In vitro fertilizing capacity of fresh and cryopreserved human spermatozoa: a comparative study of freezing and thawing procedures.

The capacity of human spermatozoa to penetrate zona-free hamster ova was used to investigate several methods of freezing and thawing sperm. No differences in postthaw sperm viability were obtained after cooling spermatozoa either with a complex egg yolk cryoprotective medium or glycerol. The in vitro fertilizing capacity of postthaw spermatozoa frozen with a moderate freezing rate was similar to that of spermatozoa frozen with a slow freezing rate. Significantly higher in vitro fertilization percentages were found when the spermatozoa were thawed at an environmental temperature of 22 degrees C (mean 51%) or 37 degrees C (mean 52%), as compared with spermatozoa thawed at 4 degrees C (mean 37%). Individual differences of in vitro fertilization percentages are discussed in relation to changes in motility. It is demonstrated that a high prefreeze in vitro fertilization percentage is no guarantee of a high postthaw fertilizing ability.