Fig. 5

Primitive endoderm behaviour during the blastocyst to egg cylinder transition. a Examples of embryos used for analysis of primitive endoderm (PE) cell shape at different stages of peri-implantation development. Representative of 20 embryos for each stage. b Quantification of PE cell aspect ratio at different developmental stages. The red lines between different columns correspond to events taking place within the trophectoderm (TE). Two-sided unpaired student’s t test; ****P < 0.0001; mean ± SEM. Pre-implantation blastocyst: n = 117 cells; Implanted blastocyst: n = 219 cells; Transitioning blastocyst: n = 117 cells; early egg cylinder: n = 184 cells. c Stills from a time lapse movie of Lifeact-GFP E4.5-implanting blastocyst. PE (cyan) acquires a columnar morphology at implantation stages and this is followed by tissue spreading through cell-shape changes upon polar TE (red) expansion as quantified in b. Red double-headed arrows: polar TE expansion. Cyan arrows: PE spreading. Stills without coloured overlay are presented in Supplementary Fig. 7b. d Representative examples of implanting blastocyst (E4.5–E4.75). Parietal endoderm migration is completed before the expansion of the TE (double-headed arrows in bottom row) and before PE (red outline in 2D images) spreading over the lateral sides of the epiblast (bottom row). Red arrows indicate the front of the migrating parietal endoderm. e Representative examples of transitioning blastocysts (E5.0) displaying expanded polar trophectoderm and early egg cylinders. Primitive endoderm cells are found enclosed between the embryonic and Reichert’s basement membrane during blastocyst to egg cylinder transformation. arrowheads: Reichert’s membrane. Arrows: embryonic basement membrane. Dots: proximal cells. Representative images of 20 embryos per stage. Source data are provided as a Source Data file. Scale bars = 20 µm