Boar sperm: features and storage methods (review)

Swine husbandry is a major and continuously evolving agricultural sector due to the high fecundity, meat productivity, early maturity, and omnivorous nature of the animals. The development of swine husbandry is facilitated in part by artificial insemination which constitutes up to 90 % in most countries (D. Waberski et al., 2019). Artificial insemination contributes to the improvement of fertility, genetic pool and productivity of animals (R.V. Knox, 2016). Therefore, the use of boars with high fertility and cryoresistance of sperm is pertinent. Boar sperm possesses several species-specific characteristics. The average ejaculate volume in boars is 100-150 ml which is due to more developed accessory sex glands compared to other animals (S.G. Smolin, 2023). Ejaculate volume depends on breed, age of the animals, and season. Additionally, there is a relationship between ejaculate volume and sperm morphology. Spermatozoa in smaller ejaculates have longer and wider heads (K. Górski et al., 2017). A significant feature of boar sperm affecting storage is its low resistance to freezing. The low cryoresistance of boar sperm is attributed to high phospholipid concentration and low cholesterol concentration in the sperm plasma membrane (Y. Wang et al., 2021). During cryopreservation, spermatozoa undergo osmotic stress and dehydration, accumulating reactive oxygen and nitrogen species, leading to a sharp decrease in motility and viability after thawing. Therefore, boar sperm is optimally stored at a temperature of 17 to 25 °C for no more than 120 hours (H. Henning et al., 2022). When choosing a protocol (with or without semen fraction), it is necessary to take into account the composition of the semen, as well as the centrifugation and freezing mode. Sperm centrifuged at 2400 rpm for 3 minutes is damaged to a lesser extent that increased its cold resistance (K. Wasilewska et al., 2017). The use of extenders such as Androhep (Minitube, Germany) or Cryoguard (Minitube, Germany) improves the preservation of spermatozoa during cryopreservation (A. Dziekońska et al., 2015; De A. Andrade et al., 2022). To increase sperm viability, pentaisomaltose (O. Simonik et al., 2022), trehalose (R. Athurupana et al., 2015), apegenin (Y. Pei et al., 2018), resveratrol are added (K. Kaeoket et al., 2023). The cooling time before cryopreservation ensures better sperm preservation. The established protocols of sperm cooling before freezing are 2-4 hours at 5 °C (J. Shäfer et al., 2017) and 24 hours at 17 °C (M.A. Torres et al., 2019). The success of cryopreservation is largely determined by the freezing mode which can be manual or automated. The optimal protocols for manual freezing are to place sperm straws horizontally 4 cm above the liquid nitrogen level for 20 minutes (V. Khophloiklang et al., 2023), followed by immersion in liquid nitrogen, or at 2 and 5 cm for 15 and 20 minutes, respectively (S. Bang et al., 2022; S.L. Soares et al., 2020). The use of automatic sperm freezing reduces the level of free radicals and lipid peroxidation, which leads to a better preservation of cell motility after thawing. The optimal protocol for automatic sperm freezing, according to research, is to lower the temperature from 5 to -5 °C, and then -39.80 °C·min-1 for 113 seconds. The sperm is then held at -80 °C for 30 seconds and then frozen at -60 °C for 70 seconds to eventually drop to -150 °C. Current protocols for thawing boar semen are as follows: 60 °C for 8 s (Z. Zhu et al., 2022), 70 °C for 8 s (R.A. Gonzales-Castro et al., 2022), 37.5 °C for 25 s (R. Osman et al., 2023). To reduce levels of apoptosis and reactive oxygen species, it is recommended to store thawed spermatozoa at 17 °C (J. Li et al., 2023). Thus, given the species characteristics of boar sperm, strict adherence to protocols for pre-preparation and deep freezing of sperm is necessary. A proper protocol of thawing, followed by a short-term storage will also increase the effectiveness of artificial insemination with cryopreserved sperm.