The wall reaction to electric micro-injury at branching sites of mesenteric arteries of the rat: an electron microscopic study of intimal cushions.

Intimal cushions consist of small bundles of longitudinally oriented smooth-muscle cells (SMC) which are interposed between the endothelium and the lamina elastica interna (LEI) of arteries. They are consistently present on the mouths of artherial branchings, at sites of haemodynamic stress. In structure and location, they closely resemble early artherosclerotic lesions. In previous experiments, we studied the local consequences of a standardized microthrombosis induced in non-branching segments of mesenteric arteries of rats. In this experimental model, the endothelium gets reconstituted within 24 h while in the underlying media the SMC exhibit a marked growth reaction which reaches a peak on the third day and eventually terminates by the sixth day with full restitution to normal. In the present work, the experiment was repeated at sites of arterial branchings. It was seen that in this location the SMC growth reaction was greatly enhanced and led to a marked increase in size of the intimal cushions. In addition their endothelial lining, although reconstituted, showed a persistent tendency to segmental breakdown. These results support the view that intimal cushions develop as a consequence of local haemodynamic stress which, by causing endothelial-cell loss, triggers off the sequence of blood-platelet adhesion, aggregation and release of mitogenic factors, the latter being ultimately responsible for an ever-recurring stimulation of myointimal growth. The same mechanisms are likely to operate in atherogenesis.