Figure 4.

Patterns of mechanical tension in the wing disc. (A) A schematic representation of the different mechanisms in which a cell (marked in orange) could elongate, starting from the isotropic configuration in the centre. (1) Local cell-autonomous extension of yellow junctions. (2) Local cell autonomous constriction of red junctions. (3) Global non cell-autonomous compression forces leading to constriction of the red junctions. (4) Global non cell-autonomous stretching forces leading to extension of yellow junctions. (B) In the wing disc, the junctions highlighted in yellow are the proximal/distal (P/D) junctions, and the junctions in red are the lateral junctions. The orientation of the wing disc is highlighted by the direction of the distal (centre) and the proximal (edge). (C) E-cadherin::GFP wing disc with the regions used for laser ablation in (D, E) highlighted. Red: distal centre, blue: proximal edge. (D) P/D junctions. Plot of increase in distance (μm) between the vertices of the cut junction (D−D₀) against time (s) after laser cut, mean±s.e.m. Blue=P/D junctions in the proximal (edge) region. Red=P/D junctions in the distal (centre). (E) Lateral junctions, as in (D). Green=lateral junctions in the proximal (edge) region. Magenta=lateral junctions in the distal (centre). (F–F′) Snapshots of an example laser ablation of a junction at the indicated time points (see Supplementary Movie 2). The cut was performed at 3.405 s and the recoil imaged for at least 15 s. (F″) An overlay of the junction before cut (red) and 11 s after cut (green) is shown. (G) The initial (maximum) recoil velocity of the vertices after the cut. Represented as mean±s.e.m. For cells at the proximal (edge) of the disc: P/D junctions (blue): velocity=1.09±0.20 μm/s, n=47; lateral junctions (green): velocity=0.58±0.18 μm/s, n=30. For cells at the distal centre: P/D junctions (red): velocity=0.73±0.16 μm/s, n=53; lateral junctions (magenta): velocity=0.52±0.2 μm/s, n=28. The average ratio of P/D to lateral junctions at the proximal edge is 1.87, higher than the average ratio of 1.4 at the distal centre of the pouch. Ablation experiments were performed at ∼100 h AEL.