Reduced number of CD41-expressing hematopoietic cells and increased remodeling in PDGF-BB/PDGFR-β–activated yolk sacs. (A) Whole-mount, somite stage–matched yolk sacs from E9.5 D849N PDGFR-β (top panels) and Rosa26-PDGF-BB (middle panels) embryos were compared with their respective stage-matched wild-type yolk sacs in immunofluorescent analyses of CD41 and VEGFR-2. Bars represent 50 μm. The number of CD41-expressing cells/field in 6 to 8 fields in the E9.5 yolk sacs (n = 8 for each of D849N PDGFR-β and wild-type C57BL/6; n = 10 for each of Rosa26-PDGF-BB and wild-type SvJ129) are shown as mean ± SD fold induction. ***P = .001. *P = .042. (B) VEGFR-2/Hoechst 33342 costaining of wild-type and Rosa26-PDGF-BB yolk sacs shows an increased number of cytoplasmic extensions (arrows) and branching of the vascular plexus. Bars represent 20 μm.