Abstract
Branching morphogenesis orchestrates organogenesis in many tissues, including the kidney, where ureteric bud (UB) branching determines kidney size and shape and the final nephron number. Molecular regulation of UB branching is rather well studied, whereas the cellular mechanisms and tissue organization during UB arborization are less understood. Here, we characterized epithelial cell morphology in three dimensions (3D), studied mechanisms regulating cell shape changes, and analyzed their contribution to novel branch initiation in normal and branching-incompetent bud tips. Unbiased machine-learning-based segmentation of tip epithelia identified geometrical round-to-elliptical transformation as a key cellular mechanism facilitating growth direction changes to gain optimal branching complexity. Cell shape and molecular analyses in branching-incompetent epithelia demonstrated a distinct failure to condense cell size and modify its conformation. This, together with changes in adhesive forces, defective actin dynamics, and disorganization in myosin-9 (MYH9)-based microtubules, suggests altered biophysical properties in tip cells, where branch point decisions are made and actualized. The data demonstrate that dynamic changes in volume and morphology of individual epithelial cells, together with optimal traction stress, facilitate novel branch formation in the UB tip niche. Based on these results, we propose a model where epithelial cell crowding, in tandem with stretching, transforms individual cells into elliptical and elongated shapes. This creates local curvatures that drive new branch formation during the ampulla-to-asymmetric ampulla transition of the UB.