Semen cryopreservation: Successes and persistent problems in farm species

Artificial insemination has arguably been the most important practice contributing to the advancement of animal production. The numerous advantages of artificial insemination are augmented when the semen is cryopreserved and conserved for long periods. Unfortunately, the utility of cryopreserved semen is limited because for most mammals, even cattle, a considerable proportion of sperm loose their fertility during freezing-thawing. In many species, this loss of fertility is substantial, rendering cryopreserved semen impractical for routine use. Our goals are to characterize the nature of the sublethal sperm damage caused by cryopreservation, and to develop a method to improve the functional fertility of thawed semen. This review describes our theory that sperm are prematurely activated by cryopreservation ("cryo-capacitation") and discusses strategies for preventing this damage.