Structure of coupled and uncoupled cell junctions

Cells of Chironomus salivary glands and Malpighian tubules have junctions of the "septate" kind. This is the only kind of junction discerned which is large enough to effect the existing degree of intercellular communication. The electron microscopic observations of the "septate" junction conform to a honeycomb structure, with 80-A-thick electron-opaque walls and 90-A-wide transparent cores, connecting the cellular surface membranes. A projection pattern of light and dark bands (the "septa") with a 150-A periodicity results when the electron beam is directed normal to any set of honeycomb walls. Treatment of the salivary gland cells with media, which interrupt cellular communication (without noticeable alteration of cellular adhesion) by reducing junctional membrane permeability or perijunctional insulation, produces no alterations in the junctional structure discernible in electron micrographs of glutaraldehyde-fixed cell material.     

STUDIES ON AN EPITHELIAL (GLAND) CELL JUNCTION : I. Modifications of Surface Membrane Permeability

Membrane permeability of an epithelial cell junction (Drosophila salivary gland) was examined with intracellular microelectrodes and with fluorescent tracers. In contrast to the non-junctional cell membrane surface, which has a low permeability to ions (10^-4 mho/cm²), the junctional membrane surface is highly permeable. In fact, it introduces no substantial restriction to ion flow beyond that in the cytoplasm; the resistance through a chain of cells (150 Ω cm) is only slightly greater than in extruded cytoplasm (100 Ω cm). The diffusion resistance along the intercellular space to the exterior, on the other hand, is very high. Here, there exists an ion barrier of, at least, 10⁴Ω cm². As a result, small ions and fluorescein move rather freely from one cell to the next, but do not leak appreciably through the intercellular space to the exterior. The organ here, rather than the single cell, appears to be the unit of ion environment. The possible underlying structural aspects are discussed.     

Diurnal Variation of Tight Junction Integrity Associates Inversely with Matrix Metalloproteinase Expression in Xenopus laevis Corneal Epithelium: Implications for Circadian Regulation of Homeostatic Surface Cell Desquamation

Background and Objectives

The corneal epithelium provides a protective barrier against pathogen entrance and abrasive forces, largely due to the intercellular junctional complexes between neighboring cells. After a prescribed duration at the corneal surface, tight junctions between squamous surface cells must be disrupted to enable them to desquamate as a component of the tissue homeostatic renewal. We hypothesize that matrix metalloproteinase (MMPs) are secreted by corneal epithelial cells and cleave intercellular junctional proteins extracellularly at the epithelial surface. The purpose of this study was to examine the expression of specific MMPs and tight junction proteins during both the light and dark phases of the circadian cycle, and to assess their temporal and spatial relationships in the Xenopus laevis corneal epithelium.

Methodology/Principal Findings

Expression of MMP-2, tissue inhibitor of MMP-2 (TIMP-2), membrane type 1-MMP (MT1-MMP) and the tight junction proteins occludin and claudin-4 were examined by confocal double-label immunohistochemistry on corneas obtained from Xenopus frogs at different circadian times. Occludin and claudin-4 expression was generally uniformly intact on the surface corneal epithelial cell lateral membranes during the daytime, but was frequently disrupted in small clusters of cells at night. Concomitantly, MMP-2 expression was often elevated in a mosaic pattern at nighttime and associated with clusters of desquamating surface cells. The MMP-2 binding partners, TIMP-2 and MT1-MMP were also localized to surface corneal epithelial cells during both the light and dark phases, with TIMP-2 tending to be elevated during the daytime.

Conclusions/Significance

MMP-2 protein expression is elevated in a mosaic pattern in surface corneal epithelial cells during the nighttime in Xenopus laevis, and may play a role in homeostatic surface cell desquamation by disrupting intercellular junctional proteins. The sequence of MMP secretion and activation, tight junction protein cleavage, and subsequent surface cell desquamation and renewal may be orchestrated by nocturnal circadian signals.     

Plasma membrane of the rat parotid gland: preparation and partial characterization of a fraction containing the secretory surface

A plasma membrane fraction from the rat parotid gland has been prepared by a procedure which selectively enriches for large membrane sheets. This fraction appears to have preserved several ultrastructural features of the acinar cell surface observed in situ. Regions of membrane resembling the acinar luminal border appear as compartments containing microvillar invaginations, bounded by elements of the junctional complex, and from which basolateral membranes extend beyond the junctional complex either to contact other apical compartments or to terminate as free ends. Several additional morphological features of the apical compartments suggest that they are primarily derived from the surface of acinar cells, rather than from the minority of other salivary gland cell types. Enzymatic activities characteristically associated with other cellular organelles are found at only low levels in the plasma membrane fraction. The fraction is highly enriched in two enzyme activities--K+ -dependent p-nitrophenyl phosphatase (K+ -NPPase, shown to be Na+/K+ adenosine triphosphatase; 20-fold) and gamma-glutamyl transpeptidase (GGTPase; 26-fold)--both known to mark plasma membranes in other tissues. These activities exhibit different patterns of recovery during fractionation, suggesting their distinct distributions among parotid cellular membranes. Secretion granule membranes also exhibit GGTPase, but no detectable K+ -NPPase. Since Na+/K+ adenosine triphosphatase and GGTPase, respectively, mark the basolateral and apical cellular surfaces in other epithelia, we hypothesize that these two enzymes mark distinct domains in the parotid plasmalemma, and that GGTPase, as the putative apical marker, may signify a compositional overlap between the two types of membranes which fuse during exocytosis.     

FINE STRUCTURE AND MORPHOGENIC MOVEMENTS IN THE GASTRULA OF THE TREEFROG, HYLA REGILLA

The blastoporal groove of the early gastrula of the treefrog, Hyla regilla, was examined with the electron microscope. The innermost extension of the groove is lined with invaginating flask- and wedge-shaped cells of entoderm and mesoderm. The distal surfaces of these cells bear microvilli which are underlain with an electron-opaque layer composed of fine granular material and fibrils. The dense layer and masses of vesicles proximal to it fill the necks of the cells. In flask cells bordering the forming archenteron the vesicles are replaced by large vacuoles surrounded by layers of membranes. The cells lining the groove are tightly joined at their distal ends in the region of the dense layer. Proximally, the cell bodies are separated by wide intercellular spaces. The cell body, which is migrating toward the interior of the gastrula, contains the nucleus plus other organalles and inclusions common to amphibian gastrular cells. A dense layer of granular material, vesicles, and membranes lies beneath the surface of the cell body and extends into pseudopodium-like processes and surface undulations which cross the intercellular spaces. A special mesodermal cell observed in the dorsal lining of the groove is smaller and denser than the surrounding presumptive chordamesodermal cells. A long finger of cytoplasm, filled with a dense layer, vesicles and membranes, extends from its distal surface along the edge of the groove, ending in a tight interlocking with another mesodermal cell. Some correlations between fine structure and the mechanics of gastrulation are discussed, and a theory of invagination is proposed, based on contraction and expansion of the dense layer and the tight junctions at distal cell surfaces.     

THE FINE STRUCTURE OF THE GALL BLADDER EPITHELIUM OF THE MOUSE

Sections of mouse gall bladder epithelium fixed by perfusion with buffered osmium tetroxide have been studied in the electron microscope as an example of simple columnar epithelium. The free surface presents many microvilli, each presenting a dense tip, the capitulum, and displaying a radiating corona of delicate filaments, the antennulae microvillares. Very small pit-like depressions, representing caveolae intracellulares, are encountered along the cell membrane of the microvilli. The free cell surface between microvilli shows larger cave-like depressions, likewise representing caveolae intracellulares, containing a dense material. The lateral cell borders are extensively folded into pleats, which do not interdigitate extensively with corresponding folds of the adjacent cell membrane. The terminal bars are shown to consist of thickened densities of the cell membrane itself in the region of insertion of the lateral cell wall with the free cell surface. This thickening is associated with an accumulation of dense cytoplasmic material in the immediate vicinity. The terminal bar is thus largely a cytoplasmic and cell membrane structure, rather than being primarily intercellular in nature. The basal cell membrane is relatively straight except for a conical eminence near the center of the cell, projecting slightly into the underlying tunica propria. The basal cell membrane itself is overlain by a delicate limiting membrane, which does not follow the lateral contours of the cell. Unmyelinated intercellular nerve terminals with synaptic vesicles have been encountered between the lateral walls of epithelial cells. A division of the gall bladder epithelial cell into five zones according to Ferner has been found to be convenient for this study. The following cytoplasmic components have been noted, and their distribution and appearance described: dense absorption granules, mitochondria, Golgi or agranular membranes, endoplasmic reticulum or ergastoplasm, ring figures, and irregular dense bodies, perhaps lipoid in nature. The nucleus of these cells is also described.     

DIFFERENTIATION OF THE JUNCTIONAL COMPLEX OF SURFACE CELLS IN THE DEVELOPING FUNDULUS BLASTODERM

The structure of the junctional complex between surface cells was investigated in blastula, mid gastrula, late gastrula, and early embryo of the teleost fish Fundulus heteroclitus. In blastulae, the intercellular complex is simple and consists of an apical region where the adjacent membranes are closely apposed (40–60 A) and in places touch, an intermediate zone with a wider intercellular space (> 100 A), and incipient desmosomes. In gastrulae, there are frequent points of fusion of membranes along the apical zone of the complex. Dilatations and an increased number of desmosomes in different stages of development are found along the intermediate zone. In mid gastrula, a close or gap junction with an intercellular space of 20 A occurs below the level of the desmosomes. In late gastrula, the gap junction is reduced in extent and desmosomes are better developed. In the early embryo, the basic organization of the complex is the same, although the deeply situated close junctions are no longer apparent and desmosomes and their associated system of filaments are well developed. At this time, the junctional complex is comparable to that of many epithelia and consists of an apical zonula occludens, a short zonula adherens, and deeply situated maculae adherentes.     

Tight junction of sinus endothelial cells of the rat spleen

The fine structure of the tight junctions between sinus endothelial cells of the rat spleen and the permeability of such sinus endothelial cells were examined by transmission electron microscopy, using freeze-fracture, triton extraction, and lanthanum-tracer techniques. In freeze-fracture replicas, the segmented strands and grooves of the tight junctions were frequently observed on the basolateral surfaces of the sinus endothelial cells irrespective of the location of the ring fiber. There were one or two wavy-strands or grooves which were, for the most part, oriented parallel to the long cell axis thus forming networks at places. In addition, some strands or grooves were discontinuous while some networks of the junctional strands were not closed. These strands also occasionally lacked intramembranous particles in the tight junctions. The junctional strands run apicobasically at certain sites. In the vertical sections of the sinus endothelial cells treated with lanthanum nitrate, although no tight junctions were observed wherever the endothelial cells were apposed, most of them were situated on the basal part of the lateral surfaces of the adjacent endothelial cells. Several fusions of the junctional membranes were observed in a vertical section of the lateral surfaces of the adjacent endothelial cells. The intercellular spaces of the adjacent endothelial cells except for the fusion of the junctional membranes, were electron dense and the infiltration of lanthanum nitrate was found not to be interrupted by these tight junctions. Based on these observations, the molecular 'fence' and paracellular 'gate' functions of the tight junctions in the sinus endothelial cells are discussed.     

A scanning electron microscope study of the extraembryonic endoderm of the 8th-day mouse embryo

Abstract. Late primitive streak embryos were dissected to reveal the junction between the visceral (VE) and parietal (PE) extraembryonic endoderm. Scanning electron microscopy showed that the two cell types differ markedly in their surface morphology and intercellular organization: the VE cells have numerous apical microvilli and form part of a continuous epithelial layer, while the smoother PE cells are scattered individually over the surface of Reichert's membrane. One interpretation of the morphology of the junction between the two tissues is that visceral endoderm cells in this region are detaching from the epithelial layer, migrating on to Reichert's membrane and differentiating into parietal endoderm. Preparatory to this, the visceral endoderm cells in the junctional zone may undergo extensive reorganization of their surface membranes.     

Anatomy and ultrastructure of dermal glands in an adult water mite,Teutonia cometes (Koch, 1837) (Acariformes: Hydrachnidia: Teutoniidae)

Organization of dermal glands in adult water mites Teutonia cometes (Koch, 1837) was studied using light-optical, SEM and TEM methods for the first time. These glands are large and occur in a total number of ten pairs at the dorsal, ventral and lateral sides of the body. The slit-like external openings of the glands (glandularia) are provided with a cone-shaped sclerite, and are combined with a single small trichoid seta (hair sensillum), which is always situated slightly apart from the anterior aspect of the gland opening. Each gland is formed by an epithelium encompassing a very large lumen (central cavity) normally filled with secretion that stains in varying intensity on toluidine blue stained sections. The epithelium is composed of irregularly shaped secretory cells with an electron-dense cytoplasm and infolded basal portions. The cells possess a large irregularly shaped nucleus and are filled with tightly packed slightly dilated cisterns and vesicles of rough endoplasmic reticulum (RER) with electron lucent contents. Dense vesicles are also present in the apical cell zone. Some cells undergo dissolution, occupy an upper position within the epithelium and have a lighter cytoplasm with disorganized RER. Muscle fibers are regularly present in the deep folds of the basal cell portions and may serve to squeeze the gland and eject the secretion into the external milieu. The structure of these dermal glands is compared with the previously described idiosomal glands of the same species and a tentative correlation with the glandularia system of water mites is given. Possible functions of the dermal glands of T. cometes are discussed.     

Protective effect of corticosteroids on intercellular junction

Actions of adrenocortical hormones on intercellular communication in salivary gland cells of Chironomus plumosus were studied by means of electrophysiological measurements. Glucocorticoids have an effect maintaining junctional communication, while mineralocorticoids have an effect blocking it. When glucocorticoids in their proper concentration are previously applied to the cells, they can prevent the block of junctional communication which should be produced by EGTA, lipase and trypsin. Mineralocorticoids prevail in competitive action between glucocorticoids and mineralocorticoids on junctional communication. The interaction between glucocorticoids and uncoupling agents, and between glucocorticoids and mineralocorticoids on junctional communication were discussed at the level of the cell membrane.     

Anatomy and fine structure of pedicellar glands in phoretic deutonymphs of uropodid mites (Acari: Mesostigmata)

Phoretic deutonymphs of uropodid mites are attached to their carrier via an anal pedicel which is formed by a secretion from the pedicellar gland. Since the ultrastructure of the pedicel and pedicellar gland has never been investigated, we studied these structures in three species, Uropoda orbicularis (Müller), Uroobovella marginata (Koch), and Trichouropoda ovalis (Koch) by light (LM) and electron microscopy (TEM, SEM). In addition, the pedicel in Uroobovella nova (Oudemans) was documented in SEM.The pedicellar gland is a distinct globular structure comprised of three types of secretory cells (A-, B-, and C-types) with apical parts directed towards a junctional zone of postcolon and anal atrium. Secretory cells of the A-type are located dorsally, whereas B-type cells are central, and C-type cells are distributed ventrally or both ventrally and dorsally. Protrusions of visceral muscle cells are distributed on the external surface of the gland. The cuticle-lined anal atrium is large and located between the gland and dorso-anal muscles. The pedicel is composed of a main stalk and two extended extremities: one adhering to the anal region of the deutonymph and a second connected to the cuticle of the carrier. In each case, the anal atria were empty, whereas the pedicellar material was located outside of the mite body.     

Fine Structure of the Esophagus of Males of Sarisodera hydrophila (Heteroderoidea)

The fine structure of the esophagus, including procorpus, metacorpus, isthmus, gland lobe, and esophago-intestinal junction, is examined in males of Sarisodera hydrophila. A cuticle-lined lumen extends most of the length of the esophagus, broadens to form a pump chamber in the metacorpus, and posteriorly is continuous with junctional complexes among four esophago-intestinal cells. These four cells are partially enveloped by the gland lobe which basically consists of three gland cells, one dorsal and two subventral. Each gland cell has an anterior process which opens into the lumen of the esophagus through a cuticle-lined duct. The dorsal gland joins the lumen in the anterior portion of the procorpus, whereas ducts of the subventral glands terminate at the base of the metacorpus pump chamber. The subventral glands are predominant in the posterior portion of the gland lobe and are partially ensheathed by a narrow portion of the dorsal gland which extends to within 5 μm of the posterior terminus of the gland lobe. Contents of the dorsal gland include primarily electron dense granules, although rough endoplasmic reticulum (RER) is predominant posteriorly. Secretory granules within the subventral glands vary in morphology and are evenly distributed throughout the two ceils among other organelles, including RER and a large Golgi apparatus. Innervation of the esophagus includes nerve processes which originate from several perikaryons (cell bodies) located in the anterior portion of the gland lobe. The esophagus of males of S. hydrophila is compared with that of other Heteroderoidea, Heterodera glycines and Meloidogyne incognita.     

An ultrastructural analysis of cell and matrix differentiation during early limb development in Xenopus laevis

Early development of the hind limb of Xenopus (stages 44–48) has been analyzed at the level of ultrastructure with emphasis on differentiation of extracellular matrix components and intercellular contacts. By stages 44–45, mesenchyme is separated from prospective bud epithelium by numerous adepidermal granules in a subepithelial compartment (the lamina lucida), a continuous basal lamina and several layers of collagen (the basement lamella). Tricomplex stabilization of amphoteric phospholipid demonstrates that each adepidermal granule consists of several membranelike layers (electron-lucent band 25–30 Å; electron-dense band 20–40 Å), which are usually parallel to the basal surface of adjacent epithelial cells. Collagen fibrils are interconnected by filaments (35 Å in diameter) which stain with ruthenium red. Epithelial cells possess junctional complexes at their superficial borders, numerous desmosomes at apposing cell membranes and hemidesmosomes at their basal surface. Mesenchymal cells predominantly exhibit close contacts (100–150 Å separation) with few focal tight junctions at various areas of their surface. By stages 47–48, adepidermal granules are absent beneath bud epithelium and layers of collagen in the basement lamella lose filamentous cross-linking elements. Filopodia of mesenchymal cells penetrate the disorganized matrix and abut the basal lamina. Hemidesmosomes disappear at the basal surface of the epidermis and mesenchymal cells immediately subjacent to epithelium exhibit focal tight junctions and gap junctions at their lateral borders. These structural changes may be instrumental in the epitheliomesenchymal interactions of early limb development. Degradation of oriented collagenous lamellae permits direct association of mesenchymal cell surfaces (filopodia) with surface-associated products of epithelial cells (organized into the basal lamina). Development of structural pathways for intercellular ion and metabolite transport in mesenchyme may coordinate events specific to limb morphogenesis.     

A UNIQUE JUNCTIONAL STRUCTURE FOUND IN BLASTULA CELLS OF THE NEWT, TRITURUS PYRRHOGASTER

The outermost cell layer of the animal half of the newt blastula ( Triturus pyrrhogaster ) was examined to investigate intercellular junctions by transmission and scanning electron microscopy. A unique structure is observed at the terminal region of the intercellular junction. The structures are cytoplasmic ridges elevated from the cell surfaces, and their inner part is filled with spaces of various sizes. It is supposed that these ridges result from the encounter of cytoplasmic folds protruding from two neighboring cells.
Below the ridges, there is a short close junctional area which is followed by a long region of intercellular space intermittently bridged by cytoplasmic projections. Microvillus-like cytoplasmic processes on the apical cell surfaces, and microfilaments and microtubules in subsurface regions are observed in this material as in many other embryonic cells of amphibians.     

Fine structure of the monkey epididymis: A correlated thin-section and freeze-cleave study

Summary The epithelium of the monkey epididymis was studied by means of freeze-fracture techniques and conventional electron microscopy. For the study of transepithelial permeability lanthanum hydroxide was used as an intercellular tracer. The epididymal epithelium consists mainly of tall columnar cells. The long stereocilia at the apical surface, similarly to microvilli, exhibit after freeze-fracture, two distinct faces: the E face, concave and with fewer membrane-associated particles, and the complementary convex P face. In the lumen unusual groups of smooth-surfaced vacuoles are present. A tight junctional network, which shows some permeability to the lanthanum tracer, is located at the apical end of the cells. Supranuclear cross-fractures clearly show the well developed Golgi cisternae and numerous vacuole profiles. The highly infolded, centrally located nucleus exhibits, after freeze-fracture, an even distribution of nuclear pores. In the perinuclear region the rough endoplasmic reticulum, which also presents pores, displays a sheet-like organization. The basal cytoplasm is filled by numerous globular profiles of membrane-bounded granules. Freeze-cleave exposes large cytoplasmic areas where the types and amount of organelles indicate an intense metabolic activity.Supported by Grant No 104.193.1.78 from PLAMIRHInvestigator of the National Council for Scientific and Technical Research (CONICET) Argentina     

Junctional types in the tissues of an onychophoran: the apparent lack of gap and tight junctions in Peripatus

The Onychophora are a rare group of primitive invertebrates, relatively little investigated. Tissues from a range of their digestive, secretory and excretory organs have been examined to establish the features of their intercellular junctions. Glutaraldehyde-fixed cells from the midgut and rectum, as well as the renal organ, mucous gland, salivary gland, epidermis, CNS and testis from specimens of Peripatus acacioi, have been studied by thin section and freeze-fracture electron microscopy. Adjacent cells in the epithelia of all these tissues are joined by apical zonulae adhaerentes, associated with a thick band of cytoskeletal fibrils. These are followed by regular intercellular junctional clefts, which, in thin sections, have the dense, relatively unstriated, appearance of smooth septate junctions (SSJ). However, freeze-fracture reveals that only the midgut has what appear to be characteristic SSJs with parallel alignments of closely-packed rows of intramembranous particles (IMPs); these IMPs are much lower in profile than is common in such junctions elsewhere. The mucous gland, testis, rectal and renal tissues exhibit, after freeze-fracture, the characteristic features of pleated septate junctions (PSJ) with undulating rows of aligned but separated junctional particles. Suggestions of tricellular septate junctions are found in replicas at the interfaces between 3 cells. In addition, renal tissues exhibit scalariform junctions in the basal regions of their cells. Between these basal scalariform and apical septate junctions, other junctions with reduced intercellular clefts are observed in these renal tissues as well as the rectum, but these appear not to be gap junctions. Such have not been unequivocally observed in any of the tissues studied from this primitive organism; the same is true of tight junctions.     

The fine structure of the salivary glands of the desert locust Schistocerca gregaria Forskål

Summary This paper describes the structure of the salivary glands of Schistocerca gregaria as seen under the electron microscope and the light microscope. The salivary glands consist of a number of acini located on both sides of the pro-, meso-, and metathoracic segments of the locust. Each acinus is drained by a duct which unites with others from the same side to form a lateral collecting duct. The ducts from the two sides join in the head capsule and open into a salivary cup on the labium. Each acinus consists of parietal cells, zymogenic cells, duct cells, tracheoblasts, sheath cells and pigment cells. The parietal and zymogenic cells are the main sites for the production of the salivary gland secretions, which pass through microvilli from the zymogenic cells to the lumen of the ducts within the acinus. Outside the acinus each duct is composed of highly specialized cells with infolded basement membranes extending about a third of the way across the cell. The cytoplasm between the membranes contains elongated mitochondria and glycogen granules. The apical border of the cell is thrown into microvilli which are closely aggregated under the cuticle lining the duct. These cells have all the features of cells previously described in vertebrates and invertebrates which are known to absorb water and/or ions. Absorption of water from the gut could allow the excretion of hypertonic saliva by the locust.