 1.36 ng/mL. Transplacentally transferred concentration of BLG and OVA in pre-term placentas increased continuously throughout perfusion time from 5.32 +/- 0.92 ng/mL at 1 min to 87.53 +/- 21.93 ng/mL at 120 min and 1.35 +/- 0.31 ng/mL at 1 min to 112.87 +/- 5.25 ng/mL at 150 min, respectively.

CONCLUSION

Allergen-specific cord blood reactivity may be attributed to low levels of allergens crossing the human placenta and providing the fetus with the necessary stimulus for T cell priming.

The phenotype of the human materno-fetal endothelial barrier: molecular occupancy of paracellular junctions dictate permeability and angiogenic plasticity

In vitro models predict that molecular occupancy of endothelial junctions may regulate both barrier function and angiogenesis. Whether this is true in human vascular beds undergoing physiological angiogenesis has not been shown. This review presents data which demonstrate there are two distinct junctional phenotypes, 'activated' and 'stable', present in the vascular tree of the human placenta taken from two distinct highly angiogenic gestational periods (first and last trimester). Stability is conferred by the presence of occludin in tight junctions and plakoglobin in adherens junctions. Their localization may be influenced by vascular endothelial growth factor and angiopoietins 1 and 2 that have a similar temporal and site-specific differential expression. The junctional phenotypes are reversible, as shown in studies with endothelial cells isolated from placental microvessels and grown in the presence/absence of cAMP-enhancing agents. Reductions in protein levels and loss of junctional localization of adhesion molecules result in increased permeability to macromolecules, whilst up-regulation and re-targeting of these molecules inhibit cell proliferation and increase transendothelial resistance. These studies suggest junctional adhesion molecules can regulate physiological angiogenesis and vascular re-modelling. Moreover, the activated junctional phenotype of placental microvessels allows them to participate in increased growth and proliferation. This junctional immaturity appears to be at the expense of barrier function resulting in sites of maximal materno-fetal solute exchange.

Comparative developmental anatomy of the murine and human definitive placentae

The placenta of eutherian mammals is a remarkable biological structure. It is composed of both zygote-derived and maternal cells, and mediates the complex interactions between the mother and the fetus that are necessary for fetal growth and survival. While the genetic basis of human placental development and function is largely unknown, its understanding is of immense clinical importance because placentopathies of unknown genetic aetiology are thought to be the cause of many types of pregnancy complications including unexplained miscarriage and intrauterine growth retardation. The mouse is the best-studied mammalian experimental genetic model system and research is not restricted by the inherent ethical and practical limitations associated with the human. As a result, knowledge about the genetic control of mouse placental development has expanded greatly in recent years. In order for this to be of benefit to medical practice, extrapolations from murine to human placentation have to be made. However, comprehensive comparisons of the placentae of these two species are rare. This review therefore compares the developmental anatomy of the placenta between humans and mice with emphasis on structures and cell types that might be analogous between the two species. This could be of particular benefit to mouse developmental geneticists who study placental development and have an interest in the possible clinical implications of their work.

Cyclic AMP and acidic fibroblast growth factor have opposing effects on tight and adherens junctions in microvascular endothelial cells in vitro

Endothelial adherens junctions (AJ) and tight junctions (TJ) are important determinants of vascular permeability and cell morphology. Here, we investigate their regulation, in primary human placental microvascular endothelial cell (HPMEC) cultures, by either aFGF plus heparin (ECGS) or elevated cAMP. The proliferation of HPMEC was weakly stimulated by ECGS, while cAMP was inhibitory. ECGS had little effect on transendothelial resistance (TER), but increased macromolecular permeability, whereas cAMP induced a twofold increase in TER and reduced macromolecular permeability. Ultrastructurally, ECGS-treated HPMEC exhibited an "activated" phenotype typified by proliferating cells, with poorly organized cell-cell junctions, whereas cAMP-treated cells appeared quiescent and markedly flattened with extended paracellular junctions, resembling endothelium in situ. The expression and localization of junctional molecules, F-actin, and junctional phosphotyrosine were examined by confocal microscopy and immunoblotting. Junctional molecules in ECGS-treated cells were less organized at lateral membranes than in control cells, whereas in cAMP-treated cells, they were highly localized at continuous contacts. These differences correlated with the intensity of junctional phosphotyrosine, being lowest with cAMP treatment. In the AJ of ECGS-treated and control cells, beta-catenin predominated but in cAMP-treated cells, gamma-catenin/plakoglobin was enriched. In addition, cAMP upregulated junctional expression of VE-cadherin and PECAM-1 and increased the levels of the TJ molecules occludin and ZO-1. The expression levels of junctional components, and their tyrosine phosphorylation, play an important role in dynamic regulation of endothelial cell-cell junctions.

Protein kinase C activity contributes to endothelial hyperpermeability during early angiogenesis in the chick chorioallantoic membrane

During angiogenesis in the chick chorioallantoic membrane (CAM), microvascular proliferation continues through day 12 of the 18-day CAM lifespan. Up to day 4.5, the neovascularization is associated with endothelial hyperpermeability and differentiation of restrictive barrier function occurs abruptly at day 5.0. Although exogenous activation of cAMP/protein kinase A (PKA) signaling served to decrease macromolecular extravasation at day 4.5, endogenous signaling cascades responsible for the temporal hyperpermeability remain uncertain. Here, we evaluated protein kinase C (PKC) function in the CAM endothelium at day 4.5 and day 5.0. The specific, broad-based PKC inhibitor calphostin C reduced basal levels of FITC-dextran 40 extravasation at day 4.5. Bisindolymaleimide (BIM), which inhibits selective PKC isoforms, also reduced temporal FITC-dextran 40 efflux, but to a lesser extent than calphostin C. Activation of PKC activity by phorbol-12, 13-didecanoate (PDD) or phorbol-12, 13-dibutyrate (PDBu) at day 5.0 served to partially de-differentiate barrier properties of the angiogenic endothelium. The associated elevation of FITC-dextran 40 extravasation occurred without interendothelial gap formation along the junctional clefts. Together, these results are consistent with the interpretation that PKC activity contributes, in part, to CAM endothelial hyperpermeability at day 4.5. Furthermore, down-regulation of PKC signaling correlates temporally with the ontogeny of restrictive barrier function at day 5.0.

Amino acid transporters in the human placenta

Amino acid transport across the human placenta is active, mediated by specific transporters in syncytiotrophoblast plasma membranes. Using functional criteria such as substrate specificity and sodium dependence, approximately 15 transport systems have been identified in the human placenta. Recently, the area of molecular biology of amino acid transporters has evolved rapidly and at least 25 cDNA clones coding for mammalian amino acid transporters or transporter subunits have been identified. The primary objective of this review is to integrate the available functional data on placental amino acid transport systems with recent molecular information on mammalian amino acid transporters. Furthermore, models for the mechanisms for net materno-fetal transfer of amino acids are discussed. Finally, the evidence to suggest that alterations in placental amino acid transport systems may play a crucial role in the regulation of fetal growth are presented briefly.

Molecular organization of tight and adherens junctions in the human placental vascular tree

Tight and adherens junctions are major determinants of endothelial integrity. Molecules present therein have been implicated in vascular permeability, stability of junctions, angiogenesis and intracellular signalling. Using immunofluorescence and confocal scanning microscopy, the adherens junctions (AJs) in human placental vessels were found to contain the entire cadherin-catenin complex predicted from in vitro studies. Vascular endothelial cadherin (VE-cadherin) clusters were co-localized with beta-catenin, an important signal transduction ligand, and with alpha-catenin, which is thought to link the complex to the peri-junctional actin. Antibodies to plakoglobin, a molecule shown to be a component of stable adherens junctions, revealed immunoreactivity in clefts of stromal villous vessels, but weak or negative immunoreactivity in intermediate and terminal villi. Tight junctional molecules demonstrated a differential surface expression. Within the same villous tree, arteries, veins and arterioles contained occludin but the exchange vessels in terminal villi were immunonegative. ZO-1, however, was present throughout. Ultrastructurally, there were no differences in frequency, position or dimension of tight junctions in these vessels. They showed a consistent 4 nm separation between outer membrane leaflets regardless of their location in the vascular tree. Occludin is not necessary for formation of tight junctions in the placenta; it may have an accessory role providing stability or added adhesiveness to tight junctions of large vessels. Its absence in terminal villous microvessels, along with the weak plakoglobin immunoreactivity in AJs, suggest that the junctions here are less stable. This may allow the increased plasticity necessary in terminal villi for continual growth, proliferation and solute exchange.

Proliferation, differentiation and apoptosis in villous trophoblast at 13-41 weeks of gestation (including observations on annulate lamellae and nuclear pore complexes)

Ultrastructural, immunochemical, fluorescence and stereological studies were undertaken on human villous trophoblast from 13 weeks of gestation to term. The aim was to describe and quantify morphological changes during proliferation, differentiation and apoptosis in cytotrophoblast and syncytial regions of non-aggregated and aggregated nuclei. Numbers of trophoblast nuclei increased continuously from 13 weeks. In term placentae, intrasyncytial differentiation was characterized ultrastructurally by gradual decreases in nuclear size and packing density accompanied by nucleolar regression, and increasing heterochromatinization, envelope convolution and packing density of nuclear pore complexes. In densely packed areas, nuclear profiles resembled interlocking jigsaw pieces. Occasionally, these 'pre-apoptotic' nuclei were associated with annulate lamellae. Rarely, nuclear changes terminated in apoptosis with a characteristic pattern of condensed peripheral chromatin, a central island of euchromatin, no nucleoli and no discernible nuclear pores. Apoptotic nuclei were seen singly and within dense nuclear aggregations. Similar spatial patterns of nuclei and chromatin were seen in propidium iodide-stained sections at 13-41 weeks. Whilst the relative incidence of intensely fluorescent nuclei remained constant, absolute numbers increased linearly during gestation and correlated positively with the volume of syncytial knots. Nuclei labelled for DNA fragmentation occurred very infrequently and were also found in nuclear clusters as well as singly. We suggest that nuclear differentiation in syncytium has two phases: on entering syncytium, nuclei become committed to a long programmed pre-apoptotic phase which leads to a short apoptotic execution phase. We propose further that clustered nuclei (pre-apoptotic and apoptotic) in syncytial knots probably represent the extrusion component of normal continuous epithelial turnover.

Microvascular permeability and number of tight junctions are modulated by cAMP

We tested the hypothesis that increased endothelial cell adenosine 3',5'-cyclic monophosphate (cAMP) decreases microvascular permeability in vivo. The effects of cAMP-specific phosphodiesterase type IV inhibition and adenylate cyclase activation on microvascular hydraulic conductivity (Lp) were investigated in intact individual capillaries and postcapillary venules in mesentery of pithed frogs (Rana pipiens). Treatment with rolipram (10 microM) and forskolin (5 microM) for 25 min decreased Lp to 37% of control. Rolipram alone also significantly decreased Lp. Isoproterenol (10 microM) decreased Lp to 27% of control within 20 min. A subgroup of eight vessels treated with rolipram and forskolin, in which mean Lp fell to 25% of control, was examined with transmission electron microscopy. The mean number of tight junctions in the treated vessels was 2.2 per cleft (303 clefts), significantly higher than in a matched control group (192 clefts), which was 1.7 per cleft. The results indicate that microvascular Lp can be modulated by intracellular cAMP and that one of the structural end points of stimulated cAMP levels is an increase in the mean number of tight-junction strands between endothelial cells.

Effect of surface charge of small unilamellar liposomes on uptake and transfer of carboxyfluorescein across the perfused human term placenta

We aim to investigate the effect of surface charge of small unilamellar liposomes on transfer and uptake of a low molecular weight, hydrophilic and polar molecule carboxyfluorescein in an in vitro model of perfused human term placenta. Carboxyfluorescein-encapsulated neutral liposomes were prepared by using an equimolar concentration of lecithin and cholesterol. Anionic and cationic liposomes were prepared by adding dicetylcholine and stearylamine, respectively. Size distribution, encapsulation efficiency, and stability of liposomes in blood-based medium were determined. The transfer kinetics of free carboxyfluorescein and liposomally encapsulated carboxyfluorescein were studied in a dually perfused isolated lobule of human term placenta. The concentration of carboxyfluorescein was measured fluorometrically. The maternal to fetal transfer and placental uptake of free carboxyfluorescein was 1.6 +/- 0.1% and 4.2 +/- 0.1% of the initial dose, respectively. This constitutes the control data. The placental transfer of carboxyfluorescein was significantly increased by neutral (2.5 +/- 0.1%; p < 0.01) and anionic liposomes (3.1 +/- 0.2%; p < 0.001), whereas cationic liposomes prevented its transfer (0.4 +/- 0.1%; p < 0.001). The placental uptake of neutral (14.9 +/- 2.3%; p < 0.001) and anionic liposomes (21.1 +/- 1.2%; p < 0.001) were significantly higher than the cationic liposomes (2.3 +/- 0.6%) and control group (p < 0.001). The placental uptake of cationic liposomes was comparable with the control data. These results indicate that placental uptake of small unilamellar liposomes depends upon their surface charge, and transfer of carboxyfluorescein is enhanced by anionic and impeded by cationic liposomes.

Mechanism of glucose transport across the human and rat placental barrier: a review

Glucose is one of the most important substances transferred from the maternal blood to the fetal circulation in the placenta, and its transport across the cellular membranes is mediated by glucose transporters. Facilitated-diffusion glucose transporter GLUT1 is abundant in the placental barrier, as is the case in other blood-tissue barriers, where GLUT1 is present at the critical plasma membranes of the barrier cells. In the human placenta, the microvillous apical and the basal plasma membranes of the syncytiotrophoblast are rich in GLUT1, which molecule seems to be responsible for the transcellular transport of glucose across the placental barrier. In the rat placental labyrinth, two layers of syncytiotrophoblasts (termed syncytiotrophoblasts I and II from the maternal side) serve as a barrier. GLUT1 is abundant at the plasma membrane of syncytiotrophoblast I facing the maternal side, and the plasma membrane of syncytiotrophoblast II facing the fetal side. Numerous gap junctions, made of connexin 26, connect syncytiotrophoblasts I and II, comprising a channel for the transfer of glucose between them. GLUT1 in combination with the gap junction, therefore, seems to serve as the structural basis for the transport of glucose across the rat placental barrier.

Structure and permeability of human placental microvasculature

Endothelial paracellular junctions are important structures for the regulation of vascular permeability, junctional organisation being systematically related to the functional properties of the endothelium. Electron microscopic studies, immunocytochemistry, and single-passage permeability measurements have established that the placental microvessels resemble the fairly tight continuous microvessels of skeletal muscle both in structure and permeability. The endothelial paracellular clefts of these microvessels contain two distinct junctional entities which may influence permeability: the tight junction and the adherens junction. These clefts impose a substantial restriction to molecules above RMM 1000 Da and large haemproteins cannot cross the clefts. The 18 nm-wide zones of the clefts possess the transmembrane adhesion molecules PECAM-1 and VE-cadherin, which have been implicated in junctional assembly and permeability. Inflammatory mediators such as histamine and tumour necrosis factor cause a redistribution of these adhesion molecules to non-junctional regions, and histamine (100 microM) causes a rapid and sustained rise in extraction of radio-labeled tracers. Electron microscopy has also revealed possible first indications of tight junctional disassembly. Both the endothelia of larger placental vessels and isolated placental microvascular endothelial cells express cadherins and PECAM-1 and contain an extensive F-actin cytoskeleton, which is implicated in changes of cell shape and junctional assembly/disassembly. Thus, the human placental endothelium, using perfusion techniques and in vitro experiments, offers a valuable model for vascular permeability studies.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Not trophoblast alone: a review of the contribution of the fetal microvasculature to transplacental exchange

The fetal microcirculation of the term human placenta offers an interesting microvascular model. A perfused placenta can be used for integrated studies of vascular permeability-structure relationships. The organization of the paracellular pathway in human placental microvessels closely resembles not only that of the guinea-pig placenta, but also that seen in typical continuous non-cerebral capillaries such as those of the myocardium. This uniformity of organization has allowed the development of a model of the organization of endothelial junctional complexes that allows testable predictions about the relationship between junctional organization and microvascular permeability. The key features of this model are: (1) molecular size restriction may be determined by a fibre matrix based on cadherin arrays in the zonula adhaerens. (2) The zonula occludens (tight junction) is discontinuous and so cannot act as a molecular sieve for solutes. It may serve as a shutter that limits the proportion of the paracellular cleft available for permeation. The main implication for placental function is that the human placental microcirculation is relatively tight and is an important restriction to diffusive permeation of the maternal-fetal barrier by large molecules.

Actin cytoskeletal isoforms in human endothelial cells in vitro: alteration with cell passage

The microfilamentous actin component of the cytoskeleton is crucial to endothelial angiogenesis and vascular permeability. Differences in actin cytoskeletal profiles in cultured human endothelial cells were explored: when first isolated, both primary human umbilical vein endothelial cells (HUVEC) and primary human placental microvascular endothelial cells (HPMEC) expressed F-actin, but not beta-actin or alpha-smooth muscle actin. A similar endothelial actin profile was observed in cryo-sections of freshly delivered term umbilical cord and placenta. In subsequent cell culture, although the actin cytoskeleton of HUVEC remained unchanged, the actin profiles of HPMEC altered after the second passage with the induction of alpha-smooth muscle actin expression, which was intercellularly heterogeneous and increased to 20% at P4. This behaviour occurred in HPMEC monolayers cultured on a variety of extracellular matrices. Comparisons with a spontaneously immortalized human microvascular cell-line, HGTEN 21, revealed that in prolonged passage, both alpha-smooth muscle actin and beta-actin were expressed, whereas HPMEC at P4 showed a lower level of beta-actin expression. Therefore, in comparison with large vessels, microvascular cells are more likely to dedifferentiate. This may reflect the ability of microvascular cells to remodel according to changing requirements for new vessel formation. In conclusion, passage of human microvascular endothelial cells, but not of larger vessel endothelial cells, alters the expression of actin isoforms. This may be important in relation to comparisons of in vitro and in vivo vascular permeability; higher passage microvascular endothelial cells should thus be used with caution in such studies.

Transport of transferrin across the blood-thymus barrier in young rats

The interaction of transferrin-peroxidase (Tf-HRP) with the capillary endothelium of the rat thymus was analyzed by diaminobenzidine (DAB) cytochemistry and electron microscopy. The thoracic aortas of young rats were cannulated and the upper bodies perfused with the Tf-HRP conjugate. In the thymus, plasmalemmal vesicles of the endothelium mediated the endocytosis and transport of Tf-HRP. Transcytosis of Tf-HRP appeared to occur by micropinocytosis, without morphological evidence of involvement by endothelial endosomes. DAB reaction product was commonly present in the subendothelial interstitial space and, in addition, was often localized in clathrin coated pits and vesicles in epithelial reticular cells that surround the thymic capillaries. In perfusions done at 4 degrees C, no binding of Tf-HRP to the lumenal membranes of capillaries was detected. The transport of Tf-HRP across the capillary endothelium in the thymus was not inhibited by competition with excess native transferrin; however, the uptake of Tf-HRP by epithelial reticular cells was completely inhibited by excess transferrin. Transferrin receptors were localized in the thymus by indirect immunocytochemistry using OX-26, a mouse anti-rat transferrin receptor monoclonal antibody. No transferrin receptors were detected on the capillary endothelium but diffuse reaction product was localized on the subjacent epithelial reticular cells. These results indicate that transport of Tf-HRP across the thymus capillary wall is independent of transferrin receptors at the level of the endothelial cells but that subsequent uptake of Tf-HRP by epithelial reticular cells is a transferrin receptor dependent process.

Isolation of endothelial cells from human term placental villi using immunomagnetic beads

Chorionic villi excised from freshly delivered human term placentae and small endothelial cell aggregates were released from them by the sequential use of collagenase and trypsin. The endothelial cells were further isolated by rosetting with magnetic polystyrene beads which were coated with QB END/40, the endothelial-specific monoclonal antibody (mAb) to thrombomodulin. Cell rosettes were plated on gelatin coated Petri dishes. The cells initially grew as discrete colonies but reached confluence within 7 days. The monolayers were sub-cultured five times, and grew to confluence each time. All the cells were immunoreactive to the endothelial markers von Willebrand factor, QB-End/40 and Ulex europaeus-1 lectin. They did not show immunoreactivity to trophoblast markers (mAbs ED341 and ED235). The isolated cells could also incorporate acetylated low-density lipoprotein. Most of the cells possessed an elongated morphology, though some were slightly spread and polygonal in shape. The cell monolayers did not resemble the typical cobblestone appearance of endothelial cells isolated from large vessels. Ultrastructurally, most of the cells resembled placental microvascular cells in shape and frequency of caveolae; undifferentiated cell-cell contacts and extracellular matrix material was observed. Human placental microvascular endothelial cells may offer an in vitro model which complements the use of the perfused term placental lobule in studies of microvascular permeability.

Non-electrolyte solute permeabilities of human placental microvillous and basal membranes

1. Permeability to non-electrolytes of isolated microvillous and basal membranes from human term placenta was measured using stopped-flow light-scattering techniques. The studied solutes were urea, ethylene glycol, glycerol, creatinine, erythritol, arabitol and mannitol. 2. At 37 degrees C, permeability of the microvillous membrane to mannitol and urea was 0.30 +/- 0.02 x 10(-6) cm/s (mean +/- S.E.M.) and 3.2 +/- 0.2 x 10(-6) cm/s, respectively. The corresponding permeabilities for the basal membrane were 1.2 +/- 0.1 x 10(-6) cm/s (mannitol) and 4.4 +/- 0.3 x 10(-6) cm/s (urea). The basal membrane was substantially more permeable to hydrophilic solutes than the microvillous membrane. This is probably due to differences in lipid composition, as illustrated by membrane cholesterol content, which was found to be approximately 50% lower in the basal as compared to the microvillous membrane. 3. Similarities between permeabilities in placental membranes and lipid bilayers and the linear relationship noted between solute hydrophobicity and placental permeability suggested that solutes permeate both human syncytiotrophoblast membranes by a solubility/diffusion mechanism. In the microvillous membrane this was supported by data obtained for activation energies (> 10 kcal/mol) and reflection coefficients (close to 1). In the basal membrane, low activation energies for glycerol and urea and a low reflection coefficient for urea indicated that these solutes may, in part, share a common pathway with water. 4. It was estimated that the placental permeability to molecules with a molecular weight under 200 observed in vivo can, to a great extent, be accounted for by transcellular permeation.

Permeability of the fetal villous microvasculature in the isolated perfused term human placenta

1. Capillary permeability-surface area (PS) products for the low molecular weight radioactive tracers, 22Na, 51Cr-EDTA (relative molecular mass 357) and 57Co-cyanocobalamin (relative molecular mass 1353) were measured in the fetal circulation of isolated dually perfused lobules of normal term human placentae using the single circulation, multiple-tracer dilution technique. 2. In lobules perfused with M199 medium, containing dextran and 5 g l-1 bovine albumin, the extractions of all three tracers decreased as the flow was increased over the range of 2-8 ml min-1, and PS products for 51Cr-EDTA and 57Co-cyanocobalamin, but not for 22Na, reached constant values at flows above 0.1 ml min-1 g-1. 3. Flow-independent PS products in the presence of albumin were 0.025 +/- 0.002 ml min-1 g-1 (mean +/- S.E.M., n = 25) for 57Co-cyanocobalamin and 0.057 +/- 0.003 ml min-1 g-1 (n = 25) for 51Cr-EDTA. The ratio of PS values (51Cr-EDTA/57Co-cyanocobalamin) was 2.28, while the ratio of the corresponding free diffusion coefficients was 1.79, indicating substantial restriction to the diffusion of the 57Co-cyanocobalamin. 4. In another series of lobules perfused in the absence of albumin, extraction values for all three test tracers were constant over the same flow range. Values at high flow rates were therefore about twice those measured in the presence of albumin, and PS products for all three tracers failed to reach diffusion-limited values. 6. Lobules perfused with and without albumin were fixed using a glutaraldehyde fixative containing 1% Alcian Blue dye. An ultrastructural examination of the endothelium showed no significant changes in cell or cleft morphology, or in the glycocalyx, in the absence of albumin which might account for the observed permeability change. 7. These data are the first physiological measurements specifically characterizing fetal microvascular permeability in the human placenta. The results suggest that permeability resembles that found in skeletal muscle and, as such, the endothelium presents a significant barrier to the diffusion of large solutes. The observed 'protein effect' indicates that albumin can interact with elements of the solute pathway to increase its restrictiveness.

Immunoelectron characterisation of the inter-endothelial junctions of human term placenta

The molecular constituents of the paracellular clefts in human placental microvessels were investigated using antibodies against PECAM-1, pan-cadherin, A-CAM (N-cadherin), cadherin-5 and two types of integrins (those recognised by antibodies to the beta 1 chain and alpha v beta 3). Ultrastructural localisation of these molecules in ultrathin frozen sections of human term placentae was attempted using colloidal gold immunocytochemistry, after establishing their presence by indirect immunofluorescence. At the light microscopical level, the endothelial paracellular clefts were found to be immunoreactive to the antibodies against PECAM-1, cadherin-5 and pan-cadherin, but not the integrins. The latter showed diffuse distribution in the endothelium and in the abluminal interstitial space. PECAM-1 and pan-cadherin were also seen in the cytoplasm and luminal surface of the endothelium. Immunoelectron studies revealed that the cadherins and PECAM-1 were present in the wide regions of the paracellular clefts, but not in tight junctional regions. Using immunocytochemistry, these wide junctional areas were found to be associated with the cytoskeletal linking molecules vinculin and alpha-actinin. These regions may therefore contain adherens-type junctions. Cadherin-5, localised by two different monoclonal antibodies, 7B4 and TEA, was the only antigen which was cleft-specific, the others also being seen in the cytoplasm of the microvascular endothelium. Cadherin-5 and pan-cadherin were co-localised in the same wide junction, but were usually seen to occupy different microdomains of, and different wide zones of, the same cleft. The cell adhesion molecules localised in the paracellular wide junctions of the human placental microvessels may play a role in maintaining the intercellular spacing between endothelial cells, and may be part of a paracellular “fibre matrix” with permeability-restricting properties.

Paracellular permeability pathways in the human placenta: a quantitative and morphological study of maternal-fetal transfer of horseradish peroxidase

Physiological data indicate that both a transcellular and a paracellular pathway are available for transfer across the human placenta but the morphological correlate of the latter is uncertain. We measured the permeability of the dually perfused human placental cotyledon to the predominantly cationic protein horseradish peroxidase (MW 40,000), to the neutral polymer 14C-dextran (MW 50-70,000) and to the extracellular space marker creatinine (MW 113). Following fixation and cytochemistry, we used brightfield microscopy to localize peroxidase reaction product within the same tissue. Steady state unidirectional maternofetal clearance (Kmf) for the peroxidase (0.90 +/- 0.27 microliters/min/g, n = 9) was not significantly different from that for 14C-dextran (0.95 +/- 0.07 microliters/min/g, n = 3) suggesting that charge does not markedly influence peroxidase permeability. The Kmf for creatinine was 13.1 +/- 2.5 microliters/min/g (n = 9); these permeability data are similar to those reported for the placenta in vivo. Microscopically, peroxidase reaction product was localized to the microvillous surface of the syncytiotrophoblast of most villi and to the trophoblastic basement membrane and connective tissue cells of the villous core in a few villi. The reaction product was also associated with fibrin-containing deposits attached to the villous core at sites of discontinuity of the syncytial epithelium. The staining pattern within the deposits was consistent with a diffusion gradient of the peroxidase. These fibrin-containing deposits at discontinuities in the syncytiotrophoblast may provide one paracellular route for peroxidase diffusion from the intervillous space into the villous core.

Interendothelial junctions during blood-brain barrier development in the rat: morphological changes at the level of individual tight junctional contacts

The endothelium of brain capillaries represents the structural basis for the blood-brain barrier in vertebrates. Individual endothelial cells are linked by a continuous belt of complex tight junctions (zonulae occludentes). Hydrophilic solutes and macromolecules are believed to cross the barrier through specific carrier mechanisms. Unspecific paracellular ionic leak is thought to be very low. In rats the blood-brain barrier is not fully developed until postnatal day 24. We investigated the ultrastructure of the developing blood-brain barrier at 5 developmental stages between embryonic day 17 and young adults. The use of high power goniometric tilting of ultrathin sections allows one to gather information about the exact relationship between two opposing membranes throughout the entire length of the cleft. Our results suggest that the maturation of blood--brain barrier interendothelial clefts is accompanied by the establishment of a characteristic ratio of 'narrow zone' (complex tight junctions) to 'wide zone' (15-20 nm), and of a typical cleft length. Membrane separation larger than 20 nm disappear and individual tight junctional contacts undergo structural changes.