Ovarian tissue cryopreservation feasible in Hong Kong

Clinical implementation of ovarian tissue cryopreservation and subsequent transplantation via xenografts is feasible in Hong Kong, according to researchers from the Chinese University of Hong Kong.

Ovarian tissue cryopreservation and subsequent transplantation, though initially considered to be experimental, represent the only fertility preservation (FP) option for prepubertal girls. Since the American Society for Reproductive Medicine removed their experimental designation in 2019, these procedures have gained acceptance worldwide, including in Belgium, Denmark, Spain, France, Japan, Singapore, the US, India, Australia, the Philippines, South Korea, and some parts of China, with >130 births reported from women who have undergone the procedures. “However, these FP technologies remain unavailable in Hong Kong,” noted the researchers. [J Assist Reprod Genet 2018;35:593-600; Hong Kong Med J 2023;doi.org/10.12809/hkmj2210220]

To examine the feasibility of ovarian tissue cryopreservation and transplantation in Hong Kong via xenografts in nude mice, a pilot study was conducted between July 2019 and December 2021 at the Prince of Wales Hospital. Fifty-two ovarian tissues were collected from 12 patients aged 29–41 years during ovarian cystectomy or salpingo-oophorectomy. The collected tissues were cut into several pieces, followed by either slow freezing or vitrification (cryopreservation) or no freezing (controls). After thawing, the human ovarian tissues were engrafted into 34 mice (nonovariectomized mice, n=9; ovariectomized nude mice with gonadotrophin injection, n=25) to investigate whether freezing and thawing cause damage to ovarian tissue.

All xenografts were successfully retrieved from the experimental mice, except for two calcified tissues in ovariectomized nude mice. “Immunohistochemical staining showed that all retrieved grafts maintained viability, with the exception of two calcified tissues,” reported the researchers. Microscopic findings consistently showed viability, as evidenced by the presence of viable nuclei and the absence of necrosis, apoptosis, and pyknotic nuclei.

Haematoxylin and eosin, cluster of differentiation 10 (CD 10), and oestrogen receptor staining results in stromal tissue showed no between-group differences (controls vs slow-frozen, fresh vs vitrified, and slow-frozen vs controls), indicating that both slow freezing and vitrification are effective and feasible. “[Moreover], primordial follicles were observed in thawed and grafted ovarian tissues, indicating that both the cryopreservation and transplantation protocols were effective. The results were unaffected by gonadotrophin stimulation,” they added.

“To our knowledge, this is the first study to demonstrate the feasibility of ovarian tissue cryopreservation and transplantation in Hong Kong,” concluded the researchers. “Our findings support the clinical implementation of these technologies at medical centres in Hong Kong.”

Ovarian tissue cryopreservation and subsequent transplantation are a promising option for FP in gynaecology patients and oncology patients undergoing gonadotoxic treatments. Unlike oocyte freezing, it allows natural conception and preservation of hundreds of primordial follicles in a single procedure. Also, primordial follicles are more resistant to cryodamage compared with mature oocytes. “Ovarian tissue cryopreservation is ideal for patients who cannot undergo ovulation stimulation necessary for oocyte or embryo freezing as well as prepubertal girls, who are ineligible for oocyte freezing,” pointed out the researchers.

Looking forward, more studies are required to clarify optimal cryopreservation and engraftment protocols. The researchers plan to conduct a randomized controlled trial to directly compare the two cryopreservation methods used in the current study.