Cryopreservation of testicular tissue from the dog (Canis familiaris) and wild boar (Sus scrofa) by slow freezing and vitrification: Differences in cryoresistance according to cell type.

Sperm cryopreservation is the most common procedure used to establish germplasm banks for endangered species - but sometimes sperm cells cannot be obtained. In such cases, freezing testicular tissue may be the only option. The testes contains germ cells at different stages of differentiation, including spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids, and spermatozoa, among which differences in cryoresistance might be expected. The present work compares the viability and DNA integrity of 'rounded' cells, and of elongated spermatids and spermatozoa, from the dog and wild boar, following the cryopreservation of testicular tissue by slow freezing or vitrification. Cell viability was analyzed by PI/SYBR14 staining, and DNA integrity via the TUNEL technique. For wild boar, no significant differences were seen between the two methods with respect to the percentage of viable cells, nor in the percentage of cells with DNA damage. In the dog, the percentage of viable rounded germ cells (65.0 ± 2.4%) was higher (P < 0.05) after vitrification than after slow freezing (45.1 ± 6.7%). No difference was found between the two methods in terms of the viability of elongated cells. For rounded cells, the percentage of intact DNA was greater (P < 0.05) after vitrification (90.5 ± 2.1%) than after slow freezing (42.6 ± 11.0%), while for elongated spermatids and spermatozoa it was higher (P < 0.05) after slow freezing (66.9 ± 6.1%) than after vitrification (50.7 ± 4.5%). Thus, the response to cryopreservation is cell type-, cryopreservation type-, and species-dependent. Vitrification would appear to be the most appropriate method for preserving dog testicular tissue given the associated high cell viability and low degree of DNA fragmentation, while in wild boar, either method might be used.