Electron microscopy of muscular arteries; pial vessels of the cat and monkey

Abstract

Pial blood vessels of the cat and monkey are qualitatively similar. The tunica media consists of a pure population of smooth muscle cells, while endothelium is the only cell type found in the tunica intima. Thus the smooth muscle is capable of organizing the connective tissue skeleton that pervades the vessel, and probably is responsible for the precursor substances that contribute to all of the formed extracellular elements. A system of basement membranes invests the cells. A single basement membrane may be shared by adjacent cells. Heavier basement membranes show tendencies to cavitate and split, thus opening up patent connective tissue spaces. Elastin and collagen appear in these spaces. To a limited extent elastin attaches to and blends with basement membranes to form a continuous system. Collagen is never directly associated with either elastin or basement membranes, but forms a separate skeletal framework, permeating the tunica media of the larger vessels. Basement membranes, when suitably stained, have a reticulated appearance as does the diffusely distributed mucopolysaccharide. Elastin apparently is formed in a mucopolysaccharide matrix by the addition of a second material. Following phosphotungstic acid staining, globular units of elastin approximately 200 A in diameter can be discerned. These fuse into a continuum, but their less dense centers remain visible with a spacing of about 200 A from center to center. The muscle cells have particularly dense masses of cytoplasm which apparently serve to anchor groups of myofilaments to the cell surface. Also, the muscle cells have large numbers of pinocytotic vesicles. The smooth muscle of these vessels is without specific innervation.