Whether either of the hypothetical lifecycle pathways described above turn out to be correct for salivary gland cells, and are generalisable to other cell-types, remains to be seen. These lifecycle pathways are based on observations of fixed tissue, antibody-stained preparations...no movement tracking was possible. There is some visual evidence that small, innexin-immunopositive vesicles (putative transport vesicles) are present at both the basal and apical ends of the region of lateral cell membrane that is occupied by gap junctions. At the basal end, small and large vesicles are observed - potentially a mix of transport and endocytic vesicles. At the apical end, only small (...transport?) vesicles are observed. Hopefully, at some stage a Drosophila lab will attempt real-time imaging to elucidate the true pathway in salivary glands or some other amenable tissue. A number of approaches are available to achieve this including; GFP-tagged innexin constructs (....such as that referred to in Lehmann et al. 2006) or tetracysteine-tagging of innexins (Gaietta et al. 2002...reference for tetracys-tagged connexins). The notion that the basal region of salivary gland cells may act as a sorting zone (similar to the sub-apical sorting zone described in some vertebrate cells (Van Ijzendoorn and Hoekstra 1999) is based on a few observations:
- - The vesicles observed at the basal pole of salivary gland cells range in size from small (possibly transport vesicles) to large (possibly annular junctions).
- - Small putative transport vesicles are mainly detected around the cell nucleus and in areas basal to the nucleus. However, no small vesicles have been observed in the cytoplasm between the nucleus and the lateral plasma membrane (at least in pupal glands). It's possible that our sampled cells did not include a time window where vesicle transport is maximal and more likely to be detected.
- - The basal region of the salivary gland cells is rich in actin filaments that could provide the infrastructure required for a sorting zone (Images: Inx2 and F-actin association and Linear arrangement of Inx2 on the cell surface).
- - Still images of the baso-lateral junction region of pupal salivary gland cells just 'looks' like it's a zone undergoing a lot of activity....the most intense innexin-staining is detected in, and around, this area.