Fig. 7. Plasma viscosity alters yolk sac remodeling. Control, low-hematocrit and low-hematocrit+hetastarch mouse embryos were prepared as described (see Materials and methods). Brightfield images show that control embryos turned (A) and yolk sac vessels remodeled into a hierarchical, branched phenotype as evident from Texas Red (TR)-dextran microangiograms (B). eGFP expression (green) from the Tg(-globin-GFP) construct shows that erythroblasts are evident in all parts of the yolk sac vasculature (C). The embryo shown in A-C was given a score of 5 for both turning and remodeling. Embryos with sequestered erythroblasts often did not turn but continued to develop after 24 hours in culture (D). The yolk sac vasculature did not remodel and retained features of an immature plexus (E). Erythroblasts remained confined to the blood islands (F). This embryo was scored a 1 for both turning and remodeling. Embryo turning was restored (score=5) (G) and yolk sac remodeling was rescued (score=5) (H) in embryos with sequestered erythroblasts (I) after injection of the hetastarch solution. Scale bar: 500 µm.