Definitive evidence for the existence of tight junctions in invertebrates

Ultrastructural differentiation of plasma membrane and cell junctions in the hindgut cells is synchronized with key developmental transitions in Porcellio scaber

Differentiation of transporting epithelial cells during development of animal organisms includes remodelling of apical and basal plasma membranes to increase the available surface for transport and formation of occluding junctions, which maintain a paracellular diffusion barrier. This study provides a detailed ultrastructural analysis of apical and basal plasma membrane remodelling and cell junction formation in hindgut cells during late embryonic and early postembryonic development of the crustacean Porcellio scaber. Hindgut cells in late-stage embryos are columnar with flat apical and basal plasma membranes. In early-stage marsupial mancae the hindgut cells begin to acquire their characteristic dome shape, the first apical membrane folding is evident and the septate junctions expand considerably, all changes being probably associated with the onset of active feeding. In postmarsupial mancae the apical labyrinth is further elaborated and the septate junctions are expanded. This coincides with the transition to an external environment and food sources. First basal infoldings appear in the anterior chamber of early-stage marsupial mancae, but in the papillate region they are mostly formed in postmarsupial mancae. In molting late-stage marsupial mancae, the plasma membrane acquires a topology characteristic of cuticle-producing arthropod epithelia and the septate junctions are considerably reduced.

Making the connection – shared molecular machinery and evolutionary links underlie the formation and plasticity of occluding junctions and synapses

The pleated septate junction (pSJ), an ancient structure for cell–cell contact in invertebrate epithelia, has protein components that are found in three more-recent junctional structures, the neuronal synapse, the paranodal region of the myelinated axon and the vertebrate epithelial tight junction. These more-recent structures appear to have evolved through alterations of the ancestral septate junction. During its formation in the developing animal, the pSJ exhibits plasticity, although the final structure is extremely robust. Similar to the immature pSJ, the synapse and tight junctions both exhibit plasticity, and we consider evidence that this plasticity comes at least in part from the interaction of members of the immunoglobulin cell adhesion molecule superfamily with highly regulated membrane-associated guanylate kinases. This plasticity regulation probably arose in order to modulate the ancestral pSJ and is maintained in the derived structures; we suggest that it would be beneficial when studying plasticity of one of these structures to consider the literature on the others. Finally, looking beyond the junctions, we highlight parallels between epithelial and synaptic membranes, which both show a polarized distribution of many of the same proteins – evidence that determinants of apicobasal polarity in epithelia also participate in patterning of the synapse.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

A novel smooth septate junction-associated membrane protein, Snakeskin, is required for intestinal barrier function in Drosophila

Septate junctions (SJs) are the membrane specializations observed between epithelial cells in invertebrates. SJs play a crucial role in epithelial barrier function by restricting free diffusion of solutes through the intercellular space. In arthropod species, two morphologically different types of SJs have been described: pleated septate junctions (pSJs) and smooth septate junctions (sSJs), which are specific to ectodermal and endodermal epithelia, respectively. In contrast to the recent understanding of pSJ-related proteins, the molecular constituents of sSJs are mostly unknown. Here we report a novel sSJ-specific membrane protein, designated ‘Snakeskin’ (Ssk). Ssk is highly concentrated in sSJs in the Drosophila midgut and Malpighian tubules. Lack of Ssk expression is embryonically lethal in Drosophila and results in defective sSJ formation accompanied by abnormal morphology of midgut epithelial cells. We also show that the barrier function of the midgut to a fluorescent tracer is impaired in Ssk-knockdown larvae. These results suggest that Ssk is required for the intestinal barrier function in Drosophila.

Diverse evolutionary paths to cell adhesion

The morphological diversity of animals, fungi, plants, and other multicellular organisms stems from the fact that each lineage acquired multicellularity independently. A prerequisite for each origin of multicellularity was the evolution of mechanisms for stable cell-cell adhesion or attachment. Recent advances in comparative genomics and phylogenetics provide critical insights into the evolutionary foundations of cell adhesion. Reconstructing the evolution of cell junction proteins in animals and their unicellular relatives exemplifies the roles of co-option and innovation. Comparative studies of volvocine algae reveal specific molecular changes that accompanied the evolution of multicellularity in Volvox. Comparisons between animals and Dictyostelium show how commonalities and differences in the biology of unicellular ancestors influenced the evolution of adhesive mechanisms. Understanding the unicellular ancestry of cell adhesion helps illuminate the basic cell biology of multicellular development in modern organisms.

Glial cells in peripheral nerves of the cockroach, Periplaneta americana

The ultrastructural organization and the junctional complexes of peripheral nerves have been investigated in the cockroach Periplaneta americana. Nerve 5 is surrounded by a layer of connective tissue, the neural lamella, beneath which is a layer of perineurial glial cells wrapping the axons. Adjacent perineurial cells are joined to one another by septate, gap and tight junctions. Septate and gap junctions were observed in freeze-fracture replicas of main trunk nerve 5. Septate junctions were found as rows of PF particles mainly in perineurial cell membranes. Gap junctions exhibited EF macular aggregates in perineurial and subperineurial glial cells. During incubations in vivo with extracellularly applied ionic lanthanum, the lanthanum did not penetrate beyond the perineurium. Where nerve 5 branches and contacts the muscle, lanthanum penetrated freely between the muscle fibres and the nerve branches. In small peripheral branches where the axons are surrounded by single a glial layer, lanthanum is unable to penetrate to the axolemma.

Cell-cell adhesion in the cnidaria: insights into the evolution of tissue morphogenesis

Cell adhesion is a major aspect of cell biology and one of the fundamental processes involved in the development of a multicellular animal. Adhesive mechanisms, both cell-cell and between cell and extracellular matrix, are intimately involved in assembling cells into the three-dimensional structures of tissues and organs. The modulation of adhesive complexes could therefore be seen as a central component in the molecular control of morphogenesis, translating information encoded within the genome into organismal form. The availability of whole genomes from early-branching metazoa such as cnidarians is providing important insights into the evolution of adhesive processes by allowing for the easy identification of the genes involved in adhesion in these organisms. Discovery of the molecular nature of cell adhesion in the early-branching groups, coupled with comparisons across the metazoa, is revealing the ways evolution has tinkered with this vital cellular process in the generation of the myriad forms seen across the animal kingdom.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Epithelial cell polarity and cell junctions in Drosophila

The polarized architecture of epithelial cells and tissues is a fundamental determinant of animal anatomy and physiology. Recent progress made in the genetic and molecular analysis of epithelial polarity and cellular junctions in Drosophila has led to the most detailed understanding of these processes in a whole animal model system to date. Asymmetry of the plasma membrane and the differentiation of membrane domains and cellular junctions are controlled by protein complexes that assemble around transmembrane proteins such as DE-cadherin, Crumbs, and Neurexin IV, or other cytoplasmic protein complexes that associate with the plasma membrane. Much remains to be learned of how these complexes assemble, establish their polarized distribution, and contribute to the asymmetric organization of epithelial cells.

Sites of lanthanum occlusion in the testis of the crayfish Procambarus paeninsulanus (Crustacea: Cambaridae)

The presence of stage-dependent occlusive junctions between adjacent Sertoli cells in the seminiferous epithelium of the crayfish testis was demonstrated by a lanthanum tracer study. The germinal epithelium did not appear to be compartmentalized, as evidenced by access of lanthanum to spermatogonia, spermatocytes, and spermatids. During late spermiogenesis, when encapsulated stage VI spermatids were concentrated in the center of an acinus, lanthanum was excluded apically, coincident with lumen formation. This is the first study examining occluding junctions using a barrier penetration method in the testis of a crustacean.

Fine structure of the blood-brain interface in the cuttlefish Sepia officinalis (Mollusca, Cephalopoda)

The blood-brain interface was studied in a cephalopod mollusc, the cuttlefish Sepia officinalis, by thin-section electron microscopy. Layers lining blood vessels in the optic and vertical lobes of the brain, counting from lumen outwards, include a layer of endothelial cells and associated basal lamina, a layer of pericytes and a second basal lamina, and perivascular glial cells. The distinction between endothelial cells and pericytes breaks down in small vessels. In the smallest microvessels, equivalent to capillaries, and in venous channels, and endothelial and pericyte layers are discontinuous, but a layer of glial cells is always interposed between blood and neural tissue, except where neurosecretory endings reach the second basal lamina. In microvessels in which cell membranes of the entire perivascular glial sheath could be followed, the glial layer was apparently 'seamless', not interrupted by an intercellular cleft, in ca 90% (27/30) of the profiles. Where a cleft did occur, it showed an elongated overlap zone between adjacent cells. The walls of venous channels are formed by lamellae of overlapping glial processes. In arterial vessels, the pericyte layer is thicker and more complete, with characteristic sinuous intercellular clefts. Arterioles are defined as vessels containing 'myofilaments' within pericytes, and arteries those in which the region of the second basal lamina is additionally expanded into a wide collagenous zone containing fibroblast-like cells and cell processes enclosing myofilaments. The 'glio-vascular channels' observed in Octopus brain are not a prominent feature of Sepia optic and vertical lobe. The organization of cell layers at the Sepia blood-brain interface suggests that it is designed to restrict permeability between blood and brain.

Structural domains of the tight junctional intramembrane fibrils

Freeze-fracture reveals intramembrane fibrils lying along the intermembrane contacts that characterize tight junctions. Tight junctions from a variety of species are reexamined here by rapid freezing prior to freeze-fracture. The tight junction fibril is uprooted alternatively from either the cytoplasmic or the exoplasmic hemibilayer during freeze-cleavage, exposing two distinct but complementary views of its hybrid structure within the same replica. When the transmembrane fibril is uprooted from the exoplasmic hemibilayer it appears on the P-fracture face as a smooth-surfaced cylinder which is sometimes resolved into periodic globular structures. The lack of indication that the P-face cylinder has been pulled out through the opposite membrane half indicates that this domain of the fibril is, in large part, buried in the hydrophobic interior of the membrane. However, when the transmembrane fibril is uprooted from the cytosolic hemibilayer it appears on the E-fracture face as a row of irregular intramembrane particles. The irregular particles on the E-face aspect of the fibril are interpreted as corresponding to transmembrane protein segments that may very well make projections onto the cytosolic surface of the bilayer. En face views of the outermost junction strand between adjacent epithelial cells show periodic lines on the bilayer on each side of the junction which are interpreted as periodic transmembrane protein segments arising from the core structure of the tight junction fibril. If the backbone of the tight junction strand is an inverted cylindrical micelle, it must typically include proteins, which might anchor it to structures outside the membrane bilayer.

Morphology of glial blood-brain barriers

Glial cells, in certain situations in the CNS, may become modified to form the structural basis of the blood-brain barrier. This occurs in more primitive vertebrates, such as the elasmobranch fish, and in some higher invertebrates. In the latter, the outermost glial sheath, often called the perineurium in avascular ganglia, substitutes functionally for the vascular endothelium of higher organisms. The intercellular junctions between the lateral borders of these modified glial or perineurial cells may be of several types. In nearly all cases, adhesive and communicating (gap) junctions are found together with an occluding junctional structure. The latter is assumed to be the morphologic basis of the observed blood-brain barrier. It varies in nature and may be one in which the adjacent cell membranes fuse, partially or completely, to form a classical tight junction, or it may be one in which the cell membranes remain separated by a distinct intercellular cleft. If the latter, the cleft may be straddled by columns or septal ribbons, between which a charged matrix substance may be found. Restrictive linker junctions, recently found to be the basis of the interglial barrier in cephalopod CNS, as well as that of myriapods, are characterized by cross-striations or columns which, in combination with charged residues, inherent either in them or in the associated extracellular matrix, slow down the entry of exogenous molecules. Septate junctions, which occur between glial cells in certain other invertebrates, exhibit intercellular septal ribbons, which do not prohibit paracellular transport of all substances but may slow down the passage of some by virtue of charged moieties. There is an association of cytoskeletal components with these septate, linker, and tight junctions; the role of the cytoskeleton in tight junctions, which can be seen by freeze fracture to be based on simple ridges in insects or a more complex network of them in arachnids, may also be important in the regulation of paracellular permeability. The structural details of the junctions in different groups are summarized and their physiologic implications discussed.

Changes in intercellular junctions during peripheral nerve regeneration in insects

Peripheral nerves of the adult cockroach have been cut and the changes in glial cells followed during the subsequent process of regeneration. After three to four weeks of regrowth, the severed tips of nerves were examined by freeze-fracture to assess the state of intercellular junctions between the perineurial sheath cells as well as the underlying glial cells. Both pleated septate and gap junctions were found in the immature state; their intramembranous particle (IMP) distribution was characteristic of junctions in the process of assembly, since the IMPs were irregularly and loosely arrayed in contrast with the parallel septate junctional IMP rows and gap junctional plaques found in the fully regenerated or control tissues. These junctional stages resembled those occurring in developing embryonic or metamorphosing insect tissues.

Ultrastructural characteristics of the contact zones between Langerhans cells and lymphocytes

The present work was carried out to look for the ultrastructural substrate of the contact zones between Langerhans cells and lymphocytes. A high resolution electron microscopic analysis of the contact zones between Langerhans cells and lymphocytes was performed. The material used for this study was obtained from experimental contact dermatitis in mice, and human cervical squamous carcinoma and mycosis fungoides. Three types of cell-cell binding were found. Type I is a junction-like structure characterized by the presence of intercellular bridges. It is suggested that this contact might represent a fixation mechanism between the two cells. Type II is characterized by a glycocalyx-glycocalyx continuity. An immunological function--recognition and antigen presentation--is proposed for this type of contact. Type III is a septilaminar tight contact area which seems to be a gap junction. It is suggested that all these types of physical contact might be the morphological expression of interaction between antigen presenting cells and lymphocytes.

Actin filaments are associated with the septate junctions of invertebrates

Septate junctions are almost ubiquitous in the tissues of invertebrates but are never found in those of vertebrates. In spite of their widespread occurrence and hence obvious importance to the invertebrates, their precise function has remained elusive although they have been variously considered to be regions of cell-cell coupling, permeability barriers or adhesion sites. This report demonstrates that elements of the cytoskeletal system insert into the cytoplasmic face of septate junctions. Actin filaments, identified by virtue of their capacity to bind the S1 subfragment of rabbit myosin, are associated with the membranes of septate junctions. Cytochalasin D, an actin depolymerizer, leads to disorganization of the intramembrane components of these junctions. These data suggest that a primary role of septate junctions could be to maintain intercellular cohesion and hence tissue integrity. The assembly and localization of these junctions may be mediated, directly or indirectly, by the cytoplasmic actin filaments associated with their lateral membranes.

The elasmobranch kidney. III. Fine structure of the peritubular sheath

In the kidney of two elasmobranch fish, the little skate (Raja erinacea) and the spiny dogfish (Squalus acanthias), each tubular bundle is wrapped by a continuous sheath of extremely flattened cells which are ordered in several closely superimposed layers. Thin sections and freeze-fracture replicas demonstrate that extensive tight junctions exist between the cells of this peritubular sheath. The sheath cells lie on a discontinuous basement membrane which suggests that they do not belong to the connective tissue. Conceivably, each peritubular sheath segregates the milieu inside the sheath (surrounding the bundle of 5 tubules and capillaries which form the countercurrent system) from the milieu outside the sheath (connective tissue matrix in which the bundles are embedded).

Functional analysis of tight junction organization

The functional basis of tight junction design has been examined from the point of view that this rate-limiting barrier to paracellular transport is a multicompartment system. Review of the osmotic sensitivity of these structures points to the need for this sort of analysis for meaningful correlation of structure and function under a range of conditions. A similar conclusion is drawn with respect to results from voltage-clamping protocols where reversal of spontaneous transmural potential difference elicits parallel changes in both structure and function in much the same way as does reversal of naturally occurring osmotic gradients. In each case, it becomes necessary to regard the junction as a functionally polarized structure to account for observations of its rectifying properties. Lastly, the details of experimentally-induced junction deformation are examined in light of current theories of its organization; arguments are presented in favor of the view that the primary components of intramembranous organization (as viewed with freeze-fracture techniques) are lipidic rather than proteinaceous.

Lack of orthogonal particle assemblies and presence of tight junctions in astrocytes of the goldfish (Carassius auratus). A freeze-fracture study

The cytoplasmic membranes of astrocytes in the optic nerve of the goldfish were investigated by means of freeze-fracture techniques. Astrocytes of normal and regenerating optic nerves did not differ in the fine structure of plasma membranes. Emphasis is placed on the following results: Astrocytic membranes of fish do not reveal the orthogonal particle assemblies that are believed to be generally characteristic for astrocytes in the white matter. Astrocytes reveal extensive membrane areas occupied by tight junctions, which to date have not been described as characteristic astrocytic structures. These junctions are astro-astrocytic and are frequently intercalated by gap junctions. Desmosomes are another characteristic type of astro-astrocytic junction. By means of freeze-fracture replicas it can be demonstrated that they occur in more or less close association with tight and gap junctions. Caveolae are also seen in the astrocytic membranes of fish: their density and distribution show distinct variations. Caveolae occur at the interface between astrocytes and the interstitial space, between astrocytes and myelin sheaths, and in astrocytic processes. It is suggested that the differences between the astrocytic membranes of fish and mammals reflect different physiological functions. They are discussed in relation to the problem of neuronal-glial interrelationships and the behavior of astrocytes during fiber regeneration in the CNS.

The ultrastructure of cerebral blood capillaries in the ratfish, Chimaera monstrosa

Sharks and skates (Chondrichthyes: Elasmobranchii) have a glial blood-brain barrier, while all other vertebrates examined so far have an endothelial barrier. For comparative reasons it is desirable to examine the blood-brain barrier in species from the other subclass of cartilaginous fish, the holocephalans. The ultrastructure of cerebral capillaries in the chimaera (Chondrichthyes: Holocephali) is described in the present study. The endothelial cells are remarkably thick. Fenestrae and transendothelial channels were not observed. The endothelial cells are joined by elaborate tight junctions. The perivascular glial processes are separated by wide spaces (15-60 nm) without obliterating junctional complexes. These findings indicate that the chimaera has an endothelial blood-brain barrier.

Unorthodox pattern of microvilli and intercellular junctions in regular retinular cells of the porcellanid crab Petrolisthes

1. Retinular fine structures in compound eyes of the porcellanid crab Petrolisthes differs significantly from two paguroid anomurans Clibanarius and Pagurus which basically conform to the usual conservative decapod crustacean retinular pattern. 2. Bidirectional orientation of microvilli has been discovered in rhabdomeres of retinular cells R1-R7 in Petrolisthes. Distally the regular rhabdom has mainly a typical banded microvillus structure (Figs. 7,8). Proximally rhabdom banding continues but uniquely all seven regular retinular cells contribute sets of alternately orthogonal microvilli to each band (Figs. 5, 6, 12). This unorthodox pattern should reduce polarization sensitivity and enhance sensitivity to unpolarized light. 3. In this special region microvillus layers are strongly elliptical in cross section with the minor axis parallel to the microvilli (Fig. 12). Hence the ends of the major axes protrude considerably from the central area of overlap (Fig. 6). 4. Retinular cell eight has bidirectional microvilli (Figs. 5-7) as usual in brachyuran crabs. Unlike the latter as well as paguroid crabs, Petrolisthes has square facets and a rectangular retinular array (Figs. 1, 3) similar to other galatheids and macruran decapods generally. It also resembles macrurans (shrimps and lobsters) in having perirhabdomal vacuoles absent or much reduced. 5. Tight junctions occur widely between adjacent retinular cells (Figs. 14, 17) especially basally immediately distal to longitudinal zonular adherentes (Figs. 6, 16) typical of compound eyes. Freeze fracture reveals in addition numerous rectangular arrays of particles on the protoplasmic face of retinular cell membrance near, but not part of, the rhabdom (Figs. 19, 20). Other authors have hypothesized polarized transfer functions for similar particle aggregates in certain vertebrate cells.

Lack of restriction at the blood-brain interface in Limulus despite atypical junctional arrangements

Tracer and freeze-fracture techniques are used to evaluate the capacity of the central and peripheral nervous system of the horseshoe crab, Limulus polyphemus to admit or exclude molecular or ionic constituents of the blood intercellularly. Both the peripheral and central nervous systems are contained within blood sinuses so there is intimate contact between the haemolymph and the neural lamella. No discrete perineurium exists so any protection afforded to the nerve cells must be provided by the ensheathing glial cells and any junctions between them. Using ionic lanthanum as a pre-fixation incubation medium the system is seen to be completely "open', with the tracer gaining access to all regions of the nervous tissue. Cellular association in the peripheral nervous system, as revealed by thin-section and freeze-fracture, consist only of small scattered gap junctions between glial cells which afford no restriction to tracer entry. Gap junctions are again present between glial cells in the C.N.S. but here they are far more numerous, sometimes forming extensive sheets of almost continuous gap junctional plaques. Between certain glial cells there also exists a junctional system of linear PF ridges and complementary EF grooves; these may associate with or surround, often discontinuous arrays, the gap junctional plaques. Given their characteristics and the freedom of tracer entry, they seem unlikely to represent either typical occluding tight junctions or septate junctions.

A vertebrate-like blood--brain barrier, with intraganglionic blood channels and occluding junctions, in the scorpion

India ink and ionic lanthanum injections have revealed that the central nervous system (CNS) of the scorpion possesses a highly vascularized cephalothoracic ganglionic mass. It, together with other abdominal ganglia which form a ventral nerve cord, are all ensheathed by an outer layer of modified glial, or perineurial, cells. These cells resemble those which line the blood channels permeating the CNS, in exhibiting both inverted gap and tight junctions. Although the latter show close or fused membrane appositions, lanthanum appears to penetrate past a number, but not all, of them. Freeze-fracturing reveals that these junctions are composed of E-face particles aligned into a network of rows, or ridges, which are frequently discontinuous, especially near the periphery of the perineurium. This produces a somewhat 'leaky' system but occlusion to tracers occurs ultimately, for in the CNS none can be found beyond the perineurium. The existence of this perineurial blood-brain barrier is also demonstrable electrophysiologically where cations such as Mg2+ are unable to penetrate beyond the perineurial layer although they can, it seems, leak in via the blood vascular system. Relative differences in tightness between the perineurium and the cells lining the blood channels may be attributed to differences in the relative number of discontinuous ridges. This is borne out by the observation that the peripheral nervous system has a highly attenuated perineurium with many fewer junctions, and some of these nerves tend to be leaky with respect to tracer penetration. In fixed material the junctional ridges may fracture on to the E-face or partly on both the EF and PF, while in unfixed tissue they are usually found on the PF. In both cases they exhibit complementary grooves that are coincident with the ridges across membrane transitions; in such cases the cell membranes are fused with concomitant obliteration of the intercellular space. These tight junctions, often closely associated with EF gap junctional particle aggregates which may be very loosely clustered, appear to form the basis of the observed blood-brain barrier in the scorpion CNS.