VEGF-mediated vascular fusion in the generation of uniluminal spheroids. Luminized vascular networks present in mouse allantoic-derived tissue spheroids (upper image) fuse when treated with VEGF to form a single central vessel (lower image). Spheroids were immunolabeled with antibodies to PECAM-1 (green) and SMαA (red). See Gentile et al., Developmental Dynamics 237:2918–2925. Developmental Dynamics 237:2918–2925, 2008. © 2008 Wiley-Liss, Inc.