Adding complexity to the system: endothelial and endothelial-derived molecules promote platelet collection. (A) The silk microtube lumen supports a confluent monolayer of human endothelial cells. (Bi) Confocal microscopy images of confluent HMVEC-d into the silk microtube lumen (green = VE-cadherin; scale bar = 100 µM). (Bii) Magnification of HMVEC-d seeded into silk microtube lumen (green = VE-cadherin; blue = nuclei; scale bar = 50 µM). (C) Statistical analysis of collected platelets after perfusion of silk microtubes in presence of endothelium or functionalized with VEGF and VCAM-1 with respect to silk microtube functionalized with ECM components only (average ± SD, n = 5, *P < .05).