Figure 1. The Continuum of Vascular Smooth Muscle Phenotypes.

The medial layer of arteries consists of a highly heterogeneous mix of cells of diverse origins. Many but not all smooth muscle cells begin as adventitial fibroblasts. These fibroblasts initially differentiate into myofibroblasts and then into smooth muscle myocytes that migrate through the medial layer. Migratory myocytes can then transform into proliferative, synthetic and contractile smooth muscle in response to growth factor stimulation. These patterns of differentiation are not terminal, and can be reversed when certain growth and stress factors are introduced in the local environment. In this manner, the artery wall is highly dynamic and heterogeneous in terms of both its structural and functional characteristics.