Neointima formation: a local affair.

Where do intimal smooth muscle cells (SMCs) come from? For many years, the idea that intimal SMCs originated from the underlying media went unchallenged.1 Then, reports2,3 began to appear that up to half of the SMCs in the intima of atherosclerotic plaques and injured arteries arose from circulating progenitor cells of bone marrow origin. This new view of intimal SMC formation was potentially important because it raised the possibility of a new class of therapeutic targets for intervention in the process of restenosis based on a bone marrow derivation of intimal SMCs. However, as other laboratories began to follow up on these intriguing initial reports, a long-term contribution of bone marrow–derived cells to intimal tissue became less tenable.4,5 In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, a careful and detailed study by Daniel et al6 seems to leave little or no room for a role of bone marrow–derived cells as progenitors for the intimal SMCs and endothelial cells that are stable residents of a mature neointima that forms after acute vascular injury.