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Existing animal models for screening tumor angiogenic process have various setbacks that necessitate further investigations. In this study, we developed an ex-ovo egg yolk angiogenesis model to screen the angiogenic potency of tumor cells (HeLa and SiHa cell lines). The egg yolk angiogenesis assay was applied to study the nitric oxide (NO) influence on switching from sprouting angiogenesis (SA) to intussusceptive angiogenesis (IA) under tumor microenvironment. Morphological analysis and SA-like or IA-like markers expression were determined during the development of chicken chorioallantoic membrane (CAM) from day 5 to 13. Expression of Notch1, Notch2, EphrinB2, and Tie2 were considered as SA-like while TEM8, CALD1, CXCR4 and HOMX1 were followed as IA-like markers. The HeLa and SiHa cell lines embedded CAM showed an increase in micro and macro blood vessels and vascular size, junction and length which are the pivotal morphological parameters of angiogenesis. Further, the study revealed that HeLa is more aggressive than SiHa in inducing tumor angiogenesis. To determine the NO signaling implication in tumor milieu, NO donor (Spermine NONOate (SPNO)), NOS inhibitor (L-nitro-L-arginine-methyl ester (L-NAME) and VEGF inhibitor (Avastin) were administrated to chick embryo vascular bed with and without HeLa cells. The results demonstrated that HeLa cells promote IA through NO signaling, VEGF and eNOS and it was documented by angiogenic morphological parameters and SA-like or IA-like markers expression. Therefore, our study claims that ex-ovo egg yolk angiogenesis model could be used to study tumor angiogenesis and NO plays a key role in switching of IA under tumor microenvironment., (Copyright © 2018 Elsevier Inc. All rights reserved.)