Immunohistochemistry, flow cytometry and cell culture procedures have
demonstrated the presence of circulating endothelial cells (CECs) and circulating endothelial
progenitors (CEPs) in the blood of vertebrates. CECs and CEPs are currently being investigated
in a variety of diseases as markers of vascular turnover or damage and, also in the case of
CEPs, vasculogenesis. CEPs appear to have a “catalytic” role in different steps of cancer
progression and recurrence after therapy, and there are preclinical and clinical data suggesting
that CEC enumeration might be useful to select and predict clinical response in patients who
are candidates for anti-angiogenic treatments. In some types of cancer, CECs and CEPs might
be one of the possible hidden identities of cancer stem cells. The definition of CEC and CEP
phenotype and the standardization of CEC and CEP enumeration strategies are highly desirable
goals in order to exploit these cells as reliable biomarkers in oncology clinical trials.
