The dental pulp is a highly vascularized and innervated loose connective tissue surrounded by hard dental tissues - enamel and dentine. With the primary dentin formation and the closure of the root apex, the conditions in the dental pulp change and pulp tissue compliance are reduced. Endothelial cells of pulpal blood vessels are highly differentiated and are capable of adaptation to changes in the environment. We aimed to evaluate the phenotypic plasticity of endothelial cells of pulpal blood vessels in permanent premolars with open (N = 6) or closed root apex (N = 30). The pulp tissue was stained with hematoxylin-eosin (HE) for the histological analysis, and immunohistochemically for a cluster of differentiation 31 (CD31), a cluster of differentiation 34 (CD34), and for smooth muscle actin (SMA) to detect vessels with CD31, CD34, and SMA positive endothelial cells. We used the student's t-test and Pearson correlation test for the statistical analysis. We found a significantly higher percentage of the vessels with CD31 (P = 0.005) and CD34 (P = 0.000) positive endothelial cells in the group of teeth with closed root apex compared to the group with open apex. A significant positive correlation between SMA positive and CD31 positive vessels (P = 0.003) and between CD31 positive and CD34 positive vessels (P = 0.031) was also found. We conclude that the endothelial cells of dental pulp vessels express a small amount of CD31, but have a pronounced expression of SMA and CD34, which indicates their progenitor potential and contractile ability.