Cardiac Histology (IX): Atrioventricular valves (Mitral and Tricuspid part 2)

G . Histologic examination of the valve leaflet shows that the relative proportion of the three layers that form the valve vary with the topography. The fibrosa is the most consistent, being found from the base to the free border. The spongiosa is prominent from the base to the middle third. The ventricularis, with abundant elastic fibers, is clearly larger toward the free border. This arrangement is consistent with the need of the valve for elasticity near the annulus and at the insertion site of the strongest of the chordae. Whereas the proteoglycans (green) are abundant in the free border region of the leaflet where they cushion the cyclic trauma of closure of the leaflets. These changes are exaggerated as a function of age, with fibrosis (sclerosis) being the predominant change at the base and myxoid degeneration at the free edge. The chordae tendineae are seen as dense (yellow), collagenous structures in the ventricular side of the leaflet and are continuous with the fibrosa. H. and I. Micrographs of the junction of the basal third and the middle third of the valve show mostly dense collagen bundles in the valves of two aging individuals. (H&E and Masson trichrome 20X)

Besides working cardiac myocytes, conduction system myocytes and valvular tissue there are other cell types which are structural components of the heart and can play different roles in cardiac diseases.