Fig. 5. Formation of the aorta and molecular characterization of somite angioblasts. (A-E) Cross-sections from caudal (A) to cephalic (E) levels in an 18-somite quail embryo. QH1 staining merged to Nomarski's interferential contrast. (A) Primitive streak level. QH1⁺ angioblasts visible at a lateral position (arrowhead) aggregate against the endoderm and organize into groups of cells. (B) Segmental plate level. QH1⁺ groups organize into vessels that progressively fuse. These structures remain positioned lateral to the segmental plate. (C) Nascent somite level. The paired aortae have formed. A few QH1⁺ angioblasts are now detected in a dorsal position. (D) Epithelialized somite level. The paired aortae are now underneath the somites. Dorsal QH1⁺ angioblasts become more numerous. (E) Dermomyotome and sclerotome have separated. Conspicuous QH1⁺ cells are around the Wolffian duct (arrow) and in close association with the aortic roof (arrowhead). Scale bar: 100 µm. (F-I). EC-specific markers in chick (F-H) and quail (I) somites. (F-H) GATA2, VEGFR2 and SCL/TAL1 in situ hybridization. All markers delineate a quadrant of cells in the dorsolateral aspect of the epithelial somite (arrow). GATA2 is also present in the epidermis (Sheng and Stern, 1999; Minko et al., 2003). (I) QH1 immunohistochemistry. Scale bar: 50 µm. D, dermomyotome; E, ectoderm; En, endoderm; M, mesoderm; Sc, sclerotome; So, somatopleural mesoderm; Sp, splanchnopleural mesoderm; SP, segmental plate; WD, Wolffian duct.