  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.
