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.