X-Message-Number: 27759
From: "Basie" <>
Subject: Tissues are unaffected by the freeze thaw process 
Date: Sat, 25 Mar 2006 13:34:22 -0500

26 APRIL 2002 VOL 296 SCIENCE www.sciencemag.org656fact, studies 
demonstrating that the morpho-logical and biomechanical properties of 
tis-sues are unaffected by the freeze-thaw pro-cess contradict this 
hypothesis (2).Comparisons between freezing and vitrifi-cation are often 
misleading, whereas eithertechnique can fail spectacularly or 
succeedbrilliantly, depending on the exact protocolused for preservation. 
Protocol developmentinvolves the adjustment of a multitude of pro-cess 
parameters, and the optimal processingrequirements are tissue-specific. As 
reportedby Kaiser, researchers at Organ RecoverySystems have achieved an 
important mile-stone in successfully vitrifying rabbit veins.However, their 
claim that "vitrification worksbetter than freezing" is based on the 
rel-ative performances of a fine-tuned vitri-fication protocol and an 
unoptimizedfreezing protocol (3), an inappropriatecomparison.For large 
organs, heat and masstransfer limitations become a signifi-cant obstacle to 
vitrification, which re-quires high cryoprotectant concentra-tions and rapid 
rates of temperaturechange. In contrast, freezing tech-niques use relatively 
dilute cryoprotec-tant solutions and low cooling rates.Moreover, the 
preservation process is assist-ed by ice formation: Ice sequesters 
watermolecules, causing a gradual and relativelyuniform concentration of 
cryoprotectants,even in large specimens. Paradoxically,when tissue is 
frozen, its biological compo-nents are actually vitrified in an 
amorphousmatrix that envelops the crystals.The behavior of tissue during 
freezing ismore complex than during vitrification, andoptimization of 
freezing procedures maytherefore be more challenging. 
Nonetheless,optimization of freezing protocols for cellshas benefited 
greatly from the developmentof mathematical models of the process. Re-cent 
efforts to model tissue freezing may thusultimately improve our ability to 
optimizefreezing procedures for organs (4).JENSO. M. KARLSSONDepartment of 
Mechanical Engineering, Universityof Illinois at Chicago, Room 2057 ERF (M/C 
251),842 West Taylor Street, Chicago, IL 60607-7022,USA. 

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