Follicle cells form two different types of polarized actin-rich protrusions. (A and B) Single frames of spinning disc microscopy time-lapse movies of ex vivo–cultured egg chambers expressing LifeAct-EGFP in the follicle epithelium using the GR1-Gal4 driver. Bars, 5 µm. Stage 6/7 (A) and stage 7/8 egg chamber (B). Time intervals are indicated. Arrows mark the direction of collective cell migration. The closed arrowheads mark whip-like protrusions formed at tricellular junctions whereas open arrowheads in (A) mark smaller filopodial protrusions along the cell front. Cell protrusions extend in the same direction as follicle cell movement perpendicular to the A-P axis. See also Video 2, A–C. (C and D) SIM images from the basal surface of stage 6/7 wild-type follicle cells. Bars, 5 µm. (C) Basal actin structures of follicle cells are poorly preserved by standard fixation methods. (D) Numerous filopodial and whip-like protrusions are preserved in acute preparations after fixation without any subsequent permeabilization and washing steps. C′ and D′ show entire SIM images, and the white boxes highlight the area of magnified insets shown in C and D. (E) Single frame of a spinning disc microscopy movie of an ex vivo–cultured egg chamber expressing LifeAct-EGFP in the follicle epithelium using the da-Gal4 driver. Arrows mark the direction of collective cell migration. The closed arrowheads mark whip-like protrusions, and open arrowheads mark smaller filopodial protrusions along the basal cell front and at the apical side. Bar, 10 µm. See also Video 3 C. The white box marks the region selected for 4D reconstruction shown in (E1–4). Time intervals are indicated. Basal is up, and apical is down. Bars, 5 µm. The closed arrowheads mark whip-like protrusions formed at tricellular junctions whereas open arrowheads mark smaller filopodial protrusions along the basal cell front and at the apical side. (F) Schematic drawing illustrates a 3D morphology of marked follicle cells in E4. WT, wild type.