Cryopreservation of bovine oocytes: current status and recent developments.

The cryopreservation of oocytes of most animal species remains a challenge due to their complex structure. Equilibrium freezing is not satisfactory because oocytes seem to be damaged by exposure for several minutes to temperatures near 0 degrees C. Therefore, cryopreservation of oocytes by vitrification--especially the use of very high cooling rates of oocytes suspended in extremely small volumes of various cryoprotective additive modifications--seems the most appropriate method.