Limited blood flow in E8.25 to E8.75 embryos.

In the top row, embryos proper and yolk sacs remain attached. In the bottom row, embryos proper were dissected free of extraembryonic tissues. Before 8 sp, an embryo proper is initially curved with its back arched. From 8 to 12 sp, the embryo proper then turns into the more traditional fetal position. Distribution of βH1 globin mRNA-containing erythroblasts was detected by in situ hybridization (A-F), and distribution of PECAM-expressing endothelial cells was detected by immunohistochemistry (G-H). (A-B) At 6 sp, most of the red blood cells were in the yolk sac and particularly concentrated at the proximal edge (red arrowhead). The few erythroblasts (red arrows) in the embryo proper were seen in the head and aorta. (C-D) At 8 sp, more red blood cells were seen throughout the yolk sac and within the aorta and head regions. (E-H) By 12 sp, while most blood was still in the yolk sac, increased numbers of red blood cells were seen in the embryo proper and were found throughout the length of the aortae. However, blood was not seen throughout the entire embryonic endothelial network (compare panels F and H). For example, blood was rarely seen in the intersomitic arteries (s) that were observed at this time. v indicates vitelline connections of yolk sac and embryo proper vasculature. Original magnification A-D, × 75; E-H, × 50.