Recent developments and concepts in the cryopreservation of spermatozoa and the assessment of their post-thawing function.

New research on the cooling and cryopreservation of mammalian spermatozoa is reviewed in the context of the older literature. Cryoinjury to a variety of cell organelles is regarded as being due to the two major stresses of cryopreservation, i.e., the change in temperature, and the formation and dissolution of ice and its consequences. Since the cryopreservation process involves departure of the cells from and return to body temperature, both cold shock and warm shock are included as potential stresses to be considered, as well as the stages involving cooling below the freezing point of the medium. The causes of cryoinjury are reconsidered and new concepts concerning the influence of osmotic stress are presented. Heterogeneity of the sperm population is discussed in the context of the success with which spermatozoa can be cryopreserved between and within ejaculates and individuals. The functional state of frozen and thawed spermatozoa is examined on the basis of published results of structural and functional tests of sperm competence. The hypothesis is advanced that cryopreserved mammalian spermatozoa are in a state resembling capacitation, which accounts for their relatively reduced longevity and their readiness to undergo egg penetration without incubation. The importance of this to the utilization of cryopreserved spermatozoa is examined, and proposals are made for new avenues of research to overcome these problems.