Is glycerol a good cryoprotectant for sperm cells? New exploration of its toxicity using avian model

Glycerol is a cryoprotectant used widely for the cryopreservation of animal sperm, but it is linked to a decrease in fertility. The mechanism underlying the negative effects of glycerol remains unclear. Therefore, in this study, we aimed to gain a better understanding by using the chicken model. First, we investigated the impact of increasing the concentration of glycerol during insemination on hen fertility. Our findings revealed that 2% glycerol resulted in partial infertility, while 6% glycerol led to complete infertility. Subsequently, we examined the ability of sperm to colonize sperm storage tubules (SST) during in vivo insemination and in vitro incubation. The sperm used in the experiment were stained with Hoechst and contained 0, 2, or 6% glycerol. Furthermore, we conducted perivitelline membrane lysis tests and investigated sperm motility, mitochondrial function, ATP concentration, membrane integrity, and apoptosis after 60 min of incubation with different glycerol concentrations (0%, 1%, 2%, 6%, and 11%) at two temperatures to simulate pre-freezing (4 °C) and post-insemination (41 °C) conditions. Whereas 2% glycerol significantly reduced 50% of sperm containing SST, 6% glycerol completely inhibited SST colonization in vivo. On the other hand, in vitro incubation of sperm with SST revealed no effect of 2% glycerol, and 6% glycerol showed only a 17% reduction in sperm-filled SST. Moreover, glycerol reduced sperm-egg penetration rates and also affected sperm motility, bioenergetic metabolism, and cell death at 4 °C. These effects were observed when the concentration of glycerol exceeded 6%. Furthermore, at 41 °C, glycerol caused even greater damage, particularly in terms of reducing sperm motility. These data altogether reveal important effects of glycerol on sperm biology, sperm migration, SST colonization, and oocyte penetration. This suggests that glycerol plays a role in reducing fertility and presents opportunities for improving sperm cryopreservation.