Table 2.

Summary of Changes Seen in Aging and Myxomatous Disease

Property	Age Related Changes	Disease Related Changes
Mechanical behavior and gross anatomy	Reduced extensibility and increased elastic modulus Thickening of leaflet cusps	Increased extensibility and reduced strength Both: Leaflets become opaque and thickened (especially along coaptation edge) Dogs: Greyish-white, smooth glistening nodules / plaques on atrial surface of leaflets and in rough zone Humans: Larger region affected throughout leaflets and chordae
Layers (microstructure)	Thickening of fibrosa layer Layers more delineated	Disruption of atrialis layer, expansion of spongiosa layer and destruction of fibrosa layer
Cells	VICs more activated (remodeling phenotype) Decrease in cell density	VICs and myofibroblasts activated phenotype Cell density (DNA concentration) unaltered
Collagen and MMPs	Increased production and remodeling Reduced crimp	Mbr1>Loss of fiber bundle organization but increase in immature collagen MMP-2 and -9 increased MMP-2 decreased MMP-14, TIMP-2 and -3 increased with severity and age
Elastin	Decreases Increase in collagen reinforcement of elastic fibers	Increase in number but appear more granular
PGs and GAGs	Increased abundance in fibrosa layer Decorin and biglycan greater expression Altered expression of certain sulfated GAGs Increase of chondroitin sulfate / dermatan sulfate ratio	Accumulate in leaflets and chordae