Endothelial regeneration. II. Restitution of endothelial continuity.

With a modified balloon catheter the endothelium of rat thoracic aortae was completely removed to study the interaction between two important responses of the vessel wall to intimal injury: endothelial regeneration and formation of an intimal fibrocellular thickening. Endothelial cells deriving from the uninjured intercostal arteries regenerated by migration followed by proliferation and proceeded as a continuous sheet at a rate of approximately 0.07 mm. per day in the circumferential direction and approximately 6 times faster in the axial direction. Smooth muscle cells appeared in the intima only in areas which were not covered by regenerating endothelium 7 days after injury. The smooth muscle cells formed a multilayered fibrocellular intimal lesion which reached the maximal thickness after 3 weeks. The continuous sheet of regenerating endothelium covered the intimal smooth muscle cells at a slower rate; 6 weeks after injury large areas located most distant from the source of regenerating endothelium still showed modified smooth muscle cells lining the lumen. However, platelets did not adhere to these smooth muscle cells, and the total amount of intimal thickening did not increase between 3 and 6 weeks after injury. We conclude that, in response to intimal injury, endothelial regeneration precedes the accumulation of intimal smooth muscle cells, and that injured intimal areas, which are rapidly covered by continuous endothelium, are protected from the development of a fibrocellular intimal lesion.