Histological sections of lacZ-stained hematopoietic and vascular tissues in SCL^lacZ/w embryos. (A) Sagittal section through the dorsal aorta of an E9.5 embryo demonstrating lacZ staining of variable intensity in circulating hematopoietic cells. (B) Cytocentrifuge preparation of E13.5 embryonic blood showing lacZ staining in a definitive erythrocyte. (C) Endothelial lacZ expression in a cerebral artery of an E13.5 embryo. (D) The dorsal aorta of an E11.5 embryo demonstrating lacZ staining in the endothelium and in a ventral aortic wall cluster of hematopoietic cells (arrow). All the cells in the cluster were positive, but this cannot be appreciated in a single focal plane. (E) and (F) LacZ staining in E11.5 fetal liver of a wt (E) and heterozygous SCL^lacZ/w embryo (F) showing specific staining in large megakaryocytes and smaller erythroblasts. Original magnification: (A, C through F), ×400; (B), ×1,000.