In situ hybridization with a GATA-1 antisense probe. (A) Mid-primitive streak stage conceptus with no localized signal above background. (B) Late-primitive streak stage conceptus with GATA-1 mRNA expression present in the mesoderm cell mass (closed arrow). There is expression evident in extraembryonic ectoderm cells of the chorion (open arrow). (C) Early somite stage conceptus with localized signal in yolk sac blood islands (arrow). (D) Higher magnification of an early somite stage conceptus, with GATA-1 mRNA present in yolk sac blood islands (arrow). Low levels of GATA-1 transcripts are also evident in extraembryonic ectoderm cells of the chorion (open arrow) compared to yolk sac endoderm (closed arrow). GATA-1 sense probes gave similar high generalized background with no localized signal (data not shown). ep, embryo proper; m, mesoderm; pc, proamniotic cavity. Each scale bar represents 100 μm.