Tumour neovascularization acquires vessels through a number of processes, including angiogenesis, vasculogenesis, vascular remodelling, intussusception, and possibly vascular mimicry in certain tumours. The end result of the tumour vasculature has been quantified by counting the number of immunohistochemically identified microvessels in areas of maximal vascularity so-called hot spots. Other techniques have been developed, such as Chalkley counting and the use of image analysis systems that are robust and reproducible as well as more objective. Many of the molecular pathways that govern tumour neovascularization have been identified, and many reagents are now available to study these tissue sections. These include angiogenic growth factors and their receptors, cell adhesion molecules, proteases, and markers of activated, proliferating, cytokine-stimulated, or angiogenic vessels, such as CD105. It is also possible to differentiate quiescent from active vessels. Other reagents that can identify proteins involved in microenvironmental influences such as hypoxia have also been generated. Although the histological assessment of tumour vascularity is used mostly in the research context, it may also have clinical applications if appropriate methodology and trained observers perform the studies.
