In situ hybridization with an rbtn2 antisense probe. (A) Mid-late primitive streak stage conceptus with rbtn2 mRNA expression in both extraembryonic mesoderm (closed arrow) and posterior embryonic mesoderm (open arrow). (B) Transverse oblique section of a mid-primitive streak stage conceptus with rbtn2 mRNA confined to mesoderm cells located between the inner epiblast cells (e) and the outer visceral endoderm cells (ve). rbtn2 transcripts are present both in the axial mesoderm and in the mesodermal wings (arrowheads). (C) Late-primitive streak stage conceptus with rbtn2-positive extraembryonic mesoderm cells (arrow). (D) Neural plate stage conceptus with rbtn2 transcripts present in both mesodermal cell masses (closed arrow) and posterior embryonic mesoderm (open arrow). (E) Early somite stage yolk sac with rbtn2 mRNA accumulation in cells within blood islands (closed arrows), but not in the yolk sac endoderm (open arrow). There was no signal above background in the sense controls. ac, amniotic cavity; e, epiblast; ec, ectoplacental cavity; ex, exocoelomic cavity; pc, proamniotic cavity; ve, visceral endoderm. Each scale bar represents 100 μm.