Figure 3. Schematic representation of changes to the collagen network in HHD.

In the normal heart, thin layers of perimysium and endomysium surround myocardial bundles and myocytes, respectively. The walls of the blood vessels also contain adventitial fibroblasts that create an endomysial network. In HHD, there is hypertrophy of cardiomyocytes and transition of fibroblasts to myofibroblasts. These changes are associated in early disease with increases in ECM manifest by perivascular fibrosis and fibrosis of the endomysium and perimysium.