Figure 7. An anterior to posterior gradient of proliferation counteracts ingression in the anterior primitive streak (PS) epiblast.

(A) Quantification of the number of SOX2/T double-positive cells in chicken embryos from stage 4HH to 35-somite (n = 38 embryos). (B) Snapshots of the posterior region of chicken embryos from stage 5HH to 20-somite stained in whole-mount with an anti-phosphorylated histone H3 (pH3) antibody. (C) Quantifications of the mitotic index along the PS in the boxes shown in (B). Orange box: 250 µm from node; green box: 750 µm from node; blue box: 1200 µm from node (n = 13 embryos). Unpaired t-test; **p=0.0017 and 0.0022; *p=0.0187, p=0.0278. (D) Quantification of the number of dividing cells in H2B-Cherry transgenic quails at stage 4+/5HH in the anterior and posterior PS (n = 6). Paired t-test; ***p=0.0001. (E) Tracks and (F) quantification of trajectories of the neuromesodermal progenitor (NMP) (gold) and lateral plate progenitor (LPP) (gray) cells during PS regression (n = 159 cells, 80 posterior, 79 anterior in seven embryos). (G) Quantification of the time interval between two rounds of division in cells of the NMP region measured in time-lapse movies (n = 12 inter-division events in four embryos). (B, E) Dorsal views, anterior to the top. Arrowhead shows Hensen’s node position. Scale bar: 100 µm.

Figure 7—figure supplement 1. Analysis of cell division profiles in the SOX2/T region.

(A) Tracks of three rounds of cell division of an epiblast cell of the SOX2/T-positive territory in an H2B-RFP electroporated chicken embryo imaged for 12 hr starting at stage 5HH (corresponding to Video 6). d1, d2, d3: the division events. (B) Based on the location of daughter cells after cell division, we defined the cell division as symmetric (symmetric neuromesodermal progenitor [NMP]: when both daughter cells remain in the primitive streak [PS]; symmetric mesoderm: when both daughter cells subsequently join the mesoderm [d3]) or asymmetric (when one daughter cell remains in the PS and the other daughter cell joins the mesoderm) (d1, d2). We identified 33 division events in seven embryos, which are shown in (A). Scale bars: 100 µm.