The architecture of the medial smooth muscle of arterial vessels in normal human brains was investigated using scanning electron microscopy. We could divide the arterial vessels into four subdivisions according to the number of the circular muscle cells. The arteries ( \u3e 100 μm in diameter) had 4-20 layers of circular smooth muscle cells; individual circular muscle cells were spindle-shaped and occasionally had branches at their ends. Multidirectional muscle cells were observed in the medio-adventitial border only at the branching sites in large arteries (\u3e300 μm), but at both branching and non-branching sites in the small arteries (100-300 μm). The nonterminal arterioles (30-100 μm) had 2-3 layers of circular muscle cells; most of the circular muscle cells had nodular or rod-like processes at their branched ends. Multidirectional muscle cells were most frequently observed in the medio-adventitial border in this subdivision at both branching and non-branching sites. The terminal arterioles (10-30 μm) had a single layer of circular muscle cells. Multipolar (stellate in appearance) smooth muscle cells were mainly seen in the medio-adventitial border at branching sites. The precapillary arterioles (7-10 μm) had a single layer of branched muscle cells; individual muscle cells had 2-4 circular branches on both sides of the central bulges
