Early arterial injury-induced myointimal proliferation in canine pulmonary arteries.

Transmission electron microscopy was used to study the early vascular response induced by arterial damage with heartworm infection. The pulmonary arteries were examined in dogs 4 days after experimental transplantation of 6 to 8 Dirofilaria immitis adults. Evan's blue dye was given IV and followed in 60 minutes by perfusion fixation with 1% glutaraldehyde. Endothelial cell junctions were disrupted and rounded endothelial cells were observed. Macrophages and neutrophils adhered to abnormal endothelial cells. Focal areas had platelet aggregates adhered to exposed subendothelial structures. Platelets were activated and degranulated. Areas of the internal elastic lamina appeared to be disrupted, and smooth muscle cell processes from the tunica media extended through these regions of disruption. Smooth muscle cells were oriented toward the arterial lumen and some had migrated to the surface. These findings are compatible with the response to injury theory of myointimal proliferation.