Fig. 3. Normal and auxetic nuclei.

(a) Depiction of stresses (arrows in the second column) acting on small and large nuclei as defined in Fig.s 2f-i (small and large black circles, respectively) for cells untreated and treated with Cytochalasin D. Size of arrows indicates relative force. The normal and auxetic nucleus response is depicted in the third and fourth column, respectively, where the red shapes representing the deformed nuclei, are overlapped with the original nuclear shape (black circle). (b) Correlation between transverse and axial nuclear strain for normal and auxetic nuclei. The dotted arrows are guides for the eye denoting the trend from small to large nuclei (small and large symbols, respectively). Designation of small and large nuclei is determined from the cut-off shown in Fig. 2(f)-(i). N- and P- (inset) nuclei (squares and upward triangles, respectively) present a “normal” response: upon increasing the nuclear size (from small to large symbol) the nuclear S_T becomes more negative while the nuclear S_A increases. This is qualitatively in agreement with simple biaxial compression models (dashed dotted line, for information about models see Supplemental Information). T-nuclei (circles) clearly exhibit an auxetic behavior: upon increasing the nuclear size the nuclear S_T changes sign from positive for small nuclei to negative for large nuclei, whilst the ratio of S_A for small to large nuclei is nearly 2. S_A presents the same trend in T nuclei treated with Cytochalasin D (olive triangles). Error bars are reported in Fig. S10. (c)-(d) Bright field and epifluorescence images of a T-ESC after fluorescein incubation, imaged through a 60× objective (NA = 1.4). Scale bar: 5 μm. (e)-(h) Epifluorescence images and corresponding gray value profiles (measured along the dashed line) of a T-ESC loaded with fluorescein before and upon entering the microfluidic channel, respectively. The apparent signal is higher in the center of the cell where this is thicker. Ratio between the profile standard deviations σ (i) and kurtosis k (j) (inside/outside channel) for naïve (black) and transition (red) cells (cf Fig. S15). (M,m) are (3,270) and (3,317), respectively. Scale bars: 5 μm. The decrease in σ and increase in k for small T-ESCs suggest that fluorophores are driven towards the center of the stretched cells, i.e. the nuclear signal increases at the expense of the signal from the cytoplasm.