The principal variables of cryopreservation: solutions, temperatures, and rate changes.

OBJECTIVE: To describe several fundamental variables that influence ultimate survival of oocytes and embryos when they are cryopreserved.
DESIGN: The literature describing fundamental and applied aspects of cryobiological variables that determine the responses of oocytes and embryos has been reviewed. CONCLUSION(S): When oocytes and embryos are to be cryopreserved, they are suspended in a solution of one of several low-molecular-weight solutes. The permeability of oocytes and embryos to these various low-molecular-weight compounds differs. These differences determine how these compounds are taken up by cells. That, in turn, influences how these compounds act to protect cells against damage when the cells are subjected to cryopreservation. Because of those protective effects, the compounds are referred to as cryoprotective additives. Another principal variable that influences oocyte and embryo survival is the rate at which the cells are cooled to subzero temperatures. After being stored for some time at -196°C in liquid nitrogen, the cryopreserved oocytes and embryos are warmed to liquefy the medium. The rate at which the specimens are warmed is at least as important, if not more important, in determining the ultimate survival of the oocytes and embryos. The effects of these physical variables on cell survival also are described.