Abstract
Vitrification is now the main route to the cryopreservation of human and animal oocytes and preimplantation embryos. A central belief is that for success, the cells must be placed in very high concentrations of cryoprotective solutes and must be cooled extremely rapidly. We have shown recently that these beliefs are incorrect. Over 90% of mouse oocytes and embryos survive being cooled relatively slowly even in solutions containing only 1/3(rd) the normal solute concentrations, provided that they are warmed ultra-rapidly at 10(7)°C/min by a laser pulse. Nearly all vitrification solutions contain both permeating and non-permeating solutes, and an important question is whether the former protect because they permeate the cells and promote intracellular vitrification (as is almost universally believed), or because they osmotically withdraw a large fraction of intracellular water prior to cooling. The answer for the mouse system is clearly the latter. When oocytes or embryos are placed in 1 molal concentrations of the impermeable solute sucrose, they osmotically lose ~85% of their cellular water in less than 2 minutes. If the cells are then cooled rapidly to -196°C, nearly 90% remain viable after warming, again provided that the warming is ultra rapid.