[Testicular tissue vitrification: evolution or revolution?]

Preservation of reproductive health is a major concern for patient long-term quality of life. While sperm freezing has proven to be effective to preserve fertility after puberty, cryopreservation of immature testicular tissue (ITT) is emerging as a promising approach for fertility preservation in young boys. Slow-freezing (SF) is the conventional method used to preserve ITT and has resulted in the birth of mice offspring. In humans, methods to preserve ITT are still at the research stage. Controlled SF using dimethyl sulfoxide showed preservation of proliferative spermatogonia after thawing in a xenotransplantation model used to evaluate the efficiency of freezing and thawing procedures. However, spermatogonial recovery was low and normal differentiation could not be achieved. Both freezing/thawing and the environment of the xenotransplantation model may be implicated. Indeed, with SF, ice crystal formation could damage tissue and cells. For this reason, vitrification, leading to solidification of a liquid without crystallization, may be a promising alternative. ITT vitrification has been investigated in different species and shown spermatogonial survival and differentiation to the round or elongated spermatids stage. Offspring were also recently obtained after vitrification and allotransplantation in avians, confirming the potential of vitrification for fertility preservation. In humans, vitrification appears to be as efficient as SF in terms of spermatogonial survival and initiation of differentiation after xenotransplantation. However, before validation of such fertility preservation methods, completion of normal spermatogenesis and the fertilization capacity of sperm retrieved from cryopreserved and transplanted tissue should be fully investigated.