Distribution of extracellular matrix components in normal and degenerated canine tricuspid valve leaflets

The aim of the present study was to investigate the composition and distribution of various extracellular matrix (ECM) components in normal canine tricuspid valves (TVs) and in TVs affected by chronic valvular disease (CVD). The parietal (pTV) and septal (sTV) leaflets of the TVs from 27 dogs were investigated immunohistochemically for expression of collagen types I, III, IV and VI, elastin, laminin, fibronectin and heparan sulphate. Normal pTV consisted mainly of elastin and collagen VI in the atrialis, fibronectin in the thin spongiosa and mixed collagens in the fibrosa. The layered structure was less distinct in sTV, with numerous adipocytes and proteoglycans in the spongiosa and collagen III predominating in the fibrosa. The earliest stages of CVD affecting the pTV were recognized in the spongiosa and progression to advanced disease was characterized by nodular accumulation of proteoglycans within the free edge of the leaflet. These nodular lesions of the pTV contained more fibronectin, elastin and collagens I and VI than those affecting the sTV. These findings contrast with those reported in CVD affecting the mitral valve (MV) in which the early lesions affect the atrialis and advanced disease involves the entire leaflet. The pathogenesis of CVD in TV may involve initial alterations of the tricuspid annulus that lead to early lesions within the spongiosa, resulting in further shear stress and proteoglycan accumulation at the free edge of the pTV.