A High Incidence of Sperm with Cytoplasmic Droplets Affects the Response to Bicarbonate in Preserved Boar Semen

SIMPLE SUMMARY: The loss of sperm quality during liquid preservation of boar semen may depend on the initial characteristics of the raw semen. Retained cytoplasmic droplets (CD) at the sperm tail caused by incomplete sperm maturation in the epididymis are the most common morphological abnormality in boar semen. The aim of the present study was to examine whether the presence of CD affects the quality of boar sperm during long-term liquid storage. For this, sperm integrity and function were evaluated in semen samples containing different amounts of sperm with CD. It was found that the ability of sperm to perform an essential preparative step for fertilization, named capacitation, was impaired in semen samples rich in CD. Moreover, the enhanced presence of CD caused a higher incidence of sperm destabilization in semen doses preserved for long term (96 h). In contrast, sperm motility and viability were not affected by the presence of CD. The proportion of sperm with CD did not decrease during semen storage in the semen extender. It is concluded that a disturbed maturation of sperm in the epididymis impairs essential steps of fertilization and causes a faster sperm destabilization during in vitro storage. ABSTRACT: Retained cytoplasmic droplets (CD) are the most frequent sperm abnormality in boar semen. A high incidence of CD is associated with subfertility, but the underlaying reasons are not well understood. The storage of extended semen might augment the adverse effects of CD on essential steps towards fertilization, such as capacitation. The aim of this study was to examine whether the enhanced presence of CD in boar semen influences sperm’s response to the capacitation stimulus bicarbonate during long-term semen storage. Extended semen samples (n = 78) from 13 artificial insemination centers were analyzed using a flow cytometric calcium influx assay. Samples with >15% of CD showed a reduced specific response to bicarbonate and a higher non-specific destabilization after storage for 96 h and subsequent incubation at 38 °C in three variants of Tyrode’s medium (p < 0.05). The size of the bicarbonate-responsive sperm population was inversely correlated with the presence of CD-bearing sperm (r = −0.61, p < 0.01). Samples with ≤15% and samples with >15% of CD did not differ in motility or viability and acrosome integrity during semen storage. In conclusion, incomplete epididymal sperm maturation impairs the in vitro capacitation ability and promotes sperm destabilization in stored boar semen.