Intercellular junctions in the hepatopancreas of the lobster Nephrops norvegicus

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Abstract
The hepatopancreas of the lobster was recently found to be a rich source of material from which to isolate arthopod gap junctions biochemically. The features of these intercellular junctions and any others that may be present were assessed, in vivo. The tissue consists of columnar epithelial cells which possess apical microvilli and basal infoldings. In thin sections the lateral borders of these cells are characterized by desmosomes and smooth septate junctions as well as by gap junctions. The desmosomes exhibit no apparent freeze fracture profile but the septate junctions display parallel rows of ridges or aligned intramembranous particles (IMP) with complementary grooves on the other membrane half; these IMP shift in their preferential fracturing plane depending on whether the tissue was first fixed, always remaining on the EF [exoplasmic fracture face] if unfixed. The IMP or connexons, of which the gap junctions are composed, fracture onto the EF, leaving complementary pits on the P face [protoplasmic], regardless of whether the tissue is fixed or not. At the base of the pancreatic cells, the lateral borders are thrown into interdigitating folds which display endocytotic profiles and possible internalization of junction-bearing membranes. This phenomenon, which is readily visualized both after tracer incubation and in replicas, may represent junctional degradation relating to membrane turnover.