During development, epithelial tubes often need to grow while still maintaining their barrier properties. How can cells divide without disrupting the integrity of the tubular epithelium? On p. , Markus Affolter and colleagues address this question in the Drosophila larval tracheal system. In a particular subset of tracheal tubes, there is extensive remodelling during the early third instar, such that unicellular tubes, in which a single cell encircles the lumen and creates junctions with itself, transform into multicellular tubes, a process accompanied by proliferation. The authors demonstrate that this transition involves cell intercalation to replace the autocellular junctions with intercellular ones. Depending on cell length, mitosis may occur either before or after this junctional remodelling is complete, thus generating two major classes of cytokinesis events. In both cases, cytokinesis is asymmetric, with the new membrane extending from the side of the cell where the nucleus is located. In rare cases, this can lead to the formation of a binucleate and an anucleate daughter. The authors further find that Dpp signalling is required for appropriate junctional remodelling and cell division. Together, these data provide insights into how barrier integrity can be maintained through cell division in these tubular structures.
