CELL CONTACTS IN THE MOUSE MAMMARY GLAND : I. Normal Gland in Postnatal Development and the Secretory Cycle

The nature and distribution of cell contacts have been examined in thin sections and freeze-fracture replicas of mammary gland samples from female C3H/Crgl mice at stages from birth through pregnancy, lactation, and postweaning involution. Epithelial cells of major mammary ducts at all stages examined are linked at their luminal borders by junctional complexes consisting of tight junctions, variable intermediate junctions, occasional small gap junctions, and one or more series of desmosomes. Scattered desmosomes and gap junctions link ductal epithelial and myoepithelial cells in all combinations; hemidesmosomes attach myoepithelial cells to the basal lamina. Freeze-fracture replicas confirm the erratic distribution of gap junctions and reveal a loose, irregular network of ridges comprising the continuous tight-junctional belts. Alveoli develop early in gestation and initially resemble ducts. Later, as alveoli and small ducts become actively secretory, they lose all desmosomes and most intermediate junctions, whereas tight and gap junctions persist, The tight-junctional network becomes compact and orderly, its undulating ridges oriented predominantly parallel to the luminal surface. It is suggested that these changes in junctional morphology, occurring in secretory cells around parturition, may be related to the greatly enhanced rate of movement of milk precursors and products through the lactating epithelium, or to the profound and recurrent changes in shape of secretory cells that occur in relation to myoepithelial cell contraction, or to both.     

Tight junctions in the choroid plexus epithelium. A freeze-fracture study including complementary replicas

The tight junctions of the choroid plexus epithelium of rats were studied by freeze-fracture. In glutaraldehyde-fixed material, the junctions exhibited rows of aligned particles and short bars on P-faces, the E-faces showing grooves bearing relatively many particles. A particulate nature of the junctional strands could be established by using unfixed material. The mean values of junctional strands from the lateral, third, and fourth ventricles of Lewis rats were 7.5 +/- 2.6, 7.4 +/- 2.2, and 7.5 +/- 2.4; and of Sprague-Dawley rats 7.7 +/- 3.4, 7.4 +/- 2.3, and 7.3 +/- 1.6. Examination of complementary replicas (of fixed tissue) showed that discomtinuities are present in the junctional strands: 42.2 +/- 4.6% of the length of measured P-face ridges were discontinuities, and the total amount of complementary particles in E-face grooves constituted 17.8 +/- 4.4% of the total length of the grooves, thus approximately 25% of the junctional strands can be considered to be discontinuous. The average width of the discontinuities, when corrected for complementary particles in E-face grooves, was 7.7 +/- 4.5 nm. In control experiments with a "tighter" tight junction (small intestine), complementary replicas revealed that the junctional fibrils are rather continuous and that the very few particles in E-face grooves mostly filled out discontinuities in the P-face ridges. Approximately 5% of the strands were found to be discontinuous. These data support the notion that the presence of pores in the junctional strands of the choroid plexus epithelium may explain the high transepithelial conductance in a "leaky" epithelium having a high number of junctional strands. However, loss of junctional material during fracturing is also considered as an alternative explanation of the present results.     

The tight junctions of rabbit choroid plexus and ciliary body epithelia. A comparative study using the double replica freeze-fracture technique

The spatial arrangement of tight junctions in choroid plexus and ciliary body rabbit epithelia has been determined by studying freeze-fracture complementary replicas. In the choroid plexus epithelium, the interruptions of the junctional P-face fibrils were measured to be 14% of their total length. In the ciliary body epithelium, where the fibrils were found to be more fragmented than in the choroid plexus, the P-face fibril interruptions accounted for 12 % of the total length of the zonulae occludentes sealing the non-pigmented cells and 30% in the focal linear tight junctions connecting the non-pigmented and pigmented cells at their apices. In both epithelia, the interruptions of the ridges are precisely complemented by particles or short bars of similar length found in the E-face furrows. Consequently, it is possible to conclude that the junctional fibrils are continuous in these two epithelia. For the zonulae occludentes, this continuity appears to be inconsistent with the ‘leaky’ properties of these epithelia shown by some physiological investigations.     

Rhombic particle arrays in gill epithelium of a mollusc, Aplysia californica.

Gill epithelia from adult and juvenile Aplysia were examined by conventional thin section and freeze-fracture methods. Freeze-fracture replicas of adult gill epithelium revealed septate and gap junctions, which served as membrane markers for the epithelial cells. In these same cell membranes, non-junctional rhombic arrays of intramembranous particles were observed on prominent ridges on the membrane P fracture face of some epithelial cells. In thin sections of adult epithelium, nerve terminals were observed abutting the lateral plasma membranes near the basal lamina of some epithelial cells. Correlative areas of plasma membrane in freeze-fracture replicas showed a close association between rhombic particle arrays and abutting nerve terminals. In thin sections of juvenile Aplysia, nerve terminals abutting the epithelial cells were not recognizable, and rhombic arrays were not observed in freeze-fracture replicas. This suggested that a developmental association existed between the appearance of rhombic arrays in adult epithelia and their innervation. It is not known with certainty if, in invertebrates, rhombic arrays are an essential structural entity of all innervated cell membranes; however, in the cells thus far studied, there appears to be an associative condition. In the case of the gill epithelium of Aplysia, rhombic arrays are located in the same vicinity as the abutting nerve terminals. Similar arrays of intramembranous particles have been observed in myoneural postjunctional complexes of other invertebrates and have been interpreted to be the morphological expression of neurotransmitter receptors. An analogous explanation is put forth, namely that rhombic arrays may represent the structural correlates of neurotransmitter receptors and/or ionic channels in innervated membranes of invertebrates.     

Structural integrity of hepatocyte tight junctions

The significance of discontinuities frequently found in freeze-fracture replicas of the tight junction was evaluated using complementary replicas of hepatocyte junctions from control and bile duct-ligated rats. An extensive analysis of complementary replicas using rotary platinum shadowing indicates that discontinuities in the protoplasmic (P) fracture face do not represent structural breaks in the tight- junctional network. In no case did P-face discontinuities correspond with interruptions in the groove network on the complementary extracellular (E) face. Quantitative analysis of replicas shows that P- face discontinuities result in part from "transfer" of material to the complementary E face (approximately 7% of the junctional length). However, many P-face discontinuities (7-30% of the junctional length) are matched only by a groove on the complementary E face. This finding demonstrates that a significant amount of material can be lost during freeze-fracture. An analysis of junctions from bile duct-ligated rats, which are known to have an increased paracellular permeability, shows comparable transfer and loss of material. However, the number of junctional elements and the tight-junction network density was significantly reduced by bile duct ligation. These observations indicate that discontinuities in tight-junctional elements result during the preparation of freeze-fracture replicas and are not physiologically important features of the junctional barrier. Variation in the number of elements provides the best explanation for observed differences in tight-junction permeability.     

Occluding-like junctions at mesaxons of central myelin in Anolis carolinensis are not 'tight'. A freeze-fracture-protein tracer analysis.

The junctional complexes of the myelin sheath of central nervous system axons in the American chameleon, Anolis carolinensis, exhibit an intramembrane ridge and groove construction in freeze-fracture replicas that has usually been interpreted in other organisms as evidence for an occluding or tight intercellular junction. Close examination of PF fracture face ridges, however, shows them to be made up of discontinuous rows of particles of variable length separated by frequent gaps of non-uniform width. Introduction of horseradish peroxidase into the intercellular milieu of the lizard central nervous system is followed by appearance of this protein in interlamellar spaces of the myelin sheath and in the intercellular spaces containing focal membrane fusions that correspond precisely in position and center-to-center spacing to the ridges and grooves in platinum replicas of the same tissue. Since the junctional ridges on PF fracture faces in these mesaxonal junctional complexes are conspicuously discontinuous and since the areas within the myelin sheath where these junctional complexes are located inner and outer mesaxons) are readily permeated by exogenous protein tracer, it is concluded that the junctional complexes of central myelin mesaxons, heretofore incorrectly interpreted as functionally tight, are actually very leaky and probably contribute only to the structural stability of the myelin sheath architecture.     

The structure and function of the smooth septate junction in a transporting epithelium: The malpighian tubules of the new zealand glow-worm Arachnocampa luminosa

Junctional complexes between the epithelial cells in the four distinct regions of the glow-worm Malpighian tubule were investigated by electron microscopy using thin sectioning, freeze-fracturing, osmotic disruption and tracer techniques. The lateral plasma membranes of all four cell types are joined by smooth septate junctions but the extent of the complex across the cell depth varies in the four different regions. The width of the septa, the interseptal spacing and the separation between the outer leaflets of the adjacent plasma membranes are different for each cell type. Gap junctions were identified only in the junctional complex between Type IV cells and were intercalated amongst large lateral sinuses. In oblique sections of lanthanum infiltrated tissue, the electron-lucent septa at the basal side of the junction are outlined by the tracer as it penetrates. In the Junctional complexes of all four regions the septa appear as short, distinct, linear bars. In tangential sections of gap junctions between Type IV cells, the junctions appear as a hexagonal array of intermembrane particles with a centre to centre spacing of 18 nm. Horseradish peroxidase did not penetrate the junctional complexes very far but readily passed through the basal lamina into the spaces between extracellular invaginations of the basement membrane of the cells. Junctional complexes in all four areas of the tubule have similar freeze-fracture faces. In freeze-fracture replicas of fixed tissue continuous ridges of fused particles are seen on the P face and complementary furrows are found on the E face. Junctional response to osmotically adjusted Ringer solutions was similar in all four cell types. Distortion or ‘blistering’ of the intercellular space between the septa of the junction occurred when the tissue was bathed in or injected with a hypertonic Ringer solution. The structure of these junctions, visualized by the different techniques, and the role of the septate junction in a transporting epithelium, are discussed.     

Ultrastructural features of degeneration of the gallbladder during lamprey biliary atresia

Thin sections and freeze-fracture replicas were used to examine the fine structural features of degeneration of the gallbladder during lamprey biliary atresia. The cells of the epithelium undergo a progressive accumulation of dense bodies and vacuoles, loss of glycogen, condensation of the filamentous ectoplasm, fragmentation of microvilli, and dilation of cisternae of rough endoplasmic reticulum but eventually disappear by stage 4 of metamorphosis. Zonulae occludentes in the epithelium show a progressive increase in apical-basal depth as the junctional strands fragment. The possibility of an influence of transformed, subepithelial cells on degeneration of epithelial cells is suggested by close contact of the former with the thickened, highly pleated, epithelial basal lamina. The smooth muscle cells of the larval gallbladder are believed to transform during lamprey metamorphosis into these subepithelial cells which shed their external lamina, become intimately associated with collagen and other microfibrils, and which may be capable of phagocytosis. The events of gallbladder degeneration during lamprey metamorphosis show features of apoptosis.     

The Sertoli cell junctional complex: structure and permeability to filipin in the neonatal and adult guinea pig

The development and maintenance of the Sertoli cell junctional complex were investigated in prepubertal and adult guinea pigs. To correlate the structure of the blood-testis barrier with its permeability, the polyene antibiotic filipin (a cholesterol-binding agent of low molecular weight: 570.70) was added to the fixative as a tracer visible in freeze-fracture replicas. Discontinuous zonules, intermediate junctions (i.e., adhering fasciae) and gap junctions all proved permeable to filipin in the two age groups. Only the continuous occluding zonules characteristic of the adult guinea pig's testis were impermeable to the tracer. In pubertal animals, the establishment of the blood-testis barrier coincided with the completion of the junctional strands in occluding zonules. The formation of occluding zonules was similar in the newborn and the adult. In the adult, the Sertoli cell junctional complexes contained three types of cell junctions: occluding, adhering, and gap junctions. The sequence of occluding and adhering junctions from the base to the apex of the epithelium was the reverse of that demonstrated in most epithelia. The impermeable continuous occluding zonules at the base showed parallel patterns of uninterrupted junctional strands, whereas the permeable discontinuous zonules found higher in the epithelium showed a meandering pattern of broken strands. Our observations indicate that (1) Sertoli cell junctional complexes form near the young germinal cells at the base of the seminiferous epithelium and break down near the older germinal cells toward the apex; (2) the various patterns and orientations of the junctional strands reflect, respectively, the different stages of disintegration of the occluding zonules and the conformation of the mature Sertoli cell to the irregular contours of the germinal cells; (3) there is no relationship between permeability and junctional strand orientation; and (4) the cellular contacts between Sertoli cells and germinal cells situated below the blood-testis barrier may represent the early stages of formation of junctional elements which ultimately become incorporated into the Sertoli cell junctional complex.     

Cell junctions in the rapidly moving edge of chick and quail blastoderms during gastrulation

Summary The intercellular contacts of the migrating edge of chick and quail blastoderms during gastrulation were studied by transmission electron microscopy of thin sections and of freeze-fracture replicas. Tight junctions and gap junctions as well as desmosomes were found. Tight junctions were organized as single junctional strands or as a complex of numerous junctional strands interposed between the lamellae and the bodies of the cells building up the margin of overgrowth. The function of these intercellular junctions is considered in relation to the locomotion of the margin of overgrowth cells.     

Gap junctions in the heart of the adult Protopterus aethiopicus

In thin sections and in freeze-fracture replicas small and sparse gap junctions appear to be developed on the longitudinal plasma membrane of Protopterus cardiac cells near a macula or fascia adhaerens. By thin-section electron microscopy, they had septalaminar profiles with a length between 0.042 and 0.260 micron. In freeze-fracture images they appear on the P-fracture face as maculate particle aggregations with complementary pits on the E-fracture face. Particles with a central intercellular channel could be observed. The average center-to-center distance between neighbouring particles or pits is 10.05 +/- 1.87 nm (N = 2429). The diameter of the junctional maculae in replicas lies between 0.037 and 0.229 nm. The particle packing density increases in larger maculate aggregations, while particle-free areas emerge which could be related to the degradation or reformation of gap junctions Atypical configurations of gap junctions observed in the myocardium of lower vertebrates are rarely encountered in this primitive vertebrate.     

The structure of tight junctions in the tracheal epithelium may not correlate with permeability

Summary To test the hypothesis that cigarette smoke produces changes in the morphology of tight junctions guinea pigs were exposed to cigarette smoke or air in a previously standardized fashion (Simani et al. 1974). Permeability is greatest one half hour following exposure to cigarette smoke (Hulbert et al. 1981). The animals were sacrificed at that time. The tracheal epithelium was studied using both thin-section and freeze-fracture techniques. A quantitative analysis of the organization and integrity of junctional complexes was performed for each animal. Organization was assessed by measuring and comparing areas delimited by PF fibers and EF furrows. PF fiber integrity was assessed by measuring uninterrupted lengths of fibers and furrows from freeze-fracture replicas. This assessment did not demonstrate a change in tight-junction morphology following exposure to cigarette smoke.     

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.     

Morphology of tight junctions in the ciliary epithelium of rabbits during arachidonic acid-induced breakdown of the blood-aqueous barrier

Summary A reversible breakdown of the blood-aqueous barrier in the iridial processes of rabbits has been induced by arachidonic acid as demonstrated by the passage of horseradish peroxidase at places through the tight junctions. Freeze-fracture images reveal very discontinuous Pface ridges. However, the analysis of complementary replicas demonstrates that discontinuities of P-face ridges are always complemented by particles or short bars found in the E-face furrows. Though the problem exists of correlating freeze-fracture images of the junctional structure to the focal passage of horseradish peroxidase, the data suggest that the discontinuities of P-face ridges cannot be the structural counterpart of the passage of horseradish peroxidase. Alternative pathways of horseradish peroxidase are discussed in context with the offset bifibrillary model of the junction.Supported by a research fellowship from the Deutsche Forschungsgemeinschaft Present address: Universitäts Augen- und Poliklinik der Freien Universität, Klinikum Steglitz, Hindenburgdamm 30, 1000 Berlin 45This paper was presented in part at the International Symposium on Membrane Transport Mechanisms in the Eye, September 1984, Berlin     

Junctional diversity in two regions of the epidermis of Oikopleura dioica (Tunicata,Larvacea)

Summary A simple continuous epithelium surrounds the body of the pelagic larvacean. It consists of two zones of cells: oikoplast cells and flattened cells. The oikoplast cells are columnar and produce a thick extracellular house that ensheathes the body of the organism. These cells are joined laterally by wide tight junctions (zonulae occludentes). The tail of the animal is surrounded by exceedingly thin cells which are joined by narrow tight junctions under which lie intermediate junctions (zonulae adhaerentes) and gap junctions. A web of fibrous material inserts into the intermediate junctions. The transitional cells between the two epithelial zones have one lateral border with a wide tight junction, and the other lateral border with a narrow tight junction and a wide intermediate junction. In freeze-fracture replicas, the wide tight junction has a number of anastomosing ridges, in comparison with the narrow tight junction, which usually consists of only a single row of intramembranous particles. In replicas, the thin epithelial cells show unusual parallel arrays of particles in clusters on their apical plasma membranes. This simple epithelium, therefore, exhibits striking differences between the two cellular zones, in the structural characteristics of both the lateral borders and the apical membrane.     

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     

Freeze-fracture analysis of gap junction disruption in rat ovarian granulosa cells

The effects of chemical dissociation on rat ovarian granulosa cell gap junctions has been studied using freeze-fracture electron microscopy. Sequential exposure of granulosa cells within follicles to solutions containing 6·8 mM EGTA [ethylene-bis-(β-aminoethyl ether)-N,N′-tetra acetic acid] and 0·5 M sucrose results in extensive cellular dissociation of the follicular epithelium. Freeze-fracture replicas made from fixed, control or EGTA-treated ovarian follicles exhibit extensive gap junctions between granulosa cells that are characterized by a range of packing order of constituent P-face particles or E-face pits. In contrast, exposure to 0·5 M sucrose containing 1·8 mM EGTA for as little as 1 min results in a consistently close packing of particles or pits which is accompanied by splitting of gap junctions between granulosa cells. The process of junction splitting was studied in detail in replicas prepared from follicles treated sequentially for various periods of time with EGTA and sucrose solutions. Initially, large gap junctions lose their regular shape and fragment into numerous tightly packed aggregates of P-face particles or E-face pits which are separated by unspecialized areas of plasma membrane. Subsequent to junction fragmentation, individual junction plaques separate at sites of cell contact and generate hemijunctions that border the intercellular space, Hemijunctions undergo particle dispersion of the P fracture face which results in an increased density of large intramembrane particles; no corresponding change in E-face pits is discernible at this stage. Morphometric analysis of replicas of tissue undergoing junction splitting indicates that junctional surface area decreases to 10–20% of control levels during this same treatment and so further supports the qualitative observations on junction fragmentation. Viabilities of granulosa cells obtained by these techniques also agree with the sequence observed in the morphometric analysis of the replicas. Finally, within 15 min after placing ovaries in isotonic, Ca²⁺-containing salt solutions, gap junction reformation occurs by aggregation of particles at sites of intercellular contact. These sites are distinguished by the appearance of short surface protrusions or indentations on their respective P and E fracture faces. The data suggest a mechanism for EGTA-sucrose mediated cellular dissociation in the follicular epithelium in which gap junctional particles are free to move in the plane of the plasma membrane and may be re-utilized to form gap junctions in the presence of extracellular calcium.     

Membrane events involved in fusion of uterine epithelial cells in pseudopregnant rabbits

Summary During pseudopregnancy in the rabbit some uterine epithelial cells undergo conversion into symplasmata. This event serves as a model for studies of membrane apposition, fusion and fission of the lateral membranes with the use of different ultrastructural techniques. Apposition of lateral membranes occurs by means of proliferation of the tight-junctional belt and macular tight junctions. Membrane fusion is characterized in freeze-fracture replicas by continuously running fracture planes between neighboring membrane leaflets of epithelial cells, in general without reorganization of the particles. It is suggested that the reorganization of particles as well as the blebs or vesicles of smooth membranes, which are occasionally observed, may be artefacts. Membrane fission occurs simultaneously with fusion resulting in irregularly shaped membrane holes on freeze-fracture replicas. These events are rarely seen in thin sections. Staining with tannic acid reveals that only the layers of the plasma membrane are accessible to this agent. The fusion-fission process starts in the lower region of the lateral membranes, whereas the luminal portion with the broad tight-junctional belt remains intact.Dedicated to Professor Dr. med. Dr. phil. Karl-Heinrich Knese, Stuttgart-Hohenheim, in honour of his 70th birthday     

Cyclic modulation of Sertoli cell junctional complexes in a seasonal breeder: the mink (Mustela vison)

The development and modulation of Sertoli cell junctions was studied in newborn and adult mink during the active and inactive spermatogenic phases. The techniques used were electron microscopy of freeze-fractured replicas and thin sections of tissues infused with horseradish peroxidase as a junction permeability tracer. In the newborn, freeze-fractured developing junctions had either spherical or fibrillar particles. In addition, junctional domains where particles were associated preferentially with the E-face, and others where particles were associated preferentially with the P-face, were found developing either singly or conjointly within a given membrane segment, thus yielding a heterogeneous junctional segment. Coincidently with the development of a tubular lumen and the establishment of a competent blood-testis barrier, junctional strands were composed primarily of particulate elements associated preferentially with the E-face. In adult mink during active spermatogenesis, cell junctions were found on the entire lateral Sertoli cell plasma membrane from the basal to the luminal pole of the cell. In the basal third of the Sertoli cell, membranous segments that faced a spermatogonium or a migrating spermatocyte displayed forming tight, gap, and adherens junctions. In the middle third, abutting membrane segments localized above germ cells were involved in continuous zonules and in adherens junctions. In the apical or luminal third, the zonules were discontinuous, and the association of junctional particles with the E-face furrow was lost. Gap junctions increased in both size and numbers. Junctional vesicles that appeared as annular gap and tight-junction profiles in thin sections or as hemispheres in freeze-fracture replicas were present. Reflexive tight and gap junctions were formed through the interaction of plasma membrane segments of the same Sertoli cell. Internalized junctional vesicles were also present in mature spermatids. During the inactive spermatogenic phase, cell junctions were localized principally in the basal third of the Sertoli cell; junctional strands resembled those of the newborn mink. During the active spermatogenic phase, continuous zonules were competent in blocking passage of the protein tracer. During the inactive phase the blood-testis barrier was incompetent in blocking entry of the tracer into the seminiferous epithelium. It is proposed that modulation of the Sertoli cell zonules being formed at the base and dismantled at the apex of the seminiferous epithelium follows the direction of germ cell migration and opposes the apicobasal direction of junction formation reported for most epithelia.(ABSTRACT TRUNCATED AT 400 WORDS)     

Definitive Evidence for the existence of tight junctions in invertebrates

Extensive and unequivocal tight junctions are here reported between the lateral borders of the cellular layer that circumscribes the arachnid (spider) central nervous system. This account details the features of these structures, which form a beltlike reticulum that is more complex than the simple linear tight junctions hitherto found in invertebrate tissues and which bear many of the characteristics of vertebrate zonulae occludentes. We also provide evidence that these junctions form the basis of a permeability barrier to exogenous compounds. In thin sections, the tight junctions are identifiable as punctate points of membrane apposition; they are seen to exclude the stain and appear as election- lucent moniliform strands along the lines of membrane fusion in en face views of uranyl-calcium-treated tissues. In freeze-fracture replicas, the regions of close membrane apposition exhibit P-face (PF) ridges and complementary E-face (EF) furrows that are coincident across face transitions, although slightly offset with respect to one another. The free inward diffusion of both ionic and colloidal lanthanum is inhibited by these punctate tight junctions so that they appear to form the basis of a circumferential blood-brain barrier. These results support the contention that tight junctions exist in the tissues of the invertebrata in spite of earlier suggestions that (a) they are unique to vertebrates and (b) septate junctions are the equivalent invertebrate occluding structure. The component tight junctional 8- to 10-nm-particulate PF ridges are intimately intercalated with, but clearly distinct from, inverted gap junctions possessing the 13-nm EF particles typical of arthropods. Hence, no confusion can occur as to which particles belong to each of the two junctional types, as commonly happens with vertebrate tissues, especially in the analysis of developing junctions. Indeed, their coexistance in this way supports the idea, over which there has been some controversy, that the intramembrane particles making up these two junctional types must be quite distinct entities rather than products of a common precursor.