Cryopreserved male testicular tissue can be reimplanted after more than 20 years and will continue to produce viable sperm, according to a new rodent study published in the journal Open Access. Biology Plus by Eoin Whelan of the University of Pennsylvania School of Veterinary Medicine and colleagues for publication on May 10. But the long delay comes with lower fertility compared to tissue that is frozen only for a short time. The findings may have important implications for treating boys with cancer, for whom chemotherapy may be preceded by harvesting and freezing testicular tissue for eventual reimplantation.
The survival rate for childhood cancers has increased dramatically in the past several decades, but a serious side effect of treatment is diminished fertility later in life. A potential treatment would be to harvest, freeze and re-implant testicular tissue, which contains stem cells, a procedure recently demonstrated in the macaque model of fertility restoration, at least after short-term freezing.
But for prepubescent boys with cancer, re-implantation may not be possible for a decade or more after harvest, raising the question of how long spermatogenetic stem cells (SSCs) can remain viable. To explore this question, the researchers thawed SSCs from rats that had been cryopreserved in their lab for more than 23 years, and transplanted them into so-called nude mice, which lack an immune response that would reject foreign tissue. They compared the ability of long-frozen SSCs to generate viable sperm to SSCs frozen for only a few months, and freshly harvested SSCs, all from a single mouse colony maintained over several decades.
The authors found that long-frozen CSCs were able to successfully colonize the testicles of mice and generate all of the cell types needed to produce sperm, but not as powerfully as CSCs from any of the recently harvested tissue samples. While SSCs that were immobilized for a long time had similar profiles of gene expression changes compared to the other samples, they made fewer elongated spermatids, which go on to form swimming spermatids.
These findings have several important implications. First, they note the importance of in situ testing for SSC viability, rather than relying on biochemical or cellular biomarkers, in determining the cryopreserved potential of cells, which may not reflect the actual loss of stem cell potential over time. Second, while there are currently no protocols that can expand human SSCs for reimplantation—a requirement for the clinical development of this therapy—such protocols may need to consider time-dependent deterioration of viability, assuming that human SSCs mimic those of mice. Finally, and this is the good news, viability is in no way lost during long-term cryopreservation, suggesting that it may be possible to identify and mitigate key drivers of loss of viability, in order to improve reproductive options for boys whose childhood cancers are treated. successfully.
Whelan adds: “Our study showed that rat sperm stem cells can be successfully frozen for more than 20 years, transplanted into an infertile recipient animal and regenerated the ability to produce sperm, albeit at a low rate. This could provide a way to restore the loss of fertility in boys. before puberty who were treated for cancer.”
Future fertility: Giving hope to men treated for childhood cancer
Sperm reconstitution after more than 20 years of cryopreservation of rat sperm stem cells reveals an important effect on differentiation capacity, Plus Biology (2022). DOI: 10.1371 / journal.pbio.3001618
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