Developmental biology of the vascular smooth muscle cell: building a multilayered vessel wall

The assembly of the vessel wall from its cellular and extracellular matrix components is a critical process in the development and maturation of the cardiovascular system. However, fundamental questions concerning the origin of vessel wall cells and the mechanisms that regulate their development and differentiation remain unanswered. The initial step of vessel wall morphogenesis is formation of a primary vascular network, comprised of nascent endothelial cell tubes, via the processes of vasculogenesis and angiogenesis. Subsequently, primordial vascular smooth muscle cells (VSMCs) are recruited to the endothelium to form a multilayered vessel wall. During the course of development and maturation, the VSMC plays diverse roles: it is a biosynthetic, proliferative, and contractile component of the vessel wall. Although the field of vascular development has blossomed in the past decade, the molecules and mechanisms that regulate this developmental pathway are not well defined. The focus of this review is on those facets of VSMC development important for transforming a nascent endothelial tube into a multilayered structure. We discuss the primordial VSMC with particular attention to its purported origins, the components of the extracellular milieu that contribute to its development, and the contribution of embryonic hemodynamics to vessel wall assembly.