Homozygous P735R-knockin embryos develop distended vascular sprouts and spontaneous hemorrhages in the brain. (A) Comparison of wt/wt and mt/mt embryos at different developmental stages. Blood is visible in the hearts of the older embryos. Asterisks indicate hemorrhages in the brain ventricles, spinal cord, and spinal canal of E11.5 mt/mt embryos. The E11.5 and E12.5 mt/mt embryos are dead and appear pale as blood circulation has ceased. (B) Hematoxylin-eosin staining of coronal section of the wt/wt and mt/mt embryos’ heads at E11.5. Arrows indicate hemorrhagic lesions, which are shown at higher magnification in (C). Hemorrhages are visible in both the brain parenchyma as well as in the ventricles. Erythrocytes are nucleated at this stage of development. (D) Confocal images of angiogenic sprouting into the ventricle from PNVP of the diencephalon (coronal section) of E11 and (E) E11.5 embryos. (F) Confocal images of diencephalon distended sprouts and (G) accumulated eMKs in mt/mt embryos at E11. Neural, brain tissue. PNVP, perineural vascular plexus. ECL intensity reflects the degree of asialylation. In E11.5 mt/mt embryos, the hemorrhage penetrated the brain tissue and spread into the ventricle. IB4 marks the vasculature. CD41 marks eMKs and platelets (mostly eMKs at this stage). (H) Confocal microscopy images of wholemount stained E10.5 yolk sac with CD31 near the vitelline vein and (I) capillary plexus. Scale bar, 100 μm (B, D, F, H, I); 25 μm (C, E, G).