scholarly journals Septate junctions in imaginal disks of Drosophila: a model for the redistribution of septa during cell rearrangement.

1982 ◽  
Vol 94 (1) ◽  
pp. 77-87 ◽  
Author(s):  
D K Fristrom
Keyword(s):  
Freeze Fracture ◽  
Septate Junction ◽  
Lateral Boundary ◽  
Progressive Change ◽  
Septate Junctions ◽  
Thin Sections ◽  
Cell Rearrangement ◽  
Imaginal Disks ◽  
Discrete Domains ◽  
Small Channels

The organization of septate junctions during morphogenesis of imaginal disks is described from freeze-fracture replicas and thin sections with a view to understanding junction modulation during rearrangements of cells in epithelia. The septate junctions of each epithelial cell of the disk are distributed in a number of discrete domains equal to the number of neighboring cells. Individual septa traverse domains of contact between pairs of adjacent cells, turn downwards at the lateral boundary of the domain and run parallel to the intersection with a third cell. This arrangement leaves small channels at three-cell intersections that are occupied by specialized structures termed "tricellular plugs." Cell rearrangement involves a progressive change in the width of contact domains between adjacent cells, until old contacts are broken and new ones established. It is proposed that the septate junction adjusts to the changing width of domains by the compaction or extension of existing septa. This redistribution of septa theoretically allows a transepithelial barrier to be maintained during cell rearrangements. The applicability of this model to other epithelial tissues is discussed.

Evidence for septate junctions in the syzygy of the protozoon Gregarina polymorpha (Protozoa, Apicomplexa)

1988 ◽  
Vol 89 (2) ◽  
pp. 217-224
Author(s):  
ROMANO DALLAI ◽  
MARIA VEGNI TALLURI
Keyword(s):  
Plasma Membrane ◽  
Intercellular Space ◽  
Plasma Membranes ◽  
Freeze Fracture ◽  
Septate Junction ◽  
Fracture Face ◽  
Septate Junctions ◽  
Thin Sections ◽  
Intramembranous Particles ◽  
Lanthanum Tracer

A septate junction is described in reproductive pairs of the protozoon Gregarina polymorpha, using conventional thin sections, lanthanum tracer and freeze-fracture techniques. The septate junction is established between the plasma membranes at the tips of the joined epicytic folds. It is characterized by an intercellular space of 14–17 nm traversed by septa with a repeat of 15–25 nm. Lanthanum-treated material exhibits transparent curves forming a meshwork. Freeze-fracture replicas show membrane modifications in the shape of short rows of intramembranous particles on the E fracture face of the plasma membrane. The significance of the finding of such a septate junction between protozoan cells is discussed.

scholarly journals A freeze-fracture and lanthanum tracer study of the complex junction between Sertoli cells of the canine testis.

1978 ◽  
Vol 76 (1) ◽  
pp. 57-75 ◽  
Author(s):  
C J Connell
Keyword(s):  
Tight Junctions ◽  
Sertoli Cells ◽  
Freeze Fracture ◽  
Intercellular Junctions ◽  
Septate Junction ◽  
Septate Junctions ◽  
Thin Sections ◽  
En Face ◽  
Lanthanum Tracer ◽  
Complex Junction

What appear to be true septate junctions by all techniques currently available for the cytological identification of intercellular junctions are part of a complex junction that interconnects the Sertoli cells of the canine testis. In the seminiferous epithelium, septate junctions are located basal to belts of tight junctions. In thin sections, septate junctions appear as double, parallel, transverse connections or septa spanning an approximately 90-A intercellular space between adjacent Sertoli cells. In en face sections of lanthanum-aldehyde-perfused specimens, the septa themselves exclude lanthanum and appear as electron-lucent lines arranged in a series of double, parallel rows on a background of electron-dense lanthanum. In freeze-fracture replicas this vertebrate septate junction appears as double, parallel rows of individual or fused particles which conform to the distribution of the intercellular septa. Septate junctions can be clearly distinguished from tight junctions as tight junctions prevent the movement of lanthanum tracer toward the lumen, appear as single rows of individual or fused particles in interlacing patterns within freeze-fracture replicas, and are seen as areas of close membrane apposition in thin sections. Both the septate junction and the tight junction are associated with specializations of the Sertoli cell cytoplasm. This is the first demonstration in a vertebrate tissue of a true septate junction.

Topographical variations in the structure of the smooth septate junction

1979 ◽  
Vol 37 (1) ◽  
pp. 373-389
Author(s):  
H.B. Skaer ◽  
J.B. Harrison ◽  
W.M. Lee
Keyword(s):  
Musca Domestica ◽  
Functional Significance ◽  
Freeze Fracture ◽  
Rhodnius Prolixus ◽  
Malpighian Tubules ◽  
Septate Junction ◽  
Lanthanum Hydroxide ◽  
Septate Junctions ◽  
Standard Fixation

Smooth septate junctions in the midgut of Musca domestica and in Malpighian tubules of both Musca and Rhodnius prolixus are described. Details of the structures revealed after standard fixation, fixation in the presence of the stain, lanthanum hydroxide, and after freeze-fracture are discussed in the light of models previously put forward to explain the interrelations of the images obtained by these different methods. The organization of the junction between cells of the midgut varies in the apical-to-basal axis. At the apical border the septa (or ridges in freeze-fracture replicas) are packed tightly and follow an undulating but strictly parallel course. This packing loosens towards the middle of the junction until, at its basal extremity, the septa (ridges in replicas) are widely separated and follow independent meandering courses. That these features are found both in lanthanum-infiltrated specimens and freeze-fracture replicas allows a correlation to be made between the septa and the freeze-fracture ridges. The functional significance of these smooth septate junctions is discussed.

Stages in the assembly of pleated and smooth septate junctions in developing insect embryos

1982 ◽  
Vol 56 (1) ◽  
pp. 245-262 ◽  
Author(s):  
N.J. Lane ◽  
L.S. Swales
Keyword(s):  
Embryonic Development ◽  
Freeze Fracture ◽  
Organizational Capacity ◽  
Intramembranous Particle ◽  
Septate Junctions ◽  
Thin Sections ◽  
Intramembranous Particles ◽  
Initial Event ◽  
Random Arrays ◽  
Insect Embryos

The stages that occur during the assembly of both pleated and smooth septate junctions in developing insect tissues have been examined. The oesophagus and mid-gut of the embryonic moth, and the oesophagus and central nervous system (CNS) of the locust embryo, have been investigated in thin sections and by freeze-fracture during the course of membrane biogenesis. The smooth septate junctions developing between the lateral borders of the mid-gut exhibit, in the early stages, individual intramembranous particles becoming aligned into short ridges. These ultimately migrate over the membrane face and fuse into longer arrays, which become stacked in parallel with other ridges to form the characteristic mature form of the junction just before hatching. Pleated septate junctions occur between the cells both of the oesophagus and of the perineurium, which ensheathes the neurones and the neuroglial cells in the locust CNS; these are also fully formed by the end of embryonic development. The pleated junctions appear to be assembled during the later stages of CNS or gut differentiation, arising first in embryos about two-thirds of the way through development. During their maturation, the initial event seems to be a membrane depression in the P face, which occurs in patches over the presumptive junctional membrane. Into these depressed regions or ‘formation-plaque’ areas, 8–10 nm particles appear to be inserted intramembranously in apparently random arrays. These particles are the most common elements but larger particles are also present; the former ultimately become aligned in a row. With time, other intramembranous particles come to lie in rows parallel to the original one. By hatching, the typical undulating stacks of parallel intramembranous particle rows are fully formed. Gap junctions also form between the same perineurial or oesophageal cells, usually before, but in some cases at the same time, or just after, the septate junctions have been assembled. Tricellular associations between cells also appear around the same time in embryonic development. The simultaneous assembly of these different junctions reflects a high degree of organizational capacity at the membrane level.

scholarly journals The intramembrane structure of septate junctions based on direct freezing

1986 ◽  
Vol 80 (1) ◽  
pp. 13-28
Author(s):  
B. Kachar ◽  
N.A. Christakis ◽  
T.S. Reese ◽  
N.J. Lane
Keyword(s):  
Horseshoe Crab ◽  
Freeze Fracture ◽  
Transmembrane Proteins ◽  
Periodic Variation ◽  
Septate Junction ◽  
Fracture Face ◽  
Copper Block ◽  
Septate Junctions ◽  
The Relationship ◽  
Protoplasmic Fracture Face

Smooth septate junctions from the midgut of the cricket, Acheta, and the horseshoe crab, Limulus, as well as Hydra-type septate junctions from the epidermis of Hydra have been studied by freeze-fracture after direct freezing using the liquid helium-cooled copper block/slam freezing method. The exoplasmic fracture face at both types of septate junction exhibits rows of closely packed but irregularly shaped intramembrane particles. Complementary to these particle rows, on the protoplasmic fracture face, are sharply defined grooves with a periodic variation in depth and width that was conspicuous in Hydra but less well defined in arthropods. The closely packed, irregular particles on the exoplasmic faces could represent plastically deformed portions of transmembrane proteins pulled through the bilayer during freeze-fracture. On the basis of this interpretation, the grooves on the protoplasmic faces represent a confluence of the bilayer disruptions occurring during fracturing. The structures observed here are different from those reported in replicas of glutaraldehyde-fixed and glycerol-cryoprotected tissue, in which the intramembrane junctional components partition with the protoplasmic face and often assume the appearance of continuous cylinders. This comparison illustrates some of the artifacts associated with freeze-fracturing and shadowing. On the basis of a comparison of freeze-fracture replicas and sections of lanthanum-infiltrated tissues, the relationship between intramembrane junctional components and intercellular septal elements is analysed.

scholarly journals Development of cell junctions in sea-urchin embryos

1983 ◽  
Vol 62 (1) ◽  
pp. 27-48
Author(s):  
E. Spiegel ◽  
L. Howard
Keyword(s):  
Epithelial Cells ◽  
Basement Membrane ◽  
Digestive Tract ◽  
Sea Urchin ◽  
Freeze Fracture ◽  
Cell Junctions ◽  
Septate Junctions ◽  
Thin Sections ◽  
Sea Urchin Embryos ◽  
Lanthanum Tracer

The development of cell junctions in sea-urchin embryos has been investigated using thin sections, lanthanum-tracer and freeze-fracture techniques. Three types of desmosomes are present: belt desmosomes and spot desmosomes, which attach cells to each other, and hemi-desmosomes, which attach cells to the basement membrane. Two types of septate junctions are present: the straight, unbranched, double-septum septate, which is present in epithelial cells throughout embryogenesis, and the pleated, anastomosing, single-septum septate. The latter is formed only on cells that have invaginated to the interior of the embryo to form the digestive tract. The pleated junctions are shown to replace the straight junctions that were originally present before the cells migrated to the interior. It is suggested that these pleated septates may be specialized for digestive processes, since they are developed just prior to feeding and are retained in the adult intestine. Tricellular junctions, which join the bicellular junctions of three adjoining cells, have been identified in the embryo and in the adult intestine. Evidence for the presence of gap junctions was not obtained, but there are indications of their presence.

Plasma membranes, cell junctions and cuticle of the rectal chloride epithelia of the larval dragonfly Aeshna cyanea

1983 ◽  
Vol 59 (1) ◽  
pp. 159-182
Author(s):  
J. Kukulies ◽  
H. Komnick
Keyword(s):  
Gap Junctions ◽  
Structural Control ◽  
Plasma Membranes ◽  
Freeze Fracture ◽  
Septate Junction ◽  
Cell Junctions ◽  
Junctional Complex ◽  
Apical Region ◽  
Thin Sections ◽  
Aeshna Cyanea

The cell membranes and cell junctions of the rectal chloride epithelia of the larval dragonfly Aeshna cyanea were examined in thin sections and by freeze-fracture. These epithelia function in active ion absorption and maintain a high concentration gradient between the haemolymph and the fresh-water environment. Freeze-fracturing reveals fine-structural differences in the intramembraneous particles of the luminal and contraluminal plasma membranes of these epithelia, reflecting the functional diversity of the two membranes, which are separated by the junctional complex. The particle frequency of the basolateral plasma membranes is reduced after transfer of the larvae into high concentrations of environmental salinity. The junctional complex is located in the apical region and composed of three types of cell junctions: the zonula adhaerens, seen in freeze-fracture as a nearly particle-free zone; the extended and highly convoluted pleated septate junction and randomly interspersed gap junctions of the inverted type. Gap junctions also occur between the basolateral plasma membranes. They provide short-cuts in the diffusion pathway for direct and rapid co-ordination of the interconnected cell processes. Colloidal and ionic lanthanum tracer solutions applied in vivo from the luminal side penetrate through the cuticle via epicuticular depressions, but invade only the apical portion of the junctional complex. This indicates that the pleated septate junction constitutes a structural control of the paracellular pathway across the chloride epithelia, which are devoid of tight junctions. The structure of the pleated septate junctions is interpreted as a device for the extension of the diffusion distance, which is inversely related to the net diffusion. A conservative estimate of the total length of the junction, and the number and extension of septa reveals that the paracellular route exceeds the transcellular route by a factor of 50.

Variations of separate junction structure in the invertebrates

1978 ◽  
Vol 36 (2) ◽  
pp. 338-339
Author(s):  
Colin R. Green
Keyword(s):  
Thin Section ◽  
Freeze Fracture ◽  
Epidermal Cells ◽  
Septate Junction ◽  
Septate Junctions ◽  
Wide Range ◽  
Lanthanum Tracer ◽  
Endodermal Cells ◽  
Junction Structure

Three main variations of the invertebrate septate junction are now generally accepted; the Hydra type, the pleated septate and the smooth septate junctions. A junctional study of many members of a wide range of invertebrate phyla using thin section, lanthanum tracer and freeze-fracture techniques has however revealed at least eight distinct septate junction types, including two anastomosing septate junctions in the higher invertebrate phyla.In the Coelenterata three forms of septate junction occur. The Hydra type found in Hydrozoa (Fig 1), a pegged junction seen in the epidermal cells of Anthozoa and a ladder-like junction seen in the endodermal cells of Anthozoa. The pegged Anthozoa junction consists of septa with distinct short pegs branching at right angles mainly from one side (fig 2). Where two septa run close together, the pegs may form crossbars linking them. The ladder junction has a pegged double septum with crossbars linking the two parts of each septum (fig 3).

Phylogenetic Relationships within Theinvertebrata in Relation to Thestructure of Septate Junctions and the development of ‘Occluding’ Junctional Types

1982 ◽  
Vol 53 (1) ◽  
pp. 279-305 ◽  
Author(s):  
COLIN R. GREEN ◽  
PATRICIA R. BERGQUIST
Keyword(s):  
Tight Junction ◽  
Phylogenetic Relationships ◽  
Freeze Fracture ◽  
Intercellular Junctions ◽  
Septate Junction ◽  
Simple Type ◽  
Septate Junctions ◽  
Exterior Surface ◽  
Lanthanum Tracer ◽  
To Come

The structures of 13 variants of invertebrate septate junction are reviewed on the basis offreeze-fracture, lanthanum tracer and thin-section studies. In addition, a simple type ofoccluding junction in the phylum Porifera, a variation of tight junction in the phylum Tunicateand the vertebrate tight junction are covered. All the junctions considered form a belt around the apical circumference of cells lining a lumen or an exterior surface. The large number of these junctions now recognized permits discussion relating to invertebrate classification and suggested phylogenetic relationships, and to the development of intercellular junctions. The relationships revealed are discussed under three headings: Coelenterates and lower invertebrates, Proterostomia (the annelid, molluscan and arthropod lineage) and the Deuterostomia(the echinoderm and chordate lineage). It is proposed that the pleated septate junction of the lower invertebrates resembles that of the hydrozoan rather than anthozoan Coelenterates. This lower invertebrate pleated septate junction occurs in several lower invertebrate phyla including the Annelida (of the proterostome lineage), but also occurs in the Sipunculoidea, a group supposedly on the deuterostome lineage.The proterostome line includes the molluscs and the arthropods, which have the molluscarthropodpleated septate junction. Several variations of the smooth septate junction are alsoseen in Arthropoda. Among the deuterostomes the Chaetognatha have both a paired septatejunction and a pleated junction and are therefore considered to be not very far removed fromthe Sipunculoidea. The echinoderms and hemichordates also have double-septum septatejunctions. In addition however, these two phyla have anastomosing septate junctions thatare very similar, varying only in their final configuration. Of the two, the echinoderm anastomosingseptate junction most closely resembles the tight junction seen in the tunicates, and the Hemichordata are therefore considered to be a lateral development from the main lineof chordate evolution. The tunicates have a tight junction similar to that seen in vertebrates;it is however more ‘leaky’ and has distinctive freeze-fracture characteristics.In the phylum Porifera a form of simple parallel membrane junction appears to serve anoccluding function. This junction has regular intercellular spacing in the absence of any septaand it is suggested that the spacing in septate junctions is probably not dictated by the septa.This interpretation is reasonable particularly when the diversity of septal types in conjunctionwith stable intercellular spacing is considered. Finally, a theory is put forward suggesting thatin evolution a change from the septate to the tight junction could simply involve a modificationof a ‘membrane spacing factor’, which allows the membranes of adjacent cells to come together at intervals, in the normal tight junction pattern.

Isolation and characterization of invertebrate smooth septate junctions

1983 ◽  
Vol 62 (1) ◽  
pp. 351-370
Author(s):  
C.R. Green ◽  
C. Noirot-Timothee ◽  
C. Noirot
Keyword(s):  
Molecular Weight ◽  
Gap Junctions ◽  
Freeze Fracture ◽  
Intercellular Junctions ◽  
Septate Junction ◽  
Major Protein ◽  
Septate Junctions ◽  
Isolation And Characterization ◽  
Significant Difference ◽  
Freeze Fracture Electron Microscopy

Using modifications of techniques used for the isolation of macula type intercellular junctions (gap junctions and desmosomes) the arthropod smooth septate junction has been isolated from insect midgut tissue. Midguts from cockroaches or mealworms were used and membrane fractions were obtained by sucrose gradient and ultracentrifugation techniques. Preparations with reasonable concentrations of septate junction were obtained and have been studied by thin-section, negative-stain and freeze-fracture electron microscopy. The junctions appeared to be well preserved, although there was evidence that the junction strands were able to slide within the plane of the membrane. Septa were seen to have a cross-striated appearance when viewed after negative staining but their exact structure remained difficult to determine. Polyacrylamide gel electrophoretic studies demonstrated the reproducibility of the isolation procedure and showed that septa may have a 47 000 molecular weight glycoprotein component. Gel electrophoresis also gave some indication of the intramembrane biochemistry of the smooth septate junction, with proteins of 31 000 and 32 000 molecular weight always occurring in the junction fractions. The junctions were, however, very sensitive to both mechanical and chemical treatments, the septa were destroyed by rough homogenization or by treatment with urea at a concentration as low as 1 M. Freeze-fracture of untreated, isolated junctions demonstrated no differences from junctions in intact tissue, while replicas of urea-treated material were more difficult to interpret as the component parts of the junctions became separated once the septa had been destroyed. Gap junctions were also obtained and resisted both mechanical and chemical treatment, which destroyed the septate junctions. Their major protein component appeared to have a molecular weight of 36 000. Attempts to isolate pleated septate junctions (from insects, molluscs and annelids) by the same techniques failed, implying a significant difference in the structures of the two types of septate junction.

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