Innexin-immunopositive objects near the basal (non-lumen) cell surface of salivary gland cells may be annular junctions.
A, Image from just below the basal surface of a pupal salivary gland cell antibody-stained for Inx2 (green). Some of the spots appear to be linear (arrowheads) while others have a hole in the middle and look like donuts (arrows). The latter morphology suggests that they are innexin-bounded vesicles. B, The “donut” shaped immunopositive objects have been observed using antibodies against Ogre, Inx2 (arrows) and Inx3 (not shown). These are presumably annular junctions and the largest of these vesicles are approximately 1.5-2μm in diameter. C, Very rarely, small immunopositive donut-shaped regions are observed at the cell plasma membrane (arrow) at the same cell depth as gap junction plaques (arrowhead). Because of their scarcity it is unclear whether these correspond to a step in the formation of the putative annular junctions or are transport vesicles incorporating newly synthesised hemichannels into the cell membrane. Hopefully, labs with appropriate live-fluorescent-imaging resources and tagged-innexin molecules can shed light on these vesicles.