Expression ofRunx1 mRNA during gastrulation in the mouse embryo (E7.25-E8.25).

(A) Mid-to-late primitive streak stage embryo (E7.25) withRunx1 mRNA accumulation in mesoderm cells adjacent to visceral endoderm in the forming yolk sac (arrowhead). (B) Early neural plate embryo (E7.5) with increased Runx1 transcripts in the yolk sac mesoderm (arrowhead). (C) Late neural plate embryo (E7.75) reveals Runx1 expression in nascent yolk sac blood islands (arrowhead) and in the mesoderm component of the chorion (open triangle). (D) Early somite-stage embryo (E8.25) displaysRunx1 mRNA accumulation in expanding blood islands of the yolk sac (arrowheads) and in the chorion (open triangles). a indicates amniotic cavity.