Factors affecting the survival of sheep embryos during-freezing and thawing.

The survival of sheep morulae and early blastocysts after deep freezing has been investigated. With a dimethyl sulphoxide concentration of 1.5 M in a phosphate-buffered saline medium, a freezing rate of 1 degreees C/min down to 120 degees C allowed embryos to survival only if rapid thawing (approximately 360 degrees C/min) was employed, whereas with a freezing rate of 0.3 degrees C/min, survival was obtained with both rapid and slow (10 degrees C or 4 degrees C/min) thawing. With even slower freezing (0.1 degrees C/min over intervals between -30 and -60 degrees C) slow thawing was no longer optional but mandatory. With the latter technique survival rates of the order of 80% were obtained. The results of transfer experiments with frozen-thawed sheep and cattle embryos are briefly presented in an Appendix.