Ultrasound-assisted cryopreservation of gametes.

Cryobanking of gametes for later use in assisted reproduction may support the preservation of endangered species. Since the application ultrasound during the freezing process is known to alter the structure of ice crystals, we attempt here to reduce the mechanical stress during the freezing process by low-frequency sonification. We used domestic animals (bovine, porcine) as models to determine the highest tolerable ultrasound intensities for sperm. Continuous application of ultrasound during the freezing process significantly increased the survival rate of bull sperm compared to non-sonified control samples frozen under similar conditions. By applying short ultrasonic pulses at sub-zero temperatures, we were able to induce a defined crystallisation process at definite temperatures in contrast to spontaneous crystallisation random temperatures. The survival rate of bull and boar sperm was, however, not significantly dependent on the seeding temperature and could not be improved. Whether the defined ultrasound-assisted seeding is advantageous for larger cells like oocytes, remains to be investigated.