Figure 2.

Schematic drawing of definitive endoderm origin. (A) Precursors of the primitive streak have been mapped to Koller's sickle (KS) at prestreak stages (green dots), and (B) are within the primitive streak at stage 2 (red, PS). (C) The rostral third of the extending streak (pale red) gives rise to the definitive endoderm, which ingresses displacing the hypoblast laterally and eventually forming the whole lower layer of the embryo (arrows to D). (D) Gene expression is regionally restricted within the lower layer. The rostral germ cell crescent and displaced hypoblast (pale green hexagons) and junctional mesendoderm (pale green bars) are Gata4/5/6 positive. Rostral definitive endoderm is Sox17 positive (yellow), with more caudal definitive endoderm expressing only Gata6 transcripts by stage 4 (green in area pellucida). The area of overlap at the boundary between Sox17 and Gata6 is marked by asterisks (blue).