Scientists have increasingly recognized that the cells lining blood vessels, known as endothelial cells, exhibit considerable diversity in different parts of the body, both in normal and diseased conditions. This diversity influences the architecture, structure, and function of blood vessels. For example, endothelial cells in the brain are sealed by specialized junctions to form the highly selective blood-brain barrier. In contrast, endothelial cells in the liver, kidney, and spleen are discontinuous and have pores to enable the rapid exchange of fluids, particles, and cells. The varied forms that endothelial cells take on suggest they are highly adaptable, indicating that their diversity is a key feature that allows them to perform specific tasks in different tissues. However, the molecular basis for tissue-specific endothelial differentiation and heterogeneity remains largely unknown.