Characterization of cationic amino acid transporters and expression of endothelial nitric oxide synthase in human placental microvascular endothelial cells

We investigated the expression and activity of arginine transporters and endothelial nitric oxide synthase (eNOS) in human placental microvascular endothelial cells (HPMEC). Using RT-PCR amplification products for eNOS, CAT1, CAT2A, CAT2B, CAT4, 4F2hc (CD98), rBAT and the light chains y+LAT1, y+LAT2, and b0+T1 were detected in HPMEC, but not B0+. Immunohistochemistry and Western blotting confirmed the presence of 4F2hc and CAT1 protein in HPMEC. 4F2hc-light chain dimers were indicated by a shift in molecular mass detected under nonreducing conditions. L-Arginine transport into HPMEC was independent of Na+ or Cl- and was inhibited by the neutral amino acid glutamine, but not by cystine. The Ki for glutamine inhibition was greater in the absence of Na+. Kinetic analysis supported a two-transporter model attributed to system y+L and system y+. Expression of eNOS in HPMEC was detectable by immunohistochemistry and ELISA but not by Western blotting. Activity of eNOS in HPMEC, measured over 48 h, either as the basal production of nitric oxide (NO) or as the accumulation of intracellular cGMP was not detectable. We conclude that HPMEC transport cationic amino acids by systems y+ and y+L and that basal eNOS expression and activity in these cells is low.

Endothelial barriers: from hypothetical pores to membrane proteins

The anatomical counterpart of the physiologically defined small pore system of capillary endothelia has proved difficult to establish. In non-brain continuous capillaries, the contributions of caveolar and transmembrane pathways are likely to be small and paracellular clefts are probably the dominant routes. Analogy with epithelial paracellular pathways suggests that tight junctions may be the most restrictive elements. However, structural features of tight junction-based models are incompatible with physiological data; it is more likely that the tight junction acts as a shutter limiting the available cleft area. Proposed molecular sieves elsewhere in the paracellular pathway include the glycocalyx and the cadherin-based complexes of the adherens junctions. The molecular architecture of tight junctions and adherens junctions is moderately well defined in terms of molecular species, and there are differences at both sites between the endothelial and epithelial spectra of protein expression. However, definition of the size-restricting pore remains elusive and may require structural biology approaches to the spatial arrangements and interactions of the membrane molecular complexes surrounding the endothelial paracellular clefts.

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.

Phenotype of the endothelium in the human term placenta

The placental endothelium contributes to regulating transplacental exchange and maintaining the immunological maternofetal barrier. We characterized the endothelial phenotype in human normal term placentae with a panel of antibodies to endothelial antigens using a standardized immunofluorescence method. Placental endothelium strongly expressed vWF, PAL-E, H-antigen, thrombomodulin, PECAM-1, CD34, CD36, ICAM-1, CD44, thy-1, A10/33-1, VE-cadherin, caveolin-1 and HLA-G, whereas occludin, claudin-1, eNOS, angiotensin converting enzyme (ACE), ICAM-2, endoglin and integrin-alphathetabeta(3)were weakly expressed. PGI(2)synthase, tissue factor, E-selectin and VCAM-1 were not detected. Some antigens were heterogenously expressed along the vascular tree or within individual villi. Expression of ACE, eNOS, vWF, P-selectin, E-selectin, integrin alpha(v)beta(3)and endoglin was stronger in the maternal decidual vessels, while PECAM-1, CD44, thy-1 and caveolin-1 expression was stronger in fetal vessels. Some endothelial markers were present in trophoblasts and stroma. Endothelial proliferation was apparent in mature intermediate and terminal villi. There was limited inflammatory response to TNFalpha in explants, characterized by upregulation of vWF, P-selectin, PECAM-1 and CD44, downregulation of thrombomodulin, but no increase in ICAM-1 expression, nor induction of E-selectin, VCAM-1 or tissue factor. These patterns of heterogeneity, proliferative activity and inflammatory activation may underlie the specific physiological roles of the placental endothelium.

Endothelial glycoconjugates: a comparative lectin study of the brain, retina and myocardium

There is evidence that the endothelial cell (EC) glycocalyx is a significant determinant of vascular permeability, acting as a charge-size filter to permeant molecules. We have therefore examined its oligosaccharide composition in 3 classes of microvessel with differing permeabilities. EC in rat brain, retina and myocardium were labelled with a panel of lectins and subjected to a semiquantitative analysis. Surprisingly, no substantial differences were evident for any lectin labelling between the 3 microvessel types despite their marked morphophysiological diversity. In particular, all showed substantial sialic acid expression, with Maackia amurensis (MAA) labelling sialic acid in an alpha2-3 linkage to beta-galactose and Sambucus nigra (SNA) recognising sialic acid in an alpha2-6 linkage to beta-galactose. Arachis hypogaea (PNA) binding after neuraminidase digestion indicated the presence of Gal beta1-3GalNAc attached to terminal sialic acid. The results therefore show that the sequences NeuNAc alpha2-3Gal beta1-3GalNAc and NeuNAc alpha2-6Gal beta1-3GalNAc are strongly expressed in the 3 microvessel types irrespective of their permeability properties. This homogeneity suggests that these lectin ligands may be involved in a common set of EC functions, e.g. cell:cell and cell:matrix interactions. However, we cannot rule out the possibility that glycocalyx differences may exist between vessels in the paracellular cleft which may alter its filtration properties.

Development of endothelial paracellular clefts and their tight junctions in the pial microvessels of the rat

The microvessels of the pia mater lack an investment with astrocyte processes but nonetheless have a high transendothelial electrical resistance which has caused them to be regarded as part of the blood-brain barrier. This high resistance is known to be acquired in the perinatal period. The aim of our study was to relate the known physiological changes with differentiation of the endothelial paracellular clefts and especially of their tight junctions which provide the basis for the high transendothelial resistance of blood-brain barrier vessels. Tight junctions of endothelial cell paracellular clefts in pial microvessels were examined by transmission electron microscopy using goniometric tilting to reveal and measure membrane separations at tight junctions in fetal, postnatal and adult rats. These tight junctional membrane separations narrowed over the period (E16: 6.3 nm, D1: 6.4 nm, D7: 5.4 nm) and differentiated into two groups by the adult stage: one with a membrane separation of 2.8 nm and the staining characteristics of non-brain endothelial junctions, and the other with no detectable membrane separation and the staining characteristics of blood-brain barrier endothelial junctions. This patchy and incomplete differentiation of pial tight junctions into a blood-brain barrier-like form could result either from non-uniform exposure to inductive signals or to local variation in responsiveness to such agents. Although these changes in junction organization may be related to the known increase in pial transendothelial resistance in the perinatal period, we have not yet identified any sharply defined structural change which coincides with this physiological event.

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.

Differential distribution of an endothelial barrier antigen between the pial and cortical microvessels of the rat

An endothelial barrier antigen (EBA), reported to be a marker for endothelial cells (EC) displaying blood-brain barrier (BBB) characteristics, was probed with a monoclonal antibody in pial and cortical microvessels in rat brain. In contrast to the uniform labelling of EC in cortical vessels, pial microvessels showed a heterogeneity in EBA expression. Most pial vessels consisted of a mixture of EBA positive and EBA negative cells whereas a smaller number of vessels were either completely negative or uniformly positive. Significantly, in vessels showing incomplete expression it was typically EC furthest from the brain surface that did not express EBA. Although the function of EBA is unknown, the variable expression in pial microvascular EC may be related to their incomplete barrier characteristics.

Ontogeny of four blood-brain barrier markers: an immunocytochemical comparison of pial and cerebral cortical microvessels

Pial and cortical microvessels possess many blood-brain barrier (BBB) properties in common, including impermeability to electron dense tracers, high transendothelial electrical resistance and specialised endothelial cell ultrastructural features. To compare pial and cortical microvessels further, a developmental, immunocytochemical study was undertaken of 4 BBB markers in the rat: OX-47, EBA, GLUT-1 and s-laminin. The appearance of the markers was monitored from embryonic d 16, to postnatal and adult stages. Each of the 4 markers appeared simultaneously in both pial and cortical vessels. GLUT-1 and OX-47 were present in endothelial cells of the BBB from E 16 to the adult. EBA and s-laminin appeared from postnatal d 7 through to the adult. Pial microvessels lack the ensheathment of astrocytes which may be involved in the induction and/or maintenance of BBB markers in the cortex. It is possible that astrocyte-derived factors diffusing from the brain surface are responsible for induction of BBB properties in the pial microvessels.

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.

Effect of histamine on endothelial permeability and structure and adhesion molecules of the paracellular junctions of perfused human placental microvessels

The microvessels of the human placenta resemble those of skeletal muscle, both in endothelial cell junctional organization and in the single passage extraction of radiolabeled tracers. Addition of histamine (100 microM) to the fetal perfusate in an isolated term lobule resulted in a rapid and sustained rise (40-80%) over a 30-min perfusion period in the single circulation extraction values of 57Co-labeled cyanocobalamin and of 51Cr-labeled EDTA, but not of 22Na. Extraction values for 125I-albumin were not increased in histamine-perfused vessels nor was any focal leakage of label observed in serial cryostat sections of lobules perfused with rhodamine-conjugated albumin. There was no electron microscopic evidence of transendothelial channels in the microvascular bed; no gaps were seen at the paracellular cleft regions or in neighboring cytoplasm in any microvessels. Tilting of sections on a goniometric stage showed a significant increase in the separation between adjoining endothelial membrane leaflets at tight junctional regions (from 4.1 to 6.1 nm) although the dimensions of the wide zones remained unchanged. Placental microvessels contain the endothelial adhesion molecules PECAM-1 and VE-cadherin in the wide regions of paracellular clefts: PECAM-1 is also localized on the luminal membrane. Histamine-stimulated microvessels showed an altered staining pattern for both of these molecules, at the expense of the cleft regions. These changes in adhesion molecule distribution and tight junctional membrane separation may be parts of a series of events which leads to increased permeability during inflammatory events.

Have declining clinical necropsy rates reduced the contribution of necropsy to medical research?

AIMS

To examine trends in necropsy based research output for a period of 27 years during which there has been a progressive decline in clinical necropsy rates.

METHODS

The numbers of necropsy based research papers published between 1966 and 1993 were determined using the CD-Plus Medline computed literature database.

RESULTS

The number of necropsy based research papers containing necropsy or a synonym in the title increased by 220% between 1966 and 1993. When papers including necropsy or a synonym in the abstract, but not in the title, were included, the proportion of all indexed papers increased from 0.35% in 1975, when abstracts were first included, to 0.53% in 1993. Analysis of the subject material indicated that necropsy based research has constantly reflected trends and advances in clinical medicine. Neuroscience related research represented the largest subject category which may reflect the difficulties in obtaining human tissue from sources other than necropsy.

CONCLUSIONS

The modern necropsy continues to provide valuable information for all clinical and laboratory based disciplines. The decline in clinical necropsy rates would not yet appear to have undermined the contribution of the necropsy to research.

Advances in understanding permeability in fetal capillaries of the human placenta: a review of organization of the endothelial paracellular clefts and their junctional complexes

A review is presented of the evidence that the capillaries of the fetal-placental circulations of man and the guinea-pig are typical members of class of continuous non-brain capillaries. Their permeability is similar to that of muscle capillaries, and there is much evidence that the main permeation route for hydrophilic solutes in capillaries of this class is through the paracellular clefts of the endothelium. The properties of this paracellular route are based on discontinuous tight junctions which may serve to restrict the fraction of the paracellular cleft available to solutes, and on a molecular sieve which may be based on arrays of adhesion molecules in the zonula adhaerens of the junctional complex. The characteristics of this paracellular route and of overall microvascular permeability in these types of placenta make it clear that fetal capillaries form a significant component of the total transplacental permeability restriction.

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.

Focal and regional variations in the composition of the glycocalyx of large vessel endothelium

The glycocalyx of the endothelium of the systemic arteries and vena cava of the rabbit was visualised by in situ perfusion fixation with glutaraldehyde containing Alcian blue. The thickness of the layer ranged from 45 +/- 1 nm in the coronary artery to 81 +/- 2 nm in the carotid. The glycocalyx was 20 +/- 1.5 nm thicker on the downstream side of intercostal ostia than on the upstream side. Changes in the staining pattern with increasing concentrations of MgCl2 indicated that carboxyl groups made the major contribution to the surface charge, though sulphate groups were also present, particularly in the aortic arch and carotid artery. Segments of the thoracic aorta and carotid artery were also stained in vitro with fluorescence labelled wheat germ agglutinin, and fluorescence intensity in histological sections was quantified using a video microscope equipped with a microcomputer-based image analysis system. The fluorescence intensity in the carotid was 1.65 +/- 0.15 times that in the aorta. Pretreatment with neuraminidase reduced fluorescence intensity by 60 +/- 4% in the carotid and 53 +/- 2% on the upstream side of intercostal ostia, but only by 37 +/- 3% on the downstream side. Chondroitinase and heparanase both reduced binding and when used together their effect was additive, reducing fluorescence by 27 +/- 3, 51 +/- 4, and 32 +/- 3% at the three sites, respectively. Though the interpretation of the lectin binding experiments is complicated by a number of factors, these results support previous reports that sialyl groups are abundant in the endothelial glycocalyx. Glycosaminoglycans are also present, however, in significant amounts.

Adenosine triphosphate-lead histochemical reactions in ependymal epithelia of murine brains do not represent calcium transport adenosine triphosphatase

The strong enzyme histochemical reactions for adenosine triphosphatase (ATPase) seen in ependymal tanycytes after incubation in calcium-containing media have previously been reported as calcium transport ATPase. Investigation of these reactions showed that: (1) any nucleoside triphosphate can serve as a substrate; (2) diphosphates and monophosphates cannot replace triphosphates; this includes p-nitrophenyl phosphate which is readily hydrolysed by plasma membrane transport ATPases; (3) strong localization occurs in the presence of millimolar concentrations of either calcium or magnesium ions; there is no absolute requirement for calcium ions; (4) they are not inhibited by sulphydryl inhibitors or calmodulin antagonists; (5) lead phosphate precipitates are localized almost entirely on the external face of tanycyte plasma membranes. In addition, the technique gives strong localization to vessels in the choroid plexus but not to the choroidal epithelium. Immunohistochemistry with a primary antibody raised against Ca2+, Mg2(+)-ATPase stains the choroidal epithelium but not the vessels or the ependymal tanycytes. These results are inconsistent with identification of the reaction as calcium transport ATPase but support characterization as an ecto-ATPase.

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.

Immunohistochemical localization of adherens junction components in blood-brain barrier microvessels of the rat

The morphology and molecular composition of intercellular adherens junctions have most frequently been described in epithelial cells and the fascia adhaerens of the intercalated disc. A group of cytoplasmic molecules is known to be associated with adherens junctions. The intercellular bond is mediated by cadherins which bridge the cells by homophilic binding. Recently, endothelial cells have also been shown to form intercellular junctions of the adherens-type. However, they are morphologically less distinct and little is known about their molecular components. In this study we report the localization of some adherens junction components in intact microvessels of the blood-brain barrier in the rat. We used antibodies raised against alpha-actinin, vinculin, zyxin, cadherin (antipan-cadherin antibody) and A-CAM (N-cadherin) in immunohistochemical experiments at light and electron microscopical levels. Microvessel walls reacted positively for all antigens throughout postnatal development. All antigens were localised, though not necessarily exclusively, to interendothelial junctions. At the ultrastructural level, pan-cadherin reactivity was present throughout the entire length of the cleft. These results could mean that in blood-brain barrier endothelial cells the complex tight junction is embedded in an adherens junction which occupies the entire length of the cleft.

Junctions between pericytes and the endothelium in rat myocardial capillaries: a morphometric and immunogold study

Pericytes are cells of mesodermal origin which are closely associated with the microvasculature. Despite numerous studies little is known about their function. We have studied the relationship between pericytes and the endothelium in rat myocardial capillaries employing ultrastructural and immunogold techniques. 14% of the subendothelial cell membrane is covered by comparatively small pericytic cell processes. About half of these processes are completely embedded in basement membrane material, whereas the remaining half forms closer contacts with the endothelium. These contacts are devoid of anti-laminin immunogold label, a marker for basement membranes. A small fraction of these contacts has been identified as tight junctions resembling those seen between endothelial cells in capillaries of the same tissue. The remaining majority of junctions reveals a cleft of approximately 18 nm between the apposed membranes in which a succession of cleft-spanning structures can often be detected. It was also found that pericytic processes are preferentially located close to interendothelial junctions. We suggest that the high frequency of intimate junctions between pericytes and the endothelium and the preferential localisation near paracellular clefts may have functional significance.

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.

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

Selected lobules of human term placentae were extracorporeally perfused for a recovery period of 20 min, fixed by perfusion and mordanted with ferrocyanide prior to processing for transmission electron microscopy. The lateral membranes of the endothelial cells of the terminal villous capillaries were found to be separated by paracellular clefts of mean width 15.6 nm. At tight junctional regions (1-4 sites per cleft) the two membranes approached each other more closely and frequently appeared to fuse. However, tilting of the sections in the electron microscope stage showed that the membranes were separated by a gap of mean width 4.1 nm in at least 94% of tight junctional profiles. When individual tight junctions were studied by a combination of serial sectioning and goniometric tilting, they were seen to widen abruptly within a distance of three to seven consecutive thin sections, indicating they were not continuous throughout the axial length of the capillaries. The wide regions of the clefts usually showed linkers, strands of glycocalyx-like material spanning the gap. Linkers may contribute to cell adhesion and possibly form part of a filter within the tortuous paracellular pathway provided by the discontinuous network of tight junctional strands. Human term placental capillaries appear to resemble closely other continuous non-brain capillaries.

The interendothelial junction in myocardial capillaries: evidence for the existence of regularly spaced, cleft-spanning structures

Water and hydrophilic solutes cross the endothelium of continuous capillaries via the paracellular cleft and possibly other routes. This pathway shows a selectivity to molecule size and charge. However, it is not yet known which systems confer this selectivity. Isolated rat hearts were perfusion-fixed through the coronary circulation, stained with lanthanum or tannic acid, and further processed for transmission electron microscopy. Thin sections viewed at × 160,000 magnification revealed regularly spaced, cleft-spanning structures in the wider zone of a small percentage of clefts in addition to at least one zonula occludens. Goniometric tilting of the specimen in steps of 5 degrees perpendicular to the plane of the wide zone showed that such “linkers” can be revealed in at least 40% of all clefts. They become visible at some tilt angles, although the same area of the cleft is featureless at other angles. Single linker spacing measurements were obtained using a computerized image analysis system, and compiled in a frequency distribution chart. On the basis of these data, two models of a regular linker distribution within the cleft are illustrated. Our results provide evidence for the presence of regularly spaced, cleft-spanning structures within the interendothelial cleft which may have implications for endothelial cell-cell adhesion and permeability.

Immunocytochemical and labelled tracer approaches to uptake and intracellular routing of immunoglobulin-G (IgG) in the human placenta

Isolated lobules of freshly delivered human term placenta were (a) subjected to an indirect immunoelectron ultracryo method in which the immunoreactivity of endogenous Immunoglobulin-G (IgG) to rabbit anti-human IgG antibody was localized with protein-A-colloidal gold and (b) extracorporeally perfused and human IgG molecules complexed to horseradish peroxidase (HRP) added to the maternal perfusate and the uptake of IgG-HRP over different perfusion durations visualized ultrastructurally by using diaminobenzidine cytochemistry. Immunoreactivity to anti-human IgG antibody was localized all along the apical plasmalemma, in apical coated and uncoated vesicles, in apical and juxtanuclear multivesicular bodies, and in basal vesicles of the syncytiotrophoblast layer of the placenta. The stroma separating the syncytiotrophoblast from the foetal endothelium as well as vesicles within the endothelium were immunoreactive. No immunoreactivity was localized in paracellular clefts of endothelia. A similar distribution of exogenous IgG-HRP was observed for the perfused placentae. When bovine IgG-HRP or HRP alone were used as control tracers no uptake was seen for the former whilst the latter was observed only in early endosomal vesicles of the syncytiotrophoblast. The pattern of localization visualized in both studies is consistent with receptor-mediated uptake of IgG by the syncytiotrophoblast and a vesicular transport of IgG across the foetal endothelium.

Uptake and intracellular routing of peroxidase-conjugated immunoglobulin-G by the perfused human placenta

Selected lobules of term human placenta were extracorporeally perfused and human immunoglobulin-G complexed to horseradish peroxidase (IgG-HRP) was added to the maternal perfusate. After different durations of perfusion IgG-HRP was visualised by use of diamino-benzidine cytochemistry. Within the first 10 min of perfusion IgG-HRP was found bound to microvilli and coated pits of the syncytiotrophoblast; internalisation into coated vesicles and tubulo-vesicular bodies was also observed. Subsequently, IgG-HRP was found in multivesicular bodies and by 30 min appeared in basal vesicles, the frequency of the latter event increasing with time. No routing of IgG-HRP into Golgi regions or lysosomes could be detected. by 60 min IgG-HRP was found in a few caveolae of fetal endothelium of both terminal and intermediate villi. IgG-HRP was not found in intercellular clefts of the endothelium. The pattern of uptake and routing observed suggests a receptor-mediated transcytosis of IgG-HRP across the syncytiotrophoblast and a transcellular pathway through the endothelium.

Effect of hypoxia on endothelial morphology and interendothelial junctions in the isolated perfused rat heart

Isolated perfused rat hearts were used to assess the effect of 30 or 60 mins of hypoxia on the ultrastructure of the capillary endothelium and particularly on the interendothelial junctions. Perfusions were carried out both in the presence and absence of albumin. Albumin had no effect on ultrastructure or membrane spacing in the interendothelial clefts, neither in oxygenated controls nor in hypoxic hearts. After 30 and 60 mins of hypoxia some capillaries showed endothelial swelling while after 60 mins the endothelium of others was attenuated. The wide regions of the intercellular cleft were not affected by hypoxia but the "narrow zone" gap between membranes became significantly smaller. We conclude that factors within the clefts other than the "tightness" of the narrow zones are responsible for changes in permeability in hypoxia and with albumin.

Immunogold localisation of endogenous immunoglobulin-G in ultrathin frozen sections of the human placenta

Endogenous immunoglobulin-G was localised in ultrathin frozen sections of human term placenta by use of an indirect immuno electron-histochemical methodology. Immunoreactivity of endogenous IgG to rabbit anti-human immunoglobulin-G antibody was visualised by use of protein-A--colloidal gold complex. Gold marked the syncytiotrophoblast in both coated and uncoated regions of the apical plasmalemma, in vesicles and multivesicular bodies, and in vesicles near the basal plasmalemma. Immunoreactivity was also seen in the interstitial space between the trophoblast and the fetal endothelial layer as well as in various types of vesicles within the endothelial cells. No immunoreactivity was seen in the intercellular clefts of the endothelium. The pattern of localisation observed is consistent with receptor-mediated uptake of immunoglobulin-G into the syncytiotrophoblast of the human placenta followed by release into the interstitial space and then vesicular transport through the endothelium.

Effects of starvation, feeding, and time of day on the activity of proton transport adenosine triphosphatase in the parietal cells of the mouse gastric glands

The effects of starvation, feeding, and time of day on mouse gastric glands were studied by means of an enzyme histochemical method for K+-dependent p-nitrophenyl phosphatase (K+-NPPase), a partial reaction of the proton pump ATPase which drives gastric acid secretion. The stomachs of mice starved for 24 h showed very low levels of parietal cell K+-NPPase histochemical reaction. However, a brief meal following such a period of starvation produced an abrupt increase in K+-NPPase reaction within most of the parietal cell-containing glands though not all parietal cells were equally susceptible to stimulation. The number of glands containing K+-NPPase-reactive parietal cells fell slowly in the hours following a feeding stimulus. These changes were shown to be caused by feeding rather than by general arousal and to follow the feeding cycle in ad libitum fed animals. The reasons that parietal cells in the basal parts of mouse gastric glands cannot be induced to show K+-NPPase reactivity by a feeding stimulus are not understood.

Interendothelial junctions of cardiac capillaries in rats: their structure and permeability properties

The isolated perfused heart model was used to examine the structure of rat cardiac capillaries and their permeability to macromolecules of various sizes. Haemoglobin (diameter 6.4 nm) and catalase (10.4 nm) did not cross the endothelium but remained on the luminal side. Cytochrome C (3 nm) and horseradish peroxidase (6 nm) both crossed the endothelium to the subendothelial space and filled the caveolae on the abluminal side as well as the entire length of the lateral intercellular spaces. The membranes of the endothelial cells are separated by an intercellular gap of mean width 18.2 nm. At one or more zonular regions within each lateral intercellular space the two membranes approach each other more closely and frequently appear to fuse. However, tilting the specimen shows that, in these regions, there is a gap of mean width 5.4 nm (in lanthanum- and tannic acid-treated tissue, 3.8 nm in ferrocyanide-treated tissue) between the membranes. We conclude that these narrow regions sieve macromolecules on the basis of size although other factors may determine their permeability properties.

Quantitative assessment of Na+,K+-ATPase localization by direct and indirect p-nitrophenyl phosphatase methods

Na+,K+-ATPase histochemistry, using direct (Pb) and indirect (Mg/Sr-Co) p-nitrophenyl phosphatase methods, was assessed by scanning integrative microdensitometry of three classes of tubules in mouse and guinea pig kidneys. The methods yielded similar and appropriate patterns of activity distribution and inhibitor response. The indirect method gave preferable results, in that non-enzymic background was lower and rate of reaction product accumulation was considerably higher.

Histochemical localization of phosphatases in the pig placenta: II. Potassium-dependent and potassium-independent p-nitrophenyl phosphatases at high pH; relation to sodium-potassium-dependent adenosine triphosphatase

Histochemical localization by Mg2+ capture methods of K+-dependent, ouabain-sensitive phosphatase activity in the pig placenta shows that strong Na+,K+-dependent adenosine triphosphatase (Na+,K+-ATPase) activity is restricted to the basal zone of the columnar epithelium covering the areolar chorionic villi. It is proposed that active Na+ absorption at this epithelium may be the source of the ouabain-sensitive, fetal-side-positive potential difference which can be measured across the placental membrane in vitro. The one-step procedure for Na+,K+-ATPase localization is unsatisfactory in this organ as any specific ATPase reaction is swamped by activity probably attributable to uteroferrin and other non-specific phosphatases.

Histochemical localization of phosphatases in the pig placenta: I. Non-specific phosphatases and their relation to uteroferrin

The uterine glands, glandular secretions and areolar chorionic epithelium of the pig placenta in late gestation show high levels of histochemical phosphatase activity at acid pH, particularly towards aromatic monophosphates. The properties of this phosphatase allow its identification with the placental iron carrier protein, uteroferrin, rather than with lysosomal acid phosphatase. Alkaline phosphatase activity is restricted to the microvasculature on the maternal side of the placenta and is entirely distinct from uteroferrin phosphatase activity.

A comparison of the effects of different perfusion regimes on the structure of the isolated human placental lobule

Isolated lobules of normal term human placentas were perfused using two different procedures. In the first more conventional system, open-circuit perfusion of both the maternal and the fetal circulations with Earle's solution containing dextran was established and maintained for either 30 min or 1 h. In the second series of experiments both circulations were perfused in separate closed circuits with a mixture of fresh autologous fetal blood and Earle's solution for 0, 1, 2 or 3 h. In both series the lobule was then fixed by perfusion through the fetal circulation. Light- and electron-microscopic examination of a set of tissue samples from each perfused lobule showed substantial differences between the effects of these two types of perfusion procedure. Tissue from lobules perfused by the open-circuit blood-free procedure showed patchy but severe cell swelling and vacuolation of the trophoblast after only one hour's perfusion. Particularly striking was swelling and disruption of a large proportion of the mitochondria in all placental cell types. By contrast, placental tissue from the closed-circuit perfusion with blood-containing medium showed little change over a period of two hours, while after three hours it showed oedema and microvillous damage, but no sign of cell swelling and little mitochondrial damage. It is concluded that the viability of the perfused human placental lobule depends on the type of perfusate used, and that the use of a fetal blood-enriched perfusate is of considerable value in maintenance of the preparation as assessed by structural criteria.

Cytochemical evidence for functional zonation of parietal cells within the gastric glands of the mouse

Parietal cells in the luminal segments of mouse gastric glands show high activity of acid-secreting potassium-dependent adenosine triphosphatase (H+, K+-ATPase) and of nicotinamide adenine dinucleotide-linked isocitrate dehydrogenase (NAD-ICDHase) and malate dehydrogenase (MDHase) but low activity of succinate dehydrogenase (SDHase). This pattern of activity is reversed in the basal segments of the same glands. These results and previous morphological findings support the conclusion that luminal segment parietal cells are much more active in hydrochloric acid secretion than those of the basal segment. The origin of this zonation may be either cellular deterioration with age or some more specific form of regulation of parietal cell metabolism.

The intercellular junctions of guinea-pig placental capillaries: a possible structural basis for endothelial solute permeability

The endothelial cell junction in guinea-pig placental capillaries consists of a continuous ribbon desmosome (zonula adherens) within which lies a particulate tight junction consisting of between one and five anastomosing strands. The intercellular space at these tight junctions is narrowed and is subdivided by junctional bars which are probably continuous with the intramembrane particle rows seen in freeze-fracture replicas of the junctions. Perfusion with lanthanum salts shows the gaps between the junctional bars to be lanthanum-filled and the entire junction to be lanthanum permeable. The estimated size of the spaces between the junctional bars is consistent with the junctional pore size indicated by previous ultrastructural tracer studies. The wider lateral intercellular space of the ribbon desmosome is spanned by more widely spaced "linkers" which may act as a coarser three-dimensional filter in series with size-limiting pores between the tight junctional bars.

Molecular charge as a determinant of macromolecule permeability across the fetal capillary endothelium of the guinea-pig placenta

The effect of molecular charge on the permeability to macromolecules of the fetal capillary endothelium of the guinea-pig placenta has been investigated. In a series of experiments for horseradish peroxidase (HRP) isoenzymes with different isoelectric points were perfused through the fetal circulations of guinea-pig placentae, which were then prepared for ultracytochemical localization of peroxidase activity. It was found that the two anionic HRP tracers did not penetrate the endothelial cell layer of the fetal capillaries. By contrast, both cationic HRP tracers did penetrate; reaction product was found in the subendothelial space underlying the endothelial cell layer. The route of transendothelial penetration was via the lateral intercellular spaces (LIS) between endothelial cells. In further experiments cationic and anionic ferritins were perfused. Cationic ferritins were found apparently bound in clusters to the luminal surface of the endothelial cell layer in the luminal ends of the LIS whereas very few anionic ferritin molecules were present. We conclude that molecular charge is an important determinant of macromolecule permeability through the fetal capillary endothelium of the guinea-pig placenta and may be related to the presence of anionic sites on the luminal plasma membrane of the endothelial cells.

Permeability of the foetal capillary endothelium of the guinea-pig placenta to haem proteins of various molecular sizes

Haem proteins of different molecular sizes were perfused into the foetal circulation of the guinea-pig placenta to study the permeability of the foetal endothelium. The smallest molecules tested, microperoxidase (ae 1.0 nm) and cytochrome C (ae 1.5 nm), readily penetrated the endothelium; tracer-reaction product was found in the subendothelial space of the capillaries. However, there was no uptake of these two tracers into the syncytiotrophoblast layer of the placenta. An intermediate-sized molecule, myoglobin (ae 1.7 nm) produced only a weak reaction product in the subendothelial space even when perfused at high concentration. The largest molecule tested, haemoglobin (ae 2.8 nm), did not penetrate the foetal endothelium at any of the concentrations employed. The foetal capillary endothelium thus provided a barrier to protein penetration from the foetal circulation, dependent on molecular size. There was evidence that the site of this barrier was located in the lateral intercellular spaces between the endothelial cells. The syncytiotrophoblast of this haemomonochorial placenta provided an almost absolute barrier to protein penetration from the foetal circulation. As other workers have described maternal-to-foetal transmission of proteins across this layer in the guinea-pig, a working hypothesis of the role of endothelium and syncytiotrophoblast in maternal/foetal protein exchange is discussed.

Ultrastructural study of the permeability of the guinea-pig placenta to horseradish peroxidase

Horseradish peroxidase (HRP) was used to study macromolecule permeation into the guinea-pig placenta perfused in situ. When tissue culture medium 199 (TC 199) was used as fetal-side perfusate, the tracer reaction product was found only lining the fetal endothelium. When a longer period of perfusion with HRP in TC 199 was used, a small amount of reaction product was found in the subendothelial space and syncytiotrophoblastic vesicles, but not in maternal lacunae. In similar experiments using a Krebs bicarbonate Ringer (KRBG) as perfusate the tracer was found (i) lining the fetal endothelium, (ii) in the laternal intercellular spaces of the endothelium, (iii) in the subendothelial space, and (iv) in the maternal lacunae. It is therefore evident that the vehicle influenced the permeability of the guinea-pig placenta to horseradish peroxidase. As other studies have shown that perfusion of the fetal side with salt solution increases pore size, the results with TC 199 are regarded as more representative of the situation in the intact animal. It is therefore suggested that the fetal endothelium of the guinea-pig placenta may be largely impermeable to molecules of the size of horseradish peroxidase (4 nm) or larger.

Gastric proton pump localization. Application of triphosphatase and monophosphatase techniques

Potassium-dependent phosphatase activity can be demonstrated in unfixed frozen sections of mouse stomach using either adenosine triphosphate (ATP) or p-nitrophenyl phosphate (NPP) as substrate. In both cases the potassium-dependent reaction is confined to oxyntic cells, but with ATP, a strong, potassium-independent reaction occurs in the connective tissue of the lamina propria and elsewhere. In the NPP system potassium-independent reaction is very slight, and the oxyntic cell reaction shows responses to inhibitors that differentiate it from Na+, K+-ATPase and that are consistent with its identification with the dephosphorylation step of the proton pump enzyme H+, K+-ATPase, recognized as the active transport component in gastric acid secretion.

Polarity of the blood-brain barrier: distribution of enzymes between the luminal and antiluminal membranes of brain capillary endothelial cells

The subcellular distribution in brain capillaries of alkaline phosphatase and Na+, K+-ATPase was investigated by two methods. Cytochemical studies using whole brain perfusion and electron microscopic examination indicated that alkaline phosphatase activity was located in both the luminal and antiluminal cytoplasmic membranes of the brain capillary endothelial cells. By contrast, the K+-dependent phosphatase activity associated with Na+, K+-ATPase was located in only the antiluminal membrane. Biochemical studies using membranes prepared by homogenization of isolated brain capillaries and density gradient centrifugation resulted in identification of two plasma membrane fractions. The light fraction contained alkaline phosphatase but very little Na+, K+-ATPase while the heavier fraction contained both enzyme activities. In addition, gamma-glutamyl transpeptidase showed a distribution similar to alkaline phosphatase while 5'-nucleotidase activity was distributed with the Na+, K+-ATPase activity. We conclude that the luminal and antiluminal membranes of brain capillaries are biochemically and functionally different. This polarity should permit active solute transport across brain capillary endothelial cells which are the cells responsible for the blood-brain barrier.

The role of gap junctions in trophoblastic cell fusion in the guinea-pig placenta

Fusion of cytotrophoblast cells in the guinea-pig placenta occurs at regions of plasma membrane interdigitation where the cells are attached to one another by complex arrays of gap junctions and desmosomes. Fusion begins at the gap junctions, which are lost in this process. The desmosomes play no obvious part in the fusion mechanism and remain after fusion as sites of attachment of syncytiotrophoblast membrane to itself. It is proposed that a major role of gap junctions in placental development is to bring trophoblast plasma membranes into a close relationship which may act as a starting point for cell fusion.

Reliability and specificity of membrane adenosine triphosphatase localizations

The main development in ATPase cytochemistry in recent years is the development of three methodologically independent approaches to the localization of Na+, K+-ATPase. These methods in general support one another well and have led to significant insights into the mechanisms of bulk cation transport at several important sites. The area most in need of development is now the problem of specific localization of divalent ion-dependent ATPases at the plasma membrane. The value of the traditional Wachstein-Meisel methods appears to be very limited, and progress will depend on the introduction of more specific techniques.

The localization and properties of membrane adenosine triphosphatases in the guinea-pig placenta

The distribution and properties of cytochemically demonstrable phosphatases in the near-term guinea-pig placenta were examined using a strontium capture technique for sodium- and potassium-dependent adenosine triphosphatase (Na+, K+-ATPase) and a lead capture technique for magnesium-dependent adenosine triphosphatase (Mg2+-ATPase). Localizations with the strontium technique in the presence of an alkaline phosphatase inhibitor were mainly on the syncytiotrophoblast plasma membranes; the reaction was potassium-dependent and ouabain-sensitive. Reaction product using the lead capture method was found on both trophoblast and endothelial cell plasma membranes and was independent of magnesium and insensitive to p-hydroxymercuribenzoate (POHMB), an inhibitor of membrane ATPases. However, a very large proportion of this reaction could be blocked by an alkaline phosphatase inhibitor. It is concluded that the strontium capture technique gave a reliable localization for Na+, K+-ATPase. However, the lead capture method mainly demonstrated alkaline phosphatase, and does not offer a useful approach to specific ATPase studies in this particular system.

Structural features and quantitative age-dependent changes in the intervascular barrier of the guinea-pig haemochorial placenta

The haemomonochorial placenta of the guinea-pig undergoes several quantitative changes between the 49th and 64th days of gestation, all of which are in such a direction as to increase the efficiency of transplacental transport. The fetal vessels become larger, the maternal vessels increase in surface area by proliferation of microvilli, and the effective mean distance between the two vessel sets decreases. The magnitude of these changes suggests that the efficiency of transport of hydrophilic solutes across the maternal-fetal interface could double, although changes in the number of permeation sites per unit area may modify this relationship. The presence of open intercellular spaces and fenestrations in the fetal endothelium suggests that this layer may not be a major permeability barrier in the guinea-pig, but may create an unstirred layer of extracellular fluid between endothelium and syncytiotrophoblast.

Cytochemical localization of the K+ regulation interface between blood and brain

Phosphatase activity identified with Na+-K+-ATPase was localized at the basal surface of cerebral cortical capillary endothelium by perfusion with a p-nitrophenyl phosphate-strontium medium. The relationship of this to the blood-brain barrier to K+ is discussed.