Live imaging of Drosophila neuroblast delamination reveals two stages with differential cytoskeletal dynamics. Yan Yan^1,2, Chris Doe². 1) Division of Life Science, HKUST, Hong Kong, Kowloon, Hong Kong; 2) Institute of Neuroscience/HHMI, University of Oregon, Eugene, OR.

   Ingression is a conserved morphogenetic process across species. During embryogenesis, individual cells frequently emigrate from an epithelial sheet and give rise to various cell types. In adult epithelial tissues, individual cell extrusion is utilized to maintain epithelial homeostasis. Here we document the neural stem cell (neuroblast) delamination process with high spatiotemporal resolution during Drosophila embryogenesis. We find that neuroblast delamination is a stereotyped process with two distinctive stages. In a first fast stage, delaminating neuroblasts decrease their apical domain incrementally correlated with medial myosin activity. In a second slow stage, the adherens junctions undergo prolonged remodeling associated with junctional myosin activity in the delaminating neuroblasts. Through analyzing Notch signaling mutants in which all the neuroectoderm cells attempt to delaminate, we find that the first fast stage can occur cell autonomously while the second stage is sensitive to aberrant cell-cell communication. Our analysis provided the foundation for further investigation of the molecular machineries that orchestrate the critical steps of cell ingression.