C-22: The next frontier: Vascularized tissues

Cryopreservation of thin tissues such as small animal ovaries, organ slices, skin, and blood vessels has been studied for many years. As long ago as 1980, three dozen organized tissues could be listed that had been successfully cryopreserved by freezing, including partial successes with some organs. But the cryopreservation of vascularized tissues adds another level of complexity to tissue preservation, and requires consideration of vascular dynamic issues that must be added to other common factors in tissue cryopreservation. Most early studies illustrate well the pitfalls of delivering and removing cryoprotectants by perfusion, but fortunately, much has been learned since then, and it now appears that the preservation of even whole organs will be possible by vitrification in the foreseeable future. Extension of current work on kidneys to hearts and other organs and limbs could become and remain a new frontier for decades to come. In the meantime, there are at least four lessons learned that may be of assistance for the preservation of non-vascularized tissues as well. First, models of optimized tissue treatment with cryoprotectants developed for organs may be more applicable to non-vascularized tissues than to organs. Second, whole organ research has highlighted the importance of, and possible remedies for, chilling injury which is a factor that is rarely considered in conventional tissue freezing or vitirfication protocols. Chilling injury may depend on cell volume and hence on levels of and gradients in tissue CPA concentration. Third, conventional tissue freezing usually ignores the possibility of cryoprotectant toxicity in the frozen state, a problem that may be reduced by using low-toxicity cryoprotectant mixtures of the kind necessary for vitrifying kidneys. Recent studies indicate that even polymeric agents appear to permeate well enough into unperfused tissues to be beneficial. Finally, use of synthetic ice blockers also appears applicable to non-perfused tissues, and may have benefits even during freezing for the prevention of damaging recrystallization.