Abnormal angiogenesis in Bmpr2 KD mice. Representative H&E-stained sections of Matrigel loaded with 200 ng/mL bFGF 7 days after implantation into the WT or Bmpr2 KD mice are shown. Gel implants in WT mice mount robust neoangiogenesis and form well-defined, cell-lined vessels (arrows), whereas specimens from Bmpr2 KD mice show increased hemorrhage and congestion (arrowhead), which were partially alleviated by the presence of exogenously supplied BMP4. Bar graph at lower right shows average number of RBC-containing vessels and tubular structures (black bar) or large blood pools (red bar) counted per × 400 microscopic field. Values are means plus or minus SD. *P < .05.