Asymmetry in the frequency and position of mitosis in the mouse embryo epiblast at gastrulation

A

Z‐projection of an E7.25 embryo mosaically labelled through OH‐tamoxifen injection, imaged by two‐photon microscopy. Scale bar: 50 μm. Data information: n embryos = 54. (A′) Mouse line strategy used to produce embryos ubiquitous for membrane tdTomato (grey) and mosaic for membrane GFP (mGFP, green) in the epiblast.

B

Z‐projections of stills from live imaging recording of a bottle‐shaped cell (indicated by a white arrow) that delaminates through the basal membrane at time 0 min, 25 min, 50 min, 1 h 15 min and 1 h 40 min. Scale bar: 10 μm. (B′) Top: bottle shape events: GFP⁺ bottle shape cells over total GFP⁺ cells, expressed in percentage, in anterior versus posterior region of the epiblast. The posterior region includes the PS region, as it could not be precisely discriminated. Data information: Mean ± SEM, n embryos = 40, n GFP⁺ cells: anterior = 551, posterior = 540. Bottom: Percentage of bottle shape cells exiting the epiblast among total number of GFP⁺ bottle shape cells, in anterior versus posterior epiblast. Data information: Mean ± SEM, n embryos = 17, n GFP⁺ bottle shape cells: anterior = 19, posterior = 32. Normality was assessed using a Shapiro–Wilk test. Accordingly, samples were compared using non‐parametric Mann–Whitney. *P‐value ≤ 0.05, ***P‐value ≤ 0.001.

C

Z‐projection (C) and 3D reconstruction (C′) of stills from live imaging recording showing an apical division without basal contact, followed by daughter cell dispersion at 0 min, 25 min, 50 min, 1 h 15 min and 1 h 40 min. Scale bar: 10 μm. In (B, C and C′), white dotted lines outline basal (top) and apical (bottom) borders of the epiblast. (C″) Left: frequency of apical division (GFP⁺ apical division over total GFP⁺ cells, expressed in percentage) in anterior and posterior epiblast. Data information: Mean ± SEM, n embryos = 40, n GFP⁺ cells: anterior = 551, posterior = 540. Middle: apical division with or without persistence of contact with the epiblast basal pole, normalized by the total number of GFP⁺ apical division, in anterior versus posterior epiblast. Data information: Mean ± SEM, n embryos = 24, n GFP⁺ apical divisions: anterior = 50, posterior = 29. Right: cell dispersion (defined as loss of basolateral contact between daughter cells after reinsertion in the epiblast layer) after apical division in anterior and posterior epiblast. Data information: n embryos = 18, n GFP⁺ apical divisions: anterior = 32, posterior = 26. Normality was assessed using a Shapiro–Wilk test. Accordingly, samples were compared using non‐parametric Mann–Whitney. ns, non‐significant.

Figure 2. Mosaic epiblast labelling allows tracking of bottle‐shaped cells and cell division.