Figure 2.

Progressive Formation of the LPM and Concomitant Patterning by Hox Genes

(A) Time series showing LPM formation from stage 5 to stage 11. LPM precursors are electroporated with H2b-RFP (red) and GFP (green). White brackets outline the presumptive LPM in the PS; white asterisks, Hensen’s node; PS, primitive streak; prLPM, presumptive LPM; HF, head folds. See also Video S1.

(B) Position of electroporated cells in the LPM (y axis) as a function of their timing of ingression (x axis); n = 57 tracked cells, 8 embryos (see Video S2).

(C) Timeline of Hoxb4 (red), Hoxb7 (light blue), and Hoxb9 (blue) activation in the presumptive LPM with regard to LPM formation (black arrows). For detailed expression data, see Figure S2.

(D) Schematic summarizing anterior (red) and posterior (blue) Hox genes expression in the LPM.

(E–G) Expression of Hoxb4 (E), Hoxb7 (F), and Hoxb9 (G) at stage 13. (E’)–(G’) are higher magnifications of (E)–(G), showing Hox genes posterior (Hoxb4; E’) or anterior (Hoxb7 and Hoxb9; F’ and G’, respectively) border of expression in the LPM (dashed black line). Red asterisks mark the somite 20.

(H–L) Stage 13 embryos, electroporated at stage 4 with GFP (H), Hoxb4/GFP (I), Hoxb7/GFP (J), Hoxb9/GFP (K), or Hoxb4 dn-GFP (L). Yellow brackets highlight different distribution of electroporated cells in the LPM.

(M and M’) Hoxb4 endogenous expression (M’) in a Hoxb4 dn-GFP electroporated embryo (M). Note the decrease in Hoxb4 expression on the electroporated side (red brackets, n = 13/21 embryos) compared to the control side (black dashed line).

(N) Distribution of electroporated cells along the A-P axis for GFP-only (gray, 16 embryos, 6,994 cells), Hoxb4/GFP (red, 12 embryos, 3,203 cells), Hoxb7/GFP (green, 11 embryos, 2,359 cells), Hoxb9/GFP (blue, 15 embryos, 3,922 cells), and Hoxb4 dn-GFP (dashed red, 22 embryos, 20,528 cells) electroporated embryos. Distribution is presented as mean ± SEM.

Scale bars are 100 μm. See also Figures S1, S2, and S3A and Videos S1, S2, and S3.