STAGE-DEPENDENT SENSITIVITY OF OOCYTES AND EMBRYOS TO LOW-TEMPERATURES

The cryopreservation of mammalian oocytes and embryos has become an in tegral part of assisted reproduction in animals, especially of domesti c species. Embryo cryopreservation of mice and cattle has been remarka ble, tens of thousands of live young having been produced by transfer of frozen-thawed embryos, As methods to produce animals in the laborat ory by in vitro methods from ovarian oocytes have been improved, embry o cryopreservation has become even more important as a logistical tool ; live calves produced from cryopreserved, in vitro-derived embryos ar e evidence of success, Nevertheless, it is also clear that there is su bstantial room to improve cryopreservation, in particular as applied t o oocytes of most species, porcine embryos, and in vitro-derived bovin e embryos. To define the problems more clearly, experiments have been conducted to examine physiological characteristics of oocytes and embr yos that render them especially difficult to cryopreserve.