Leukocytes, platelets, and surface microstructure of spontaneously degenerated porcine bioprosthetic valves.

The microstructure of 33 spontaneously degenerated porcine bioprosthetic valves was assessed by scanning electron microscopy in order to gather insight regarding the degenerative process. Twenty-four mitral and 9 aortic valves were removed from 32 patients. The duration of insertion was 7.7 +/- 2.4 years (mean +/- SD). All valves showed denudation of endothelial cells and exposure of the subendothelial fibrous tissue. Fibroblastlike cells were occasionally seen. Platelet deposits were observed on 22 of 33 valves (67%). Leukocytes were observed on the surface of 27 of 33 valves (82%). Mononuclear leukocytes were the most common category of cells (66%). Crystalline material was present on the surface of some leukocytes, suggesting that they may serve as a nidus for calcification. Transmission electron microscopy showed leukocytes in the process of phagocytizing collagen fibers. Macrophages, by exerting their scavenger function seem to contribute to destruction of the collagen framework of the valves. Whether the observed lymphocytes and plasma cells reflect an immunological involvement is unclear.