During embryogenesis, defective or dying epithelial cells need to be removed from the epithelium without compromising its integrity. In Drosophila, apoptotic cells are mostly extruded basally and engulfed by macrophages. A key pathway that regulates epithelial homeostasis in Drosophila development is epidermal growth factor receptor (EGFR) signalling; however, its exact role in regulating cell extrusion is unclear. In this study, Shigeo Hayashi and Kentaro Yoshida investigate the effects of EGFR depletion on epithelial integrity in the Drosophila embryo. Here, EGFR mutant embryos undergo epithelial collapse, which begins with sporadic apical extrusion of both apoptotic and non-apoptotic cells in the head region, which then sweeps along the ventral body wall. This epithelial collapse is apoptosis dependent, since removal of pro-apoptotic genes in EGFR mutant embryos abrogates this phenotype. EGFR mutants also showed clustering of apoptotic cells, particularly in regions undergoing invagination and detachment from the vitelline membrane (which provides structural support to the epithelium). Forced detachment of the vitelline membrane also caused rapid collapse of EGFR mutant epithelia. Together, these findings demonstrate that EGFR signalling, as well as promoting cell survival, maintains epithelial integrity during Drosophila embryogenesis by preventing inappropriate apical extrusion of cells.
