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Ann Thorac Surg 2007;83:1921-1922
© 2007 The Society of Thoracic Surgeons

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^a Cardiovascular Research Laboratory, University of California Los Angeles, David Geffen School of Medicine, 675 Charles E. Young Dr S, MRL 3-579, Los Angeles, CA 90095-1760
^b Department of Medical Physics and Biophysics, Charité University Hospital and Department of Cardiac Surgery, Heart Centre Brandenburg, Bernau, Berlin, Germany

(Email: kschenkelayland{at}mednet.ucla.edu; ulrich.stock{at}charite.de).

To the Editor:

We would like to thank Drs Dohmen and Konertz [1] for their interest in our work. As addressed in our article [2], cryopreservation by controlled freezing using cryoprotectant solutions is indeed the primary preservation method for cardiovascular organ banking. However, as shown in our study and several other studies [2–5], this technique does not adequately preserve complex multi-cellular tissues, such as heart valves or blood vessel, and it has a number of limitations. Extracellular matrix (ECM) damage induced by cryopreservation has (in large part) been ascribed to the phenomenon of interstitial ice formation, predisposing valvular grafts to calcification in vivo; therefore causing early structural deterioration and dysfunction of cryopreserved allogeneic homografts [6]. The extent of ECM damage due to ice crystal formation depends on the amount of free water in the tissue and the ability of that water to crystallize during freezing [5, 6]. In our study we showed direct ECM structure damage using high-resolution, nondestructive multiphoton-induced autofluorescence and second harmonic generation (SHG) imaging rather than ice crystal formation, which can only be visualized using cryosubstitution techniques [3, 5]. To study the impact of conventional cryopreservation on the ECM structures of valvular tissues we used slaughterhouse valves. We agree that care has to be taken when handling those tissues; therefore in our study the cold ischemic time during transport on ice did not exceed 45 minutes, and the warm ischemic time during tissue processing was less than 30 minutes. As further outlined in our article, we processed all valves according to cryopreservation protocols established for clinical use, which includes that prior to processing of the valve tissues, all ice had to be visibly absent. Therefore thawing fractures, due to early manipulation before ice had completely melted, can be excluded as the cause of artifactual ECM damage.

The evolving fields of regenerative medicine and tissue engineering for living cell-based therapies embody a wide variety of enabling technologies that include the need for effective methods of long-term preservation. Even though conventional cryopreservation remains the method of choice for long-term storage of cardiac valves, it has clear limitations and therefore it cannot be considered the "gold standard." For the best possible preservation of cardiovascular tissues and organs, it is crucial to address the current issues including ECM damage, and improvements need to be made.

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1. Dohmen PM, Konertz W. Can cryopreservation destroy the extracellular matrix of pulmonary allografts?(letter) Ann Thorac Surg 2007;83:1921.[Free Full Text]
2. Schenke-Layland K, Madershahian N, Riemann I, et al. Impact of cryopreservation on extracellular matrix structures of heart valve leaflets Ann Thorac Surg 2006;81:918-927.[Abstract/Free Full Text]
3. Brockbank KG, Taylor MJ. Tissue preservationIn: Baust JG, Baust JM, editors. Advances in biopreservation. Boca Raton, FL: CRC Press/Taylor & Francis Group; 2006. pp. 157-192.
4. Thakrar RR, Patel VP, Hamilton G, Fuller BJ, Seifalian AM. Vitreous cryopreservation maintains the viscoelastic property of human vascular grafts FASEB J 2006;20:874-881.[Abstract/Free Full Text]
5. Dahl SL, Chen Z, Solan AK, Brockbank KG, Niklason LE, Song YC. Feasibility of vitrification as a storage method for tissue-engineered blood vessels Tissue Eng 2006;12:291-300.[Medline]
6. Brockbank KG, Song YC. Mechanisms of bioprosthetic heart valve calcification Transplantation 2003;75:1133-1135.[Medline]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Ulrich A. Stock Permission Requests Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Schenke-Layland, K. Articles by Stock, U. A. Search for Related Content PubMed Articles by Schenke-Layland, K. Articles by Stock, U. A. Related Collections Related Article