Ultrarapid freezing (URF) of mouse and human preembryos: a modified approach

Ultrarapid freezing (URF) employs a short one-step exposure of preembryos to cryoprotectant followed by immersion into liquid nitrogen. Higher concentrations of dimethyl sulfoxide (DMSO) have been associated with improved embryo survival. We compared two different concentrations (3.5 and 4.5 M) of DMSO for freezing of mouse preembryos. The postthaw survival for mouse preembryos was 88% (3.5 M DMSO) and 93% (4.5 M DMSO), respectively (ns). Blastocyst formation occurred in 88% of the surviving URF preembryos. We also used URF to freeze a small series of human preembryos. Four of the thirty-eight patients became pregnant following transfer of URF preembryos. Two were term pregnancies and two are ongoing. URF is simpler and less expensive than other methods and its use with human preembryos warrants further evaluation.