Tension-oriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly

Epithelial spreading is a common and fundamental aspect of various developmental and disease-related processes such as epithelial closure and wound healing. A key challenge for epithelial tissues undergoing spreading is to increase their surface area without disrupting epithelial integrity. Here we show that orienting cell divisions by tension constitutes an efficient mechanism by which the Enveloping Cell Layer (EVL) releases anisotropic tension while undergoing spreading during zebrafish epiboly. The control of EVL cell-division orientation by tension involves cell elongation and requires myosin II activity to align the mitotic spindle with the main tension axis. We also found that in the absence of tension-oriented cell divisions and in the presence of increased tissue tension, EVL cells undergo ectopic fusions, suggesting that the reduction of tension anisotropy by oriented cell divisions is required to prevent EVL cells from fusing. We conclude that cell-division orientation by tension constitutes a key mechanism for limiting tension anisotropy and thus promoting tissue spreading during EVL epiboly.

💡 Research Summary

The paper investigates how the Enveloping Cell Layer (EVL) of the zebrafish embryo spreads during epiboly while preserving epithelial integrity, focusing on the role of tension‑oriented cell divisions. Using laser ablation, particle tracking, and high‑resolution live imaging, the authors first map the mechanical stress field across the EVL and discover a pronounced anisotropy: tension is higher along the animal‑to‑vegetal axis at the leading edge and diminishes toward the yolk syncytial layer. This anisotropic tension drives cell elongation in the direction of maximal stress, creating a morphological cue that precedes mitosis.

The study then probes the mechanistic link between tissue tension and spindle orientation. By quantifying the alignment between the long axis of elongating cells and the mitotic spindle, the authors show that, under normal conditions, the spindle aligns closely (within ~15°) with the principal tension axis. Pharmacological inhibition of myosin II with blebbistatin, as well as genetic disruption of the non‑muscle myosin II heavy chain gene myh9b, uncouple this alignment: cells still elongate but the spindle orientation becomes random, indicating that myosin II activity is required for translating mechanical cues into spindle positioning.

Functional consequences of disrupting tension‑oriented divisions are examined by monitoring cell fusion events. In both myosin‑II‑inhibited embryos and myh9b mutants, ectopic cell fusions appear dramatically, especially in regions of high residual tension. Conversely, artificially increasing tissue tension (e.g., by external compression) without allowing proper spindle alignment also elevates fusion frequency. These observations suggest that oriented divisions relieve anisotropic stress, thereby preventing excessive membrane tension that would otherwise trigger cell–cell rupture and fusion.

To integrate the experimental data, the authors construct a continuum mechanical model in which each division redistributes stress along the division plane, reducing the deviatoric component of the stress tensor. Simulations reveal that when at least 80 % of divisions are correctly oriented, the average tissue tension remains below a critical threshold, epiboly proceeds at a normal rate, and fusion events are minimized. The model quantitatively predicts the observed relationship between division orientation efficiency, tension anisotropy, and tissue spreading speed.

Overall, the work identifies a feedback loop in which anisotropic tissue tension biases cell shape, myosin‑II‑dependent mechanisms align the mitotic spindle with the tension axis, and the resulting oriented divisions act as a stress‑relief mechanism. This loop ensures that the EVL can expand its surface area rapidly without compromising epithelial continuity. The findings have broader implications for other developmental processes, wound healing, and pathological contexts where tissues must enlarge while maintaining mechanical integrity, highlighting tension‑guided cell division as a general strategy for managing tissue‑scale forces.