The effect of cryopreservation on functional correlates of embryo integrity.

To assess possible functional effects of the procedures involved in freezing/thawing of mammalian embryos, uptake of 3H-glycine and 125I-BSA (bovine serum albumin) was measured in mouse two-cell embryos and morulae before and after freezing. None of the procedures of dehydration, rehydration, or freezing affected glycine uptake significantly. Earlier results for uptake of 125I-BSA by fresh embryos which indicated an increase in endocytic activity between morula and blastocyst stages were confirmed, and hydrolysis of endocytosed 125I-BSA by morulae but not two-cell embryos was observed. Only 53 +/- 9% of two-cell embryos but 83 +/- 7% of morulae were apparently normal after freezing and thawing. Apparently normal frozen/thawed two-cell embryos were twice as active in 125I-BSA uptake as controls. This was not the case for morulae. The results suggest that freezing/thawing effects endocytotic processes in two-cell embryos but not morulae.