Salivary gland cells are conveniently large enough to facilitate three-dimensional reconstruction of plaques from series of two-dimensional optical sections (other examples of 3D reconstructions of gap junction plaque regions can be found on the Innexin subcellular distribution - summary diagram page). But salivary gland cells are highly specialised and it remains to be seen whether the plaques shown above are representative of those found in other Drosophila tissues. The innexin-plaques generally extend in an oblong shape in an apical-basal orientation..although there was quite a lot of variability, some if which might be due to 'stretching' of cells between microscope slide and cover-slip. It would be interesting to discover whether the elongated shape arises due to the incorporation of newly synthesised innexin channels at only one end (apical or basal) of the existing plaques. Maybe real-time live-imaging experiments could demonstrate that these plaques grow (are replenished) from one end? Alternatively, the plaques could be internalised for degradation preferentially from one end leading to their extended shape? In vitro real-time experiments reveal that connexin-based plaques are replenished laterally and internalised from the middle (Gaietta et al. 2002). Connexin-containing plaques can appear disc-like in shape or they can have a more disorganised morphology in the cell membrane (Falk, 2000). So far, disc-shaped innexin-containing plaques are observed infrequently in Drosophila salivary gland cells. In summary, there may be morphological differences between the plaques constructed from connexins and those from innexins and these may arise through different modes of channel replenishment/degradation.
The morphology of innexin-based gap junction plaques
Figure legend: What do innexin-based gap junction plaques actually look like? The confocal images above show a side-view of such plaques reconstituted from a Z-series of single optical sections through the boundary of two pupal salivary gland cells in the region of the grey box in the cell diagram. This region, at the basal (non-lumenal) pole of the cells, contains the greatest density of plaques. The tissue was antibody stained to detect Inx2 protein (green) and the reconstructed Inx2-containing plaques are white.