Histological and immunohistological study of cryopreserved aortic valve grafts: the possibility of a clinical application for cryopreserved aortic valve xenograft

Cryopreserved tissue engineered mucosa

Single cells suspensions used for grafting in the clinical setting may be reliably cryopreserved by established protocols. However, for tissue engineered constructs which now also get used as grafts in the clinic such established protocols and assays which indicate graft viability and their function as graft do not exist. a) The purpose was to develop a cryoprotocol and an animal model to test the efficacy of tissue engineered to act as graft after cryopreservation. b) Therefore, tissue engineered mucosa grafts consisting of keratinocytes and fibroblasts grown in a collagen sponge were cryopreserved and grafted in the nude rat to test its efficacy to function as mucosa graft. At different points after cryopreservation the mucosa was grafted into the nude rats. Healing of grafts was allowed for one or three weeks. c) Sufficient cells survived the cryopreservation allowing for the development of epithelial-fibroblast tissue in the collagen sponge. After three weeks of healing the formation of mucosa tissue was more complete and more collagen sponge had disappeared. d) The nude rat model is suitable to assess the efficacy of tissue engineered mucosa to function as graft after cryopreservation. The formation of human epithelial-fibroblast tissue in vivo has to be interpreted as proof of principle that the approach of cryopreservation of tissue engineered grafts is working.

Time-related Histopathologic Analyses of Immunologically Untreated Porcine Valved Conduits Implanted in a Porcine-to-Goat Model

This study was performed to evaluate the clinical feasibility of use of immunologically nontreated xenogenic valves, using a porcine-to-goat pulmonary valved conduit implantation model. Porcine pulmonary valve conduits were prepared with no specific immunological treatment and implanted in the right ventricular outflow tract of goats under cardiopulmonary bypass. The goats were assigned at predetermined intervals (1 day, 1 week, and 3, 6, and 12 months) as two animals for each interval. Echocardiographic examinations of the valves were performed before sacrifice. Upon retrieving the xenograft specimens, they were inspected visually and microscopically. Ten of the 12 animals survived the predetermined observation periods. Variable degrees of pulmonary regurgitation were the main findings on echocardiographic evaluations. On gross examination of the explanted specimens, all leaflets, except in one animal that prematurely died, were fairly well preserved. They were slightly shortened but free of thrombosis or vegetation. Aneurysmal dilatations of the anterior wall of the implanted pulmonary artery were observed in one of 12-month-survival animals and in another one of 3-month-survival animals. Microscopically, the three components of implanted xenografts (the pulmonary artery, valve, and infundibulum) were shown to be gradually replaced with host cells in time, while maintaining structural integrity. The nuclei of the donor tissue disappeared through pyknosis and karyolysis. In conclusion, immnunologically untreated xenogenic pulmonary valved conduits can be an alternative potential as valve substitutes with distinctive advantages of providing self-healing potential, despite a few problems observed in the current study such as occurrences of pulmonary regurgitation and sporadic cases of aortic aneurysm.

In vivo recellularization of plain decellularized xenografts with specific cell characterization in the systemic circulation: histological and immunohistochemical study

Decellularized biological scaffolds have been used for the tissue engineering of heart valves with good results in the pulmonary circulation. However, little information is available on the recellularization of plain decellularized biological scaffolds in the systemic circulation. The aim of this study was to determine whether plain decellularized xenografts (PDXs) can recellularize with specific cell characterization in the systemic circulation. The xenogenic aortic valved conduit grafts of rabbits were implanted in the abdominal aorta of dogs after decellularization. The grafts were explanted at 4, 12, or 24 weeks after implantation for histological, immunohistochemical examination, scanning electron microscope, and Western blot analysis. Although the valvular structures were completely lost after implantation, supravalvular conduits showed normal shapes and endothelialization. The PDXs were repopulated with basic vascular cell components in approximate natural proportions without immunological responses. The PDXs had been recellularized with specific cell characterization in the systemic circulation.

Effect of Cryopreservation Techniques on Aortic Valve Glycosaminoglycans

This study was designed to evaluate the effect of cryopreservation on the glycosaminoglycan (GAG) content of the aortic allografts. Twenty-one porcine aortic valves were obtained. Five aortic roots were immediately analyzed without cryopreservation, eight were cryopreserved in closed leaflet position, and eight in open leaflet position. The groups were compared in terms of GAG concentration and subclass proportion in three different zones including the aortic root wall, the commissures, and the leaflets. GAG content at the commissures was significantly lower in the closed leaflet group than in the other groups (P = 0.001). The electrophoretic analysis did not show any significant difference in the zonal distribution of GAG classes between groups. Quantitative analysis in various aortic valve zones suggests that cryopreservation can alter the GAG content. Cryopreservation of the aortic valve in an open leaflet position can preserve the matrix more efficiently and might prolong the durability of the aortic allograft.

Impact of Cryopreservation on Extracellular Matrix Structures of Heart Valve Leaflets

BACKGROUND

Transplantation of cryopreserved allografts represents a well-established valve replacement option. Despite their clinical use for more than 40 years, the integrity of the extracellular matrix (ECM) of these valves after thawing has not been determined. The purpose of this study was to investigate and compare ECM structures of fresh and cryopreserved porcine heart valve leaflets with special emphasis on the condition of collagenous and elastic fibers.

METHODS

Pulmonary valves were excised from unprocessed porcine hearts under sterile conditions. After treatment with antibiotics, the valves were incubated in a cryoprotective solution, cryopreserved stepwise, and stored at -196 degrees C for 1 week. Two groups of heart valves (fresh untreated and thawed cryopreserved [each, n = 8]) were analyzed using biochemical (collagen, elastin, desmosine), histologic (hematoxylin-eosin, Movat-pentachrome, resorcin-fuchsin), and immunohistochemical (antibodies against collagen I, III, IV, and elastin) methods. Near-infrared femtosecond multiphoton laser scanning microscopy and second harmonic generation were used for high-resolution three-dimensional imaging of ECM structures.

RESULTS

Biochemical testing demonstrated similar amounts of collagen and desmosine, but a minor loss of elastin in the cryopreserved specimens. Conventional histology revealed almost comparable cell and ECM formations in fresh and cryopreserved valve leaflets. In contrast, laser-induced autofluorescence imaging showed substantial ultrastructural deterioration and disintegration of most collagenous structures. Second harmonic generation was not inducible.

CONCLUSIONS

Conventional cryopreservation of heart valves is accompanied by serious alterations and destruction of leaflet ECM structures, specifically demonstrated by multiphoton imaging. Further in-depth studies to clarify the impact of alternative cryopreservation techniques proposed for clinical use, such as vitrification, are crucial.