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

A

Z‐projection (A) and 3D cell reconstruction (A′) of stills from two‐photon live imaging of an E7.25 embryo mosaically labelled for epiblast cells showing a division giving rise to an epiblast and a mesoderm cell that will exit from the epiblast layer. White dotted lines outline basal (top) and apical (bottom) borders of the epiblast at 0 min, 25 min, 50 min, 1 h 15 min and 1 h 40 min. Scale bar: 10 μm.

B, C

Z‐projection of stills from two‐photon live imaging of an E7.25 embryo mosaically labelled for epiblast cells. Non‐apical division can be observed in the posterior epiblast only, without (B) or with (C) attachment to the apical side. Stills were taken at 0 min, 25 min, 50 min, 1 h 15 min and 1 h 40 min for (B) and 50 min, 1 h 15 min, 1 h 40 min, 2 h 05 min and 2 h 30 min for (C). White dotted lines outline basal (top) and apical (bottom) borders of the epiblast. Scale bar: 10 μm.

D

Mitotic Index (GFP⁺ dividing cells over total number of GFP⁺ cells) in the anterior and posterior region, as well as among cells exiting the epiblast at the PS. The posterior region includes the PS region, as it could not be precisely discriminated. Data information (D–F): Mean ± SEM, n embryos = 40; n GFP⁺ cells: anterior = 551, posterior = 540, exiting cells = 45. Normality was assessed using a Shapiro–Wilk test followed by a Kruskal–Wallis test. ns, non‐significant.

E

Ratio of non‐apical rounding (non‐apical GFP⁺ rounding divided by the total GFP⁺ rounding) and expressed in percentage for the anterior and posterior regions. Normality was assessed using a Shapiro–Wilk test followed by a Mann–Whitney test. *P‐value ≤ 0.05.

F

Ratio of non‐apical division (non‐apical GFP⁺ division over total GFP⁺ division) and expressed in percentage for the anterior and posterior region of the embryo. Normality was assessed using a Shapiro–Wilk test followed by a Mann–Whitney test. **P‐value ≤ 0.01.

G

Quantification of posterior cell division outcome for apical and non‐apical divisions. Outcomes are two epiblast daughter cells (epi/epi), one epiblast and one mesoderm daughter cells (epi/meso) or two mesoderm daughter cells (meso/meso). Data information: Mean ± SEM, n embryos = 41; n posterior divisions: apical = 35, non‐apical = 17. Normality was assessed using a Shapiro–Wilk test followed by Kruskal–Wallis test with post hoc Dunn's multiple column comparison test. ns, non‐significant, ***P‐value ≤ 0.001.

Figure 3. Non‐apical division occurs in the posterior epiblast and can give rise to one or two mesoderm cells.