Colony-forming units. (a) Endothelial cell colony-forming unit. Non-adherent mononuclear cells grown on fibronectin form colonies after 4 to 9 days. (b) 'Late outgrowth' colony-forming unit. Adherent mononuclear cells grown on collagen I display a cobblestone morphology after 7 to 21 days. Magnifications: 40× (a) and 20× (b). Figure b provided courtesy of Kate Williamson.