eLife assessment
This study provides insights into the developmental origin of endothelial cells found in blood vessels called pial collaterals. The work is important, as collateral capacity can strongly influence the trajectory of outcomes with vascular blockage, and the approaches are novel and overall convincing; however, some mechanistic claims are only partially supported, and collateral characterization is incomplete. Given the clear positive correlation between pial collateral flow and improved stroke outcome, this study will be of interest to vascular biologists and clinicians caring for stroke patients.