Two new septate junctions in the phylum Coelenterata

1980 ◽  
Vol 42 (1) ◽  
pp. 43-59
Author(s):  
C.R. Green ◽  
N.E. Flower

Freeze-fracture of fixed and unfixed tissue, lanthanum tracer and conventional thin-section studies have revealed 2 new types of septate junction in the class Anthozoa, phylum Coelenterata. These new junctions have the 15–18-nm intercellular spacing of all other described septate junctions and are found around the apical circumference of cells lining a lumen or outside edge. However, in freeze-fracture replicas and tangential views of lanthanum-impregnated tissue, they are seen to be quite different from other known septate junction types. One of the new junctions is found in endothelial tissue such as that lining the gut or the inside of the tentacles. In tangential view it is seen to consist of relatively short, straight, double septa, again with lateral projections. In feeeze-fracture of unfixed tissue, the junction consists of double rows of particles on the P face, the particles of one row being rounded, those of the other being elongated at right angles to the line of the septum. This dichotomy in particle size is unexpected, as the 2 halves of the septa as seen in tangential view are symmetrical. In freeze-fracture of fixed material the particle arrays remain on the P face and appear similar to those of unfixed material, but never as clear. In fixed tissue, some distortion had occurred and in extreme cases septa appear as a single broad jumbled row of particles. In this double septa junction, the rows of particles seen in freeze-fracture are occasionally seen to anastomose with a septum dividing into 2 and a third row of particles aligning with the 2 new septa to form their double particle rows. In both fixed and unfixed tissues, the E face of the junction consists of wide, shallow grooves. The second of the new junctions occurs in epithelial tissue, such as around the outer edge of sea-anemone tentacles, and consists of long wavy septa with lateral projections. In views where these projections appear longest, they arise predominantly from one side of the septa. In freeze-fracture of both fixed and unfixed tissue, this junction appears as rows of closely spaced particles on the P face. Occasionally rows of particles are seen on the E face, but usually this face is characterized by shallow grooves. In some aspects these 2 new junctions have features in common with the Hydra type junction also found in the Coelenterata. In all 3 types septa are relatively straight, rather than pleated, and there are lateral projections on the septa.

Author(s):  
Colin R. Green

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).


1988 ◽  
Vol 89 (2) ◽  
pp. 217-224
Author(s):  
ROMANO DALLAI ◽  
MARIA VEGNI TALLURI

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.


1982 ◽  
Vol 53 (1) ◽  
pp. 279-305 ◽  
Author(s):  
COLIN R. GREEN ◽  
PATRICIA R. BERGQUIST

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.


1978 ◽  
Vol 76 (1) ◽  
pp. 57-75 ◽  
Author(s):  
C J Connell

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.


1975 ◽  
Vol 17 (1) ◽  
pp. 221-239
Author(s):  
N.E. Flower ◽  
B.K. Filshie

The junctional structures present between the midgut cells of 3 lepidopteran caterpillars have been examined using freeze-etching, conventional staining and lanthanum tracer techniques. The bonding junction present in this type of tissue is the so-called continuous junction. Septa have only occassionally been reported in conventionally strained cross-sections of these junctions. During the present study septa have been observed in such sections but were more readily located in tissue treated with lanthanum tracer. Tangential sections of lanthanum-impregnated tissue show that these septa are parallel-sided, in contrast to the honeycomb appearance of the septa in septate junctions. The septa in freeze-etch replicas of glutaraldehyde-fixed tissue often show continuous rods on one membrane face, suggesting that the continuous junction may be more akin to the tight junction than to the normal septate junction. However, freeze-etch replicas of unfixed tissue appear much more like replicas of normal septate junctions. The main differences between septate and continuous junctions appear to be that the inter-membrane septa of the continuous junction are parallel-sided in tangential section, as against the honeycomb appearance of the septate junction, and that the particles which delineate the septa in freeze-etched preparations appear to be both somewhat differentl bonded and closer together in the continuous junction. A diagram is presented showing the internal and inter-membrane structures of the 2 types of junction based on the present study, and suggesting a possible explanation of the fact that septa are more readily seen in cross-sections of septate junctions than in continuous junctions. As septa are present in both types of junction, and because their freeze-etch appearances are not very different, it is suggested that the two types of septate junction be called ‘smooth septate’ and ‘pleated septate’ junctions, to indicate their characteristic appearances in tangential section.


1979 ◽  
Vol 37 (1) ◽  
pp. 373-389
Author(s):  
H.B. Skaer ◽  
J.B. Harrison ◽  
W.M. Lee

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.


1982 ◽  
Vol 94 (1) ◽  
pp. 77-87 ◽  
Author(s):  
D K Fristrom

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.


1986 ◽  
Vol 80 (1) ◽  
pp. 13-28
Author(s):  
B. Kachar ◽  
N.A. Christakis ◽  
T.S. Reese ◽  
N.J. Lane

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.


1983 ◽  
Vol 62 (1) ◽  
pp. 27-48
Author(s):  
E. Spiegel ◽  
L. Howard

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.


1977 ◽  
Vol 23 (1) ◽  
pp. 151-172
Author(s):  
B.K. Filshie ◽  
N.E. Flower

The sealing and communicating junctions present in hydra have been examined using conventional staining, lanthanum tracer and freeze-fracturing techniques. The presence of distinct types of gap and septate junctions has been confirmed. Combined lanthanum tracer and freeze-fracture results have provided a more detailed understanding of these junctional structures. A model has been constructed which demonstrates the various aspects of the junction seen at different sectioning angles. The probable lengths of septa within septate junctions and the junctional ‘maze’ formed by them is discussed because of its bearing on the ‘sealing’ nature of the junction and also, to some extent, on its permeability to tracers such as lanthanum.


Sign in / Sign up

Export Citation Format

Share Document