43. Cell-based modeling of mechanical and chemical stress in tissues during cryoprotocols

Abstract

Since Mazur’s work a half century ago, modeling of the biophysical processes of single cell cryopreservation has been successful in providing a theoretical groundwork for cryopreservation induced damage hypotheses. Important contributions have included both biophysical explanations of osmotic damage during CPA equilibration due to too aggressive or overly conservative equilibration strategies and estimation of cell injury due to intracellular ice formation through simple or complex ice-growth models. Follow up work by Mazur, Liebo and Chu presented the archetypical single cell protocol optimization using modeling, and subsequent investigators have for the most part followed in their footsteps with more sophisticated models of mass transport, cell damage and ice growth. Expanding on this work, a number of investigators have modeled unique damage pathways for cryopreserved tissues, raising important and challenging questions while providing steps towards optimal protocols. Models have included spatially dependent heat and mass transport, solidification fronts, and thermal and mechanical stresses, and a few models have combined tissue level transport with cell level damage estimators in sparsely celled articular cartilage and in simple spherically symmetrical tissue types. Most tissue modeling paradigms are cell unaware until modeling is complete: either cells are ignored in a continuum hypothesis, or are overlaid in a one-way model of tissue level transport to cell interaction. This approach precludes modeling a number of important cryobiological endpoints that are well understood at the single cell level including cell-to-cell mass transport, intercellular ice propagation, and strains on intercellular adhesion. As computational capabilities increase, cell- or agent-based multiscale modeling paradigms are becoming more realistic for understanding cell interactions, growth, and death in vivo. In these models cell interactions with their environment including cell–cell adhesive forces, transport, and growth and death through a number of mechanisms can be modeled in conjunction with classical heat and mass transport of critical solutes. Importantly from the cryobiological perspective, this approach allows modeling of cell-level tissue damage due to osmotic volume regulation, cell-to-cell ice propagation, and excitingly, the impacts of accumulated damage due to the cryopreservation process on post implantation survival at the single cell level via modeling of apoptotic, necrotic and other pathways. Here we summarize classical tissue cryopreservation modeling and successful approaches to individual cell damage modeling and optimization, and present our vision for the future of tissue and organ cryopreservation modeling and optimization.