Abstract
We uncover the mechanisms allowing the generation of two opposing apical epithelial surfaces within the centre of an initially unpolarised, solid organ. Through functional inhibition of NOK/Pals1/Mpp5a and Rab11a we infer that Crumbs protein plays a dual role in coordinating the generation of ipsilateral junctional belts whilst simultaneously releasing contralateral adhesions across the centre of the tissue. We further suggest that Nok/Rab11a/Crumbs-mediated resolution of cell-cell adhesions is both necessary for midline lumen opening and contributes to later maintenance of epithelial organisation. In light of a recent conflicting publication, we also clarify the important role of cell division in aligning both ipsilateral and contralateral junctions at the midline of the tissue.