Ultrastructural substrates of dystrophic calcification in porcine bioprosthetic valve failure

Calcific degeneration is the main cause of porcine bioprosthetic valve failure. This dystrophic phenomenon has been studied by transmission electron microscopy in 26 explants; six normally processed unimplanted xenografts and a pig aortic valve from the slaughterhouse served as controls. Loss of endothelial lining and proteoglycans were a regular finding in all the commercially processed valves. In order to detect initial calcifications, we investigated in particular areas apparently devoid of mineralization at x-ray. Three main ultramicroscopic features were found: 1) intracytoplasmic and interstitial calcospherulae in 22 explants, 2) calcified collagen fibrils in 15, and 3) platelike calcium deposits upon amorphous material in 3. X-ray diffraction and energy-dispersive microanalysis identified Ca2+ deposits as crystals of hydroxyapatite. From these findings there is evidence that debris and membrane fragments of the pig cusp cells represent one of the initial nuclei of calcification.