Kinetics of adventitial repair in the rat carotid model

BACKGROUND: Discrepancies between success in experimental animals with a variety of pharmacologic strategies and failure with such agents in clinical trials have raised questions concerning the mechanism of restenosis. Recent observations suggest a potential implication for the adventitial (Adv) layer in neointimal formation. METHODS: The purpose of this study was to examine the Adv changes in the rat carotid artery subjected to balloon injury. These changes were characterized by morphometric, immunohistochemical, and electron microscopy analyses, with special attention devoted to early time-points post-injury. RESULTS: We report that the most important adventitial changes occurred in the first 48 h post-injury. Within 2 h there was extensive cell-loss by apoptosis and oncosis in the Adv and in the media; this was followed by the rapid onset of proliferation and a parallel slow increase in Adv thickening, reaching a maximum at 7 days. We further demonstrate an early migration of these Adv cells to the media and neointima. Moreover, we characterize the Adv cell phenotype with a panel of antibodies. Within 48 h after injury, a population of Adv cells expressed alpha-actin and vinculin with a maximum expression 7 days post-injury. At that time, these Adv cells started to express smooth muscle myosin heavy chain, a specific marker of smooth muscle cells. In parallel, we report an impaired production of elastic fibres in the Adv and medial layer. CONCLUSIONS: We reported a detailed time-course of adventitial changes after rat carotid injury (cell death, proliferation, migration and differentiation) that supports an important role of adventitia in neointima formation.