Intercellular junctions in myriapods
Dipartimento di Biologia Evolutiva, Universita' di Siena, via Mattioli, 4, 53100 Siena, Italy. Tissue and Cell
(Impact Factor: 1.25).
02/1990; 22(3):359-69. DOI: 10.1016/0040-8166(90)90010-7
Tissue from the intestinal tract of myriapods, including millipedes, centipedes and pauropods were examined in tracer-impregnated sections and freeze-fracture replicas. The foregut and hindgut of all three classes exhibit pleated septate junctions; these display undulating intercellular ribbons in thin sections. In replicas they show discrete intramembranous particle (IMP) arrays aligned in rows in parallel; with one another. The tissues of the hindgut also possess scalariform junctions, characterized by cross-striated intercellular clefts in sections and IMP-enriched membranes in replicas. Gap junctions occur in all groups, but they are atypical in replicas in that their component IMPs do not always fracture onto the E face, as is characteristic of other arthropods; some IMPs cleave to the P face and others to the E face. The midgut of these organisms exhibits smooth septate junctions with conventional straight septal ribbons and occasional interseptal columns. However the intramembranous appearance in replicas is variable, particularly in centipedes, in that the rows of IMPs in chemically-unfixed propanecryofixed tissues, are prominent and adhere preferentially to the E face, with complementary P face grooves, while in fixed tissues the IMPs are much less distinct and fracture to either P face or E face. They tend not to protrude far beyond the mid-plane of the membrane bilayer and lie in rows which commonly take on the form of a network. Individual rows of the network sometimes curve to run beside a second row, over a short distance, before bending away into another part of the network. The aligned particle rows, which are much more prominent in millipedes, where they frequently lie in close parallel appositions, do not fuse into ridges as often occurs in insect tissues. The myriapod junctions, therefore, are of the same general kind as are found in the gut tract of other arthropod groups, but differ with respect to the subtleties of their intramembranous organization and disposition.
Available from: Andrew Donini
- "However, tracer impregnation techniques made it possible to show SJ structure in tangentially cut sections and substantial differences in the fine details of SJ intercellular elaboration was revealed. Based on tangential views, SJs have been subdivided into several types and some animals are reported to possess multiple types of SJs specific to different epithelia (Staehelin 1974; Green and Bergquist 1982; Dallai et al. 1990; Xué and Dallai 1992). The most obvious variation in SJs is in the conformations of the septa themselves which can be double or single and arranged in parallel linear rows, pleated sheets or anastomosing networks similar to TJ strands (see Figs. 2, 5; Green and Bergquist 1982; Green and Bergquistsssss 1979). "
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ABSTRACT: Invertebrate diversity and architecture is
immense. This is achieved by the organization and function
of four tissue types found in most metazoan phyla—epithelial,
connective, muscle and nervous tissue. Epithelial tissue
is found in all extant animals (parazoan and metazoan
alike). Epithelial cells form cellular sheets that cover internal
or external surfaces and regulate the passage of material
between separated compartments. The transepithelial
movement of biological material between compartments
can occur across the transcellular pathway (i.e. across cells)
or the paracellular pathway (i.e. between cells) and the latter
is regulated by occluding junctions that typically link
cells in a subapical domain. In this review, information on
occluding junctions of invertebrate epithelia is consolidated
and discussed in the context of morphology, ultrastructure
and physiology. In addition, an overview of what is currently
known about invertebrate occluding junction proteins
and their role in maintaining the integrity of invertebrate
epithelia and regulating the barrier properties of these tissues is presented.
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ABSTRACT: The main cell junctions in the intestinal tract of a small group of apterygotan insects, Protura, were examined in conventional thin sections, tracer-infiltrated sections and freeze-fracture replicas. The smooth septate junctions in the midgut of collembolan Tomocerus minor were also studied for comparison. Pleated septate junctions are found in foregut, hindgut and Malpighian papillae. They exhibit regular septa crossing the intercellular clefts in thin sections; and the septa with a pronounced zig-zag appearance run parallel to form a honeycomb structure in tracer-impregnated sections. After freeze-fracture undulating rows of intramembranous particles (IMPs) are visible on the P face. Smooth septate junctions are observed in the midgut. The intercellular septa often run in pairs for long tracts and exhibit a wavy course in lanthanum impregnated sections. The IMPs exhibited on the E face are usually separated one from another. Twin arrangement of particle rows is also apparent on the replicas. Gap junctions are frequent in both the midgut and hindgut and possess the conventional characteristics of 'inverted gap junction' with E face connexons. These results provide further evidence relating Protura closely to Collembola as well as to primitive arthropods.
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ABSTRACT: 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.
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