Paracellular pathway in the shell epithelium of Anodonta cygnea

Ultrastructural study of cell-cell connections in the outer mantle epithelium (OME) on high-pressure-frozen specimens revealed zonula adherens, septate junctions and gap junctions in Anodonta cygnea. In order to evaluate the permeability of the paracellular pathway, the OME was incubated under gradients of lanthanum and calcium. After lanthanum incubation (4 mM) from the basal side, the septate junctions were penetrated completely by this tracer. When applied from the apical side, lanthanum deposits were located similarly over the entire length of the septate junctions up to the first dilatations of the intercellular space. Calcium deposits were also present in paracellular areas only when OME had been incubated simultaneously with calcium (6 mM) and lanthanum (4 mM) gradients. Lanthanum and calcium deposits were detected with ESI (Electron Spectroscopic Imaging) and identified with EELS (Electron Energy Loss Spectroscopy). On the other hand, electrophysiological observations showed a 48% reduction of conductance when the OME was bathed on both sides with solutions containing lanthanum (4 mM) and calcium (6 mM), compared to bathing with lanthanum-free solution (control). The conductance reduction was 52% when calcium was removed from the control solution. Supported by morphological and physiological evidence, it appears that, under in vivo conditions, calcium ions may diffuse paracellularly from the haemolymph towards the extrapallial fluid and vice-versa across the septate junctions in the OME of A. cygnea. Permeability of the septate junctions depended proportionally on the calcium concentration in fluids.