Pulmonary vascular morphology of vehicle-treated (a) or SU5416-treated (b) rats exposed to Denver altitude conditions for 3 weeks. Note the increase in medial thickness in the SU5416-treated rat lungs, which extends into preacinar vessels (arrow). On the other hand, normal pulmonary arteries have well defined medial muscular layer (arrow), which becomes progressively thinner (dashed arrow) in the preacinar region.