The von Willebrand factor is heterogeneously distributed though out the vasculature and is used as a negative prognostic factor for many solid tumors [26]. Basement membrane F8 expression is widely used as an endothelial marker for the analysis of physiological and pathological angiogenesis [27]. Increased F8 expression is an early sign of activation of the endothelium [26]. Immunohistochemical analysis of CAMs reveals high degree of F8 expression along platelet aggregations cites such as endothelial lining and epithelial membrane surfaces. The high F8 activity on these tissues is consistent with the results of the vascular density analysis and
sprout formation where it was shown that restriction of blood vessel branch point and capillary elongation is due to the high expression of F8 in the CAM. F8 has been found to bind [28] and regulate the internalization of v3 integrin levels in vitro [29]. In addition, F8 regulates Ang-2 release in endothelial cells [29]. In this sense, Ang-2 regulates VEGF related stimulation of VEGF sprouting and migration [30]. Similarly, it was noted [29] that deficient levels of F8 in vivo led to increased angiogenesis and high blood vessel density. Therefore, it is possible that the restriction of blood vessel branch points and capillary elongation as well as the low vascular density obtained in the current study is a consequence to the increase in F8 levels in CAM endothelial cells and epithelial
membranes. Further immunohistochemical analysis of the CAM indicated non-reactivity of the tissues to anti-human EMA antibody. EMA is used for routine diagnosis of tumor angiogenesis and marrow micrometastases [31]. It is suggested then that the possible mechanism of action in the observed angio-suppression in CAMs exposed to different ratios of 1a – 1c is partly due to the role of von Willebrand factor in regulating angiogenesis. Further, a very low concentration of the pure isolates from 1a – 1c mixtures used in the assay compared to its angio-suppressive activity indicates a high impact bioactivity of the substance.