The functional integrity and fate of cryopreserved ram spermatozoa in the female tract.

Cryopreservation advances capacitation-like changes in ram spermatozoa. These changes are reflected in an increased fertilizing ability compared with fresh spermatozoa, followed by an accelerated decline in fertilizing ability after incubation in vitro or in vivo. Furthermore, frozen-thawed spermatozoa are released earlier than fresh spermatozoa after binding to oviduct cells in vitro, confirming their physiological readiness to participate in fertilization despite their short lifespan. After insemination large numbers of spermatozoa are lost from the female reproductive tract of the ewe via the vagina. Frozen-thawed spermatozoa are expelled faster than fresh spermatozoa. The advanced membrane status of frozen-thawed spermatozoa may provoke their rapid loss and possibly makes them more vulnerable to attack by uterine leucocytes, or by some other mechanism, as a high proportion of spermatozoa lost from the tract are decapitated. The observed destabilization of the membranes of cryopreserved spermatozoa is accompanied by impaired sperm transport, associated with mitochondrial injury, necessitating intrauterine deposition of frozen-thawed semen to obtain satisfactory fertility after artificial insemination. However, the frozen-thawed spermatozoa that can participate in fertilization may contribute to increased embryonic loss by the advancement of cleavage or through a direct effect of cryopreservation on the male genome.