Neur is required for apical constriction and acts in part via Sdt. (A and A′) Loss of neur activity in neur^IF65 MARCM clones (myr-RFP, red) led to defective apical constriction in cells located at the medial edge of the TZ as well as to the persistence of E-cad (white) in mutant NSCs. (B–B″) Inhibition of Neur by Brd^R in the Optix domain (nuclear RFP, red) induced a loss of apical constriction of epi-NSCs (Neur-GFP, green) and persistence of apical E-cad (white) past the epi-NSCs (arrow). (C) Analysis of apical area in cells expressing Brd^R and in control wild-type cells of the Vsx1 domain, plotted as in Fig. 2 B, showed that inhibition of Neur led to reduced apical constriction (n = 7 brains; means ± SEM were calculated using 34–210 cells per binned distance; ***, P = 8 × 10⁻²⁹, two-tailed Student’s t test). (D and D′) Expression of only Neur-resistant isoforms of Sdt (loss of Sdt-GFP3), following excision of exon 3 of the sdt^GFP3 allele, interfered with apical constriction of epi-NSCs (E-cad, white in D′5; compare apical area of cells at the medial edge of the TZ in the Optix vs. control Vsx1 domains). (E) Quantification of apical area in cells of the Optix domain expressing Neur-resistant isoforms of Sdt compared with control cells from the Vsx1 domain, plotted as in Fig. 2 B. The failure to down-regulate Sdt in the epi-NSCs (located at the 0–2-µm position along the x axis) resulted in intermediate level of apical constriction (n = 8 brains, means ± SEM were calculated using 82–319 cells per binned distance; ***, P = 5.9 × 10⁻²⁶, two-tailed Student’s t test). (F–F″) In wild-type brains, MyoII (MyoII-TagRFP, red) is enriched at straight interfaces along the medial edge of the TZ where Neur-sensitive isoforms of Sdt (Sdt-GFP3, green) are down-regulated. The regions boxed in F–F″ are shown at higher magnification below. (G–H″) When only Neur-resistant isoforms of Sdt are expressed, upon FLP mediated excision of sdt exon 3 (G–G″), both increased MyoII levels (MyoII-TagRFP, red) and straight interfaces seen in wild-type brains (H–H″) were less clearly observed at the boundary between epi-NSCs (Neur-GFP, green) and adjacent TZ cells (see arrows). (I) Quantification of junctional MyoII along different cell–cell contacts (data plotted as in Fig. 2, B and C; see Fig. 2 C for the wild-type control; n = 9 brains; ns, nonsignificant difference, one-way ANOVA). Scale bars = 10 μm.