The role of the pericytes of the adventitial microcirculation in the arterial intimal thickening

Segments of rat femoral arteries, with one collateral each, occluded between ligatures and dissected from surrounding tissue, developed intimal thickening, with or without ligation of their collaterals. Numerous newly-formed capillaries from the surrounding arterial microcirculation growing into the adventitia, tunica media and intimal thickening were demonstrated by means of serial longitudinal sections, predominantly in the ostium of the collateral. When the ligatures were applied without damaging the microcirculation surrounding the artery and the normal continuity of the adventitial vessels was unchanged, earlier presence of intimal thickening was observed. When the fibrous layers of the adventitia were removed at the moment of the arterial ligation, the continuity between newly-formed vessels of the neoadventitia and those growing into the media and neointima was much more evident. It was then noted that the pericytes constituted a major component of the intimal thickening. The introduction of contrast material in microcirculation confirmed the connections between newly-formed adventitial and intimal vessels. At the beginning of the experiment, autoradiographic studies showed an increased DNA synthesis in the cells of preformed postcapillary venules and capillaries of surrounding arterial microcirculation and later in those of the newly-formed vessels growing into the arterial wall. These results indicate that newly-formed capillaries derived from surrounding arterial microcirculation penetrate the wall of the occluded arterial segments and contribute to the intimal thickening formation. It is likely that the pericytes and endothelial cells (EC) of these ingrowing vessels are sources of myointimal cells at the intimal thickening and of endothelium at the luminal surface, respectively.