Fig 3. Schematic illustration of endothelial cell transitions.

The arrows show the transitions that each endothelial cell phenotype may experience. Phalanx cells (E) may transition to tip cells if the concentration of VEGF is greater than a threshold α_V and is greater than d_ES away from the closest tip cell (T) or to a stalk cell (S) if the distance to an activated tip cell is less than d_SE. Stalk cells may divide after time τ_SP and then have a growing period τ_gs.