Cryopreservation of semen from domestic livestock.

Fifty years after the first successful cryopreservation of spermatozoa, the technique is an integral part of the cattle breeding industry but has failed to establish itself commercially in the production of other breeds of domestic livestock. New assessment techniques have shown that the ejaculate consists of a heterogeneous population of cells, which achieve their full fertility potential at different rates within the female tract and thus maximize the chances of a fertile spermatozoon successfully combining with an egg. It is becoming apparent that the freeze-thaw process results in a more homogeneous cell population, which may be functionally compromised. One aspect of sperm function that has been demonstrated to be affected by cryopreservation is the process of capacitation. Chlortetracycline staining has shown that frozen-thawed spermatozoa undergo an accelerated 'capacitation-like' process which has implications for their interaction with the female tract, ability to establish sperm reservoirs in vivo and hence for their life expectancy after insemination. In addition to heterogeneity within the ejaculate, there is increasing evidence for variation between individuals in the success of sperm freezing. Post-thaw sperm survival may be consistently poor for certain individual animals even though pre-freeze parameters appear normal. The mechanisms that may underlie such differences in cryosensitivity remain unclear. A greater role for the use of frozen semen in livestock production can come only from an improvement in the preservation of the functional competence of the cryopreserved spermatozoon after insemination into the female tract.