Incubation of boar spermatozoa in viscous media by addition of methylcellulose improves sperm quality and penetration rates during in vitro fertilization

This work was designed to study whether viscous media can improve the in vitro sperm functionality in pigs by using methylcellulose as a thickener. Viscosity of porcine oviductal fluid (POF) was compared with culture medium (Tyrode's) supplemented with methylcellulose (MET 0, 0.5 and 1% w/v). Spermatozoa were incubated in the different media (0, 1 and 2 h) and sperm motion parameters, lipid membrane disorder, plasma membrane integrity and reactive oxygen species (ROS) formation were assessed. Fertilization results were assessed i) preincubating spermatozoa in the viscous media followed by gamete coculture in a non-viscous medium; and ii) gamete coculture in the viscous media. Viscosity of POF from early luteal phase was higher than late follicular phase. Medium without methylcellulose presented constant viscosity with increased shear rate, while viscosity of the POF and media with methylcellulose was reduced by increased shear rates. Methylcellulose improved sperm linearity, straightness and the proportion of fast-linear spermatozoa. Moreover, methylcellulose increased the rate of viable spermatozoa with intact acrosome and low lipid disorder, reducing the ROS generation. Preincubation in viscous media increased the penetration rate and the mean number of spermatozoa bound to the zona pellucida (both with 0.5 and 1% MET) and reduced monospermy with 1% MET. On the other hand fertilization in the viscous media reduced penetration rate and increased monospermy. The efficiency of the IVF system was not improved with the use of viscous media. The results show the relevance of increasing viscosity thus making the in vitro media more comparable to physiological conditions.