Cryopreservation of human embryos.

Studies on the cryopreservation of ninety-seven 4-cell and 8-cell human embryos indicate that morphologic survival can be achieved by means of two different cryoprotectants and two different freezing procedures. To date, pregnancies can be achieved after freezing and thawing of 8-cell human embryos cooled at 0.3 degree C per minute to -80 degrees C in the presence of 1.5 molar dimethyl sulfoxide (DMSO) and thawed at +8 degrees C per minute from -80 degrees C to +4 degrees C. By this procedure, 27 of 47 (57%) embryos frozen survived with 50% or more of their blastomeres intact. The transfer of these 27 embryos to 22 patients resulted in five pregnancies (22%). Morphologic survival of 4-cell and 8-cell human embryos after freezing and thawing is not affected by slight irregularities 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.