Roles of intracellular ice formation, vitrification of cell water, and recrystallisation of intracellular ice on the survival of mouse embryos and oocytes

Mazur and collaborators began examining the validity of initial views regarding mouse oocyte and embryo vitrification and found that most are partially or fully wrong. First, the relative effects of warming and cooling rates on the survival of mouse oocytes subjected to a vitrification procedure were determined. The high sensitivity to warming rate strongly suggests that the lethality of slow warming is a consequence of either the crystallisation of intracellular glassy water during warming or the recrystallisation during slow warming of small intracellular crystals that had formed during cooling. Warming rates of 107 degrees C min(-1) were achieved in 0.1-mL drops of ethylene glycol-acetamide-Ficoll-sucrose (EAFS) solution plus a small amount of India ink on Cryotops warmed using an infrared laser pulse. Under these conditions, survival rates of 90% were obtained even when mouse oocytes were suspended in 0.3 x EAFS, a concentration that falls in the range that many cells can tolerate. A second important finding was that the survival of oocytes is more dependent on the osmotic withdrawal of much of the intracellular water before vitrification than it is on the penetration of cryoprotective solutes into the cells. Herein we review the roles of internal ice formation, vitrification and recrystallisation. It remains to be seen how widely these findings will be applicable to other types of cells and tissues from other species.