Fig. 2. Extrinsic cues orient the neuroblast division axis over multiple cell cycles. (A) Series of still images from a time-lapse DIC movie showing six consecutive telophase figures from a single clustered or (B) isolated neuroblast in primary cell culture. The red arrow points towards the developing GMC along the neuroblast intrinsic apicobasal polarity axis. (A,B) Diagrams on the right summarize GMC position at telophase from all six consecutive divisions, showing tight grouping of GMCs in clustered neuroblasts versus dispersal of GMCs in isolated neuroblasts. Maximum distance between two sister GMCs is uncovered by a 53° angle in clustered neuroblasts and 141° angle in isolated neuroblasts. (C) Quantitation of GMC position at telophase during successive divisions in clustered and isolated neuroblasts. Only neuroblasts dividing more than three times were scored. (D) Three serial confocal sections of a cell cluster fixed and stained for neuroblast and epithelial markers (as indicated). The Insc-positive neuroblast cortex directly contacts several DE-cadherin-positive epithelial cells. Brackets indicate the same neuroblast in the three sections, asterisks indicate the DE-cad positive epithelium only in the left panel and arrows indicate localized DE-cadherin in the middle and right panels. Scale bar: 10 µm.