Calcific degeneration of pericardial valvular xenografts implanted subcutaneously in rats

Tissue valve dystrophic calcification, resulting in hemodynamic regurgitation and/or stenosis, is the most serious complication when bioprosthetic heart valves are concerned. The objective of this investigation was to define morphologically the sequential development of calcific deposits in pericardial tissue cusps of unimplanted cardiac bioprostheses implanted subcutaneously in rats. Small samples of pericardium were implanted subcutaneously in the dorsal area of young rats weighing 60-80 g. The animals were selected randomly and sacrificed at days 1, 2, 7, 14 and 28 after implantation. The specimens were retrieved and studied morphologically. Calcific deposits were seen grossly as small punctate white masses from day 7 after implantation, progressively becoming more extensive. The light and electron microscopic studies showed that: calcific deposits occurred as early as 24 hours after implantation, the calcific degenerative process was progressive with time, and the mineralization was diffuse, although irregular in degree. The ultrastructural findings revealed that cytoplasmic organelles (mitochondria in particular) and plasma membrane of connective tissue cells appear to serve as initial sites of the process of calcification, which then progresses in the interfibrillar spaces, adjacent to collagen fibrils and elastic fibers.