Architecture of the media of the arterial vessels in the dog brain: a scanning electron-microscopic study.

The architecture of the media of arterial vessels in dog brain was investigated using scanning electron microscopy. The arrangement and shape of the circularly-oriented smooth muscle cells varied with vessel diameter: The arteries (greater than 100 microns in diameter) had 4-10 layers of spindle-shaped smooth muscle cells; the muscular arterioles (30-100 microns), 2-3 layers of spindle-shaped smooth muscle cells; the terminal arterioles (10-30 microns), a compact layer of spindle-shaped smooth muscle cells with more dominant nodular or rod-like processes and thin lateral processes; and the precapillary arterioles (5-15 microns), a less compact layer of branched smooth muscle cells. Longitudinally-oriented muscles were observed in the medio-adventitial border. The distribution and arrangement of these muscles varied with vessel size: in the large arteries (greater than 300 microns in diameter), at the branching sites only; in the small arteries (100-300 microns), at both the branching and non-branching sites; in the muscular arterioles, at both the branching and non-branching sites in a reticular arrangement with some muscle cells having an "asteroid" appearance; in the terminal arterioles, only "asteroid-like" muscle cells were found at the branching and non-branching sites.