Effect of stage of development on survival of mouse embryos frozen--thawed rapidly.

Embryos were recovered on Day 4 of pregnancy from superovulated random-bred OF1 Swiss albino mice. They were classified into four categories based on their stage of development: expanding blastocyst, blastocyst, early blastocyst, and compacted morula. They were then cooled at 2 degrees C/min from -7 to -25 degrees C in a freezing medium containing 1.36 M glycerol and 0.25 M sucrose in phosphate-buffered saline (PBS). At -25 degrees C, they were plunged into LN2 and thawed a few hours later in water at 20 degrees C. After washing in PBS, recovered embryos were cultured for 20 to 24 hr and the number of embryos that had developed normally was recorded. The results showed a clear effect of the stage of development on survival. Survival of expanding blastocysts and blastocysts was very low (1.4 and 21.8%, respectively) compared to that of early blastocysts and compacted morulae (69.4 and 73.5%). The more differentiated stage of the blastocyst (two kinds of cells) and the presence of a blastocoelic cavity may explain the differences observed under our cooling conditions. As a further test of viability, 93 blastocysts that had developed in culture for 20 hr from 153 frozen-thawed early blastocysts and compacted morulae (60.8%) were transferred to 8 recipient mice. Seven became pregnant, yielding 38/82 normal live young (46.3%).