Frozen testicular tissue deemed viable after two decades

Friday, 13 May, 2022

Frozen testicular tissue deemed viable after two decades

Male testis tissue that is cryopreserved can be reimplanted after more than 20 years and will go on to make viable sperm, according to a new study in rodents. The study results may have important implications for treatment of boys with cancer, for whom chemotherapy may be preceded by harvesting and freezing of testicular tissue for eventual reimplantation.

The rate of survival for childhood cancers has increased dramatically in the last several decades, but a serious side effect of treatment is diminished fertility later in life. A potential treatment would be to harvest, freeze and reimplant testicular tissue, which contains stem cells — a procedure which has been shown in a macaque model to restore fertility after short-term freezing.

For pre-pubertal boys with cancer, reimplantation may not be feasible for a decade or more after harvesting, which raised the question of how long frozen spermatogenic stem cells (SSCs) can remain viable. This question was explored by researchers at the University of Pennsylvania, with their results published in the journal PLOS Biology.

The study authors thawed rat SSCs that had been cryopreserved in their laboratory for more than 23 years and implanted them in so-called nude mice, which lack an immune response that would otherwise reject the foreign tissue. They compared the ability of the long-frozen SSCs to generate viable sperm to SSCs frozen for only a few months and to freshly harvested SSCs, all from a single rat colony maintained over several decades.

The authors found that the long-frozen SSCs were able to colonise the mouse testis and generate all of the necessary cell types for successful sperm production, but not as robustly as SSCs from either of the more recently harvested tissue samples. While the long-frozen SSCs had similar profiles of gene expression changes compared to the other samples, they made fewer elongating spermatids, which go on to form swimming sperm.

These results have several implications. First, they point out the importance of in situ testing of SSC viability, rather than relying on biochemical or cellular biomarkers, in determining the potential of cryopreserved cells, which may not reflect the actual loss of stem cell potential over time. Second, while there currently are no protocols that can expand human SSCs for reimplantation — a requirement for clinical development of this treatment — such protocols may need to consider time-dependent degradation of viability. Finally, viability is by no means lost during long-term cryopreservation, suggesting that it may be possible to identify and mitigate the key drivers of loss of viability, in order to improve the reproductive options of boys whose childhood cancers are successfully treated.

“Our study showed that rat spermatogonial stem cells can be successfully frozen for over 20 years, transplanted into an infertile recipient animal and regenerate the ability to produce sperm, albeit at a reduced rate,” said senior research investigator Eoin Whelan. “This could provide a method to recover the loss of fertility in pre-pubertal boys treated for cancer.”

Image caption: Cross-section of infertile mouse testis showing previously frozen transplanted rat germ cells and sperm. Image credit: Eoin Whelan, Whelan et al., 2022, PLOS Biology, CC-BY 4.0

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