Studies on intra-arterial cushions. I. Morphology of the cushions at the origins of intercostal arteries in mice.

The origins of intercostal arteries in mice are surrounded by cushions which project into the aortal lumen. A boat-like structure, whose bow is directed against the blood stream, is formed by the cushions. The shape of this formation depends on the dilatation of the aortal wall, becoming flatter when the aortal wall is stretched. Grooves direct the blood into the initial segment of the branching artery which is narrowed in moderately distended or relaxed states of the aorta. The cushions are covered by an endothelial layer which continues to the endothelium of the aorta and of the branching artery. Ramified smooth muscle cells are found in the stroma of the cushions, while connective tissue are absent. Ramifications of the lamina elastica interna project into the stroma and faint elastic fibres reach the apical margins of the cushions. Basement membranes of ramified smooth muscle cells form a three-dimensional network which is connected with the basement membrane of the endothelium. Contacts between processes of endothelial cells and those of ramified smoot; muscle cells occur. Collagen fibrols are absent or extremely rare. In the interstitial space clusters of electron dense granules (mean diameter 500 A) are found which might be secreted by the specialized ramified smooth muscle cells. This peculiar morphology characterizes the intraaortal cushions as autonomous formations and excludes their being regarded as simple retroextension of the branching artery.