Refining organ architecture by branch elongation and maturation. (A–C) Schematics of stalk elongation by cell elongation and intercalation in Drosophila trachea. (A) In response to the pulling force of the migrating tip cell, stalk cells elongate and intercalate, resulting in >2-fold increased stalk length. (B) Schematics of stalk elongation by rosette-based convergent extension in vertebrate kidney collecting duct. (C) Schematics of branch stalk elongation by cell proliferation in most mammalian epithelial organs. Some cells divide within the stalk, whereas some cells are deposited by the proliferating tip to elongate the stalk. (D) Schematics of the acinar branch tip in adult mouse salivary gland. During maturation, cells at the branch tip differentiate to become columnar cells that surround a relatively small cavity to form an acinus for saliva secretion. Homeostasis of the acinar cells is maintained by self-renewal with little contribution from the stalk. (E) Schematics of the alveolar branch tip in adult mouse lung. During maturation, lung cells at the branch tip differentiate into flat, squamous AT1 cells and columnar AT2 cells. AT2 cell–mediated self-renewal can be triggered by AT1 injury.