Morphogenesis of calcification in porcine bioprosthesis: insight from high resolution electron microscopic investigation at molecular and atomic resolution.

High resolution electron microscopy (HREM) coupled with energy dispersive X-ray microanalysis were used to investigate the morphogenesis of calcification in 28 explanted porcine bioprosthetic heart valves. Different morphology and compositions of calcific deposits were consistently observed in various valvular structures of all explanted porcine bioprostheses. The most common forms of calcific deposits were present as spherical particles and needle-shaped structures. HREM investigation of spherical particles of calcium phosphate revealed very fine electron-dense dots about 0.3-1.0 nm in diameter which were packed to form sphere-like clusters of about 2.0-3.5 nm in diameter in the particles. HREM observations of the microneedle-shaped structures even as small as 1.9 nm in thickness could identify the lattice fringes with center-to-center spacings of about 0.8-0.9 nm. Energy dispersive X-ray microanalysis defined the components of needle-shaped calcification as calcium and phosphorus in a ratio typical of hydroxyapatite crystals. The calcific deposits in the collagen tissue have been shown to be at discrete sites on or in the collagen fibrils as the spherical particles or homogeneous electron-dense masses. Based on HREM observations of different types of morphogenesis of calcification it is suggested that chemical processes of calcification accompanying the deposition of calcium in the implanted bioprosthetic heart valves are specific to each substrate system. The formation of amorphous calcium phosphate precursors and direct deposition of microcrystalline hydroxyapatites are considered to be the primary mechanisms of calcification occurring in implanted porcine bioprostheses.