Mammalian fertility preservation through cryobiology: value of classical comparative studies and the need for new preservation options

Abstract

Human-related fertility preservation strategies have enormous potential for helping sustain and protect other species, especially to assist managing or ‘rescuing’ the genomes of genetically valuable individuals, including endangered species. However, wider-scale applications are limited by significant physiological variations among species, as well as a lack of fundamental knowledge of basic reproductive traits and cryosensitivity. Systematic and comparative cryopreservation studies (e.g. on membrane biophysical properties and resilience to freezing temperatures) are required to successfully recover gametes and gonadal tissues after thawing and eventually produce healthy offspring. Such data are currently available for humans and a few laboratory and livestock animals, with virtually all other species, including wildlife, having gone unstudied. Interestingly, there also are commonalities among taxa that allow a protocol developed for one species to provide useful information or guidance for another. However, when a rare animal unexpectedly dies there is no time for a prospective understanding of that species’ biophysical traits. Because the odds of success will be much lower in such instances, it is essential that more fundamental studies be directed at more species. But also worthwhile is thinking beyond these systematic characterisations to consider the potential of a ‘universal preservation protocol’ for animal biomaterials.

Biobanking efforts and new advances in male fertility preservation for rare and endangered species

“… 50 However, there appear to be significant species variance in tissue cryosensitivity. For example, we have found that felid testicular tissue survives vitrification better than the same tissue treated the same way from laboratory rodents (based on both structural and functional assays 13 ). Interestingly, testicular tissues from ungulates (deer, gazelles, and antelopes) appear to survive slow freezing better than vitrification.…”

“…There also are many examples of “milestone” births using frozen–thawed spermatozoa 11 12 with the incidence of success completely dependent on having an excellent understanding of the female reproductive physiology in the target species. 4 13 …”

“…As an alternative to mature spermatozoa, testicular tissue banking has become a promising option in vertebrates. 13 Such biomaterials are sources of early stage germ cells that can be grown in vitro or grafted to an appropriate host, both for the purpose of eventually producing mature cells useful for assisted reproduction. Importantly, gametes can be rescued from adult individuals that die unexpectedly or even from prepubertal animals.…”

“…However, substantial basic research is still required, largely because of the complexity of testicular tissue structure and cell heterogeneity, highlighting the similarities and differences among diverse species. 13 Testicular fragments (0.5–1.0 mm 3 ) from different mammal species have been frozen in cryovials using a programmable unit after equilibrating in glycerol (human; 49 ) or dimethyl sulfoxide (DMSO; mouse, hamster and marmoset 50 ). These studies have been deemed successful based on favorable postthaw histology or by measuring resumption of gametogenesis after xenografting.…”

Presence of sucrose in the vitrification solution and exposure for longer periods of time improve post‐warming follicle integrity in cat ovarian tissues

“…In addition to fertility preservation for women, ovarian tissue cryopreservation can be used for conservation of wildlife species through gamete rescue (Comizzoli & Wildt, ). Ovarian tissue cryopreservation by slow freezing has been successfully performed in the domestic cat (Tanpradit, Comizzoli, Srisuwatanasagul, & Chatdarong, ) and in lions (Wiedemann et al., ), while ovarian tissue vitrification has yielded less successful results in the domestic cat (Luvoni et al., ; Tanpradit et al., ).…”