"Spontaneous" endothelial injury in the rat caudal artery.

Numerous spontaneous lesions, characterized principally by a gap in the internal elastic lamina, form with age in the caudal artery of the young, male Wistar rat. Such newly formed lesions were studied, with special reference to the state of the endothelium, using three techniques: "en face" examination of silver-stained caudal artery, [3H]thymidine autoradiography, and light and electron microscopy. In many, but perhaps not all cases, areas of endothelium are damaged or removed either simultaneously or shortly after the formation of the break in the internal elastic lamina and some underlying smooth muscle cells are damaged. Polymorphonuclear cells and monocytes are attracted to the site of injury. A repair process rapidly ensues including rapid regeneration of the endothelium and proliferation of smooth muscle cells in the underlying media and of some adventitial fibroblasts. This process results in the restitution of the endothelial monolayer and the laying down of one or several layers of longitudinal smooth muscle and collagen and elastic fibers in the subendothelial space. This demonstration of naturally occurring damage to fairly large areas of endothelium and the events which occur in the repair process is of interest in view of the "response-to-injury" theory of atherogenesis; observations are discussed in the light of previous studies performed on experimentally removed endothelium.