Fig. 6. Pecam1 identifies a nascent vascular scaffolding that is missing in T^C/T^C allantoises. (A) Whole-mount Pecam1 (brown) and Flk1 (X-gal, blue) in Flk^lacZ conceptuses (see Materials and methods). (a) Pecam1 (arrows) was detected in a small cluster as early as the LB stage (inset), whereas lacZ-positive cells were dispersed in the allantois (arrowheads in this and all panels in A). By the EHF stage, Pecam1 defined a central vessel that extended distally and proximally (a-c), and ultimately converged upon the developing embryonic dorsal aortae (da; c). Flk1-positive cells were not localized to a patterned central vessel until 6-s (d). (B) Pecam1 detection in +/+ (a) (inset shows localization on the cell surface), T^C/+ (b,c), and T^C/T^C (d) 2-s conceptuses. Formation of the Pecam1-positive central vessel (arrows in a-c) was variable in T^C/+ allantoises. A single Pecam1-positive cell was detected in the T^C/T^C allantois (arrow in d). Pecam1 was not detected in the posterior embryonic region of T^C/+ or T^C/T^C conceptuses (arrowheads mark the site where continuity with the embryonic dorsal aortae was expected). Scale bar in B, part d: 100 µm for A, parts a,b, B parts a-d; 250 µm for A, parts c,d; 25 µm for inset in A, part a; 50 µm for inset in B, part a.