Ultrastructure of the fibrous ring in patient with discrete subaortic stenosis.

Histologic and ultrastructural studies disclosed the presence of five tissue layers in fibrous rings excised at operation from five patients with discrete subaortic stenosis. These layers were: (1) a surface monolayer of endothelial cells; (2) a subendothelial layer rich in acid mucopolysaccharides and basement membrane-like material; (3) a fibroelastic layer containing collagen and small (1 micrometer or less in diameter) elastic fibers; (4) a layer of smooth muscle cells with thickened basement membranes, and (5) a central fibrous layer with large amounts of collagen and small amounts of elastic fibers. The connective tissue layers in the rings frequently were discontinuous. The layered arrangement of these rings resembles that of normal endocardium in the left ventricular outflow tract.