A novel adhering junction in the apical ciliary apparatus of the rotifer Brachionus plicatilis (Rotifera, Monogononta).
ABSTRACT Cultures of the rotifer Brachionus plicatilis were examined with regard to their interepithelial junctions after infiltration with the extracellular tracer lanthanum, freeze-fracturing or quick-freeze deep-etching. The lateral borders between ciliated cells have an unusual apical adhering junction. This apical part of their intercellular cleft looks desmosome-like, but it is characterized by unusual intramembranous E-face clusters of particles. Deep-etching reveals that these are packed together in short rows which lie parallel to one another in orderly arrays. The true membrane surface in these areas features filaments in the form of short ribbons; these are produced by projections, possibly part of the glycocalyx, emerging from the membranes, between which the electron-dense tracer lanthanum permeates. These projections appear to overlap with each other in the centre of the intercellular cleft; this would provide a particularly flexible adaptation to maintain cell-cell contact and coordination as a consequence. The filamentous ribbons may be held in position by the intramembranous particle arrays since both have a similar size and distribution. These contacts are quite different from desmosomes and appear to represent a distinct new category of adhesive cell-cell junction. Beneath these novel structures, conventional pleated septate junctions are found, exhibiting the undulating intercellular ribbons typical of this junctional type, as well as the usual parallel alignments of intramembranous rows of EF grooves and PF particles. Below these are found gap junctions as close-packed plaques of intramembranous particles on either the P-face or E-face. After freeze-fracturing, the complementary fracture face to the particles shows pits, usually on the P-face, arrayed with a very precise hexagonal pattern.
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ABSTRACT: The receptor-rich postsynaptic membrane of the elasmobranch electric organ was fixed by quick-freezing and then viewed by freeze-fracture, deep-etching and rotary-replication. Traditional freeze-fracture revealed a distinct, geometrical pattern of shallow 8.5-nm bumps on the E fracture-face, similar to the lattice which has been seen before in chemically fixed material, but seen less clearly than after quick-freezing. Fracture plus deep-etching brought into view on the true outside of this membrane a similar geometrical pattern of 8.5-nm projections rising out of the membrane surface. The individual projections looked like structures that have been seen in negatively stained or deep-etched membrane fragments and have been identified as individual acetylcholine receptor molecules. The surface protrusions were twice as abundant as the large intramembrane particles that characterize the fracture faces of this membrane, which have also been considered to be receptor molecules. Particle counts have always been too low to match the estimates of postsynaptic receptor density derived from physiological and biochemical studies; counts of surface projections, however, more closely matched these estimates. Rotary-replication of quick-frozen, etched postsynaptic membranes enhanced the visibility of these surface protuberances and illustrated that they often occur in dimers, tetramers, and ordered rows. The variations in these surface patterns suggested that in vivo, receptors in the postsynaptic membrane may tend to pack into "liquid crystals" which constantly appear, flow, and disappear in the fluid environment of the membrane. Additionally, deep-etching revealed a distinct web of cytoplasmic filaments beneath the postsynaptic membrane, and revealed the basal lamina above it; and delineated possible points of contact between these structures and the membrane proper.The Journal of Cell Biology 08/1979; 82(1):150-73. · 10.82 Impact Factor