Recent advances in the studies on cryopreservation of ovarian tissues and oocytes in fish species

Fish germplasm conservation is crucial for sustainable aquaculture and biodiversity preservation. In contrast to the well-established technique of sperm cryopreservation, the cryopreservation of oocytes remains a major challenge due to their large size, high yolk content, and complex cellular structure. This review systematically synthesizes the current status, persistent challenges, and recent advances in the cryopreservation of fish ovarian tissues and oocytes, with the ultimate aim to provide a theoretical foundation for establishing efficient and standardized cryopreservation protocols. Cryopreservation efficiency for fish ovarian tissues and oocytes is mainly determined by biological characteristics and technical variables. Biologically, the oocyte developmental stage, tissue sample size, and species-specific physiology are crucial. Technically, the composition, concentration, and toxicity of cryoprotectants (CPAs), the choice of cryopreservation protocol, along with the cooling and warming rates, also have significant influence on cellular dehydration, ice crystal formation, survival rate and developmental capacity. The freeze-thaw process and CPAs can induce various cryoinjuries on ovarian cells. These include mechanical damage from ice crystals and membrane lipid phase transition, profound oxidative stress resulting from metabolic disruption and CPA toxicity, as well as a range of cellular and subcellular damage such as disruption of membrane integrity, mitochondrial dysfunction, DNA damage, and chromatin disorganization. To address these challenges, a variety of optimization strategies has been developed. These mainly encompass cellular preconditioning (e.g., membrane lipid modulation, antioxidant application) and targeted mitochondrial protection (e.g., via metabolic inhibition or exogenous supplementation). Concurrently, technological advances focus on ice-crystal control through novel CPAs or ultra-rapid warming, the use of biomaterial-based carriers for enhanced stability, and the adoption of automated platforms like microfluidics to improve standardization and scalability. In conclusion, considerable progress has been made in elucidating cryodamage mechanisms and devising protective strategies for fish ovarian materials. In order to establish a comprehensive fish maternal germplasm biobank, several proposals were advanced, including prioritize studying subcellular cryo-response mechanisms, extending preservation limits, standardizing protocols, and improving in vitro culture systems. Thus, this work would provide the basis for the sustainable development of aquaculture and the conservation of genetic resources in endangered aquatic animals.