We show that endothelial cell (EC)-generated vascular guidance tunnels (ie, matrix spaces created during tube formation) serve as conduits for the recruitment and motility of pericytes along EC ablumenal surfaces to facilitate vessel maturation events, including vascular basement membrane matrix assembly and restriction of EC tube diameter. During quail development, pericyte recruitment along microvascular tubes directly correlates with vascular basement membrane matrix deposition. Pericyte recruitment to EC tubes leads to specific induction of fibronectin and nidogen-1 (ie, matrixbridging proteins that link together basement membrane components) as well as perlecan and laminin isoforms. Coincident with these events, up-regulation of integrins, ␣ 5 ␤ 1 , ␣ 3 ␤ 1 , ␣ 6 ␤ 1 , and ␣ 1 ␤ 1 , which bind fibronectin, nidogens, laminin isoforms, and collagen type IV, occurs in EC-pericyte cocultures, but not EC-only cultures. Integrin-blocking antibodies to these receptors, disruption of fibronectin matrix assembly, and small interfering RNA suppression of pericyte tissue inhibitor of metalloproteinase (TIMP) - IntroductionConsiderable interest has focused on determining how support cells such as pericytes affect the vasculature during development and in various disease states. 1-3 An important step in vascular morphogenesis is the recruitment of pericytes, which, in conjunction with endothelial cells (ECs), establish conditions to facilitate tube stabilization. 2,4-11 EC factors such as platelet-derived growth factor-BB play a critical role in these events, and failure to recruit pericytes during development leads to vascular instability and regression. 4,[12][13][14] Thus, abnormalities in EC-pericyte interactions lead to embryonic death due to failures in vascular remodeling and stabilization. 2,11,12 Recently, we reported that pericyte recruitment to EC tubes induced stabilization by affecting the production and function of EC-derived tissue inhibitor of metalloproteinase (TIMP)-2 and pericyte-derived TIMP-3, which led to inhibition of both tube regression and morphogenic events through blockade of particular matrix metalloproteinases (MMPs). 15 The molecular mechanisms controlling how pericytes affect vascular tube stabilization are being elucidated and include the identification of key growth factors regulating these events, such as angiopoietin-1, vascular endothelial growth factor (VEGF), and transforming growth factor-␤ (TGF-␤), signaling pathways involving Notch and Ephrins, as well as the presentation of MMP inhibitors such as [7][8][9]11,[15][16][17][18][19][20][21][22][23] Recent work from our laboratory has identified a key regulatory step in vessel formation, which is a requirement for membrane type 1 (MT1)-MMP in both EC lumen and vascular guidance tunnel formation. 24 Vascular guidance tunnels are generated in conjunction with EC tube morphogenesis and represent physical spaces throughout the matrix that serve as conduits for tube assembly, remodeling, and recruitment of other cell types such as pericy...

Recently, we reported a novel system whereby human pericytes are recruited to endothelial cell (EC)-lined tubes in 3-dimensional (3D) extracellular matrices to stimulate vascular maturation including basement membrane matrix assembly. Through the use of this serum-free, defined system, we demonstrate that pericyte motility within 3D collagen matrices is dependent on the copresence of ECs.