Tight Junctions in Arthropod Tissues

Cell junctions in amphioxus (Cephalochordata): a thin section and freeze-fracture study

Tissues from the epidermis, alimentary tract and notochord of the cephalochordate Branchiostoma lanceolatum have been examined in both thin sections and freeze-fracture replicas to ascertain the nature of the intercellular junctions that characterize their cell borders. The columnar epithelial cells from the branchial chamber (pharynx), as well as from the anterior and posterior intestine, all feature cilia and microvilli on their luminal surfaces. However, their lateral surfaces exhibit zonulae adhaerentes only. No gap junctions have been observed, nor any tight junctions (as are a feature of the gut of urochordates and higher vertebrates), nor unequivocal septate junctions (as are typical of the gut of invertebrates). The basal intercellular borders are likewise held together by zonulae adhaerentes while hemidesmosomes occur along the basal surface where the cells abut against the basal lamina. The lateral cell surfaces, where the adhesive junctions occur, at both luminal and basal borders, do not exhibit any specialized arrangement of intramembrane particles (IMPs), as visualized by freeze-fracture. The IMPs are scattered at random over the cell membranes, being particularly prevalent on the P-face. The only distinctive IMPs arrays are those found on the ciliary shafts in the form of ciliary necklaces and IMP clusters. With regard to these ciliary modifications, cephalochordates closely resemble the cells of the branchial tract of ascidians (urochordates). However, the absence of distinct junctions other than zonulae adhaerentes makes them exceptions to the situation generally encountered in both vertebrates and urochordates, as well as in the invertebrates. Infiltration with tracers such as lanthanum corroborates this finding; the lanthanum fills the extracellular spaces between the cells of the intestine since there are no junctions present to restrict its entry or to act even as a partial barrier. Junctions are likewise absent from the membranes of the notochord; the membranes of its lamellae and vesicles exhibit irregular clusters of IMPs which may be related to the association between the membranes and the notochordal filaments. Epidermis and glial cells from the nervous system possess extensive desmosomal-like associations or zonulae adhaerentes, but no other junctional type is obvious in thin sections, apart from very occasional cross-striations deemed by some previous investigators to represent 'poorly developed' septate junctions.

Development of the alimentary tract during morphogenesis of the metacercariae of Levinseniella brachysoma

For the first time the development of the alimentary tract of Levinseniella brachysoma metacercaria (Trematoda: Microphallidae) obtained experimentally from Gammarus oceanicus has been described. The foregut primordium in 16-day-old metacercariae is represented by a syncytial cylindrical cord, resulting from the fusion of embryonic cells. Non-fused parts of the plasma membranes of adjacent cells are revealed as gap cavities within the cord. Later (30th day post infection) the lumen of the foregut is formed as a result of both partial vacuolization of the cytoplasm and by a broadening of the gap cavities, resulting from a thinning of the cytoplasmic spaces between them. Besides the usual organelles, the foregut of the mature metacercaria (42nd day p.i.) contains dense secretory granules in the apical cytoplasm region and numerous microtubules in basal areas. The cellular gastrodermis is formed later than the foregut syncytium (on day 30 p.i.); its large cells contain well-developed Golgi complexes, RER and mitochondria. A noteable inclusion of the gastrodermal cells of mature metacercariae are spherical granules of moderate electron density measuring up to 3 μm in diameter. On the basis of an analysis of the ultrastructural data the possible functioning of the metacercarial alimentary tract is discussed.

Recognition, calcium and the control of desmosome formation

Since desmosome formation requires the participation of two adjacent cells, a crucial initiating event must be recognition between desmosomal adhesion molecules. Studies of mutual desmosome formation between different cell types suggest that the recognition mechanisms are highly conserved between different tissues and different species of animals. A further requirement for desmosome formation is an adequate extracellular concentration of Ca2+ (greater than 0.1 mM). Keratinocytes, MDCK cells and MDBK cells all show Ca2+-induced desmosome formation. The desmosomes of these cells also show variable stability to reduction in [Ca2+] and Ca2+ chelation. Desmosome formation at low [Ca2+] is triggered by tunicamycin in keratinocytes, suggesting that the carbohydrate moieties of desmosomal glycoproteins may be involved in the Ca2+ control mechanism. The desmosomal glycoproteins appear to bind Ca2+, while the desmosomal adhesion molecules known as desmocollins, like other Ca2+-dependent adhesion molecules, yield a soluble fragment on trypsinization in the presence of Ca2+. For desmocollins the soluble fragment has a relative molecular mass of 42,000.

The different structures containing tight junction proteins in epidermal and other stratified epithelial cells, including squamous cell metaplasia

In stratified squamous epithelia constituent proteins of tight junctions (TJs) are not restricted to the zonula occludens-related structures of the uppermost living cell layer such as the stratum granulosum of the epidermis but TJ membrane proteins such as occludin and certain members of the claudin family as well as TJ plaque proteins, notably cingulin and protein ZO-1, have also been identified by immunofluorescence and immunoelectron microscopy in more basal layers where they form special cell-cell-connecting structures such as the "lamellated" and the "sandwich" junctions. In the present study, we describe another TJ protein-containing structure, the very small puncta occludentia ("stud junctions"), as the smallest identifiable TJ-like unit that occurs in most, perhaps all strata. We have also determined the specific distributions of TJ proteins in the cell layers of squamous cell metaplasias of the human bronchial tract. Moreover, we show that the occludin-related tetraspanin protein tricellulin-alpha connects and seals the membranes of adjacent "three corner" cell structures of the uppermost layer in keratinocytes growing in culture. We hypothesize the possible occurrence of tricellulin-beta in more basal cell layers of keratinocyte cultures and the general occurrence of different tricellulin splice forms in stratified epithelia in situ, and discuss the possible functions of TJ proteins in stratified epithelia and tumors derived therefrom.

Sealing the live part of the skin: the integrated meshwork of desmosomes, tight junctions and curvilinear ridge structures in the cells of the uppermost granular layer of the human epidermis

In the literature the question of whether a system structurally and functionally related to the barrier function of the tight junctions (TJs) of polarized epithelia exists in the epidermis has been and still is controversially discussed. We have systematically addressed this question in a study of the granular layer of fetal and adult human epidermis, combining different light and electron microscopic methods. We show that the lateral membranes of the cells of the stratum granulosum are connected by an extended subapical complex system integrating desmosomes and TJ structures identified as sites of close membrane-membrane contact and as regions of membrane-to-membrane apposition that in immunoelectron microscopy are positive for TJ marker proteins, notably occludin, indicative of an extended, probably continuous TJ barrier. In addition, we have noted in freeze-fractures of the apical membrane attaching this layer to the basalmost membrane of the stratum corneum an extended system integrating desmosomes with intramembraneous ridge configurations that appear as strands, circles, lariats or complex meshworks showing numerous continuities with the desmosomes. In some regions this system interconnecting desmosomes with curvilinear ridge structures occupies the major part of the plasma membrane. The molecular organizations and possible functional contributions of both structural systems positioned at the border between the living portion of the epidermis and the corneal layer are discussed, in particular in relation to the formation of a stable association between the two layers and of a barrier to the paracellular flow of molecules and particles. It is also discussed whether similar structures occur in other keratinizing stratified squamous epithelia, in squamous metaplasias and in tumors derived from such tissues.

Tight junctions and compositionally related junctional structures in mammalian stratified epithelia and cell cultures derived therefrom

The occurrence of extended tight junction (TJ) structures, including zonulae occludentes (ZO), and the spatial arrangement of TJ proteins in stratified mammalian epithelia has long been controversially discussed. Therefore, we have systematically examined the localization of TJ proteins in diverse stratified epithelial tissues (e.g., epidermis, heel pad, snout, gingiva, tongue, esophagus, exocervix, vagina, urothelium, cornea) of various species (human, bovine, rodents) as well as in human cell culture lines derived from stratified epithelia, by electron microscopy as well as by immunocytochemistry at both the light and the electron microscopic level, using antibodies to TJ proteins such as occludin, claudins 1 and 4, protein ZO-1, cingulin and symplekin. We have found an unexpected diversity of TJ-related structures of which only those showing colocalization with the most restricted transmembrane TJ marker protein, occludin, are presented here. While in epidermis and urothelium occludin is restricted to the uppermost living cell layer, TJ-related junctions are abundant in the upper third or even in the majority of the suprabasal cell layers in other stratified epithelia. Interfollicular epidermis contains, in the stratum granulosum, extended, probably continuous ZO-like structures which can also be traced at least through the Henle cell layer of hair follicles. Similar apical ZO-like structures have been seen in the upper living cell layers of all other stratified epithelia and cell cultures examined, but in most of them we have noticed, in addition, junctional regions showing relatively broad, ribbon-like membrane contacts which in cross-section often appear pentalaminar, with an electron-dense middle lamella ("lamellated TJs", coniunctiones laminosae). In suprabasal layers of several stratified epithelia we have further observed TJ protein-containing junctions of variable sizes which are characterized by a 10-30-nm dense lamina interposed between the two membranes ("sandwich junctions"; iuncturae structae). Moreover, we have often observed variously sized regions in which the intermembrane distance is rather regularly bridged by short rod-like elements ("cross-bridged cell walls"; parietes transtillati), often in close vicinity of TJ-related structures or desmosomes. The significance of these structures and their possible biological importance are discussed.

Blood barriers of the insect

The blood-brain barrier (BBB) ensures brain function in vertebrates and insects by maintaining ionic integrity of the neuronal bathing fluid. Without this barrier, paralysis and death ensue. The structural analogs of the BBB are occlusive (pleated-sheet) septate and tight junctions between perineurial cells, glia and perineurial cells, and possibly between glia. Immature Diptera have such septate junctions (without tight junctions) while both junctional types are found in the imago. Genetic and molecular biology of these junctions are discussed, namely tight (occludin) and pleated-sheet septate (neurexin IV). A temporal succession of blood barriers form in immature Diptera. The first barrier forms in the peripheral nervous system where pleated-sheet septate junctions bond cells of the nascent (embryonic) chordotonal organs in early neurogenesis. At the end of embryonic life, the central nervous system is fully vested with a blood-brain barrier. A blood-eye barrier arises in early pupal life. Future prospects in blood-barrier research are discussed.

Membrane particle distribution in the sternal epithelia of the terrestrial isopod Porcellio scaber latr. (Crustacea, oniscidea) during CaCO(3) deposit formation and resorption, a freeze-etch analysis

The anterior sternal epithelium of terrestrial isopods transports cuticular Ca(2+) to and from large sternal CaCO(3) deposits. We analyzed the anterior and posterior sternal epithelium by the means of the freeze-etch technique and measured the size distribution and density of intramembrane particles (IMPs) during three different molting stages. At least three IMP size classes around 4.5, 7.7, and 9.4 nm can be distinguished on the P-face of the apical and basolateral plasma membrane. An additional size class of around 12.8 nm is restricted to the apical compartment. In the anterior sternal epithelium, the density of these large particles changes by a factor of 1.9 during the molt cycle, suggesting a role in CaCO(3) formation and/or resorption. The density of the smaller IMPs rises transiently by a factor of 1.3 in the posterior sternal epithelium only. The IMP density of the basolateral plasma membrane increases significantly by a factor of 1.4 and 1.3 in the anterior and posterior sternal epithelia, respectively. The results indicate that increases in the IMP density contribute to the differentiation to an increased transport activity during the cyclic enlargements of the plasma membrane surface area in the anterior sternal epithelium.

Insect Na(+)/K(+)-ATPase

Na(+)/K(+)-ATPase (sodium/potassium pump) is a P-type ion-motive ATPase found in the plasma membranes of animal cels. In vertebrates, the functions of this enzyme in nerves, heart and kidney are well characterized and characteristics a defined by different isoforms. In contrast, despite different tissue distributions, insects possess a single isoform of the alpha-subunit. A comparison of insect and vertebrate Na(+)/K(+)-ATPases reveals that although the mode of action and structure are very highly conserved, the specific roles of the enzyme in most tissues varies. However, the enzyme is essential for the function of nerve cells, and in this respect Na(+)/K(+)-ATPase appears to be fundamental in metazoan evolution.

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.

Periaxonal ensheathment of lobster giant nerve fibres as revealed by freeze-fracture and lanthanum penetration

Sheath structure and permeability have been studied in the nerve fibres of lobster (Panulirus argus) walking limbs, in particular the individually ensheathed larger giant fibres, 100-150 microns in diameter, of which there are five or six in a peripheral bundle. They are easily distinguished and can be separated from neighbouring fibre bundles in which smaller giant axons (65-80 microns diameter) and many axons of much smaller diameter (5-15 microns) are ensheathed together. Each of the larger giant axons is enveloped by a Schwann cell layer outside of which is a multilayered sheath consisting of one-cell thick belts of flattened cells and interleaved zones of collagen fibrils and extracellular matrix. The cells in each belt lack basal lamina and, after freeze-fracture, as well as in thin sections, exhibit intercellular gap junctions and incomplete, fascia type, tight junctions; their most striking aspect is an exceedingly large number of exo-endocytic profiles. Permeability to lanthanum chloride in the bathing medium studied before or during fixation both in intact nerves and in nerves with surgically breached (slit) epineurium showed penetration of lanthanum tracer between the cells around the giant fibres, but the electron-dense tracer was excluded from the Schwann cell layer and the periaxonal space unless the epineurium had been slit. The extent of lanthanum diffusion was evaluated by transmission electron microscopy of thin sections and confirmed by X-ray microanalysis (EDAX) of comparable selected areas in such sections. The results indicate structural similarities but distinct permeability differences between the multilayered sheath surrounding the lobster giant axons and the vertebrate nerve perineurium. Other ultrastructural details provided by the freeze-fracture replicas concern the distribution of intramembrane particles in the axolemma and the Schwann and sheath cell membranes.

Evidence for intercellular cohesion in the septate junction of the protozoon Gregarina

Injection of Lucifer Yellow dye was used to establish whether septate junctions formed a permeability barrier between primite and satellite gamonts in the syzygy of the protozoon Gregarina. The fluorescent dye did not pass from one cell to the other, thus suggesting that the septate junction served only for mechanical adhesion. Later on in development, when the gametocyst had formed, the septate junction vanished and interruptions were observed between opposing cell membranes. At this stage the fluorescent dye was able to pass freely into the conjugated cells.

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.

Novel arthropod cell junctions with restrictive intercellular 'linkers'

The peripheral glial cells that surround the components of the avascular CNS in certain groups of primitive arthropods are characterized by unusual intercellular junctions. In the centipedes and millipedes (Myriapoda), these glial cells are associated by interconnecting filamentous 'linkers' which produce a reduction, but not an occlusion, of the intercellular cleft; these are interposed between conventional gap junctions. In replicas, the freeze-cleave images are of loosely aggregated gap junctional connection plaques, fracturing on to the extracellular membrane half leaflet (E face), together with linear alignments of intramembranous particles (IMPs) and furrows; complementary P face ridges also occur. Exogenous tracers appear unable to penetrate beyond these junction-rich glial clefts, possibly by binding to the 'linkers' or extracellular matrix between them. Peripheral glial cells in the cerebral ganglion of the horseshoe crab, Limulus, are also characterized by linear IMP arrays; in this case they primarily exhibit E face grooves and complementary ridges of P face IMPs, which also do not produce complete membrane fusion. These, too, are intimately associated with gap junctional plaques of E face particles or P face pits. These intramembranous particle rows are novel structural modifications, called here 'linker' junctions, and are quite distinct from conventional tight or septate junctions found between the outer glial cells in more highly evolved arthropods such as the insects and arachnids. They seem to represent a new category of intercellular junction.

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 lack of a structured blood-brain barrier in the onychophoran Peripatus acacioi

Onychophorans are 'living fossils' frequently purported to have evolved from the same ancestor as the arthropods and annelids. In the CNS of Peripatus acacioi, beneath an outer acellular neural lamella, glial cells ensheath the cerebral ganglion and the nerve cords. These glial cells are, however, attenuated and rather few in number and, although they interdigitate with one another, they seem to lack intercellular junctions. Exogenous tracers penetrate between them and into the underlying neuropile, suggesting that there is no structural blood-brain barrier. Throughout the nervous tissue, extracellular spaces occur which contain banded collagen fibrils embedded in a matrix material. Thin glial cell processes, characterized by dense filaments, surround these regions and frequently form hemi-desmosomes with the extracellular matrix. The peripheral nerve cell bodies have a range of diameters; some have the characteristics of neurosecretory neurons. Granules in such neurons are produced by the Golgi saccules and associated fenestrated membranes which also possess many coated vesicles. Comparable granules are also found in axonal tracts, but no distinct peripheral neurohaemal areas have been found. Lysosomes are common in the nerve cell bodies and are frequently in the form of multivesicular bodies or large phagocytic vacuoles. Beneath the outer nerve cells lie many tracheae, arranged as a ring around the central neuropile which consists of glial processes, extracellular matrix, axons and nerve terminals. These nerve terminals occur throughout the central neuropile and are characterized by dense pyramidal presynaptic specializations and postsynaptic subsurface cisternae. The nervous system of Peripatus is relatively simple in its organization, in the lack of glial intercellular junctions and in the ready accessibility of substances from the external milieu.

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).

Plasma membrane specialization at the discharge site of the excretory poreless vacuole of the ciliate Euplotes raikovi

The contractile vacuole of Euplotes raikovi consists basically of a membrane-delimited cistern that has no direct communication with the exterior of the cell. Contiguous alveoli interpose between the cisterna and the plasma membrane and probably receive the cisternal contents before being expelled. The area of the plasma membrane that interacts with the outer membrane of the alveoli overlying the cistern has been found to carry a special region of intramembranous particles organized in 100-200 strands, showing varied sizes and a tendency to align parallel with one another. The region is also characterized by a smooth central core and by characteristic deformations that have been interpreted as focal sites of discharge of the contractile vacuole.

Physiological and ultrastructural evidence for an extracellular anion matrix in the central nervous system of an insect (Periplaneta americana)

The efflux of radiocations (22Na, 2K and 45Ca) and of radiochloride occur as two-stage processes from intact cockroach nerve cords. It is suggested that the initial, fast fraction of efflux comes mainly from the superficial connective tissue layer, the neural lamella, and the clefts between the underlying layer of neuroglia, the perineurium. This is deduced from the lack of effect of a metabolic inhibitor and sodium-transport inhibitors on the fast component of 22Na efflux (which contrast with their effects both on the size an the half-time of the slow component) and from the typically extracellular ratios between the fast components of substantial increase in the fast fractions of 22Na and 45Ca efflux but only a small increase in 36Cl efflux: effects which would be expected if the addition to the fast fraction consisted of ions maintained in Donnan equilibrium with fixed anionic sites within the extracellular system. The presence of such anionic sites is also indicated by lanthanum-binding in the extracellular matrix and by the previous histochemical demonstration of hyaluronic acid in the matrix by Ashhurst and Costin. It is suggested that the anionic glycosaminoglycans provide an extracellular cation reservoir which could serve a role in short-term ionic homeostasis of the brain microenvironment.

Differential accessibility to two insect neurones does not account for differences in sensitivity to alpha-bungarotoxin

The nicotinic acetylcholine receptor probe alpha-bungarotoxin (1.0 x 10(-7) M) blocks the depolarising response to ionophoretic application of acetylcholine onto the cell body membrane of the fast coxal depressor motoneurone (Df) of desheathed cockroach (Periplaneta americana) metathoracic ganglia, but at the same concentration is completely ineffective in blocking the depolarising action af acetylcholine on dorsal unpaired median (DUM) neurones in the same ganglion. The possibility that this is due to differences in accessibility of the toxin to the neurones has been tested by a combination of ionophoretic injection of horseradish peroxidase into single neurones with a study of the distribution of the exogenous tracer lanthanum, which is of similar effective size to alpha-bungarotoxin. The peripherally located cell body membranes and the fine axonal processes of Df and DUM neurones of desheathed metathoracic ganglia are equally accessible to lanthanum. Differential accessibility to the two cell types does not account therefore for the differences in sensitivity to alpha-bungarotoxin.

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.