Extended Data Figure 1.

Loss of AP gradients of supracellular stresses and cell and nuclear shape anisotropy in N-cadherin mutants. a, ActinRed^™ staining of F-actin in the PSM of control (cdh2^{+/+, +/−}) and mutant (cdh2^−/−) embryos at the 10-somite stage. Cell shapes are visibly elongated along the mediolateral (ML) direction in control (cdh2^{+/+, +/−}) embryos. Cell shape anisotropy is largely lost in cdh2^−/− embryos. b, DAPI staining showing higher nuclear ML elongation in the PSM of control embryos compared to cdh2 mutants. c, Frequency of nuclear major axis orientations in the MPZ and PSM (A-PSM and P-PSM). In control embryos (cdh2^{+/+, +/−}), nuclei in the PSM are elongated along the mediolateral direction, whereas nuclei are oriented randomly in the MPZ. The observed nuclear anisotropy along the ML direction in the PSM of control embryos is decreased in cdh2 mutants (cdh2^−/−). For cdh2^{+/+, +/−}: n=695 (A-PSM), n=752 (P-PSM) and n=732 (MPZ), obtained in 6 embryos per region. For cdh2^−/−: n=833 in 5 embryos (A-PSM), n=538 in 6 embryos (P-PSM), n=336 in 4 embryos (MPZ). d, A posterior-to-anterior increase in the extent of nuclear elongation (quantified by the nuclear aspect ratio; see inset and Methods) is observed in control embryos (cdh2^{+/+, +/−}). No AP gradient in the extent of nuclear elongation (aspect ratio) is observed in cdh2 mutants (cdh2^−/−). For cdh2^{+/+, +/−}: n=695 (A-PSM), n=752 (P-PSM) and n=732 (MPZ), obtained in 6 embryos per region. For cdh2^−/−: n=833 in 5 embryos (A-PSM), n=538 in 6 embryos (P-PSM), n=336 in 4 embryos (MPZ). Mean ± SEM. e, Relative change of cell-cell contact length along the anterior-posterior (AP) axis and the medial-lateral (ML) axis (n=6427 and 4319 cell-cell contacts for PSM and MPZ from 5 embryos, respectively). Cell junctions are longer along the ML axis compared to the AP axis, both in the PSM and MPZ. Mean ± SEM. f, Supracellular stresses are uniform along the AP axis in cdh2 mutant embryos (n=5 for A-PSM, n=5 for P-PSM, and n=12 for MPZ). Mean ± SEM; Mann-Whitney test. The observed posterior-to-anterior increase in both supracellular stresses and nuclear elongation in control embryos (Fig. 1e and panel d), and the loss of both such gradients in cdh2 mutants (panels d and e), indicate the existence of a N-cadherin-dependent, posterior-to-anterior increase in supracellular stresses, consistent with a posterior-to-anterior increase in mediolateral constriction. Importantly, if the observed thinning of the body axis was caused by pulling forces from the MPZ on the PSM, as previously proposed¹⁷, both cells and nuclei would be elongated along the AP axis.