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.
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We are grateful to B. Aiguy, S. Eaton, S. Grill, R. Hauschild and M. Sixt for advice, and the imaging and zebrafish facilities of the IST Austria and MPI-CBG for continuous help. We are particularly grateful to J-F. Joanny for discussions regarding the theory part of this work, and B. Baum for sharing data before publication. This work was supported by the IST Austria, MPI-CBG and a grant from the Fonds zur Förderung der wissenschaftlichen Forschung (FWF) (I930-B20) to C-P.H.
Integrated supplementary information
Time-lapse of the EVL in a wild-type embryo expressing GPI–RFP to outline EVL cells and cell divisions marked in yellow; t = 0 min corresponds to sphere stage (4 hpf). Scale bar, 100 μm.
Time-lapses of cortical laser cuts of the apical actomyosin cortex perpendicular (red) and parallel (green) to the EVL margin and at the animal pole (blue), in Tg(actb2:myl12.1-eGFP) embryos at 65% epiboly also expressing GPI–RFP to outline EVL cells. Red, green and blue lines (100 μm length) mark the position of the perpendicular, parallel and animal cuts, respectively. Scale bar, 20 μm.
Time-lapse of the alignment of the cell division axis with the axis of induced tension in a Tg(actb2:myl12.1-mCherry) embryo at 40% epiboly also expressing Tau-GFP to mark spindle microtubules. Tension was induced orthogonally to the initial axis of the spindle (yellow) by creating two constricting wounds in the EVL. Scale bar, 20 μm.
Time-lapse of a typical EVL cell division (arrow) in a wild-type embryo at 50% epiboly expressing GPI-GFP to outline EVL cells. Scale bar, 20 μm.
Time-lapse of dividing EVL cells in Tg(actb2:myl12.1- mCherry) embryos between 30–50% epiboly. Embryos also express Tau-mCherry to mark spindle microtubules and were treated with either the myosin II-inhibitor Blebbistatin (right) or its inactive enantiomer (left). Scale bar, 20 μm.
Time-lapse of exemplary cortical laser cuts of the apical actomyosin cortex perpendicular (blue) or parallel (orange) to the axis of induced tension either in the presence (right) or absence (left) of an EVL cell division (white contour) oriented along the axis of tension. Blue and orange lines (50 μm length) indicate were the cuts will be performed. Tg(actb2:myl12.1-eGFP) embryos at 30–40% epiboly. Scale bar, 20 μm.
Time-lapse of the spreading displacement of an EVL cell (white cell contour) in Tg(actb2:myl12.1-eGFP) embryos at 30–40% epiboly upon induction of ectopic tension either in the presence (right) or absence (left) of a cell division oriented along the axis of tension. Red crosses mark the ablation sites where wounds were induced. Scale bar, 20 μm.
Time-lapse of a exemplary EVL cell fusion (arrowhead) in a cell division inhibitor-treated embryo, expressing both Tau-mCherry and GPI–RFP to mark the spindle microtubules and plasma membrane, respectively. Scale bar, 20 μm.
Time-lapse of an exemplary EVL cell fusion event (arrowheads) in a cylindrical Tg(actb2:GFP-utrCH) embryo from sphere stage (t = 0 min) onwards. Arrows point at a cell division, which gives rise to a daughter cell that subsequently fuses with another unrelated cell. Cell membrane and nuclei were marked by GPI–RFP and H2A-Cherry, respectively. Scale bar, 20 μm.
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Nature Communications (2017)