Asymmetric distribution of dense bands in muscle cells of mammalian arterioles.

Arterioles from viscera of mice, rats, guinea pigs, rabbits, sheep, and pigs were examined by electron microscopy. In all the organs and species investigated the muscle cells of the tunica media are arranged in tight helices and make several full turns. The myofilaments also are arranged approximately circumferentially, although there is often a little divergence between bundles of myofilaments in different areas of the same cell. The dense bands are conspicuous and anchor the myofilaments to the cell membrane; they are markedly larger and more numerous on the regions of the cell membrane facing the adventitia than on those facing the intima. This asymmetrical distribution is interpreted as essential to an efficient mechanism for constriction of the lumen.