A morphologic and permeability study of luminal smooth muscle cells after arterial injury in the rat

In the carotid air drying model of arterial endothelial injury in the stressed rat, endothelium does not always regenerate over the zone of intimal thickening; instead, a layer of modified smooth muscle cells appears to form a temporary luminal surface. We examined the properties of these luminal smooth muscle cells in injured right carotid arteries from stressed rats at intervals up to 2 months by light, scanning electron, and transmission electron microscopy. Before perfusion fixation, selected animals were given injections of Evans blue dye, ferritin, or horseradish peroxidase. Unlike adjacent endothelium, the luminal smooth muscle cells most closely resembled neighboring intimal smooth muscle cells, lacked morphologic characteristics of normal endothelium, and did not stain with rabbit antibody to rat factor viii. Unlike normal mature endothelium, this layer did not exclude horseradish peroxidase, Evans blue, or ferritin. These data demonstrate that a nonthrombogenic layer composed of modified smooth muscle cells can appear at the luminal surface of a zone of injury-induced myointimal thickening; however, this layer does not form a permeability barrier to large molecules.