Apico-basal polarisation is a crucial step in the formation of biological tubes. In model systems in which tubulogenesis occurs in cell clusters, the inner surface of each cell in the cluster differentiates into an apical domain where lumen secretion occurs, thus ensuring the formation of an unobstructed lumen. But in many developmental contexts, tubes form from simple cords of cells, which presents a unique challenge for the formation of a continuous lumen. On , Di Jiang and colleagues investigate how this challenge is overcome during tubulogenesis in the Ciona intestinalis notochord, which is made up of a single file of geometrically identical cells. The researchers show that, during early tubulogenesis, a patch that contains the highly conserved Par complex and a set of tight junctions becomes established at both ends of the notochord cells. The formation of these two apical domains, they report, is controlled by Par3. Together, these results suggest a new mechanism for tubulogenesis from a simple cell cord that requires the formation of bi-apical cells.
