Cryovial versus straw for sheep semen cryopreservation: a comparative study of surface area-to-volume ratio on post-thaw viability and in vitro embryo production.

Semen cryopreservation results in decreased viability and fertilizing ability due to temperature variation-mediated cryodamage limiting its practical application. Though many factors play a role in the success of semen cryopreservation, ice crystal formation plays an important role in post-thaw viability because of the position and surface area to semen volume ratio of the container. The present study was to compare the post-thaw cryo-survivability of sheep semen cryopreserved in cryovials with a smaller surface area to semen volume ratio in the vertical position versus straws with a larger ratio in both horizontal and vertical positions. Significantly (p < 0.05) higher post-thaw functional parameters and subsequent embryo production were observed in the semen cryopreserved in cryovials with LN2 vapour than cryopreserved in straws with a bio-freezer and LN2 vapour. The study concluded that higher post-thaw viability of the sperm cryopreserved in cryovials in the vertical position might be due to gravity-supported settling of the sperm at the bottom, creating a small air gap, and resulting in less ice crystal formation and subsequent embryo production in vitro. Whereas the straws in the horizontal position created a large air gap and faster ice crystal formation, and uneven distribution of temperature along the length of the straw in the vertical position leads to significant (p < 0.05) sperm damage for subsequent embryo production.