Asymmetric blood formation and first blood flow in early somite pair embryos.

In rodents, the embryo proper develops within the amnionic sac, which itself is within the yolk sac. In this figure, the yolk sac was photographed with the embryo proper inside, except for panels D and I, where the embryo proper was dissected free of amnion and yolk sac. (Top row) Blood flow was monitored by the redistribution of yolk sac–derived primitive erythroblasts detected by in situ hybridization to the embryonic-specific βH1 globin mRNA. (Bottom row) Endothelial development was delineated by PECAM immunohistochemistry. No staining was seen in stage-matched negative controls of in situ hybridization or immunohistochemistry (see panels I and J for examples). (A-B) At 1 sp, red blood cells were restricted to blood islands (red arrowheads) in the proximal region (farthest from the embryo proper), where vascular development was most prominent. The yolk sac vascular plexus centered at the blood islands and the embryo proper aortae (arrow) had begun to form (h indicates head; t, tail). (C-D) By 3 sp, blood was still restricted to the proximal yolk sac, whereas vascular development had spread throughout the yolk sac, and extended aortic tubes were visible (arrow). (E-G) At 4 sp, the first few dispersed red blood cells were observed (red arrow) in the distal yolk sac (E) and in the embryo proper (G). At this stage, a well-developed vascular plexus extended throughout the distal yolk sac (F), forming the vitelline connections (v) with the vasculature of the embryo proper (H). Stars in panel H indicate sections of the forming cardinal vein. Somite pairs in panels I and J are marked with small squares. Original magnification A-J, × 75.