Fig. 1. Runx1 expression before and after chorio-allantoic fusion. (A) Schematic diagram of an early headfold (EHF) stage mouse embryo, showing locations of the ectoplacental cavity (epc), chorion (ch), exocoelomic cavity (ecc), amnion (am), amniotic cavity (ac), embryo (emb), yolk sac blood islands (bi) and allantois (al). Shown in color and labeled are both embryonic and extra-embryonic endoderm layer (end), mesoderm layer (mes) and ectoderm layer (ect). The conceptus is surrounded by embryonic (emb), yolk sac (ys) and ectoplacental (ep) visceral endoderm (vis end). The chorion consists of chorionic mesoderm (ch mes) and ectoderm (ch ect), the latter of which is derived from trophectoderm. The anterior (a) and posterior (p) boundaries of the primitive streak (ps) are indicated. Compass shows anterior (ant), posterior (pos), proximal (pro) and distal (dis) directions for the extra-embryonic compartment. (B) Sagittal section of an EHF-stage Runx1^lz/+ conceptus showing Runx1 expression (blue) in the chorionic (ch) mesoderm (black arrow) and blood islands (bi). The blue arrow indicates area of punctate blue staining in ectoplacental visceral endoderm. ß-Galactosidase staining was performed for 18 hours. (C) Runx1 expression (arrowheads) at the chorio-allantoic fusion junction in a 12 s Runx1^lz/+ conceptus. (D) Detail of Runx1 expression from boxed region in C. (E) Runx1 expression in a 12 s Runx1^lz/+ conceptus at the plane of chorio-allantoic fusion (black arrow), at the junctions between the allantois and chorion (blue arrows) and in a cluster of cells within the base (boxed region). ß-Galactosidase staining was performed for 18 hours. (F) Detail of Runx1 expression from boxed region in E. (G) Runx1 expression in the distal region (arrowhead) and within the allantoic base (box) in a 16 s Runx1^lz/+ conceptus. (H) Detail of Runx1 expression (boxed region in G) in putative endothelial cells (arrowheads) lining a blood vessel within the allantoic base. (I) Section through placenta of a 9.5 dpc Runx1^lz/+ embryo showing Runx1⁺ cells in allantoic mesothelium (arrow). (J) Detail of boxed region from I. (K) Runx1 expression in endothelium and umbilical artery hematopoietic clusters in a 10.5 dpc Runx1^lz/+ conceptus. (L) Whole mount showing Runx1 expression in the chorionic disk (cd) and along the length of the umbilical artery (ua) of a 10.5 dpc Runx1^lz/+ conceptus.