Allograft heart valves: The role of apoptosis-mediated cell loss

Objective: The purpose of this study was to determine whether apoptosis of endothelial and connective tissue cells is responsible for the loss of cellularity observed in implanted aortic allograft valves. Methods: Fresh (n = 6) and cryopreserved (n = 4) aortic allograft valves were retrieved at 2 days to 20 weeks after implantation in an ovine model. Sections of these valves were studied with the use of histologic and electron microscopic methods, nick end-labeling and dual immunostaining for factor VIII-related antigen and proliferating cell nuclear antigen, followed by counterstaining for DNA and laser scanning confocal fluorescence microscopic observation. Results: The endothelial cells and cusp connective tissue cells of implanted valvular allografts showed loss of proliferating cell nuclear antigen (indicative of cessation of mitotic activity) and evidence of apoptosis (nick end labeling). The latter was manifested by nuclear condensation and pyknosis, positive nick end labeling, and formation of intra- and extracellular apoptotic bodies derived from the fragmentation of apoptotic cells, These changes began to develop at 2 days after implantation, peaking at 10 to 14 days, and became complete by 20 weeks, at which time the valves had the typical acellular morphologic features of allografts implanted for long periods of time. Conclusions: Apoptosis occurs in endothelial cells and cuspal connective tissue cells of implanted allografts and appears to be a cause of their loss of cellularity. This apoptosis may be related to various factors, including immunologic and chemical injury, and hypoxia during valve processing and reperfusion injury at the time of implantation.