Junctional diversity in two regions of the epidermis of Oikopleura dioica (Tunicata,Larvacea)

Summary A simple continuous epithelium surrounds the body of the pelagic larvacean. It consists of two zones of cells: oikoplast cells and flattened cells. The oikoplast cells are columnar and produce a thick extracellular house that ensheathes the body of the organism. These cells are joined laterally by wide tight junctions (zonulae occludentes). The tail of the animal is surrounded by exceedingly thin cells which are joined by narrow tight junctions under which lie intermediate junctions (zonulae adhaerentes) and gap junctions. A web of fibrous material inserts into the intermediate junctions. The transitional cells between the two epithelial zones have one lateral border with a wide tight junction, and the other lateral border with a narrow tight junction and a wide intermediate junction. In freeze-fracture replicas, the wide tight junction has a number of anastomosing ridges, in comparison with the narrow tight junction, which usually consists of only a single row of intramembranous particles. In replicas, the thin epithelial cells show unusual parallel arrays of particles in clusters on their apical plasma membranes. This simple epithelium, therefore, exhibits striking differences between the two cellular zones, in the structural characteristics of both the lateral borders and the apical membrane.