Friis T, Hansen AB, Houen G, Engel AM. Influence of angiogenesis inhibitors on endothelial cell morphology in vitro. APMIS 2006;114:211–24. Human umbilical vein endothelial cells (HUVEC) propagated in co-culture with fibroblasts form capillary-like networks of tubes. Here we characterize the morphology and ultrastructure of HUVEC in such co-cultures and investigate the influence of different angiogenesis inhibitors on endothelial cell morphology. Addition of angiogenesis inhibitors to the co-culture disrupted endothelial network formation and influenced endothelial cell morphology in two distinct ways. Instead of characteristic capillary-like networks, the endothelial cell morphology appeared as either short cords or compact cell clusters of variable size. Electron microscopy (EM) showed that in co-culture untreated HUVEC formed capillary-like tubes with lumina and retained important ultrastructural and physiological properties of endothelial cells in functional vessels as they contained both Weibel-Palade bodies and transport vesicles. Immuno- EM showed that the endothelial cell marker CD 31 stained endothelial membranes at cell-cell contacts, and at the luminal and abluminal side of the capillary-like tubes, although most abundantly at the luminal membranes. No ultrastructural signs of apoptosis were seen in HUVEC in inhibitor-treated co-cultures. Our results demonstrate that treatment with levamisole or anti- VEGF inhibits endothelial cell differentiation into tubes or instead induces formation of compact endothelial cell clusters. Treatment with platelet factor 4, suramin and TNP-470 results in formation of short endothelial cell cords. We discuss the implications of these findings.