A three-dimensional model of vasculogenesis

Postnatal bone marrow contains various subpopulations of resident and circulating stem cells (HSCs, BMSCs/MSCs) and progenitor cells (MAPCs, EPCs) that are capable of differentiating into one or more of the cellular components of the vascular bed in vitro as well as contribute to postnatal neo-vascularization in vivo. When rat BMSCs were seeded onto a three-dimensional (3-D) tubular scaffold engineered from topographically aligned type I collagen fibers and cultured either in vasculogenic or non-vasculogenic media for 7, 14, 21 or 28 days, the maturation and co-differentiation into endothelial and/or smooth muscle cell lineages were observed. Phenotypic induction of these substrate-grown cells was assayed at transcript level by real-time PCR and at protein level by confocal microscopy. In the present study, the observed upregulation of transcripts coding for vascular phenotypic markers is reminiscent of an in vivo expression pattern. Immunolocalization of vasculogenic lineage-associated markers revealed typical expression patterns of vascular endothelial and smooth muscle cells. These endothelial cells exhibited high metabolism of acetylated low-density lipoprotein. In addition to the induced monolayers of endothelial cells, the presence of numerous microvascular capillary-like structures was observed throughout the construct. At the level of scanning electron microscopy, smooth-walled cylindrical tube-like structures with smooth muscle cells and/or pericytes attached to its surface were elucidated. Our 3-D culture system not only induces the maturation and differentiation of BMSCs into vascular cell lineages but also supports microvessel morphogenesis. Thus, this unique in vitro model provides an excellent platform to study the temporal and spatial regulation of postnatal de novo vasculogenesis, as well as attack the lingering limit in developing engineered tissues, that is perfusion.