Figure 4. Increased pericyte coverage protects tumour blood vessels.

Although inhibition of vascular endothelial growth factor signalling pathways can lead to vessel regression, a few ‘normal-appearing’ slim and functional vessels remain; these vessels are densely and tightly covered with pericytes, and are markedly distinct from the vessels that are seen in tumours of untreated animals, which are typically dilated, tortuous and irregularly shaped, and variably covered with less closely associated pericytes. Such coating by pericytes arguably helps the tumour endothelium to survive and function, and thereby enables tumours to grow (perhaps more slowly) during the course of an anti-angiogenic therapeutic regimen.