Embryonic and extra-embryonic vasculogenesis is affected differently by a lack of FNs. Immunohistochemistry with antibody MEC 13.3 to mark PECAM-1–positive endothelial cells in wild-type (A) and FN.null-/- less severe (B) yolk sac and wild-type (C) and FN.null-/- less severe (D) dorsal aortae. In the mutant yolk sac, PECAM-1–positive cells are associated with the mesodermal layer only. In the wild-type yolk sac, endothelium lines the blood vessel. In contrast, PECAM-1–positive endothelium lines both wild-type and FN.null-/- dorsal aortae. Within the embryo, a lack of FN does not prevent organization of the blood vessel, but rather its maintenance. E, yolk sac endoderm; M, yolk sac mesosderm; BC, primitive blood cells; DA, dorsal aortae.