Deep-freezing and transfer of human embryos.

Studies on the cryopreservation of 162 four-cell and eight-cell human embryos indicate that morphological survival and pregnancies can be achieved by specific techniques of freezing and thawing. Survival rates are highest when embryos are cooled at 0.3 degrees C/min to -80 degrees C in the presence of 1.5 M dimethylsulfoxide (DMSO) and thawed at +8 degrees C/min from -80 to +4 degrees C. Morphological survival of four-cell and eight-cell human embryos after freezing and thawing is not affected by irregularites in blastomere size or the presence of small cytoplasmic fragments. Light and electron microscopic examination of fixed specimens indicates a good correlation between the appearance of frozen-thawed embryos at the dissecting microscope level and the extent of cryoinjury. Sixty-eight of 136 four-cell and eight-cell embryos (50%) survived with half or more of their blastomeres intact when cooled to low temperatures and thawed at the rates described. The transfer of these 68 embryos into 45 patients resulted in nine pregnancies.