Vascular patterning is detected in FN.null embryos even in the absence of organized blood vessels. Whole-mount in situ hybridization with antisense Flk-1 probe to mark early endothelium in wild-type and more severe class FN.null-/- at E 8.5 (A). Hybridization with antisense PECAM-1 probe to mark early endothelium in wild-type and 2 more severe class FN.null-/- at E 8.5 (B) and wild-type and less severe class FN.null-/- at E 8.5 (C and D). Note that in the more severe class, only the pattern of dorsal aortae is visible in Flk-1–positive cells. In the less severe class, PECAM-1–positive cells are organized into blood vessels. DA, dorsal aortae. Arrowheads indicate endothelial cells.