The ultrastructural basis of alveolar-capillary membrane permeability to peroxidase used as a tracer

The permeability of the alveolar-capillary membrane to a small molecular weight protein, horseradish peroxidase (HRP), was investigated by means of ultrastructural cytochemistry. Mice were injected intravenously with HRP and sacrificed at varying intervals. Experiments with intranasally instilled HRP were also carried out. The tissue was fixed in formaldehyde-glutaraldehyde fixative. Frozen sections were cut, incubated in Graham and Karnovsky's medium for demonstrating HRP activity, postfixed in OsO4, and processed for electron microscopy. 90 sec after injection, HRP had passed through endothelial junctions into underlying basement membranes, but was stopped from entering the alveolar space by zonulae occludentes between epithelial cells. HRP was demonstrated in pinocytotic vesicles of both endothelial and epithelial cells, but the role of these vesicles in net protein transport appeared to be minimal. Intranasally instilled HRP was similarly prevented from permeating the underlying basement membrane by epithelial zonulae occludentes. Pulmonary endothelial intercellular clefts stained with uranyl acetate appeared to contain maculae occludentes rather than zonulae occludentes. HRP did not alter the ultrastructure of these junctions.     

THE ULTRASTRUCTURAL BASIS OF CAPILLARY PERMEABILITY STUDIED WITH PEROXIDASE AS A TRACER

The transendothelial passage of horseradish peroxidase, injected intravenously into mice, was studied at the ultrastructural level in capillaries of cardiac and skeletal muscle. Peroxidase appeared to permeate endothelial intercellular clefts and cell junctions. Abnormal peroxidase-induced vascular leakage was excluded. Neutral lanthanum tracer gave similar results. The endothelial cell junctions were considered to be maculae occludentes, with gaps of about 40 A in width between the maculae, rather than zonulae occludentes. Some observations in favor of concurrent vesicular transport of peroxidase were also made. It is concluded that the endothelial cell junctions are most likely to be the morphological equivalent of the small pore system proposed by physiologists for the passage of small, lipid-insoluble molecules across the endothelium.     

The ultrastructural basis of the permeability of arterial endothelium to horseradish peroxidase

The thoracic aorta and basilar artery, in which the incidence of atherosclerosis is known to be different, were examined to elucidate the correlation between the structure of the intercellular cleft junction between adjacent endothelial cells and its permeability to HRP. Tannic acid or HRP in the vessel lumen passed through the intercellular clefts of the thoracic aorta into the subendothelial space, whereas in the basilar artery they were unable to penetrate beyond the tight junction of the intercellular clefts. Freeze-fracture replicas revealed that the tight junctions of the thoracic aorta consisted of one to two junctional strands in most areas of the cleaved planes, with discontinuities in some places, whereas those of the basilar artery consisted of a continuous belt-like meshwork of six anastomosing junctional strands on average. These observations confirm that the structure of endothelial junctions in arteries has a close correlation with the permeability of the intercellular clefts to HRP.     

Maturation of tight junctions in guinea-pig cecal epithelium

Summary By means of the freeze-fracture technique and in tracer studies it is demonstrated that the structure of tight junctions and the permeability to lanthanum of the guinea-pig cecal epithelium change during maturation of cells. Height and strand number of tight junctions in the apical-basal direction increase as crypt cells migrate to the surface of the epithelium. Likewise, the interlacing of continuous strands was greater in surface than in crypt junctions. The numerous free-ends, isolated individual freestrands and maculae occludentes found in crypt cells were absent in surface epithelial cells. Goblet cells, located at the bottom of crypts, displayed tight junctions similar in characteristics to those of cells located in the middle region of crypts. Cells at the surface and in middle regions of crypts possess tight junctions impermeable to lanthanum, whereas junctions between cells located at the bottom of crypts often were permeable to the tracer, indicating that permeability decreases as the epithelial cells mature. Genesis and maturation mechanisms related to structural configuration of tight junctions are discussed.     

“Tight” junctions in the sheath of normal and regenerating motor nerves of the crayfish,Orconectes virilis

Summary Tight or occluding intercellular junctions occur between adjacent glial processes in normal and regenerating crayfish motor nerve sheaths. Although infrequent, these junctions possess the ridge and groove configuration characteristic of freeze-cleaved occluding junctions. When present, nerve sheath tight junctions consist of a single, or at most a few, parallel intramembrane ridges situated on the EF membrane face of the glial plasma membrane. Consequently, such contacts are rarely recognized in thin sections of plasticembedded nerve sheaths. Crayfish nerve sheath tight junctions are of the fascia occludens type and, therefore, do not impede solute flow across the nerve sheath. Fasciae occludentes of regenerating nerve sheaths occur in close proximity to discoid plaque-like aggregates of particles assumed to represent maculae adhaerentes. This relationship, which was not observed in normal nerve sheaths, suggests a functional association between the two types of junctions, perhaps developmental transformation of one junction type into the other. Although ridges and grooves of tight junctions occur next to crossfractured trans-glial channels, no functional significance is proposed for this relationship. This study is the first report of tight intercellular junctions in crustacean glial nerve sheaths.Supported by the National Research Council of Canada     

The functional and structural borders between the CSF-and blood-dominated milieus in the choroid plexuses and the area postrema of the rat

Summary In the borderline area between the hemal milieu of the choroid plexuses (PC) and the interstitial cerebrospinal-fluid (CSF) compartment, ground substances displaying increased amounts of basal lamina-like material and containing negatively charged sulfated glycosaminoglycans appear to be endowed with selective properties. They may function as a sieve or filtration barrier gradually controlling the passage of substances between the two milieus, depending on their charge and molecular weight. Special structural features and functional properties of ependymal cells are associated with such bordering structures. These ependymal cells are transitional elements between choroid epithelium and ciliated ependymal cells. As judged from experiments with horseradish peroxidase and conventional electron microscopy, occluding junctions at the basal pole of these cells prevent a rapid alteration in the milieu conditions, enabling gradual change from hemal to CSF composition near the bases of these transitional ependymal cells. The borderline structures between the hemal milieu of the PC and the area postrema (1) are established by leptomeningeal cells which face a hemal mileu, (2) are endowed with conspicuous tight junctions, and (3) produce a flocculent substance, the light-microscopic equivalent of which is PAS positive. These structures probably establish an effective barrier between the two milieus of different composition. The functional characteristics and the morphology of the meningeal cells facing the hemal milieu of neurohemal regions resemble closely the neurothelial cells, which are interposed between the CSF milieu and the hemal milieu in the dura mater. The present results suggest that the location between the hemal and the CSF milieu is decisive for the transformation of leptomeningeal cells into neurothelial elements.Supported by the Deutsche Forschungsgemeinschaft (Grant Kr 569/5-1)Dedicated to Professor Dr. med., Dr. med. vet. h.c., Dr. phil. h.c. Andreas Oksche on the occasion of his 60th birthday     

The ultrastructural basis of transcapillary exchanges

A brief survey is given of current views correlating the ultrastructural and permeability characteristics of capillaries. Observations based on the use of peroxidase (mol wt 40,000), as an in vivo, and colloidal lanthanum, as an in vitro, ultrastructural tracer, are presented. In capillaries with "continuous" endothelium, the endothelial intercellular junctions are thought to be permeable to the tracers, and are regarded as maculae occludentes rather than zonulae occludentes, with a gap of about 40 A in width between the maculae. Some evidence for vesicular transport is also presented. It is inferred that the cell junctions are the morphological equivalent of the small-pore system, and the vesicles the equivalent of the large-pore system. Peroxidase does not apparently cross brain capillaries: the endothelial cell junctions are regarded as zonulae occludentes, and vesicles do not appear to transport across the endothelium. This is regarded as the morphological equivalent of the blood-brain barrier for relatively large molecules. The tracers appear to permeate the fenestrae of fenestrated capillaries, and the high permeability of these capillaries to large molecules is attributed to the fenestrae. Capillaries with discontinuous endothelium readily allow passage of the tracers through the intercellular gaps. A continuous basement membrane may act as a relatively coarse filter for large molecules. In general, the morphology of capillaries correlates well with physiological observations.     

Uptake of horseradish peroxidase from CSF into the choroid plexus of the rat,with special reference to transepithelial transport

Summary Protein uptake from cerebral ventricles into the epithelium of the choroid plexus, and transport across the epithelium were studied ultrastructurally in rats. Horseradish peroxidase (HRP, MW 40,000) was used as protein tracer. Steady-state ventriculo-cisternal perfusion with subatmospheric pressure (-10cm of water) in the ventricular system was applied. HRP dissolved in artificial CSF was perfused from the lateral ventricles to cisterna magna for various times, and ventriculo-cisternal perfusion, vascular perfusion or immersion fixation with a formaldehyde-glutaraldehyde solution was performed.Coated micropinocytic vesicles containing HRP were seen both connected with the apical, lateral and basal epithelial surface and within the cells. Heavily HRP-labeled vesicles were often fused with the lining membrane of slightly labeled or unlabeled intercellular spaces. Since the apical tight junctions of the epithelium never appeared open or never contained HRP in the spaces between the fusion points, and since the intercellular spaces between adjacent epithelial cells below the junctions only infrequently contained tracer after 5 min, by increasing amounts after 15–60 min of HRP perfusion, a vesicular transport of HRP from the apical epithelial surface to the intercellular spaces, bypassing the tight junctions, is suggested.In addition to the transepithelial transport, micropinocytic vesicles also transported HRP to the lysosomal apparatus of the epithelial cells. With increasing length of exposure to HRP, a sequence of HRP-labeled structures could be evaluated, from slightly labeled apical vacuoles and multivesicular bodies to very heavily labeled dense bodies.     

Freeze-fracture study of the epidermal cells of a teleost with particular reference to intercellular junctions and permeability to tracer

The plasmatic membranes, the intercellular junctions and the intercellular spaces of the epidermis of the fish Pimelodus maculatus were studied by freeze-fracture and by lanthanum methods. The observations has confirmed the presence of desmosomes. Gap junctions were not found and the tight junctions can be seen very rarely, arranged to form small discrete maculae. The finger-print pattern due to the microridges of the apical plasma membrane of the superficial cells was studied by direct replicas. The tracer penetrates all the intercellular epidermal spaces but failed to penetrate the dermis, suggesting the presence of a barrier at the dermo-epidermal level.     

Intercellular junctions during development of the definitive kidney in lampreys: freeze fracture and morphometric analyses

The changing morphology of intercellular junctions in renal morphogenesis during lamprey metamorphosis was followed using freeze-fracture replicas and morphometry. Gap junctions and particle aggregates among strands of occluding junctions are conspicuous in the differentiating podocytes of the renal corpuscle and in the early ciliated neck and proximal segments but not in the distal segment. The cells of the segmented nephron arise from alpha nephrogenic cells which have a focal aggregate (macula occludens) of 4.8 junctional strands. Upon the initiation of metamorphosis the number of strands increases to 8 in undifferentiated cells with either maculae or zonulae occludentes. Differentiation of both neck and proximal segments is accompanied by gradual transformation of a 7-strand zonula occludens to a 4-strand junction but it becomes more shallow in the latter segment. Two types of undifferentiated cell are recognized through five of the seven metamorphic stages. Cells of two types of distal segments begin differentiation at the midpoint of transformation and immediately show zonula occludens of different morphology. Distal segment I (pars recta) has 5 strands and a 0.250 mum depth whereas segment II has 8-9 strands with twice the apical-basal depth. The larval archinephric duct undergoes a moderate transformation in junctional morphology with the addition of 2 strands and no increase in apical-basal depth in zonulae occludentes during metamorphosis. Changes and development of types of intercellular junctions along the nephron in lampreys are discussed with reference to known regional functional specialization in this organism and with the morphology of renal tubular intercellular junctions in other vertebrates.     

A blood-brain barrier without tight junctions in the fly central nervous system in the early postembryonic stage

Summary The anatomical basis of the vertebrate blood-brain barrier is a series of tight junctions between endothelial cells of capillaries in the central nervous system. Over two decades ago, tight junctions were also proposed as the basis of the blood-brain barrier in insects. Currently there is a growing understanding that septate junctions might possess barrier properties in various invertebrate epithelial cells. We now examine these two views by studying the blood-brain barrier properties of the early postembryonic larva of a dipteran fly (Delia platura) by transmission electron microscopy. Newly hatched larvae possess a functioning blood-brain barrier that excludes the extracellular tracer, ionic lanthanum. This barrier is intact throughout the second instar stage as well. The ultrastructural correlate of this barrier is a series of extensive septate junctions that pervade the intercellular space between adjacent perineurial cells. No tight junctions were located in either nerve, glial or perineurial cell layers. We suggest that the overall barrier might involve septate junctions within extensive, meandering intercellular clefts.     

The permeability of the primary decidual zone in the rat uterus: an ultrastructural tracer and freeze-fracture study

The rat primary decidual zone (PDZ) is a transitory avascular region of transformed fibroblasts surrounding the implanting embryo. Studies using fluorescein-labeled tracers have shown that the PDZ is selectively permeable to macromolecules, permeability decreasing with increasing molecular weight. In the present study we investigated the morphologic basis of the permeability barrier. Horseradish peroxidase (HRP) or HRP-labeled immunoglobulin G (IgG-HRP) was administered i.v. to rats on Day 7 of pregnancy, and the animals were killed 30 min to 2 h later. The reaction product of HRP was the same density in uterine blood vessels as in the intercellular spaces of the endometrium and PDZ at 30 min and 1 h after administration. Two hours after administration, the reaction product of IgG-HRP was dense in uterine blood vessels, much less dense in the interstitial spaces of the endometrium, and was not detected in the PDZ. There was an abrupt change in the density of the IgG-HRP reaction product at the intercellular clefts between endothelial cells, where cellular junctions were observed in control tissue. This suggests that the passage of large macromolecules from blood to the implantation chamber is limited initially by cellular junctions between capillary endothelial cells. The exclusion of IgG-HRP from the PDZ indicates that an additional barrier(s) to macromolecules in this region. Lanthanum nitrate tracer was uniformly present throughout the intercellular spaces of the PDZ except at tight junctions between decidual cells. Freeze-fracture replicas of the PDZ showed tight junctions that varied from single strands to interconnected networks of strands oriented mainly parallel to the long axis of the PDZ. Some strands were discontinuous. The tight junctions between decidual cells appear to be functionally discontinuous because HRP readily penetrated the PDZ, but such junctions may retard larger macromolecules such as IgG-HRP. The biological significance of the permeability barrier of the PDZ is discussed.     

Endocytosis in the uterine luminal and glandular epithelial cells of mice during early pregnancy

We have localized horseradish peroxidase (HRP) in the mouse uterus after intravenous administration on days 1 and 5 of pregnancy in an effort to understand how serum proteins reach the uterine lumen. Direct movement of HRP into uterine and glandular lumina was blocked by the epithelial tight junctions on both days. In luminal and glandular epithelial cells at both times, HRP was localized in endocytic vesicles along the basolateral membranes, multivesicular bodies (mvb), elongated dense bodies below the nucleus (bdb), and many small vesicles near the apical surface of the cells. The uptake of HRP was most extensive in the luminal epithelium on day 1: the number of tracer-containing apical vesicles and bdb was largest, and there were also clusters of vesicles containing the tracer above the nucleus. Acid phosphatase was localized on day 1 in mvb and bdb in both cell types, indicating that these structures are lysosomes. It appeared that HRP followed two pathways after basolateral endocytosis by the epithelial cells: it was transported to the apical region of the cells, where it was present in small vesicles that may release their contents into the uterine or glandular lumina, or it was transported to lysosomes. To investigate whether macromolecules may be transported from the uterine lumen to the stroma, we also studied endocytosis at the apical pole of luminal epithelial cells after intraluminal injection of HRP. There was no detectable uptake of HRP from the lumen on day 1, and no tracer was detected in the intercellular spaces or basement membrane region. On day 5, a large amount of HRP was taken up from the lumen into apical endocytic vesicles, mvb, and dense bodies, but tracer was not present in the Golgi apparatus, lateral intercellular spaces, or the basement membrane region at the times studied. These observations indicate that there was no transport of luminal macromolecules to the uterine stroma on day 1, while the possibility of transport on day 5 requires further study.     

Cell junctions in the epithelium of Bowman's capsule

Summary The intercellular connections between the epithelial cells of Bowman's capsule were investigated. It could be demonstrated that typical zonulae occludentes (tight junctions) are present in the species (rat, hamster, and Tupaia) studied. Freeze-fracturing shows a network of anastomizing strands; some species variations are described. In the rat two strands are common. In the golden hamster mostly two to four and occasionally five strands occur. In Tupaia regularly three tight junction strands are found and also gap junctions associated with the zonulae occludentes. In thin sections the goniometric analysis confirms the freeze-fracturing results and reveals attachment zones of macular shape, which are classified as intermediate junctions and desmosomes. The functional role of these cell junctions observed in the epithelium of Bowman's capsule is discussed.     

The cell junctions of the notochord of Xenopus laevis tadpoles

The cell junctions of the notochord of Xenopus laevis tadpoles were examined with the electron microscope using thin sections, lanthanum tracer experiments, and freeze-fracture replicas. Both the peripheral and vacuolated cells of the notochord are connected by numerous spot desmosomes characterized by an intercellular desmogloea and intermediate filaments on the cytoplasmic sides. The peripheral cells also display numerous hemidesmosomes facing the underlying basal lamina. Staining with rhodamine-phalloidin for F-actin yielded negative results and suggested that adhaerens-type junctions are absent. Tracer experiments with lanthanum and freeze-fracture replicas clearly revealed the presence of gap junctions between both cell types but no indications of tight junctions were found and no intercellular barrier existed for tracer infiltration of the notochord.     

Different functions of tight junctions in the ascidian branchial basket

The stigmatal cells in the branchial basket of ascidians from a number of genera have been examined as to the nature and distribution of their intercellular junctions. The branchial wall consists of ciliated and parietal cells; the ciliated cells are arranged in seven rows and are associated by junctions with other cells in the same row as well as with those in adjacent rows. They are also associated by junctions with peripheral parietal cells. Junctions between adjacent ciliated cells in all cases exhibit tight junctions or zonulae occludentes. However, these cell borders also possess fasciae or zonulae adhaerentes if they are in the same row and the ciliary rootlets insert-into these junctions. If the cells are in adjacent rows they exhibit adhaerentes junctions only in species belonging to the orders Phlebobranchiata and Aplousobranchiata. In contrast, if the cells in adjacent rows belong to the order Stolidobranchiata. they never exhibit any adhaerentes junctions and the ciliary rootlets of the basal bodies from the cilia insert instead into the tight junctions and the non-junctional membrane below them. At the homologous junctional borders between adjacent parietal cells and also at heterologous junctional borders between parietal and ciliated cells, tight junctions alone occur, with no co-existing adhaerentes junctions along their lateral borders. Again, fibrils from ciliary rootlets insert into zonulae occludentes. This shows that tight junctions are capable both of forming permeability barriers, in that they can be seen to prevent the entry of exogenous tracers such as lanthanum, and of acting as adhesive devices.     

Junctions in the central nervous system of the cat

Interendothelial membrane contacts in different segments of brain blood vessels were investigated by the freeze-etching technique. The study demonstrated that the endothelial cells of the pre- and postcapillary segments were coupled by elaborate zonulae occludentes. These tight junction formations encompassed gap junctions of different sizes and distribution. The globules of the pre- and postcapillary tight junctions revealed a great fragility which led to an atypical distribution of the sealing elements. In the "typical" brain capillaries the endothelial cells were connected by continuous tight junctions. In contrast to the structural continuity of these formations the fenestrated segments of capillaries in the choroid plexus and area postrema demonstrated discontinuous fasciae occludentes. They were composed of rows of individual particles which should be regarded primarily as focal in nature.     

Subcellular localization of leptin in non-tumorous and adenomatous human pituitaries: an immuno-ultrastructural study.

Leptin is a key mediator in the maintenance of neuroendocrine homeostasis. Recently, leptin and leptin receptor expression were demonstrated in non-tumorous and adenomatous human pituitaries. This study was performed to determine the subcellular localization of leptin in human adenohypophyses (n = 3) and in various types of pituitary adenoma (n = 16). Immunoelectron microscopy showed leptin immunolabeling in most hormone-producing cells of the human non-tumorous adenohypophysis, but not in stellate cells. Labeling was noted over secretory granules. Immunocytochemistry using double labeling revealed leptin expression in GH-, ACTH-, TSH-, and FSH/LH-containing cells but not in PRL cells. The percentage of immunopositive cells and the intensity of immunostaining varied considerably among the various cell types. Immunoelectron microscopy with double gold labeling showed co-localization of leptin and adenohypophysial hormones in the same secretory granules. Among pituitary tumors, leptin immunolabeling was evident only in corticotroph adenomas. Compared to non-tumorous corticotrophs, leptin immunoexpression was less abundant in corticotroph adenomas. The presence of leptin and adenohypophysial hormones in the same secretory granules suggests that leptin is secreted concomitantly with various adenohypophysial hormones and that its release is under the control of hypothalamic stimulating and inhibiting hormones.     

Impermeability of hagfish cerebral capillaries to horseradish peroxidase

Summary Brain capillaries and their permeability to intravenously injected horseradish peroxidase, HRP, (MW: 40,000) were examined electron-microscopically in an attempt to find a structural explanation for the poorly developed blood-brain barrier in the hagfish, Myxine glutinosa. In particular, it was the aim of this study to examine the role of the numerous endothelial vesicles and tubules in the transport of this tracer between blood and brain. Many of the vesicles and tubules were found to be in continuity with the luminal or abluminal surfaces, but tubules generating channels through the endothelial cells were never observed. The cleft between adjacent endothelial cells was obliterated by punctate junctions. HRP, which was allowed to circulate for up to 35 min, was not found in the basal lamina or in the surrounding brain parenchyma. Few of the luminal vesicles and tubules were marked by the tracer. In the intercellular cleft HRP was stopped by the junctions. It is concluded that the hagfish like other vertebrates has a blood-brain barrier to HRP, and the numerous vesicles and tubules occurring in hagfish brain endothelium are not involved in the transendothelial transport of this macromolecule.     

An in vitro model for studies of intercellular communication in cultured rat anterior pituitary cells

The formation of intimate associations among different hormone-secreting cells within the rat adenohypophysis may serve as a possible site for physiologic regulation. In this report we describe a high density plating method which enables us to study cell-to-cell interactions within anterior pituitary cell cultures. Trypsin-dispersed pituitary cell suspensions attach rapidly (within 6 hr) and quantitatively (95-97%) to glass or plastic surfaces when plated in medium containing microM calcium concentrations (pH 7.6-7.8). Freshly plated cell suspensions obtained from female pituitary glands contained subpopulations of mammotrophs 49.3%, somatotrophs 30.3%, gonadotrophs 12.6%, corticotrophs 3.4% and thyrotrophs 1.5%. Epithelial cell colonies were formed during a 3-day culture period as the cells flattened and re-established contacts with neighboring cells. Freeze-fracture electron microscopic analysis of these colonies produced morphological evidence for direct intercellular contacts among the hormone-secreting cells. Large areas of tight junctions and small gap junctions were identified on the membranes of the epithelial cells within these colonies. Cells which contained tight junctions usually contained microvilli and morphological signs of active hormone secretion. Small junctional plaques containing tightly packed intramembrane particles were also occasionally found on the membranes of cells which were actively secreting pituitary hormones. The high density plating procedure which is described in this report provides greater opportunity for cell-cell interaction and thus may prove to be a useful model for evaluating the role of intercellular communication within this tissue.