Intima-like smooth muscle cells: developmental link between endothelium and media?

The presence of non-contractile smooth muscle cells within the arterial wal l raises questions as to their origin and function. These cells abound with in the aortae of murine and porcine neonates, but are also present within t he intimal and medial layers of adult arteries. They are largely devoid of smooth muscle-associated proteins and manifest an epithelioid form. Their m orphological resemblance to endothelial cells prompted us to explore this p otential relationship and to investigate their angiogenic properties in thr ee-dimensional collagen gels. Using well-characterized smooth muscle cell l ines, displaying either the intima-like (epithelioid) or media-like (spindl e-shaped) morphology, we were able to show that intima-like cells share sev eral features in common with endothelial ones and can transform into a medi a-like phenotype, whereby they irreversibly lose their characteristic patte rn of protein expression. Intima-like, but not media-like, vascular smooth muscle cells are capable of forming capillary tubes, and, in co-cultures, c an induce media-like ones to participate in this process. Such capillaries consist of a randomly-organized, mixed population of endothelial cells with intima-like or media-like smooth muscle ones. The functional significance of this diversity in smooth muscle cell type is not well understood, but ph enotypic plasticity could conceivably figure as an important adaptive respo nse to changes in the local environment.