The ultrastructure of mammalian arterioles and precapillary sphincters

Abstract

A combination of techniques was utilized whereby the monolayer of microcirculatory vessels of the fascia of the rabbit medial thigh muscles was studied in vivo and the same site subsequently was fixed in situ for both light and electron microscopy. The microvessels were sectioned longitudinally and the identical segment was analyzed by phase contrast microscopy as well as by low and high magnification electron microscopy. Dilated arterioles with an inner diameter of less than 100 μ, terminal arterioles, and precapillary sphincters were studied with special reference to the organization of the endothelium, the muscular and connective tissue components, and the nervous supply. A system of myoendothelial junctions was discovered which establishes close membranous contacts between endothelium and the media of the terminal arterioles and the precapillary sphincters. The functional implications of such a system is discussed, and it is proposed that it may be part of a receptor mechanism for humoral transmitter substances.