Fig. 7. Apical cap shedding requires JAK/STAT signaling and the recruitment of outer BCs. GFP-Vkg (green) egg chambers expressing Dome^cyt (A,B) or Drac1^N17 (C,D). Follicle cells are marked with phalloidin (A,C,D; red). In B, polar cells are marked with slbo-lacZ (red). (A) Blocking outer BC recruitment does not affect the formation of the apical cap at stage 9. However, shedding does not take place and a cap is still present in stage 10 Dome^cyt egg chambers (B; phenotype schematized in the upper left panel). Expression of Drac1^N17 blocks migration (D) but has no effect on outer BC recruitment and apical cap formation (C) (phenotype schematized in the lower left panel). (E) A four-step model for BC formation and migration: (1) formation of an asymmetrical apical cap (green; containing Collagen IV 1 and 2 chains, Laminin A, Perlecan) over the anterior polar cells (orange) through Drab5-dependent transcytosis; (2) polar cells send a JAK/STAT signal to recruit outer BCs (beige) from the surrounding follicle cells (light gray); (3) shedding of the apical cap is dependent on the presence of outer BCs; (4) migration of the mixed BC cluster starts, with polar cells maintaining a BM. Anterior is to the left. Scale bar: in A, 5 µM for A-D.