[Vascular endothelium (review). I. General morphology. 2B: phylogenesis of the vascular endothelium]

The phylogenetic descent of vascular endothelium from mesenchyme--derived precursors is described related to the development of a vessel--bound microcirculation. Endothelial precursors in primitive animals may have migrated into tissue clefts gradually forming vascular tubes. True microcirculatory vessels at first appear in the nemertines, a closed vascular system is present in some annelids whereas in arthropods an open lacunar system predominates. The first appearance of true endotheliocytes is under discussion; the author gives some evidence that it is present already in some annelids. Precursor of the endothelial wall of vessels may be the so called "Leydig's membrane", covered with amoebocytes and other mesenchymal cells. The molluscs exhibit many variants of endothelium. In the fishes, the vascular system begins to split into a blood and a lymphatic system. Obviously the specialization of endothelium correlates with the level of evolution. Despite the complicated course, the evolution of endothelium may be regarded as monophyletic.