Absence of endothelium in invertebrate blood vessels: significance of endothelium and sympathetic nerve/medial smooth muscle in the vertebrate vascular system.

In the course of evolution, two remarkable changes seem to have occurred in vertebrate circulation: the appearance and development of the "endothelium or endothelial tubular system" and "sympathetic nerve/medial smooth muscle system". In the present article, some relevant literature is reviewed and discussed. Absence of endothelium in the vascular wall of most invertebrates had been known and was confirmed by recent electron microscopic studies. The medial smooth muscle is rather proper to vertebrate vessels. It seems to have appeared after emergence of and in association with the endothelial tubular system. Phylogenetically, the parasympathetic nervous system is thought to be older than the sympathetic system. The former is distributed to viscera and the latter developed in close relation with the vascular system. It is assumed that during evolution, a circulatory system composed of the heart and endothelial tubular system first formed in vertebrates, medial smooth muscle then appeared for regulation of the system, and innervation of the muscle tissue took place. This sequence of development assumed for phylogenesis is actually realized in the ontogenetic processes. We thus propose a hypothesis that the "sympathetic nerve/medial smooth muscle system" may be regarded as a new neuroeffector mechanism that developed for systemic regulation of the endothelium-lined closed vascular system in vertebrates. A few implications of the hypothesis are presented.