A protocol for the vitrification of ovarian tissue, as an alternative cryopreservation method to the widely used slow freezing protocol, is presented.
Ovarian tissue cryopreservation (OTC) is an important option for fertility preservation. For patients whose gonadotoxic treatments cannot be postponed or for pre-pubertal girls, it is often the only option for fertility protection. Cryopreservation can be performed either by vitrification or by slow freezing. Slow freezing is currently the standard approach. An increasing number of studies indicate that vitrification can replace slow freezing in the state-of-the-art in vitro fertilization (IVF) laboratories, significantly improving thawing survival rates and simplifying the technical aspects of cryopreservation. A metal grid-based, high-throughput protocol for rapid vitrification of ovarian cortex tissue, suitable for clinical routine, is described. The sterilization of metal grids and liquid nitrogen ensures high quality, meeting good manufacturing practice (GMP) standards. Vitrification was conducted to ensure ultra-rapid cooling rates. Instead of slowly thawing, samples were rapidly warmed. To assess follicular viability, calcein staining was performed both prior to cryopreservation and after rapid warming. The successful application of vitrification and rapid warming using metal grids is reported. No significant differences in follicular viability were observed prior to vitrification and after rapid warming. These results substantiate the high capacity of tissue vitrification for clinical routine applications as a potential substitute for the widely used slow-freezing method.
Cryopreservation of ovarian tissue is an important option for fertility preservation. Explanted tissue containing ovarian follicles, in which oocytes are embedded, is cryopreserved. After storage, the ovarian tissue can be thawed, warmed, and reimplanted in the patient. For viable cells or tissue, two cryopreservation methods are available: slow freezing and vitrification1.
Vitrification is used to preserve biological materials, such as embryos and oocytes, with superior survival rates compared to the slow freezing protocol1,2,3,4. Slow freezing has limitations, such as ice crystal formation, which can potentially damage cell and tissue structures. However, slow freezing is an important cryopreservation approach that facilitates the long-term storage of biological samples, and the functionality of this method has been widely proven5. Vitrification induces a glassy state of aggregation, preventing ice crystal formation6,7. On a technical level, vitrification significantly simplifies the cryopreservation procedure by reducing equipment maintenance, decreasing the likelihood of technical errors, and shortening the duration of the cryopreservation process8,9. In female fertility preservation, ovarian tissue cryopreservation is a decisive approach prior to cancer treatment10. Different groups have successfully demonstrated the concept of cryopreservation, thawing, and transplantation of tissue based on the slow freezing protocol11,12,13,14, which is currently regarded as the standard approach15.
Vitrification of ovarian tissue is regarded as a promising alternative method16,17,18,19,20,21, in terms of resource-saving22, follicular survival rates, DNA fragmentation levels, and balanced angiogenic potential23,24,25,26,27. This is substantiated by successful deliveries in Japan28, the USA29, and Germany30.
Comparing the two options for ovarian tissue cryopreservation (OTC)-vitrification versus the standard procedure of slow freezing-results are partially conflicting in current meta-analyses16. Several factors may have contributed to this, as current vitrification protocols vary greatly. These differences include the choice of cryoprotectant or combination of protectants, their concentration, the composition of the OTC media, the size of tissue fragments, and the device used as a tissue carrier. Accordingly, there is no standardized warming protocol.
As the authors found a method that yields convincing results in terms of handling, viability, apoptosis onset, release of angiogenic factors, and even a report of a birth after reimplantation9,27, a very detailed description of the protocol is provided. The described method offers a valid and effective protocol that may contribute to the standardization of vitrification of ovarian tissue.
Here, a protocol for high-throughput vitrification of human ovarian cortex tissue, suitable for clinical routine, is presented. Similar to the vitrification of oocytes or embryos, the successful application of the procedure requires detailed adherence to the protocol concerning the temperature of the vitrification and warming solutions, as well as the equilibration period. Compliance with EU tissue directives37 regarding air quality and sterility is also essential.
The vitrification procedure results in a non-crystalline, amorphous, or glassy state. Overall, vitrification is a versatile process with significant implications in various scientific and technological domains. The primary benefit of vitrification is its ability to convert tissue into a glassy state, thereby preventing ice crystal formation38,39,40, which can negatively affect tissue integrity and its components.
Supplementation of cryoprotective agents (CPAs) with polyvinylpyrrolidone (PVP) allows for a reduction in CPA concentration without compromising the quality of the vitrification solutions41,42. Furthermore, the use of metal grids provides high thermal conductivity compared to plastic-based carrier systems. The grid structure also facilitates surface adhesion, ensuring the safe and secure cryopreservation of tissue samples and small cortex punches for quality measures. If cryovessels from other manufacturers are used, it is important to test the size of the metal grids beforehand to ensure stability and proper grip within the cryovessel lid, as well as a good fit into the vessel.
Critical steps to ensure successful vitrification include rapid vitrification by immersing the tissue in sterilized liquid nitrogen and performing rapid warming without delay to avoid adverse results. In terms of cost-effectiveness, tissue vitrification is less demanding compared to the slow freezing procedure, which may influence personnel deployment planning. Additionally, vitrification eliminates the need to purchase and service equipment required for slow freezing.
Biological measures and meta-analyses have demonstrated the comparability or even advantages of vitrification compared to slow freezing43. However, differences in results after vitrification may be attributed to the lack of standardization in both the vitrification device and the protocol, including the solutions used, which vary across studies. Future research should explore the potential for follicle culture from vitrified/rapid-warmed tissue to monitor growth in vitro, as successfully demonstrated in mouse ovarian tissue by several groups44,45,46,47,48,49,50.
In summary, vitrification of ovarian tissue is a significant alternative to the widely used slow freezing protocol, supported by five successful deliveries reported by Suzuki51 (Japan), Silber52 (USA), and Sänger53 (Germany). In contrast to commercially available vitrification media and kits for cells, there are few FDA/CE-approved systems for ovarian tissue, which may limit their application in clinical settings. Therefore, the development of FDA/CE-approved kits and media for the vitrification and rapid warming of ovarian tissue is recommended30.
The authors have nothing to disclose.
We thank Cara Färber for proofreading; Katharina Wollersheim, Martin Mahlberg, Lea Korte, and Jasmin Rebholz for their technical assistance.
1.8 mL vials | VWR International GmbH | 479-6837 | |
10 mL serological pipette | Sarstedt | 86.1254.001 | |
4 well plate | Gynemed | GYOOPW-FW04 | |
50 mL Tube | Sarstedt | 62.559.001 | |
6 well plates | Sarstedt | 83.3920 | |
Bacillol AF | Hartmann | 973385 | |
Calcein AM | Merck | 17783 | |
Collagenase type 1A | Merck | C2674 | |
Cryosure DMSO | WAK Chemie | WAK-DMSO-10 | |
Custodiol | Dr. Franz Köhler Chemie | 00867288 | |
DPBS CTS | Gibco Life technologies | A12856-01 | |
ErgoOne pipette aid | Starlab | S7166-0010 | |
Ethylene glycol | Sigma Aldrich | 102466 | |
Euronda sterilization container | euronda | 282021 | |
G-MOPS+ | Vitrolife | 10130 | |
Metal meshes | Sigma Aldrich | S0770 | |
Metzenbaum scissors | world precision instruments | 501262102 | |
N-Bath System | Nterilizer | N-Bath 3.0 | |
Polyvinylpyrrolidone (PVP) | SAGE | ART-4005 | |
Serum substitute supplement (SSS) | Fujifilm Irvine scientific | 99193 | |
Sterile cup | Sarstedt | 75.562.105 | |
Sterile forceps | Carl Roth | KL05.1 | |
Sucrose | Merck | S0389 |
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