Figure 3. Polarity-driven cell rearrangements can create the observed cell morphology patterns.

(A) Stress is a combination of elastic and polarity-driven stresses. Similarly, in (B), cell rearrangements occur to relax a stretched cell shape or to respond to an internal nematic cell polarity cue. In the cartoons in (A)-(B), we chose to depict scenarios where $\zeta$ and $\lambda$ are ${\displaystyle >0}$. (C–G) We apply this model to a radially symmetric tissue at steady state to approximate the wing disc. If we impose a radial polarity field (C), cell elongation is oriented in the opposite direction, according to the equation in (D) when $R_{rr}=0$ (steady state). (E) Considering force balance in the tissue, our model also predicts a pressure profile with higher pressure in the center. (F) To fit experimental data in the wing disc, we estimate the radial profile of $q_{rr}$ from Equation 4 by measuring cell elongation as a function of $r$ in the last (${\displaystyle \sim 5hr}$) of the timelapse. We solve for $q_{rr}$ by making an empirical fit to this cell elongation data (see Appendix 1 part 2). (G) The cell area distribution we observe in the wing disc is consistent with the pressure profile predicted by our model (E and Appendix 1 part 2).