Scanning electron microscopy of surgically excised human mitral valves in patients over 45 years of age.

Leaflets were obtained from 23 diseased mitral valves of patients over 45 years of age removed at the time of prosthetic valve insertion and the topography was studied with scanning electron microscopy. The clinical diagnosis in all cases was probable rheumatic heart disease. Filiform processes (Lambl's excrescences) were seen on 15 of 23 (65%) of these leaflets. Some of these processes appeared to develop when either bundles of collagen or elastic fibers in the connective tissue framework of the cusp were broken. The freed portion of the fiber then served as a nidus for thrombus formation. The endothelium on the surface of the leaflets was consistently altered. Endothelial cells were changed in shape, had microvilli scattered over their exterior, showed discontinuous cell borders and were desquamated, either singly or in groups. Elements of the peripheral blood adhered to the exposed subendothelium. Erythrocytes were usually entrapped in fibrin networks. Single activated platelets and platelet aggregates adhered to the fibers of the exposed subendothelial connective tissue. Surface mineral deposits were composed of granular spherulites, which contained both calcium and phosphorous. This study extends our understanding of the architectural modifications that contribute to the continuing pathologic process of rheumatic mitral valve disease.