Sprouting from the dorsal aorta is reduced in the absence of flt-1 (A) The 8.5-dpc embryos that were WT (+/+; i-ii), flt-1^+/- (+/-; iii), or flt-1^-/- (-/-; iv-vi) were collected and whole-mount stained with an anti-PECAM antibody. (i-iii) In 3 different normal embryos, intersomitic vessels (white arrows) branch from the dorsal aorta (black arrowheads). A magnified view in panel iii shows several intersomitic sprouts (white arrowheads) in register posterior to the vessels. (iv-vi) Two different flt-1^-/- embryos show few branches from the dorsal aorta, and those that are seen are often stunted (black arrows). The inset in panels v and vi show stunted sprouts with blunted tips. A indicates anterior; and P, posterior. Original magnification × 5. (B) Day-8 WT (i-iv) and flt-1^-/- (v-viii) ES differentiation cultures were fixed and stained with a PECAM antibody. Representative sprouts were photographed. Note that most WT sprouts are tapered at the distal end (white arrowheads), while most flt-1^-/- mutant sprouts are blunted (white arrows) at the distal end. Original magnification × 60.