Appearance and evolution of the endothelial cell

Once upon a time there was no circulatory system but the need for it arose with the development of multicellular organisms. In the animal kingdom, the vascular system first appeared around 600 million years ago. The endothelial cells evolved later, between 540–510 million year ago, when Vertebrates, the largest subphylum of the Cordate phylum, is believed to have separated from the other two Cordate subphyla, the Urochordata and Cephalochordata. At the same time of the appearance of the endothelium, endothelial heterogeneity also developed as supported by the fact that in the Hagfish, the oldest living vertebrates, the phenotype of the endothelial cells differs trough the body, from organ to organ. An endothelial cell is designated as one covering the luminal space of the vessels, forming a continuous layer having a basal-luminal polarity and these cells are kept adherent to each other, and to the basal membrane, by specialized junctional complexes.