Cervical transfer of deep frozen cattle embryos.

Cattle blastocysts were collected from 29 donors 7-8 days after estrus and frozen and stored in liquid nitrogen up to several months. Two procedures were used for freezing and thawing: -- procedure A: slow cooling to -60 degrees C (0.3 degrees C/min to -60 degrees C) and slow thawing (12 degrees C/min); -- procedure B: slow cooling to -30 degrees C (0.3 degrees C/min to -30 degrees C) and rapid thawing in a water bath at 37 degrees C. After thawing, the embryos were cultured from 8 to 12 hours before transfer; 36% of the embryos continued normal development during culture; both procedures resulted in a high pregnancy rate (procedure A: 10/15; procedure B: 11/15) after single cervical transfer of the frozen thawed embryos which developed normally in vitro. However the overall survival rate was low (25%) and varied between donors, indicating that progress must be made before the technique of freezing can be extended to applied conditions.