DEVELOPMENT INTO BLASTOCYSTS OF BOVINE OOCYTES CRYOPRESERVED BY ULTRA-RAPID COOLING

The objective of the research described was to devise an efficient pro cedure to cryopreserve in vitro-matured bovine oocytes, using in vitro fertilization (IVF) and development of resultant zygotes into blastoc ysts as criteria of oocyte survival. Oocytes at metaphase II were foun d to be extremely sensitive to chilling, Cooling them to 0 degrees C f or as little as 5 sec significantly decreased their capability to clea ve and develop further after IVF; after 80 sec at 0 degrees C, only si milar to 10% of chilled oocytes developed into blastocysts, Oocytes we re also adversely affected by brief exposures to 4 M and 5.5 M ethylen e glycol (EG) solutions supplemented with sucrose; after being suspend ed in either of these EG solutions in plastic straws and plunged direc tly into liquid nitrogen (LN(2)), few of the oocytes were fertilized a nd developed. To ''outrace'' chilling injury, oocytes contained in < 1 mu l of EG solution were placed onto electron microscope grids and pl unged directly into N-2 slush or LN(2). After such ultra-rapidly coole d oocytes were warmed, 30% of them cleaved after IVF, and half of thes e developed into blastocysts-survival rates equivalent to those for oo cytes that had been exposed to EG without any cooling, This method off ers promise as a novel way to cryopreserve bovine oocytes.