The endothelial surface of growing coronary collateral arteries. Intimal margination and diapedesis of monocytes. A combined SEM and TEM study.

Slowly progressing coronary artery stenosis leading to complete occlusion within about 3 weeks was produced in dogs. Within this time collateral vessels had enlarged sufficiently to prevent myocardial infarction. Early, intermediate, and late (1 year after occlusion) stages of collateral development were studied with the scanning and transmission electron microscope. Early after coronary occlusion the number of endothelial cells per unit inner vascular surface had markedly increased and longitudinal bulges appeared in growing collaterals as opposed to the completely flat inner surface of small normal coronary arteries. The surface of many endothelial cells appeared rough and large numbers of monocytes adhered to the inner vascular surface. The endothelial cells formed three types of patterns: streams, whorls, and nonoriented mosaics suggesting different types of flow-jets, eddies, and lowshear flow, respectively. The existence of nonlaminar flow patterns could well be explained by the extremely tortuous course of collaterals and by segmental caliber changes (microstenoses) resulting from irregularities of the internal elastic lamina. Later stages showed a tendency toward normal endothelial cell density, flattening of bulges, and absence of microstenoses. A completely normal inner surface was, however, never observed in midzone segments although the observation period extended up to 1 year after coronary occlusion.