Initiation of Murine Embryonic Erythropoiesis: A Spatial Analysis by Louise Silver, and James Palis Blood Volume 89(4):1154-1164 February 15, 1997 ©1997 by American Society of Hematology

Development of the mouse conceptus from the mid-primitive streak to early somite stages. Development of the mouse conceptus from the mid-primitive streak to early somite stages. All sections are longitudinal with the exception of (E) and (F ), which are frontal. (A) Mid-primitive streak stage conceptus (E7.25) with the proamniotic cavity (pc) lined by ectoderm cells. Mesoderm cells are beginning to form a cavity (arrow) that will expand to form the exocoelom (ex). (B) Late-primitive streak stage conceptus with an expanded exocoelomic cavity (arrow) in extraembryonic mesoderm. (C) Neural plate stage conceptus (E7.5). Extraembryonic mesoderm cells forming the lateral wall of the exocoelom are proliferating to form a mesodermal cell mass (arrow). (D) Higher magnification of a mesodermal cell mass, consisting of flattened mesothelial cells (arrowhead) lining the exocoelom and proliferating undifferentiated mesoderm cells (closed arrow). The mesodermal cell mass is apposed to the single cell layer of visceral endoderm (open arrow) with its brush border facing the yolk sac cavity. (E) Early somite stage conceptus with multiple yolk sac blood islands (2 blood islands are indicated with arrowheads). Trophoblast giant cells (arrow) are in contact with the maternal decidual tissue (d). (F ) Blood islands at the early somite stage consisting of differentiating erythroid cells (open arrow) and endothelial cells (closed arrow). ac, amniotic cavity; d, deciduum; ec, ectoplacental cavity; epc, ectoplacental cone; ex, exocoelomic cavity; pc, proamniotic cavity; ep, embryo proper. Each scale bar represents 100 μm and the magnification of (F ) is the same as (D). Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

In situ hybridization with βH1-globin antisense (A through C) and sense probes (D through F ) of late-primitive streak (A and C), neural plate (B and D) and early somite stage (C and F ) conceptuses. In situ hybridization with βH1-globin antisense (A through C) and sense probes (D through F ) of late-primitive streak (A and C), neural plate (B and D) and early somite stage (C and F ) conceptuses. (A) Late-primitive streak stage conceptus with no signal above background. (B) Neural plate stage conceptuses (E7.5), with βH1-globin transcripts localized to the mesodermal cell mass (closed arrow). (C) Early somite stage conceptuses with high levels of βH1-globin mRNA present in yolk sac blood islands (closed arrows), and blood cells within the allantois (arrowhead) and the dorsal aorta of the embryo proper (white arrow). (D through F ) Sections of equivalent staged embryos probed with sense βH1-globin gave no signal above background. ac, amniotic cavity; ex, exocoelomic cavity; m, mesoderm; pc, proamniotic cavity. Each scale bar represents 100 μm. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

In situ hybridization with a GATA-1 antisense probe. 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. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

In situ hybridization with a GATA-2 antisense probe. In situ hybridization with a GATA-2 antisense probe. (A) Mid-primitive streak stage conceptus with no detectable signal over background in the embryo proper. (B) Late-primitive streak stage conceptus with GATA-2 mRNA expression evident both in the ectoplacental cone (epc) and in trophoblast cells (arrowhead). There is also expression evident in the mesoderm cell masses (arrow). (C) Neural plate stage conceptus with GATA-2 mRNA expression in trophoblast cells (arrowhead) and in mesodermal cell masses (arrow). (D) Early somite stage conceptus with continued signal in trophoblast giant cells (arrowhead). GATA-2 mRNA is detected in the extraembryonic ectoderm of the chorion (open arrow), but has been downregulated in yolk sac blood islands (closed arrow). (E) Higher magnification of the lower region of (D), showing GATA-2 mRNA expression in the chorion (open arrow) and trophoblast giant cells (arrowhead). Sense controls had high generalized background, but no localized signal (data not shown). ac, amniotic cavity; ex, exocoelomic cavity; pc, proamniotic cavity; m, mesoderm; ep, embryo proper. Each scale bar represents 100 μm. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

Differential expression patterns of GATA-1 and GATA-2 in yolk sac blood islands and giant trophoblast cells at the early somite stage. Differential expression patterns of GATA-1 and GATA-2 in yolk sac blood islands and giant trophoblast cells at the early somite stage. (A) In situ hybridization with a GATA-1 antisense probe, showing signal in yolk sac blood islands (arrowheads), and lack of signal in trophoblast giant cells (arrows). (B) In situ hybridization with a GATA-2 antisense probe, showing lack of signal in yolk sac blood islands (arrowheads), and presence of signal in trophoblast cells (arrows). Scale bars represent 100 μm. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

In situ hybridization with a tal-1 antisense probe. In situ hybridization with a tal-1 antisense probe. (A) Mid-primitive streak stage conceptus with tal-1 mRNA expression localized to 2 extraembryonic mesoderm cells. Serial sections probed with antisense tal-1 gave similar results. (B) Late-primitive streak stage conceptus with tal-1 signal restricted to VYS mesoderm (closed arrow) and posterior embryonic mesoderm (open arrow). (C) Neural plate stage conceptus with tal-1 mRNA present in both VYS mesoderm (closed arrow) and posterior embryonic mesoderm (open arrow). (D) Early somite stage conceptus with low levels of tal-1 mRNA expression in yolk sac blood islands (closed arrow) and in regions of embryonic mesenchyme (open arrows). ac, amniotic cavity; ex, exocoelomic cavity; pc, proamniotic cavity; ep, embryo proper. Each scale bar represents 100 μm. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

In situ hybridization with an rbtn2 antisense probe. 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. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology

Summary of tal-1, rbtn2, GATA-1, GATA-2, and βH1-globin mRNA expression patterns during murine gastrulation. Summary of tal-1, rbtn2, GATA-1, GATA-2, and βH1-globin mRNA expression patterns during murine gastrulation. Captions at the top of the figure indicate developmental stages. Abbreviations at the side of the figure indicate the different tissues in which signal was detected. EM, embryonic mesoderm; EXM, extraembryonic mesoderm; EE, extraembryonic ectoderm; TR, trophoblast giant cells or their precursors. Genes expressed at a particular stage and in a particular tissue are designated in the appropriate box. Louise Silver, and James Palis Blood 1997;89:1154-1164 ©1997 by American Society of Hematology