Ex vivo model for studying endothelial tip cells: Revisiting the classical aortic-ring assay

A drug undergoes several in silico, in vitro, ex vivo and in vivo assays before entering into the clinical trials. In 2014, it was reported that only 32% of drugs are likely to make it to Phase-3 trials, and overall, only one in 10 drugs makes it to the market. Therefore, enhancing the precision of pre-clinical trial models could reduce the number of failed clinical trials and eventually time and financial burden in health sciences. In order to attempt the above, in the present study, we have shown that aortic ex-plants isolated from different stages of chick embryo and different regions of the aorta (pulmonary and systemic) have differential sprouting potential and response to angiogenesis modulatory drugs. Aorta isolated from HH37 staged chick embryo showed 16% (p < 0.001) and 11% (p < 0.001) increase in the number of tip cells at 72 h of culture compared to that of HH35 and HH29 respectively. The ascending order of the number of tip cells was found as central (Gen II), proximal (Gen I) and distal (Gen III) in a virtual zonal segmentation of endothelial sprouting. The HH37 staged aortas displayed differential responses to pro- and anti-angiogenic drugs like Vascular endothelial growth factor (VEGF), nitric oxide donor (spNO), and bevacizumab (avastin), thalidomide respectively. The human placenta tissue-culture however evinced endothelial sprouting only on day 12, with a gradual decrease in the number of tip cells until 21 days. In summary, this study provides an avant-garde angiogenic model emphasized on tip cells that would enhance the precision to test next-generation angiogenic drugs.